EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic

Field surveying and corrections: Magnetometer: (a) Torsion- (balance) type (b) Fluxgate-type (c) Resonance-type (Proton-Precession, Cesium-Vapor)

Gradiometer

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-01

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic

Historic overview: The earliest known device which responded to the Earth’s magnetic field was a magnetized spoon used by the Chinese in the 1st century AD. Compass needles were introduced for navigation around the year 1000 in China and in Europe about 200 years later. The first accurate measurement of the inclination of the Earth’s field was made at Radcliffe in London in 1576 by Robert Normal. (The Newe Attractive, 1581). Magnetometers used specifically in geophysical exploration can be classified into three groups: the torsion (and balance), fluxgate, and resonance type.

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-02

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic Torsion and balance magnetometers These types of magnetometers consist of a magnetic needle suspended on a wire (torsion) or balanced on a pivot. In the Earth’s magnetic field the needle adopts an equilibrium position. If the device is taken to another location, the needle will align itself to the new field conditions. The deflection of the needle is taken as a measure for the strength of the magnetic field. The device requires calibration, typically by using Helmholtz coils generating a given field strength, which is used a base-point for profiling.

Accuracy of these devices is 10 nT at most in exploratory work.

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-03

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic Fluxgate magnetometer The fluxgate was originally developed during WW-II to detect submarines. It consists of two parallel cores made out of high-permeability ferromagnetic material. Primary coils are wound around these cores, but in opposite directions. Secondary coils are also wound around the cores but in the opposite sense to the primary coil. A current alternating at 50 – 1000 Hz is passed through the primary coils which drives each core through a hysteresis loop to saturation at every half-cycle in the absence of an external field, as such inducing a magnetic field in each core. EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-04

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic Fluxgate magnetometer As a result, the generated alternating magnetic fields in the cores induce an inphase voltage within the secondary coils. This voltage reaches its maximum when the rate of change of the magnetic field is fastest. As the coils are wound in opposing direction around the cores, the secondary voltages are in phase but have opposite polarity, so that the sum of these voltages is always zero. If the device is now placed in an external magnetic field, a component of that field will be parallel to the orientation of the two cores. The cores will be more easily magnetized in alignment with that field and less easily magnetized in opposition to it. Hence the alternating magnetic field, and the induced output current, will be out of step with the input current. The extent to which this is the case will depend on the strength of the background magnetic field.

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-05

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic

Primary voltage

Alternating voltage required to induce magnetic saturation

The physics behind this is governed by Faraday’s law of induction, stating that a magnetic field changing in time creates a proportional electromotive force : v

v ∂B ∇× E = − ∂t

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

(EQ 2.19)

EPS435-Potential-06-06

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic

Primary voltage

Alternating voltage required to induce magnetic saturation

Induced magn. in core 1 Induced magnetization Induced magn. in core 2

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-07

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic Primary voltage

Alternating voltage required to induce magnetic saturation

Induced magn. in core 1 Induced magnetization Induced magn. in core 2 Secondary voltages in absence of external field EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

Core 1 Net outcome = 0 Core 2 EPS435-Potential-06-08

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic

Secondary voltages in presence of an external field

Secondary voltage in core 2 lags behind that of core 1

Resultant secondary voltage from sum of above

Peak voltage proportional to field strength parallel to core-axis. After Reynolds, 1997

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-09

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic

Fluxgate magnetometers have the advantage that individual components of the total field can easily be measured. They often contain two sets of coils: one to measure the horizontal, the other to measure the vertical component. Exact balancing of the instruments is essential for accurate measurements. Some portable fluxgate magnetometers suffer from temperature-drift effects, which can reduce sensitivity to only ±10–20 nT.

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-10

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic Resonance magnetometer Two general types of resonance-magnetometers exist: (a) Proton free-precession magnetometer (b) Alkali-vapour magnetometer Both types monitor the precession of atomic particles in an ambient magnetic field to provide an absolute measure of the total magnetic field. Tao understand the operation of both these instruments we need some advanced nuclear physics dealing with precession of protons and some quantum physics.

