Towed Side-Scan SONAR System Operator's Manual V1.1.0 Procedures for using and maintaining side-scan SONAR towed systems to collect, process, view, and analyze data

Part Number

SSS-2TOM-0001

This product was designed and developed by a team of engineers at Marine Sonic Technology, Ltd. © 2011 Marine Sonic Technology, Ltd., All Rights Reserved. Marine Sonic Technology, Ltd. 5508 George Washington Memorial Highway P.O. Box 730, White Marsh, VA 23183-0730 (804) 693-9602 (800) 447-4804

Technical Support For technical support call (800) 447-4804 or visit our web site at http://www.marinesonic.com.

Copyright This manual and the software described in it are copyrighted with all rights reserved. Under the copyright laws, neither this manual nor the software may be copied, in whole or in part, without the written consent of Marine Sonic Technology, Ltd., except in the normal use of the software or to make backup copies. This exception does not allow copies to be made for others.

Limitations on Warranty & Liability Marine Sonic Technology, Ltd. warrants that the disk(s) on which this software is recorded is free from defects in materials and workmanship under normal use for 90 (ninety) days after the date of original purchase. Please refer to this product's Warranty for further information concerning the limitations on warranty and liability of this product and its associated software.

Trademarks

Contents Introduction

1

Overview

................................................................................................................................... 1

Marine Sonic Technology, .......................................................................................................................................................... Ltd. Products and Services The Nature of This .......................................................................................................................................................... Manual The Sections of .......................................................................................................................................................... This Manual Contact Information ..........................................................................................................................................................

1 2 2 3

Other References ................................................................................................................................... for Related Information 3 Advisories

................................................................................................................................... 4

Planning .......................................................................................................................................................... Advance Preparation ......................................................................................................................................................... Unpacking ......................................................................................................................................................... Cleaning Procedures .........................................................................................................................................................

6 6 6 7

Notices, Patches, ................................................................................................................................... and Updates 7 Description of................................................................................................................................... a Software-Based Towed System 8 SONAR Software .......................................................................................................................................................... Capabilities 8 Understanding .......................................................................................................................................................... SONAR 10 Side-Scan Systems .......................................................................................................................................................... 10

Towed-System ................................................................................................................................... Components 14 The Topside Box .......................................................................................................................................................... Topside Processor .......................................................................................................................................................... (Computer) Tow Cable .......................................................................................................................................................... Navigational Aid .......................................................................................................................................................... (GPS) Power Source.......................................................................................................................................................... Towfish .......................................................................................................................................................... Peripherals .......................................................................................................................................................... The Depressor ......................................................................................................................................................... Vane

Operational Procedures

15 15 15 16 17 17 19 19

21

Boat Handling ................................................................................................................................... 21 Boat Tow Points ................................................................................................................................... 23 Cable Handling ................................................................................................................................... 24

Search Methodology

29

Site/Feature................................................................................................................................... Location 29 Safety

................................................................................................................................... 29

Operation ................................................................................................................................... 30 Site/Feature................................................................................................................................... Identification 31

System Features

32

Contents

User Settings ................................................................................................................................... 32 SONAR Interface .......................................................................................................................................................... SONAR Display .......................................................................................................................................................... Color Map .......................................................................................................................................................... Inverting the ......................................................................................................................................................... Color Map Moving through .......................................................................................................................................................... the Data Scrolling ......................................................................................................................................................... Channel ......................................................................................................................................................... Measurements .......................................................................................................................................................... Height ......................................................................................................................................................... Length ......................................................................................................................................................... Area .........................................................................................................................................................

32 32 32 33 34 34 34 34 34 35 36

SONAR Control ................................................................................................................................... 37 Power .......................................................................................................................................................... Range Delay .......................................................................................................................................................... Frequency .......................................................................................................................................................... Speed Control..........................................................................................................................................................

37 37 38 38

Image Adjustment ................................................................................................................................... 39 Gain

..........................................................................................................................................................

39

Navigation and ................................................................................................................................... Fathometer Interface 40 Markers and................................................................................................................................... Waypoints 41 Markers Waypoints

.......................................................................................................................................................... ..........................................................................................................................................................

41 41

Managing a ................................................................................................................................... Survey 42 Survey Folder.......................................................................................................................................................... Organization

Image Interpretation Tips Overview

42

43

................................................................................................................................... 43

Key Approaches ................................................................................................................................... to Locating Objects 43 Shadows Size Shape

.......................................................................................................................................................... .......................................................................................................................................................... ..........................................................................................................................................................

43 44 44

Conditions That ................................................................................................................................... Complicate Interpretation 45 Ghosting .......................................................................................................................................................... Crosstalk .......................................................................................................................................................... Thermoclines.......................................................................................................................................................... and Haloclines Distortion during .......................................................................................................................................................... Turns Surface Scattering .......................................................................................................................................................... Propeller Wash .......................................................................................................................................................... Noise ..........................................................................................................................................................

45 45 46 47 48 49 50

Tools and Equipment Needed for Hookup and Maintenance

52

Revision History

53

Contents

Glossary

54

Index

61

Contents

Introduction Overview Marine Sonic Technology, Ltd. Products and Services Marine Sonic Technology, Ltd. specializes in small, compact, and rugged High-Resolution SONAR systems. Our products set the industry standard for their high resolution, portability, self-contained design, and low cost. They have been successfully used in operations for search and recovery of— Drowning Victims Vehicles Sunken Boats Equipment Weapons (Rifles, Pistols) Homeland Security Ship Hull Inspections Bridge Inspections Force Protection (Harbor and Ship security, Mine Detection) Environmental Issues (Location of debris sites, hazardous waste) Locating Old-Growth Timber Insurance Fraud Surveying (Pipe lines, wharfs, piers) Artificial Reefs (Construction and Monitoring) Marine Research (Monitoring of fish populations, oyster reefs, bottom structure, biologics in the water) Sea Scan® Survey is a user-friendly PC-based SONAR data collection program for Windows™ operating systems. Sea Scan® Survey seamlessly communicates with the Sea Scan® High-Definition SONAR (HDS) towed system hardware. Visit www.marinesonic.com to download Sea Scan® Survey and experience its capabilities and ease of use. For information on our full product line, examples of images captured using Sea Scan® HDS, or testimonials from satisfied clients, please see our website at www.marinesonic.com; contact Marine Sonic Technology, Ltd. customer service at (804) 693-9602; or use our toll-free number, (800) 447-4804.

Introduction

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The Nature of This Manual Welcome to the Marine Sonic Technology, Ltd. operator’s manual for towed side-scan SONAR systems. This manual introduces first-time users, operators, and technicians to the basics of Marine Sonic Technology, Ltd. towed side-scan SONAR systems. The operator is assumed to have little or no experience with towed SONAR systems and software; therefore, the discussion is restricted to general guidelines and overviews. For more detail or specific assembly, maintenance, or trouble-shooting instructions, please refer to the operator’s manuals that accompany the products you are using. The material in this manual is organized into discrete sections for easy acquisition of the basics about how to perform a survey, collect data quickly, and interpret situational anomalies that can present themselves in the data. A reference guide to abbreviations, acronyms, and symbols commonly used in SONAR discussions is in the Glossary. As an added feature, look for italicized words in the text, as these terms are defined in the Glossary. More detailed information and discussions can be found at the Marine Sonic Technology, Ltd. website, www.marinesonic.com. The Sections of This Manual This manual is organized into the following sections: n Introduction n Description n Operational Procedures n Features n Image Interpretation Tips n Tools and Equipment n Revision History n Glossary of Acronyms and Terms n Index

Introduction

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Contact Information Marine Sonic Technology, Ltd. towed systems are thoroughly checked before shipping. Any alteration of this product after shipment automatically voids the warranty. In case of question regarding these issues, contact Marine Sonic Technology, Ltd. at the following address: Marine Sonic Technology, Ltd. 5508 George Washington Memorial Highway or P.O. Box 730, White Marsh, VA 23183-0730 For technical support: Phone: (804) 693-9602 Toll Free: (800) 447-4804 Web: www.marinesonic.com Customer Support at Marine Sonic Technology, Ltd. also welcomes your questions, comments, and corrections about this manual. Please send your communication to the above address.

Other References for Related Information A wealth of information is available that presents the concepts and practical information necessary to conduct a well-planned and executed maritime search-and-recovery operation using side-scan SONAR. Among all the available material, Marine Sonic Technology, Ltd. highly recommends the following documents as invaluable reference material for the library of any sincere and dedicated individuals wishing to understand and work with side-scan systems. Introduction to Sea Scan® Software A thorough explanation of the functions, features, and use of the Sea Scan software can be found in the Sea Scan Software Manual, Version 2.0.0, produced and distributed by Marine Sonic Technology, Ltd. Please see that document for how to use the Sea Scan® Survey PC-based software for side-scan SONAR data collection, processing, viewing, and analysis.

Introduction

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Chapman Piloting & Seamanship, 64th Edition This reference volume by Elbert S. Maloney is a comprehensive guide to small boat handling. It contains detailed explanations of every aspect of safe boat-handling procedures for the novice or experienced boater. The Coast Guard Auxiliary uses this book as a reference tool for their courses, along with applicable Coast Guard publications. It is widely available commercially. The American Practical Navigator, an Epitome of Navigation This voluminous text by Nathaniel Bowditch and the National Imagery and Mapping Agency details all aspects of boat navigation. It has been updated repeatedly since originally published in 1802; the 1995 edition includes the latest advances in electronic navigation and digital charting technology. It also covers non-electronic navigation such as celestial, plotting, and dead reckoning. The book contains numerous tables which have been valued for years by practicing navigators. This invaluable reference book is carried on the bridge of every U.S. Navy ship. It is widely available commercially. Sound Underwater Images, A Guide to the Generation and Interpretation of Side Scan SONAR Data For in-depth explanations of SONAR and side-scan systems, this book by John P. Fish and H. Arnold Carr is invaluable. The novice will come to understand the technology and experienced users will appreciate the depth of coverage. This spiral-bound text may have to be special ordered. Black Laser Learning, Second Edition, Not in the Manual Guide® To Side Scan Sonar Image Interpretation More than 90 minutes of critical SONAR training information is presented on DVD. This highly recommended guide covers every aspect of side-scan SONAR image creation and interpretation. It includes dozens of detailed examples, as well as training animations to explain hard-to-understand concepts.

Advisories WARNING

Observe standard safety precautions and wear proper safety gear to prevent personal injury during installations.

CAUTION

Turn off power before disconnecting any component from cables. Disconnecting the component without turning off power may cause a sudden over current or voltage condition that can damage the components.

Introduction

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CAUTION

Electro-static-sensitive devices can be damaged by excessive levels of voltage and/or current. To protect these devices, bring the device and everything that comes in contact with it to ground potential by providing a conductive surface and discharge paths.

THE FOLLOWING PRECAUTIONS MUST BE FOLLOWED: Keep the computer case and topside box as dry as possible. If either one gets wet, thoroughly dry all surfaces, crevices, etc. NEVER attach the connectors when the power is on. NEVER PLUG OR UNPLUG THE TOW CABLE FROM THE TOPSIDE BOX WHEN THE POWER IS ON. DOING SO MAY RESULT IN DAMAGE TO THE TOWFISH AND/OR THE TOPSIDE ELECTRONICS. Ensure that the connectors are CLEAN AND DRY before they are mated to one another. New tow cables should be uncoiled and all twists removed from the line before wrapping it into a “figure eight” that is approximately 0.9 m (3 ft) wide on each side. If used directly from the coil, the cable has a strong tendency to develop small loops, which can damage the cable under strain. Take care of the tow cable. Do not step on it, roll heavy equipment over it, or press it between hatch covers, as this may damage the cable. It is strongly advised that the cable be wound onto a cable reel and retained there when not in use. Otherwise, stow the tow cable by coiling it loosely and replace it in the cable box that was provided. If the foregoing is not practical, coil it loosely and hang it off the floor. Do not bend the tow cable beyond its minimum (12.7-cm or 5-inch) bend radius limit. Internal damage is likely if the bend radius limit is exceeded. If the bend radius limit is exceeded while the cable is under load, cable strength will be

Introduction

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adversely affected. Check the tow cable for cuts and abrasions from damage to the insulation any time you pay out or bring in the cable. Run your fingers along the length of the cable to feel for breaks in the shielding and punctures through the jacket. Clean the tow cable after each use. If the system has been used in salt water, wash the towfish and tow cable in fresh water while still connected to prevent corrosion of the components. Thoroughly dry all components, then stow them in their packing cases in between uses. Planning Advance Preparation Review industry-established practices and procedures for installation of electronic equipment before handling the Marine Sonic Technology, Ltd. towed system. Those practices include reviewing the procedures in this manual; carefully choosing the equipment locations aboard the vessel; checking for adequate space around connectors, cabling, and the towfish; and ensuring that the target location is free of fluids such as water, salt spray, or lubricants. Unpacking The Marine Sonic Technology, Ltd. towed system comes packed in rugged shipping cases. These cases contain the following: The topside box and accessories Tow cables and depressor vane The towfish Remove the topside box, tow cable, towfish, and cable accessories from the shipping cases. Check the packing slip to ensure all components are included. Check each item for external damage. Retain the packing cases for equipment storage when not in use and for returning an item to Marine Sonic Technology, Ltd. for repair or replacement.

Introduction

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Store the packing cases in a safe, dry location. Cleaning Procedures Soak the connectors in distilled water or alcohol for 5 minutes or more if the contacts have been exposed to salt water. Blow the connectors clear and dry with compressed air. If compressed air is unavailable, employ commercially available canned air. Use a water-repellant lubricant such as WD-40™ to reduce friction and expel moisture from connectors. Check the o-rings when hooking up or packing away the equipment to ensure they remain dry and well seated. Wipe down the tow cable to dry it. If it has been used in saltwater, wash the tow cable in fresh water and dry it completely to prevent corrosion of the components. To clean and dry the towfish, wash it with fresh water and dry all connectors. Re-attach all protective connector caps before stowing the equipment. Wipe down the exterior of the topside equipment (computer, topside box) after each use. Check the interior of the topside equipment (computer, topside box) for any splashes or pooled water. Carefully dry any moisture with either compressed air or a soft cloth.

Notices, Patches, and Updates Marine Sonic Technology, Ltd. engineers continually review and improve upon the company’s hardware, firmware, and software procedures and capabilities. Revisions and updates may be frequent and can significantly streamline operation. Contact Marine Sonic Technology, Ltd. Customer Service to inquire about updates, patches, and notices.

Introduction

7

Description of a Software-Based Towed System SONAR Software Capabilities Software-based side-scan SONAR systems like Sea Scan® HDS use a computer for data display, system control, and data storage. The software allows an operator to control the SONAR data-collection process, view, analyze, and save the SONAR image with all related navigational information. The program also features a Navigation Plotter to plot location and estimated acoustic coverage. The basic function of the software is to display the SONAR image on the screen. Each time the SONAR transducer pings, the reflection data is recorded and incremented as horizontal lines on a waterfall display screen. As the towfish passes over the seafloor, it continuously transmits pings perpendicular to the direction the boat is traveling, which are reflected back to the towfish transducers by objects on the bottom and in the water column. These pings fan out in a cone shape that would appear very narrow if viewed vertically and ever widening if viewed horizontally. For a typical system, this might be a 0.5º horizontal beam and a 40º vertical beam. The seafloor image builds as the reflection data is added line by line. The data is recorded directly to a streaming data file. The application then reads and indexes the streaming data file in real time. After the data is indexed, it is displayed in the waterfall display. Only a small segment of data can be viewed in the SONAR waterfall at any one time because of the practical limit of the computer to keep up with the incoming data. The Marine Sonic Technology, Ltd. Sea Scan® Survey software features a wide-dynamic-range data-collection system. The time-varied gain is software controllable and can be set during data collection and/or during post-processing. Also, Sea Scan® Survey is compatible with Microsoft Windows™ operating systems. Figure 1 graphically shows the components that make up a software-based SONAR system.

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· Figure 1. The basic components of a Sea Scan® system

Figure 2 shows the side-scan SONAR in operation. The operator can view wide tracts of the seafloor because the transducers ping along the swath width and the software records the strength of the echoes from the sea bottom. The towfish is towed just above the seafloor bottom. The sound pulses pass through the water but are reflected from the seafloor and objects, such as wreck sites that sit on the seafloor. The computer records the echo signal strengths as they return and draws the entire SONAR record line on the screen. Thus, an image is built, line by line, as the SONAR record line from each ping returns and is drawn on the screen.

Introduction

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· Figure 2. Basic Side-Scan SONAR Operation

Understanding SONAR SONAR is a coined word derived from the phrase, SOund Navigation And R anging. SONAR generally refers to the system that uses acoustic (sound) energy transmitted through water. At the heart of that system is the transducer, the device that converts electrical energy to sound and is responsible for generating the sound pulse. That same transducer is also used to receive the echo that reflects off objects encountered along its path. Thus, SONAR is a system that determines the position of unseen underwater objects by transmitting sound waves and measuring the time it takes for their echo to return after hitting the object. Side-Scan Systems Because of their innate flexibility, side-scan systems can be used in many applications, some of which involve highly sophisticated remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), or unmanned surface systems. But the most common side-scan systems are towed behind a surface vessel and comprise three elements: n the control unit containing the software n the towfish with transducers mounted on each side n the tow cable that connects the towfish to the surface vessel, which follows

Introduction

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a prescribed track or course through the water Side-scan SONAR is used extensively for many commercial, military, and leisure applications. Some examples include search-and-recovery operations, homeland security, law enforcement, pipeline and cable-route surveys, mine detection, fish finding, shipwreck and treasure hunting, marine archeology, and geological surveys. The transducer assembly, also known as the towfish, is towed on a straight heading and at a constant depth through the water. As it is towed, the assembly emits sound pulses at precise and regulated intervals. The system receives the returning echoes from the water column and seafloor shortly after emitting a pulse. This continues for a short period until the next pulse is transmitted, thus beginning a new cycle. The returning echoes from one pulse are displayed on the SONAR window as one single line, with dark and light portions of that line representing weak or strong echoes. The stronger the SONAR’s returning signal, the brighter the mark that appears on the SONAR window or waterfall. The resulting accumulated lines then form a coherent picture of the seafloor. The two transducers provide information unique to their particular side of the towfish. In between them is the boat’s track immediately below the towfish. That center display or water column loosely relates to the boat’s track. It communicates details about objects encountered before the first bottom return and can include things such as surface returns, debris, fish, and objects protruding from the seafloor. The quality of the SONAR data will depend on operator management and the data-gathering process, which involves vessel course, tow speed, towfish altitude above the bottom, sea conditions, and range settings. For example, a higher range setting yields a larger data sample with more of the seafloor displayed; but the data will not have as high a resolution. Figure 3 provides an indication of the objects a towfish can reveal on a seafloor. Figures 4–6 typify actual objects observed with the Marine Sonic Technology, SONAR systems.