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-11

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic Details of the proton-precession magnetometer The proton magnetometer has a sensor which consist of a bottle containing a proton-rich fluid (water or kerosene) around which a coil is wrapped, connected to a measuring apparatus. Each proton has a magnetic momentum (M) and (as it is always in motion) it also possesses an angular momentum (G), like a spinning top. In an ambient magnetic field (strength F), the majority of the protons align themselves parallel with this field (with the rest oriented anti-parallel). Consequently the proton-rich liquid acquires a net magnetic momentum in the direction of the ambient field [see next page for illustration]. A current applied to the coil around the liquid generates a magnetic field about 50 – 100 times larger than the ambient field and the protons align themselves along this new field. When the applied field is switched off, the protons precess around the ambient field with a specific frequency (Larmor frequency fP), which is proportional to the strength of the ambient field. EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-12

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic

From Reynolds, 1997 EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-13

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic As protons are charged particles, their precession motion induces an alternating voltage at the same Larmor frequency (fP) into the coil surrounding the bottle, which is easily measured. It takes about 2 – 3 seconds for the precession to decay, enough to measure fP accurately. To obtain a value for the magnetic field of ~0.1 nT, the frequency must be known within 0.004 Hz (this is 100-times less sensitive than what modern gravimeters require!). F = 2πfP / ΦP ΦP = 0.26753 Hz/nT and 2π/ΦP = 23.4859 nT/Hz Thus

F = 23.4859 fP

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

(EQ 2.20)

EPS435-Potential-06-14

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic Alkali-vapour magnetometer (1) These types of magnetometer are also called optical absorption magnetometer. Under normal temperature/pressure conditions, electrons exist at certain energy states (A, B) around the nucleus of the atom. According to quantum physics, it is only possible to transfer an electron from a lower state (A) into a higher state (B) in discrete jumps. If a vapour (such as rubidium or cesium) is illuminated by a light whose filament is made of the same element, the light emitted is at the correct wavelength (i.e. energy) for incident photons to be absorbed by the vapour and the low-energy state electrons get excited into the higher energy state. The device is further modified to use circular-polarized light such that only electrons in the polarized orbit will be excited (or optically pumped) to the Bstate. The excess photons are transmitted through the excited vapour and will be detected by a photocell as an increase in light intensity.

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-15

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic Alkali-vapour magnetometer (2) A small alternating current is passed through a coil at a frequency of between 90 and 300 kHz to induce a magnetic field around the alkali vapour cell. This magnetic field forces some electrons back to their A-energy state, thus light intensity at the photocell diminishes as new photons are absorbed by the alkali vapour until saturation is reached again. Photons will be continuously absorbed until all electrons are excited in the B-state and light at the photocell is at maximum intensity. Consequently, the cycled optical pumping generates a flickering light with a frequency exactly the Larmor-frequency fP. As long as the light-beam is not parallel (or anti-parallel) to the Earth’s magnetic field axis, the Larmor-frequency and thus the magnetic field strength can be accurately measured with a precision of 0.01 nT.

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-16

EPS – 435

Geophysical Applications

Potential field methods - Geomagnetic Gradiometer A gradiometer measures the difference in the total magnetic field strength between two identical magnetometers separated by a small distance. For airborne instrument a separation of 2 – 5 m is typical, whereas for groundoperated instruments, a separation of 0.5 m is typically used. A major advantage of gradiometers is that because they take differential measurements no correction for diurnal variations are required. As gradiometers measure the vertical magnetic gradient noise effect from longwavelengths features are suppressed and anomalies form shallow sources are enhanced. Both, fluxgate- and resonance-type magnetometers are used in gradiometers.

EPS435 – Fall 2008 Dr. Michael Riedel [email protected]

EPS435-Potential-06-17