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Figure 3. Side-scan operation provides insight into features or objects on the seafloor (figure courtesy of the USGS)

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· Figure 4. Sample view of a Sea Scan® Survey waterfall, here showing a shipwreck site

· Figure 5. Two upside-down submerged cars (identified with Sea Scan® PC)

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· Figure 6. Navy PB4Y-2 Privateer Aircraft (identified with Sea Scan® PC)

Towed-System Components The minimum components required for an operation include— A topside unit A topside processor (computer) A tow cable A navigation aid (GPS) A towfish A power source Peripherals

Marine Sonic Technology, Ltd. manufactures and sells Sea Scan® HDS, a complete system that can be customized to meet varying conditions or requirements. The complete system comprises all the components discussed here plus other options or configurations.

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The Topside Box The Marine Sonic Technology, Ltd. topside unit is a ruggedized box that communicates between the towfish, power unit, and topside computer running a side-scan SONAR program such as Sea Scan® Survey. It can be mounted on a bulkhead, if desired. Topside Processor (Computer) A ruggedized rack mount or laptop computer is needed to run a side-scan SONAR program such as Sea Scan® Survey to collect, display, process, and store the SONAR data during and after an operation. The laptop or personal computer will significantly streamline data acquisition and assessment during an operation when placed strategically for optimal operator viewing and guidance to the vessel’s pilot. The computer must be linked to the topside box by means of a USB cable. Once connected, the full capacity of the software is at hand during an operation and intra-communication among the attached devices is seamless and transparent. Any such unit requires the following capacity to run the Sea Scan® Survey or similar software programs: Intel-based Windows™ Operating System

XP, Client , Client 2003, Vista, Windows 7

Hard drive space

100 MB + data storage

Available RAM

512 MB minimum; 1 GB or more recommended

Processor Speed

1.6 GHz or better

Other Features

USB 2.0 port

Screen Resolution

1024 x 768 minimum or higher

Tow Cable The Marine Sonic Technology, Ltd. standard tow cables are 328 ft (100 m) and 98.4 ft (30 m) long (custom cable lengths are available upon request). They feature flexible sheathing that protects the twisted-pair internal wiring and have a 750-lb. safe working load.

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Navigational Aid (GPS) Every search-and-recovery operation should have at least one Global Positioning System (GPS) unit onboard at all times. A GPS uses the Global Navigation Satellite System (GNSS) developed by the United States Department of Defense. It uses a constellation of 24 to 32 medium-Earth-orbit satellites that transmit precise microwave signals; these signals enable GPS receivers to determine their current location, the time, and their velocity (including direction). An enhancement to the basic GPS signal, known as Differential GPS (DGPS), provides much higher precision and increased safety in its coverage areas for maritime operations. Many nations use DGPS for operations such as buoy positioning, sweeping, and dredging. This enhancement also improves harbor navigation. Differential GPS uses beacons on the shore as guides to give corrected position information. Another approach at automatic correction is the Wide-Area Augmentation System (WAAS). This system was developed for commercial airlines flying over land, thus was limited to terrestrial use. This system is now available beyond the airline industry and can be used in terrestrial waters. A third approach is with satellite correction. A private subscription service uses geostationary satellites to transmit correction signals to receivers on the ground. Base stations constantly communicate with available GPS satellites and calculate correction values. The base stations also correct for atmospheric interference locally in their immediate territory. The stations then transfer the correction data to a network control center, which checks the data, compresses it, and relays it on to the company’s geostationary satellite. Access to this method for accurate positioning requires a prepaid contractual arrangement. Experiment in advance with the device to determine the best placement of the GPS for optimal communication with overhead satellites. The accuracy of the data from the GPS will greatly influence the data-collection process, especially if sea conditions are rough and/or you are unable to establish land-based line-of-sight reference points to corroborate site locations.

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Marine Sonic Technology, Ltd. recommends highly that a GPS unit be used at all times during a survey operation. The topside unit and side-scan SONAR software programs can operate without a GPS, but the side-scan software will not be able to monitor speed over ground, which is important for ensuring optimal data collection; further, locations of sites may not be accurate enough for purposes of documentation, retrieval, or return visits. Because GPS accuracy is critical during an operation, Sea Scan® HDS indicates the quality of the incoming GPS signal. This information on the accuracy of the signal is presented at the bottom of the waterfall display throughout an operation. Power Source The computer, topside box, and towfish all require power to operate. Generally, this power is in the form of a battery unit to which you connect the components. Marine Sonic Technology, Ltd. strongly recommends that a separate power source be dedicated to operating the Sea Scan® HDS system and equipment during an operation. The use of a dedicated power source protects the boat or vessel batteries from being unduly taxed or even depleted during a survey. Follow the recommendations that accompany the Sea Scan® HDS system as to battery requirements and output. Towfish Marine Sonic Technology, Ltd. designs and configures towfish to suit many situations or needs. They can be simple units with built-in transducers or sophisticated models with multiple-frequency transducers, internal weighted keels, temperature sensors, and depth gauges. These units are towed near the bottom of a seabed while their transducers transmit and receive the returning pings. The strength of the returning signals is then interpreted by the side-scan control software. A towfish is a cylinder with nose cone and fin assembly. Some towfish designs are modular so that they can be reconfigured easily to replace transducers, insert weights, or add multiple transducers for different angles or resolutions. Generally, there will be a tow rail at the top as well as a cable connector and a shear pin release for connecting a snag release line at the shackle. A variable-angle bracket may be on some towfish designs to allow the transducer angle to be altered (useful for viewing atypical areas such as the underside of a bridge or vessel rather than the sea floor).

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A towfish may have a safety release at the top of the tow rail behind the handle that is a secondary tow cable connection point. If the towfish were to become entangled on the seafloor and the force against the tow cable became excessive, this release mechanism would engage; this action would shift the towpoint, causing the towfish to flip, nose over, which generally frees it. The fin assembly may have a similar safety release. The shear bolts on the assembly break if the towfish were to snag hard at its fin. Standard transducers are mounted on each side of the towfish. These transducers produce a very specifically defined acoustic signal. Viewed from above, the signal is very narrow; viewed from the side, the signal is wide. This shape of acoustic sound allows the transducers to view a very narrow section perpendicular to their paths of motion. As the out-going acoustic signal travels through the water, the signal strength at the wave front weakens by a variety of influences, such as absorption by the water, wave-front spreading, and scattering. These are known physical effects of acoustic energy traveling through a “lossy” medium. As a result, the amount of energy available to reflect from an object reduces as the outgoing acoustic wave travels away from the source. That is, the reflection from a distant object is not as strong as that from a like object closer to the transducer (source of the acoustic wave). Depending upon the software selected, some programs compensate for ping rate versus Speed over Ground (SOG). For example, the Sea Scan® Survey software maintains a constant geo-referenced 1:1 aspect ratio for the SONAR image. This is possible by setting the ping rate based on the current range and the SOG. The spacing of each vertical line on the screen is equivalent to a known distance for each of the ranges. Sea Scan® Survey sets the transducers to ping at these interval distances, so the resulting image has a 1:1 aspect ratio. The time between each of these known distance intervals depends on the SOG. The time interval is set based on current speed. Depending on the operating requirements and towfish configuration, Marine Sonic Technology, Ltd. can configure the towfish with low- and/or high-frequency transducers. The low-frequency transducer provides a lower axial resolution but the pings travel farther than from the high-frequency transducer. Likewise, although not able to extend as far, the high-frequency transducer will provide very high-resolution imaging at closer ranges. The following table can serve as a guide for basic starting range and frequency for different seafloor topologies.

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· Table 1. Suggested Frequency and Range Settings for Coverage of a Search Area (all ranges listed in meters/feet) Frequency (kHz)

Average Maximum Range

Target Size

150

400 / 1312

10 / 33

300

200 / 656

5 / 16

600

75 / 246

1.5 / 5

900

40 / 131

1.2 / 4

1200

25 / 82

0.5 / 1.6

1800

15 / 49

0.3 / 1.0

All maximum ranges are those detectable by the SONAR system at the listed frequency. These are typical but conservative maximum ranges that are attainable with the particular SONAR frequency in salt water with a soft, silt bottom. Longer ranges may be attainable in fresh water or harder bottoms such as sand or gravel. Some factors that affect the maximum range of the SONAR are bottom type, water salinity, water depth, water temperature, particulates, electromagnetic noise, and acoustic noise. The target size is the smallest target size that will be identifiable at that range and frequency of SONAR. Much smaller objects can easily be detected; however, targets smaller than those listed will be more difficult to identify. Peripherals Marine Sonic Technology, Ltd. ships supporting equipment and materials to support the use and operation of each unit or component. These peripherals may comprise extra pigtails, tie wraps, spanners, or other relevant supplies.

The Depressor Vane A depressor vane can be useful in situations when the towfish lifts while being towed or when the operator wants to achieve deeper depths without having to

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attach weights. The depressor vane attaches to the tow cable, sometimes at the towfish and sometimes approximately 32.8 to 65 m (10 to 20 ft) above the towfish . Third-party depressor vanes are available. Contact Marine Sonic Technology, Ltd. for recommendations.

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Operational Procedures This chapter presents tips and guidelines for an individual trained by Marine Sonic Technology, Ltd. personnel. Basic explanation is provided for operating the Standard Topside Unit. This information is coupled with rudimentary procedures for equipment handling, site identification, and data interpretation. For an in-depth explanation of how to use your SONAR-interpretation software, please see the software manual that accompanies that software. In addition, for some suggested readings pertaining to related aspects of boat operation and SONAR functions, please see the section, Other References for Related Information.

Boat Handling A myriad of resources are available that explain every aspect of safe boat operation and handling. It is important to review such texts beforehand to ensure safe boat operation as well as how to deal with current, drift, drag, towing procedures, and the correlation between boat speed and helm control. Those issues are out of the scope of this document. For purposes of this manual, it is assumed that the operator understands the basic rules of the road, rudimentary boat-handling techniques, and the unique characteristics of the vessel that will be used. With that background of understanding in mind, an operator should consider the following practices when towing the side-scan SONAR system.

Before beginning any operation, ensure that the line of communication between the vessel’s pilot, the SONAR operator, and all personnel will be unrestricted and clear. Before deploying the towfish, consider running the boat around the area with the depth sounder on to define the perimeter of the area to be examined and to get a sense of the bottom configuration. As you do so, set waypoints for the perimeter in the navigation plotter of the software you are using or mark the latitude and longitude

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of landmarks on a chart. Then reduce boat speed, deploy the towfish, and proceed with the operation. By defining the boundaries in advance, you can now stay within the specific area of interest. When evaluating an area of interest during towfish deployment, a slow boat speed may be required. Maintain as slow and steady a speed as possible that ensures forward motion and helm control. Remember, crew and vessel safety is paramount in any operation. The shallower the water, the shorter amount of tow cable that’s paid out; the result is a correlation between the boat and the towfish. As a result, if the vessel is bouncing in heavy seas, the cable will jerk also and may cause towfish movement, thereby adversely affecting image quality. In addition, the towfish might be adversely affected by the towing vessel’s propeller wash in very shallow water. In this instance, it might be advisable to tow from amidships or the bow, thereby reducing jerks on the cable while keeping the towfish away from the propeller wash.

Follow a steady course over the area being surveyed while maintaining overlapping, parallel lanes to ensure that the area of interest is well covered. Determine the lane spacing by the requirement of the operation, but make sure the lane spacing is less than the SONAR swath width to ensure overlap.

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Avoid excessive electronic interference, engine noise, or propeller wash, as these can adversely affect image quality. Cross-currents or debris such as seaweed snagged onto the towfish body can cause the towfish to roll and/or yaw and result in distorted data. When turning, the towfish tends to drop in the water column. Make smooth, wide turns while the towfish is deployed to avoid hitting the towfish on the bottom and possibly causing damage or outright loss of the towfish. When doing a search pattern, it is best to disregard the data during turns and plan accordingly to get those areas on other passes when the boat is going straight and true.

Try to maintain a straight, steady course during the search operation, as even small navigational corrections can adversely affect the SONAR data. When corrections need to be made, the SONAR operator and boat pilot should communicate constantly to maintain constant awareness of the towfish location and altitude. The pilot should be aware of the towfish layback and wait an appropriate amount of time after passing the target before making course adjustments. Avoid backing when the tow cable is deployed, as the cable could become ensnared around the rotating propeller shaft, thereby jeopardizing boat safety; or the propellers could sever the tow cable, causing the towfish to be lost.

Boat Tow Points The individual vessel design must be taken into consideration when determining the best onboard location from which to deploy the towfish and attach the cable. If a cable winch is permanently affixed to the vessel, that hardware location (generally at the bow or stern) generally will dictate the deployment location. However, alternate locations can include an outrigger or mid-vessel arrangement. Some of the following considerations can help in selecting the best boat tow point: Consider the layout on deck; it is important to avoid interference from boat hardware or equipment that could snag lines or impede safe operation. Ensure that a hard point (cleat, stanchion, reinforced handle, etc.) is nearby for

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attaching and securing lines and that an area on deck can be cleared for the towfish, cable, and other equipment. Evaluate the noise and vibration from the boat engines when choosing a tow point and whether that noise could be conducted down the tow cable. Note the location and angle of the propeller(s). An offset propeller can generate significant “prop wash” that interferes with the transducers if the cable is deployed off that side. Avoid towing the towfish close to the boat hull because the displacement of water from the hull and propeller turbulence could adversely affect towfish stability.

Cable Handling At least one individual in addition to the vessel’s pilot generally is required to oversee a towing operation because full-time vigilance and caution are required any time a vessel tows an object. For purposes of this manual, it is assumed that the cable handler understands the operation of towing equipment as well as cable-handling techniques. With that background of understanding in mind, a cable handler must consider the following practices when towing the side-scan SONAR system: If the tow cable is new, uncoil and wrap it into “figure-eight” loops of about 0.9 m (3 ft) on each side to remove the natural twist, as shown in Figure 7. If used directly from the coil, the cable has a strong tendency to develop small loops, which can damage the cable when it is under strain.

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· Figure 7. Typical Figure-8 Configuration for the Tow Cable

Secure the tow cable to the vessel at hard points (stanchions, cleats, etc.) and attach with line. If hard points are unavailable, (particularly a concern on an inflatable) find someplace secure (e.g., a handrail or line pass-through) to use as tie-off points.

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· Figure 8. Cable Stopper Strap (note over/under weave)

The connectors on each end of the tow cable are unique so there is little chance of error. However, take special care when attaching the connectors because misconnection or forcing can damage the pins, the cable’s internal conductors, and/or the topside box.

Take care of the tow cable. Do not step on it, roll heavy equipment over it, or pass it between closed hatch covers.

Do not bend the tow cable beyond its minimum 12.7-cm (5-in.) bend radius limit. Bending beyond this limit will cause the internal shielding to rupture, puncture the jacket, and expose the internal conductors to sea water.

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If a winch is used as part of a cable-handling system, do not exceed its loading capacity or the rating of the tow cable (listed on the side of the cable). Pre-check cable integrity before mounting the cable onto a winch or cable reel. Check it for visible breaks and damage by running your fingers along the length of the cable to feel for breaks in the shielding and/or punctures through the jacket. Towfish deployment is easier with two personnel, one to lower the towfish, the other to pay out cable. Deploy the towfish as far away from the propeller as possible. Never stand in a way that a snagged towfish will pull you overboard. Stand and hold the cable in a manner that you could immediately let go of the cable if you feel it snag the bottom. Pay out cable slowly, all the while monitoring progress and watching for any indication of damage to the cable shielding that may have been missed during inspection. If any damage is found, repair or replace the cable before continuing with the operation. Periodically check the cable during the towing operation to ensure that it’s not tangling or excessively rubbing against the side of the boat. If the water is deep and a significant amount of cable is paid out, be aware of the resulting catenary effect against it (the significant resistance of water, causing the cable to bow). This pressure can affect towing speed and complicate cable retrieval. The faster the boat is travelling, the greater this effect will be and the towfish depth will not significantly increase with additional cable payed out. The boat must be slowed if you are having trouble with towfish depth. Monitor towfish location, drift, and descent as the cable pays out. If the bottom configuration is unknown, proceed more slowly and observe the waterfall display to determine the nature of the seafloor before sending the towfish toward the bottom. Otherwise, pay out cable as slowly as conditions allow, monitoring in case the towfish snags or hangs up. If this occurs, try retrieving using the winch or maneuvering the vessel from differing angles to extract the towfish. Maintain minimum forward speed while retrieving the cable until the towfish is a safe

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distance off the bottom. Slowing the boat too abruptly when the towfish is at surveying altitude will result in a bottom strike and possible damage to the towfish. After the towfish is at a safe altitude, the boat can be stopped to ease retrieval. Use the same slow method for retrieving the towfish as when deploying it, monitoring for debris or seaweed that might have become attached before the cable reaches the winch. Watch also for any damage or punctures to the shielding. Take great care when hauling the towfish out of the water; never risk falling overboard during its retrieval. Always lift the towfish out of the water with the thumb of the gripping hand pointed up. This ensures that the tow cable does not bend excessively over the handler’s hand when it is pulled up over the gunnel. Clean the tow cable after each use. If the system has been used in salt water, wash the towfish and tow cable with fresh water while still connected to each other to prevent corrosion of the components. Dry all connectors, spray with a water-repellant lubricant like WD40™, then cover with their protective caps. Store the tow cable by coiling it loosely onto a cable reel and hanging the figure-8 off the floor. Otherwise, replace the loosely coiled figure-8 in the cable box that was provided and stow it out of harm’s way.

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Search Methodology A standard search involves several stages—from pre-cruise planning to site feature identification. The instructions in this manual assume that you have defined a search area, planned search lanes, set up, and deployed the towfish. We recommend the following scenario.

Site/Feature Location The initial stage of the search involves finding a site/feature location. You may already have a marked latitude/longitude (l/l) position. This position is presented in degrees:minutes:decimal minutes. In that case it should be a simple matter to locate the site. Otherwise, the search area should be methodically scanned in a straight, predetermined search pattern, as exemplified in Figure 9.

Figure 9. A Recommended Search-and Survey Pattern

Safety Any action that involves towing a device at the end of a cable at sea has an inherent danger involved. Many of the problems that occur at sea can be averted with common sense, good boat handling, and experience.

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Operation During a search, you can monitor progress because the survey vessel's position and estimated acoustic coverage are shown on the software’s navigation plotter. When you see a feature of particular interest in the SONAR record, mark it on the navigation plotter as a waypoint or marker. Then, if possible, throw a weighted acoustic target attached to a buoy with marine line off the stern on the same side of the vessel where the feature was identified. The buoy will serve as a visual frame of reference for the pilot when returning to the site. The acoustic target will show up in the image and allow the SONAR operator to relay the buoy position relative to the target to the boat pilot for successive passes.

When navigational information is available, each SONAR record line can be associated with l/l and swath coverage. Thus, any feature in the SONAR record can be associated with a known l/l position and location within the swath. Nonetheless, as a matter of good log keeping, also write the site l/l position in a logbook and make multiple passes to ensure that both transducers cover the objects so that the data can be reviewed later with the SONAR software.

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Site/Feature Identification When you have the site location, you can adjust the SONAR software to capture the feature at a higher resolution. If you are using Sea Scan® HDS, which features the state-of-the-art application, Sea Scan® Survey, you can set the acoustic range to a shorter setting. The target of interest will appear larger and have greater resolution. Sea Scan® Survey can provide very-high-resolution imaging with a frequency of 600 kHz or greater and a range of 40 m (131 ft) or less. Make a closer pass by the target of interest for a detailed image. However, the towfish should not pass directly over the target on the seafloor; rather, it should pass to one side of the target. Guide the ship's pilot so that the target will appear on either the left or right side of the SONAR record. Mark the target for future review on a chart or in the software navigation plotter and assign a distinct name or description to it.

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System Features Sea Scan® Survey contains extremely powerful, yet user-friendly software features and capability. By understanding how to optimize these tools, an operator can make the most effective use of Sea Scan® Survey and thereby achieve optimal results. The overview below is intended to maximize software utilization and benefit.

User Settings A number of settings are available to you for modifying the user interface. Select those that are comfortable for your level of expertise or your operational requirements. These are discussed below. SONAR Interface When you turn on the SONAR, the transducers automatically communicate their settings to the Sea Scan® Survey software. The program then resets itself accordingly based on that intercommunication and it presents that information in an easy-to-read display on the computer screen. The sophisticated communication process requires no user intervention; it communicates seamlessly among the SONAR, topside box, GPS, depth sounder, or any other connected equipment. That information also is immediately accessible in the display area or in convenient drop-down menus. SONAR Display The SONAR display has been designed for optimal viewing and data interpretation. The waterfall display consumes the majority of the computer screen because this is your primary workspace. Here, you can access all the tools and menus you require— adjusting color, zooming in on an area of interest, checking markers, adjusting the frequency, or monitoring SOG, to name a few. As soon as Sea Scan® Survey initiates and SONAR is activated, the waterfall display will begin to build a line-by-line image of the seafloor that you can enhance to identify targets or collect data. Using the various tools and capabilities of the software that are presented to you in the display area, you can direct the operation, collect and notate targets of interest, and gather data for immediate or later review. The entire process is easy and convenient. Color Map Color choice can dramatically affect your interpretation of the SONAR image data, as different colors bring out different features in the SONAR record. Marine Sonic Technology, Ltd. recommends the gold, bronze, or brown color maps to maximize the user’s ability to distinguish the various shades in the image.

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Inverting the Color Map Any color map may be inverted in Sea Scan® Survey to make the high-intensity returns dark and the low-intensity returns light. This has been the display setting for traditional SONAR systems since they applied black ink for acoustic returns onto light-brownish or white paper. The higher the intensity of the acoustic return, the more black ink was used. The dark-ink-on-light-paper process required that a SONAR operator had to reverse the natural intensity visualization we are used to when we see. In the normal mode (as opposed to the inverted mode), any high-intensity returns are brighter than the darker background. This provides a view of the seafloor like illuminating the dark seafloor with light and viewing from above. Objects appear brighter than the background and shadows are black. In the inverted mode, objects are darker than the background and shadows are white or light, as shown in the following figure.

· Figure 10. Sample Sea Scan® Survey Image Before and After Inverting the Color Map

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Moving through the Data Scrolling The SONAR image is recorded directly into a streaming data file. The indexed data is then displayed in sequence in the Sea Scan® Survey waterfall display. Not all of the data can be displayed concurrently because of screen size limitations and/or computer speed, but you can scroll backward or forward through all the data, much the same way that you might fast-forward through a video tape to a particular location or scene.

Channel Sea Scan® Survey allows you to display either the left channel only, the right channel only, or both the left and right channels simultaneously in the waterfall display. Depending on the operating requirements, you may want to view both channels at standard axial resolution or only one of the channels at double the axial resolution. When viewing only one of the channels, you have twice as many horizontal pixels on the screen. Thus, for any given range, twice the axial resolution may be displayed on the screen since you are displaying only one channel; bear in mind, however, that the image will be distorted in the axial direction (that is, the image will appear stretched horizontally). Measurements Height The height of a feature in the SONAR image can be measured if there is a discernible shadow behind the feature. Defining the geometry of the towfish, the object, and the object’s shadow is the means for measuring an object’s height, as exemplified in the following figure.

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Figure 11. Object Height Calculation with the Shadowed Area That the Sound Cannot Reach

An accurate height measurement requires that the object cast a visible acoustic shadow. This may be a problem in some situations, such as the object’s proximity to the transducer. Further, the end of the shadow must be visible on the SONAR image. If the shadow extends off the side of the image, then the object is either near the same height as the towfish altitude or is higher, which might indicate a hazard to the towfish on subsequent passes of the search. The height calculation assumes an ideal geometry. That is, the acoustic path from the transducer to the top of the object to the end of the shadow is assumed to be straight and the seafloor is assumed to be level. The first assumption of a straight acoustic path is valid, considering that conditions where you see the acoustic path “bend” are not common at the operating depths and ranges of the Marine Sonic Technology, Ltd. towed system. However, the second assumption of a level seafloor typically is not the case. Therefore, you must compensate based on your knowledge of the seafloor level. The acoustic shadow for an object upslope from the transducer will be abnormally shortened. Similarly, the acoustic shadow for an object downslope from the transducer will be abnormally lengthened. Length The Sea Scan® Survey waterfall length tool allows you to measure any feature in the SONAR image data with the mouse pointer. The length is presented as a total length, which is a combination of horizontal (axial) and vertical (transverse) lengths. The lengths are measured using the user-specified length units. The range and ping separation distance for each SONAR record line is sufficient to calculate the length of features in the SONAR record accurately. Figure 12 shows how easily a length measurement can be taken. Transverse length is the distance measured in a line parallel to the towfish track. Measuring the transverse length with Sea Scan® Survey depends completely on the apparent SOG because the apparent SOG (the SOG provided by a GPS or estimated by the pilot) determines the ping separation distance. Typically, the

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apparent SOG matches the true SOG. If the apparent and true SOGs were not matched, the transverse length will be inaccurate because the ping separation distance is inaccurate. The extent of the error depends entirely on the disparity between the apparent SOG and the true SOG. For example, if the apparent SOG is set at 3.6 knots and the true SOG is 4.0 knots, there is a 10% error. Thus, the ping separation distance also will be off by 10%. In fact, the pings will be 10% too short because SONAR software programs base their calculations on the boat’s moving at 3.6 knots rather than the 4.0 knots the towing vessel is actually moving. The transducers will not ping fast enough to maintain the 1:1 aspect ratio for the SONAR image, so the under-sampling will foreshorten features. Thus, when measuring the transverse length, a shortened feature will be 10% too short in the given example.

Area The Area button in the waterfall toolbar allows you to measure the area of a feature by tracing the outline of the SONAR waterfall image with the mouse pointer. The area is measured in units that you designate (for example, feet, meters, nautical miles). The range and ping separation distance for each SONAR record line is sufficient to calculate the area of features in the SONAR record. As with Length, measuring Area depends completely on the apparent SOG, which determines the ping separation distance. Thus, if the apparent SOG does not match the true SOG, the transverse length will be inaccurate because the ping separation distance is not accurate. Here, also, the extent of the error depends entirely on the disparity between the apparent SOG and the true SOG.

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SONAR Control Sea Scan® Survey has been designed to give easy access to the important parameters you require during an operation. These are discussed next. Power Turn the power on to start the data collection process. As the transducers ping, the incoming reflection data is recorded and displayed in the software waterfall.

Range Delay The most common use for range delay is to remove the water column; that is, the section of the SONAR record that displays the acoustic returns is removed in the channel immediately beneath the towfish. Typically, you are not interested in the acoustic returns produced as the SONAR beam passes through the water column. Thus, set the range delay to the same distance as the towfish distance from the bottom to “ignore” any acoustic returns as the SONAR beam passes through the water column. The software will then start collecting the acoustic returns when the SONAR beam reaches the range delay that you have set.

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The other use for the range delay is to extend the view of the SONAR, beyond the current range setting. For example, you may want to view the area from 75 to 175 m (246 to 574 ft) but are not particularly interested in the first 1 to 74 m (3.3 to 243 ft). With zero range delay, you would lose resolution at a range of 200 m (656 ft) and you would collect data for everything from 0 to200 m (656 ft). However, if you were to set the range to 100 m (328 ft) and the range delay to 75 m (246 ft), you could focus on the area of interest at a higher resolution because of the shorter range. Frequency Depending on the size of the target, you may have the opportunity to use either the lowor high-frequency transducers. But this choice will depend on whether the towfish you are using has multiple-frequency transducers. If so, the low-frequency transducers provide a lower axial resolution but the sound travels farther through the water than from the high-frequency transducers. Likewise, although not able to extend as far, the highfrequency transducers will provide very-high-resolution imaging at closer ranges.

Speed Control The ping rate of SONAR software is based on the current range and the SOG. The spacing of each vertical line on the screen is equivalent to a known distance for each of

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the ranges. The time between each of these known distance intervals depends on the SOG. The time interval is set based on current speed. This SOG is automatically adjusted correctly with the GPS. If you are not using a GPS, Sea Scan® Survey allows you to enter the SOG manually.

Image Adjustment

Gain The Sea Scan® HDS transducers produce a very specifically defined acoustic signal. Viewed from above, the signal is very narrow; viewed from the side the signal is wide. This shape of acoustic sound allows the transducer to view a very narrow section perpendicular to its path of motion. As the out-going acoustic signal travels through the water, the signal strength at the end of the cone shape weakens; this weakening is caused by a variety of influences, such as absorption, wave-front spreading, and scattering. These are known physical effects of acoustic energy traveling through a “ lossy” medium. As a result, the amount of energy available to reflect from an object reduces as the outgoing acoustic wave travels away from the source. That is, the reflection from a distant object is not as strong as that from a like object closer to the transducer (source of the acoustic wave). That is why gain is so important during a survey or operation. It is similar in concept to the volume adjustments on a hearing aid that are used when the sound is coming from farther away. The amount of gain required to adjust for signal loss is strongly proportional to, but not entirely dependent on, range. Range may be thought of as time because it takes a known time for a signal to return from any given range. Therefore, by adjusting the gain, you can give a target 150 m (492 ft) away the same echo strength as a like target that is 50 m (164 ft) away.

The transducer receives the acoustic signal that was reflected off the seafloor and any objects there. The transducer converts this acoustic signal, which is mechanical energy, into an analog electrical signal that is then amplified and digitized.

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You can approach the image adjustment process in two ways. You can permit Sea Scan® Survey to apply a calculated amount of gain compensation. This action sets the gain settings automatically to optimal settings for the current operating environment. The automatic gain process adjusts the amount of gain at each point in the active range until it has determined that the gain settings are at optimal levels. It does this by evaluating the intensity of the background signal in the active range, much as cameras set to automatic adjustment will do. Sea Scan® Survey adjusts the gain settings so that there is a constant background level throughout the entire active range, although the outcome depends on the current operating environment. The automatic gain process samples the current SONAR record line. First, it determines the intensity of the background in the active range. Then it determines the required gain adjustment change to bring the background level to an optimal level. The gain is then adjusted. Sea Scan® Survey displays the latest SONAR record line in A-mode. This viewing mode provides a strong visual reference of the acoustic returns on a single SONAR record line. This A-mode “bar chart” view exemplified in Figure 14 below displays the acoustic return intensity in the vertical axis versus range in the horizontal axis.

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Figure 14. Typical Sea Scan Survey® A-mode image.

The vertical lines along the horizontal axis represent the intensity of the acoustic returns at the respective positions along the SONAR record line. In other words, a short vertical line represents a low-intensity acoustic return. Likewise, a long vertical line represents a high-intensity acoustic return. The A-mode view of the signal response provides an accurate visual reference for the effect of Time-Varied Gain (TVG) on the raw signal. Changes to any of the image adjustment parameters can be monitored by watching the immediate effect on the signal response.

Navigation and Fathometer Interface A baud rate of 4800 is the NMEA standard. The Marine Sonic Technology, Ltd. Sea Scan® Survey reads the following NMEA standard strings for the GPS interface: RMC, GGA, GLL, VTG, HDT, HDG, DPT, DBT, DBS, DBK, and ZDA.

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The definitions for these strings can be found in most GPS manuals.

Any one of the following minimum NMEA strings will enable the minimum set of data: (1) RMC (2) GGA, VTG (3) GLL, VTG Any additional strings to the above three combinations will add redundancy and/or extra information.

Markers and Waypoints Markers Markers are invaluable for marking targets of interest during a survey. When you mark an item in the Sea Scan® Survey waterfall display or navigation plotter, the software “takes a snapshot” of the image, sequentially numbers the marker, and notes location information. The sequentially numbered markers you insert will be listed and become part of the survey record until you delete them. Waypoints Waypoints are useful for marking navigational positions (l/l) in a survey route. These waypoints are especially practical for identifying predefined sites, landmarks, or buoys. The l/l can be inserted manually or Sea Scan® Survey can automatically add that information to the markers and waypoint management list. The sequentially numbered waypoints you insert will be listed and become part of the survey record until you delete them.

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Managing a Survey Survey Folder Organization The key to organizing and keeping survey folders accessible is to name each distinctly. This opportunity is available when starting a new survey; of course, an operator can rename a survey folder at a later time. Another important action is to add a detailed description to the survey folder. When numerous SONAR runs have been done, such information will be invaluable in determining which survey to revisit.

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Image Interpretation Tips Overview The ocean floor contains a great variety of structures, debris and natural features. Some areas are covered with miles of flat mud, while others contain precipitous rock outcrops. Flat, desert-like sand deposits occur in some areas, while clay, gravel, or round stones blanket others. Side-scan SONAR is an ideal tool for mapping these areas. In a standard towing configuration, side scan delineates even the smallest change in the seabed, whether it is a very slight incline, a small depression or sinkhole, or a change in sediment deposition due to a change in the bottom structure. The structure of the seabed will affect the acoustic signal. As sound travels outward from the transducers, it can be deflected in many different directions. It may encounter an uneven water surface, air bubbles, fish, suspended sediments, or a rough bottom. In some instances—a muddy bottom, for example—the signal is partially absorbed; in others, such as a rocky bottom or one cluttered with debris or gravel, the returning signal will be stronger because hard objects are better reflectors.

Key Approaches to Locating Objects Skill in interpreting SONAR data comes only from experience. Quite simply, it takes time to become perceptive enough to sort out what are real bottom features or targets and what may be anomalies. As you gain more experience with side-scan SONAR systems, three basic approaches will yield the most reliable results. These approaches are, in order of importance: shadows, size of objects, and the shape or appearance of objects. Shadows An acoustic shadow is an area near an object on the opposite side of the object from the towfish that the sound cannot reach because the object blocks it. Shadows cast by

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objects are a function of the angle at which the SONAR beam strikes the objects. Thus, just as your body blocks the sunlight from shining immediately behind you, so does an object on the seabed block the sound wave from hitting the immediate back part of that object. Shadows are extremely important in locating objects as well as in their analysis, so attention must be paid to shadow position, shape, and strength in the waterfall display when on a search-and-recovery mission. The shadows cast by objects are a function of the angle at which the SONAR beam strikes objects as well as the shape of the objects casting them. Thus, detailed inspection of the shape of a shadow is helpful in determining the nature of the object. The location of a shadow relative to the object creating it is also an important feature of the object. Shadows that touch the object imply that the object is touching the seafloor. Shadows that are separated from their corresponding object imply that the object is above the seafloor. Also, as stated before, a shadow that reaches out beyond the edge of the image implies an object with a height that is near or above the altitude of the towfish and might pose a hazard on subsequent passes of the search or survey. Size The size of an object is the second most critical consideration. If you know the basic size of the object for which you are searching, you can focus primarily on matching found objects to that size. While searching for an object of known size, always keep in the back of your mind how big that object would look on the screen at the current range setting. This will allow your eye to look for certain objects while partially ignoring the other clutter of the seabed.

Shape The third approach in search-and-recovery missions is to focus on the shape of the object for which you are conducting the search. You will see the shape in the waterfall display and it will be distinguishable from the background in most cases. An exception is a debris-filled waterway or seabed. In this instance, an object or body lying among clutter may be hard to distinguish. In such a case, shadow and size may be the determining factors.

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Conditions That Complicate Interpretation Ghosting The outgoing pings from the transducers do not stop or dissipate at the end of the scanned range; rather, they continue out into the environment. Ghosting occurs when the acoustic signal travels beyond the designated range and bounces off distant surfaces or even the surface of the water itself (recall the ever-widening “cone shape” of the outgoing signal). The resulting return may cause anomalous shapes in the waterfall display that are usually of low intensity. When operating in shallow water and channels, you may experience ghosting caused by acoustic returns from previous SONAR pings still bouncing between targets and the SONAR receivers. To reduce these effects, use a larger range on the SONAR display, which should slow the ping rate and allow time for these echoes to dissipate. You may also see distortion on long-range targets, caused by multi-path phenomena (surface waves and chop) where sound takes a longer path by bouncing off the surface on its way back to the transducer. The effect manifests itself as multiple targets appearing close together, as shown in Figure 15.

· Figure 15. Sea Scan® Survey Waterfall Display Showing the Multipath Phenomenon

Crosstalk Crosstalk occurs when a target returns such a strong echo that it passes through the

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towfish body to the transducer on the opposite side and reverberates there. The resulting image in the waterfall display will show a mirror image on both sides; however, the false image will not be as strong. In the figure that follows, the crosstalk image is on the left.

· Figure 16. Sea Scan® PC Waterfall Display Showing the Effect of Crosstalk

Thermoclines and Haloclines Thermoclines are a frequent cause of SONAR path distortion; they occur where different levels of salinity meet. That juncture results in abutting layers of water that are of different temperatures. The upper layer will be warmer than the lower layer. Temperature differentials can distort the SONAR image. Haloclines can be experienced where mixing of fresh water with seawater occurs (for example, where a river feeds into the ocean). The SONAR path distortion causes echoes from different parts of the seafloor to arrive back at the towfish transducer at the same time. These complex echoes from different locations on the seafloor can corrupt the side-scan image and mask and hide small targets. Refraction effects can be minimized at times by simply changing the depth at which the towfish is being towed or changing the frequency of the transducers.

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·

Figure 17. Sea Scan® PC Waterfall Display Showing the Effect of a Thermocline

Distortion during Turns Vessel turns while taking data with side-scan SONAR will result in a distorted view in the waterfall display. During a turn, the transducers no longer scan the seabed in a consistent, straight line. The transducer on the inside of the turn is sending out pings and recording reflections from a much smaller area of the seabed than the transducer on the outside, thus the image on the inside of the turn is stretched and the image on the outside of the turn is compressed and distorted. The effect of such conditions is exemplified in Figure 18.

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· Figure 18. Sea Scan® Survey Waterfall Display Showing Distortion During a Turn to the Left

Therefore, it is important not to make course changes while the search-and-recovery target is being scanned. Turn the vessel well after the waterfall display reveals the full image.

Surface Scattering Surface scattering occurs when sound waves bounce back from the surface of the water as well as from the sea floor. The shallower the water, the greater the potential for surface scattering because the SONAR may reverberate off multiple surfaces multiple times before returning to the transducers. The surfaces may be the sea floor, the surface (especially if the surface is wind whipped and turbulent), debris, schools of fish, thermal conditions, as well as targets of interest.

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Wind-generated white caps on the surface also are a very good acoustic reflector, so much so that the surface return clutter from the white caps can significantly affect the returning data. These turbulent conditions may deflect the path for the outgoing pulse as well as the returns, causing them to follow a distorted or curved path, as shown in Figure 19 below.

· Figure 19. Sea Scan® Survey Waterfall Display Showing Surface Scattering

Propeller Wash Propeller wash shows up as bubbles in the water column and can be the result of gases in the vessel’s wake. These bubbles are a strong reflector of the SONAR signal, even to the point of masking the target. This situation can be particularly problematic in a busy waterway where many powercraft are traversing through the survey area while you are conducting a search-and-recovery operation. Normally, the wake or propeller wash of the towing vessel does not distort the waterfall display image because of the downward tilt of the transducers on the towfish. However, in surveys where the towing vessel is navigating lanes in shallow water and/or other powered watercraft are in the area, turbulence may show up in the SONAR image. In this case, slow the vessel to reduce the bubbles from the propeller wash. Figure 20 exemplifies the effect of propeller wash.

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· Figure 20. Sea Scan® Survey Waterfall Display Showing Bubbles in the Water Column (worse on the right side)

Noise Noise is unwanted interference in the waterfall display. It can be either electrically generated or acoustic in nature. The noise might appear as flecks of bright speckles, streaks, continuous lines, or bands. When noise problems occur from electronics, it often helps to change subcomponents or power sources, reroute cables, or re-secure the system ground. Some acoustic noise comes from the underwater environment. It may be caused by some biologics in the water (dolphins, snapping shrimp, etc.), but most comes from the electronics on your own or other vessels. This noise can be from electronic depth sounders, fish finders, or other SONARs operating nearby. In Figure 21, the multiple bright speckles in the right column are a result of noise.

Image Interpretation Tips

50

· Figure 21. Sea Scan® Survey Waterfall Display Showing the Effect of Acoustic Noise (in this case from depth sounder interference)

Image Interpretation Tips

51

Tools and Equipment Needed for Hookup and Maintenance Spanner O-rings Fuses Water-repellant lubricant (WD40™) Vice grips Side cutters Nut drivers Nuts Bolts Shackles Shear pins Brass screws Cable stopper Straps Wrenches Screws Aerosol cans Spare pigtails Spare lines Electrical tape Duct tape C-clamps Cable ties/Zip ties Screwdrivers (Phillips and Standard) Multimeter Spare batteries

Tools and Equipment Needed for Hookup and Maintenance

52

Revision History Version

Comments

1.1.0

Fixed some errors and updated the Topside Box information. Updated the contact information.

1.0.0

Initial release

Issue Date June 2011 January 2009

Revision History

53

Glossary absorption

The diminishment of the returning pulse caused by the impact of the acoustic signal against objects, materials, or the sea bottom

acoustic shadow

Literally, the shadow an object on the bottom casts to its side in relation to the transducer; the transducer must “see” this entire shadow so that the height of that object can be calculated

acoustic signal

The fan of sound emitted from the transducer

active SONAR

A system that transmits an acoustic signal through the water that reflects off objects, then is returned

angle of incidence

The angle between the SONAR pulse path to an object and the bottom

AUV

Autonomous Underwater Vehicle

axial layback

The offset distance from the position of the towfish to the navigation antenna

backscatter

Unwanted echoes

baud rate

A unit for measuring data transmission speed, where one unit equals 1 bit per second

beam spreading

The widening angle of a SONAR beam as it moves farther from the transmitter

BMP

Bitmap, a graphic file format commonly used in Microsoft Windows© applications

Bow

The front section of a boat or other vessel

buffer

To load, as in loading a file. Also, pulling information from a file and putting it into memory

Catenary

The curve on a tow cable moving through the water as a result of the forces of water drag on the cable

CD

compact disk

Glossary

54

channel

Another term used to describe the transducer acoustic track from either side of the towfish that appears in the SONAR window. The left channel refers to the left (port) transducer track; the right channel refers to the (starboard) track.

COG

Course Over Ground

color look-up table

A way of mapping data by assigning an artificial color

contrast stretching

An operation that remaps the color look-up table based on the lower and upper threshold limits the operator elects from the histogram

crosstalk

A strong return that passes through the towfish body to the transducer on the opposite side and reverberates there

CSV

Comma-Separated Values; in computers, a CSV file contains the values in a table as a series of ASCII text lines organized so that each column value is separated by a comma from the next column's value and each row starts a new line. A CSV file is a way to collect the data from any table so that it can be conveyed as input to another table-oriented application such as a relational database or spreadsheet application.

decibel

The unit of sound intensity used to describe the strength of transmitted and received underwater sound

DGPS

Differential Global Positioning System

DL

Data Loss

ENC

Electronic Navigation Chart

EULA

End User License Agreement

ETE

Estimated Time Enroute

fathometer

An acoustic device that measures the water depth

FAQ

Frequently Asked Questions

frequency

The number of sound waves that pass any specific point in one second

FS

Frequency Switch

Glossary

55

gain

Amplification of a signal

Gaussian smoothing

A widely used effect in graphics software, typically to reduce image noise. It is a spatial low-pass filter that reduces the sharpness of image detail

GB

GigaByte (1,000,000,000 Bytes) A high-resolution image format that transfers readily to other geographic applications

ghosting

False returns from acoustic signals that extend beyond the designated range and bounce off distant surfaces

GHz

GigaHertz (1,000,000,000 Hertz)

GIS

Geographic Information Services

GNSS

Global Navigation Satellite System

GPS

Global Positioning System

Hertz

A unit of measure representing one cycle per second

HDS

High-Definition SONAR

halocline

Layers of waters having different salinities

histogram

A graph that displays the number of pixels of each color in a range

home state

The indicator to the operator that the newest SONAR line is immediately visible on the screen

HTML

hypertext markup language, a set of tags and the rules for using them in developing hypertext documents

hydrophone

An instrument that transforms an underwater sound wave into an electrical signal

intensity

The strength of the returning acoustic signal

interference

Erroneous signals caused by acoustic or electronic sources

l/l

Latitude/Longitude

latitude

An imaginary horizontal line joining points on the Earth’s surface that are all of equal distance north or south of the equator

Glossary

56

lane

The straight track the surface vessel follows while towing the towfish

lateral layback

The offset distance to the left (port) or (starboard) between the navigation antenna and the towfish

layback

The x-y coordinate distance between the navigational device and the towfish

longitude

The angular distance east or west of the prime meridian that stretches from the North Pole to the South Pole and passes through Greenwich, England

lossy medium

A local condition where absorption by the water, wave-front spreading, or scattering causes physical degradation of the acoustic signal

LUT

Look-Up Table

magnetometer

An external device that measures the current total magnetic field

marker

A tag that the operator can apply on the computer to a target of interest to save it for further review

MB

MegaByte (1,000,000 Bytes)

MRU

Most Recently Used

nadir

A point along the swath that is directly beneath the towfish

NMEA

The National Marine Electronics Association

NMEA Protocol 0183

A standardized protocol that allows electronic marine instruments to transmit and receive information. This communication is based on a block transmission or groups of NMEA 0183 sentences, which are transmitted over the serial cable. Each sentence has a header that uniquely identifies the source of the data and the information contained in the data string.

noise

Extraneous acoustic sound or electrical waveforms that interfere with the SONAR signal

null point

The location of the antenna of the external navigation device

OTG

Over The Ground

Glossary

57

overlap

The bottom area covered more than once as the towfish travels a predetermined pattern or grid

PC

Personal Computer

ping

A single pulse of a SONAR system

Plotter

A software module that shows track position and acoustic coverage

port (side)

The left side of an object or ship

projector

An device that transforms electrical signals into sound waves

pulse length

The length of time that a SONAR unit is transmitting one pulse

PW

Power

RAM

Random Access Memory

range

The maximum distance from the transducers that the SONAR signal can detect usable signals; also, distance to a target

range delay

The distance (or range) the Sea Scan Survey software is told to delay after pinging before it starts to look at acoustic returns

range marker

A scale reference line shown in the SONAR window

reflectivity

The strength of the SONAR return off an object or material

RGB

Red/Green/Blue

ROV

Remotely Operated Vehicle

sea clutter

The reflections from the sea surface (waves, bubbles, wakes, etc.)

scattering

The diffusion of a SONAR beam in many directions because of sea conditions; as a result, the amount of energy available to reflect off an object reduces as the outgoing acoustic wave travels away from the source

SDS

SONAR data stream

Glossary

58

shadow

The area that the sound wave cannot reach behind an object because of the object’s protrusion above the sea floor; used to determine the height of the submerged object

side-scan SONAR

An acoustic imaging device used to provide wide-area, large-scale images of the bottom of a body of water

signal statistic

Statistical information on a ping, typically an average, minimum, or maximum level

slant range

The straight-line distance from the towfish to an object at any given location

SOG

Speed Over Ground

SONAR

SOund Navigation And Ranging

starboard (side)

The right side of an object or ship

swath

The total side-to-side coverage of the SONAR signal on each sweep of the seabed; also called a line

thermocline

Layers of waters having different temperatures

topside processor

The control unit an operator uses on the towing vessel to collect and observe the incoming data from the transducers

towfish

The device that is towed through the water on which the transducers are mounted

transducer

The projector and hydrophone that make up an active SONAR system

true range (ground range)

The horizontal distance from the towfish to the object or bottom location

TVG

Time-Varied Gain; a process where amplifier gain is changed based on time and matched with returning signals between outgoing pulses of the side-scan SONAR

.sds

SONAR Data Stream, the extension that the software attaches to the names of data files

USB

Universal Serial Bus

Glossary

59

UTM

Universal Transverse Mercator, a coordinate mapping system that maps to a grid rather than to latitude/longitude

Universal Length Unit

A standard for measurement of areas in user-specified units

WAAS

Wide-Area Augmentation System

water column

The track along the bottom immediately beneath the towfish; this track provides supplementary information to the operator, such as the altitude of the towfish relative to the water’s depth or the presence of sea clutter

waterfall

A term used to describe a display that puts the newest data at the top and scrolls the data down like a waterfall

wavelength

The distance between acoustic waves

waypoint

A position of interest made and displayed on the navigation Plotter

XTE

Cross-Track Error

Glossary

60

Index

-EEarth-orbit satellites microwave signals 16 Effects on SONAR range acoustic noise 17 bottom type 17 electromagnetic noise 17 particulates 17 water depth 17 water salinity 17 water temperature 17 Electro-static-sensitive devices

-AAnomalies 43 autonomous underwater vehicles (AUVs)

10

-BBasic search approaches shadows 43 shape or appearance of objects 43 size of objects 43 battery unit 17 Black Laser Learning, Second Edition, Not in the Manual Guide® To Side Scan Sonar Image Interpretation 3 bulkhead 15

-G-

-CChapman Piloting & Seamanship, 64th Edition Crosstalk mirror image 45 reverberation 45

-DDepartment of Defense 16 depressor vane 19 Differential GPS buoy positioning 16 dredging 16 harbor navigation 16 sweeping 16 discrete sections 2 distilled water or alcohol 7 Distortion during Turns 47

4

3

general guidelines 2 Ghosting anomalous shapes 45 low intensity 45 Global Navigation Satellite System Global Positioning System 16 Glossary abbreviations 2 acronyms 2 italicized words 2 symbols 2 ground potential conductive surface 4 discharge paths 4

16

-HHaloclines echoes 46 refraction effects 46 high-frequency transducers 17 High-Resolution SONAR system

Index 61

1

boat wake 49 bubbles 49 busy waterway 49

-IIntel-based Windows™ Operating System Client 15 Client 2003 15 Vista 15 XP 15 Introduction to Sea Scan® Software 3

-Llaptop computer 15 lossy medium absorption 17 scattering 17 wave-front spreading 17 low-frequency transducers 17

8

-NNoise acoustic 50 electronically generated

50

-OOcean floor debris 43 natural features structures 43

43

-Ppings cone shape 8 perpendicular to boat direction Propeller wash

remotely operated vehicles (ROVs) Routine maintenance compressed air 7 connector caps 7 O-rings 7 ruggedized box 15

10

-S-

-MMicrosoft Windows™

-R-

8

salt water 7 satellite correction base station 16 geostationary satellites 16 network control centers 16 private subscription service 16 Sea Scan® High-Definition SONAR (HDS) Sea Scan® Survey 1 search and recovery Artificial reefs 1 Bridge inspections 1 drowning victims 1 Environmental protection 1 equipment 1 Force protection 1 Homeland Security 1 Insurance fraud 1 marine research 1 Old-growth timber 1 sunken boats 1 Surveying 1 Vehicles 1 Weapons 1 shackle 17 Side-scan system elements control unit 10 tow cable 10 towfish 10

Index 62

1

signal strength 17 Software-based SONAR system acoustic coverage 8 analyze 8 plot location 8 save 8 streaming data file 8 time-varied gain 8 view 8 SONAR applications commercial 10 leisure 10 military 10 sophisticated towfish models depth gauges 17 internal weighted keels 17 multiple-frequency transducers 17 temperature sensors 17 SOund Navigation And Ranging acoustic (sound) energy 10 transducer 10 Sound Underwater Images, A Guide to the Generation and Interpretation of Side Scan SONAR Data 3 sound wave absorption 43 sound wave deflection 43 Speed over Ground 17 Supporting equipment other relevant supplies 19 pigtails 19 spanners 19 tie wraps 19 Surface Scattering debris 48 schools of fish 48 sea floor 48 targets of interest 48 thermal conditions 48 wind whipped and turbulent surface 48 wind-generated white caps 48 Survey basics collecting data quickly 2 interpreting situational anomalies 2 performing a survey 2

swath width

8

-TTargets 43 The American Practical Navigator, an Epitome of Navigation 3 Thermoclines salinity changes 46 SONAR path distortion 46 Tools and Equipment 52 tow cable 750-lb. safe working load 4 figure eight 4 minimum bend radius limit 4 punctures 4 shielding 4 twisted-pair internal wiring 4 towfish cable connector 17 nose cone 17 shear bolts 17 shear pin release 17 tailfin assembly 17 tow rail 17

-Uunmanned surface systems USB cable 15

-Vvariable-angle bracket

-Wwater column debris 10 fish, 10 objects 10 surface returns 10 waterfall display 8

Index 63

17

10

water-repellant lubricant 7 Wide-Area Augmentation System

16

Index 64