Frozen: The Potential and Pitfalls of Ground-Penetrating Radar for Archaeology in the Alaskan Arctic

remote sensing Article Frozen: The Potential and Pitfalls of Ground-Penetrating Radar for Archaeology in the Alaskan Arctic Thomas M. Urban 1, *, Jef...
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Frozen: The Potential and Pitfalls of Ground-Penetrating Radar for Archaeology in the Alaskan Arctic Thomas M. Urban 1, *, Jeffrey T. Rasic 2 , Claire Alix 3 , Douglas D. Anderson 4 , Sturt W. Manning 1 , Owen K. Mason 5 , Andrew H. Tremayne 6 and Christopher B. Wolff 7 1 2 3 4 5 6 7

*

Cornell Tree Ring Laboratory, Department of Classics and Cornell Institute of Archaeology and Material Studies, Cornell University, Ithaca, NY 14853, USA; [email protected] U.S. National Park Service, Gates of the Arctic National Park and Preserve, Yukon-Charley Rivers National Preserve, Fairbanks, AK 99709, USA; [email protected] Histoire de l’Art et Archéologie, Paris 1 Pantheon Sorbonne University, Paris 75005, France; [email protected] Department of Anthropology and Circumpolar Laboratory, Brown University, Providence, RI 02912, USA; [email protected] Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA; [email protected] U.S. National Park Service, Alaska Regional Office, Anchorage, AK 99501, USA; [email protected] Department of Anthropology, University at Albany, Albany, NY 12222, USA; [email protected] Correspondence: [email protected]; Tel.: +1-607-255-0447

Academic Editors: Kenneth L. Kvamme, Henrique Lorenzo and Prasad S. Thenkabail Received: 28 September 2016; Accepted: 1 December 2016; Published: 9 December 2016

Abstract: Ground-penetrating radar (GPR) offers many advantages for assessing archaeological potential in frozen and partially frozen contexts in high latitude and alpine regions. These settings pose several challenges for GPR, including extreme velocity changes at the interface of frozen and active layers, cryogenic patterns resulting in anomalies that can easily be mistaken for cultural features, and the difficulty in accessing sites and deploying equipment in remote settings. In this study we discuss some of these challenges while highlighting the potential for this method by describing recent successful investigations with GPR in the region. We draw on cases from Bering Land Bridge National Preserve, Cape Krusenstern National Monument, Kobuk Valley National Park, and Gates of the Arctic National Park and Preserve. The sites required small aircraft accessibility with light equipment loads and minimal personnel. The substrates we investigate include coastal saturated active layer over permafrost, interior well-drained active layer over permafrost, a frozen thermo-karst lake, and an alpine ice patch. These examples demonstrate that GPR is effective at mapping semi-subterranean house remains in several contexts, including houses with no surface manifestation. GPR is also shown to be effective at mapping anomalies from the skeletal remains of a late Pleistocene mammoth frozen in ice. The potential for using GPR in ice and snow patch archaeology, an area of increasing interest with global environmental change exposing new material each year, is also demonstrated. Keywords: ground-penetrating radar; Alaska; Arctic; permafrost; mammoth; Bering Land Bridge

1. Introduction Geophysical methods were first tested at archaeological sites in Alaska by J.L. Giddings and D.D. Anderson in 1960. These early investigations with proton precession magnetometry and electrical resistivity were wrought with challenges, and the methods were abandoned after one season of trials [1]. Many improvements have been made in geophysical technology since the early investigations Remote Sens. 2016, 8, 1007; doi:10.3390/rs8121007

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of Giddings and Anderson. Multiple geophysical methods, including ground-penetrating radar (GPR), 8, 1007 2 of 23 have Remote now Sens. been2016, successfully used for archaeological investigations in Alaska [2–4]; however, Arctic settings in the region are still fraught with challenges for geophysical investigations. Here we describe ground-penetrating radar (GPR), have now been successfully used for archaeological some recent applications of ground-penetrating radar in higher latitude regions of Alaska, highlighting investigations in Alaska [2–4]; however, Arctic settings in the region are still fraught with challenges for the extraordinary potential of the for site reconnaissance in a variety of environmental geophysical investigations. Heremethod we describe some recent applications of ground-penetrating radarsettings, in whilehigher also describing someof common of GPR such places.potential of the method for site latitude regions Alaska, pitfalls highlighting the in extraordinary We draw on examples multiple locations managed by the U.S. Park Service reconnaissance in a varietyfrom of environmental settings, while also describing someNational common pitfalls of GPR places. (Figure 1).inAtsuch Cape Krusenstern National Monument, GPR revealed complex patterned ground created We draw cycle on examples from multiple locations the U.S. National Serviceat this by the freeze-thaw at the Old Whaling beach ridgemanaged [3], whileby multi-year GPR dataPark collected (Figure 1). At temporal Cape Krusenstern National Monument, table. GPR revealed complex ground location illustrates variations in the permafrost In Kobuk Valley patterned National Park, a GPR created by the freeze-thaw cycle at the Old Whaling beach ridge [3], while multi-year GPR data investigation of an early contact period Inupiat settlement revealed an earlier, deeper occupation in collected at this location illustrates temporal variations in the permafrost table. In Kobuk Valley the permafrost just beneath the known site. At the Cape Espenberg locality in Bering Land Bridge National Park, a GPR investigation of an early contact period Inupiat settlement revealed an earlier, National Preserve, a deep Birnirk period house complex was investigated with GPR, with shallower deeper occupation in the permafrost just beneath the known site. At the Cape Espenberg locality in portions in the saturated active layer andadeeper portions frozen incomplex permafrost. Also at Bering Bering Land Bridge National Preserve, deep Birnirk period house was investigated withLand Bridge National Preserve, the skeletal remains of a late Pleistocene mammoth embedded in the bottom GPR, with shallower portions in the saturated active layer and deeper portions frozen in permafrost. of a thermokarst lake were successfully mapped the from the surface frozen lake. Finally, in Gates Also at Bering Land Bridge National Preserve, skeletal remainsofofthe a late Pleistocene mammoth in the bottom of a Preserve thermokarst were surface of the clear of theembedded Arctic National Park and an lake alpine icesuccessfully patch was mapped mappedfrom withthe GPR, showing frozen lake. Finally, in Gates of the Arctic National Park and settings Preserve an alpine ice in patch mapped bedding and suspended anomalies. In similar ice patch elsewhere thewas region, ancient with GPR, showing clear bedding and suspended anomalies. In similar ice patch settings elsewhere hunters targeted caribou and occasionally lost weaponry and other tools that have long been preserved in the region, ancient hunters targeted caribou and occasionally lost weaponry and other tools that in frozen conditions. With such features now melting, artifacts of these hunters are being revealed, have long been preserved in frozen conditions. With such features now melting, artifacts of these often in association with deposits of caribou dung that might be potentially identified with GPR before hunters are being revealed, often in association with deposits of caribou dung that might be the deposits melt. This latter case lays the groundwork for how GPR may be integrated into such potentially identified with GPR before the deposits melt. This latter case lays the groundwork for investigations, andbecontribute the management of and invaluable cultural resources, particularly how GPR may integrated to into such investigations, contribute to the management of invaluablethose that face the resources, threat of environmental change. cultural particularly those that face the threat of environmental change.

Figure 1. Study location. The boundaries of the National Parks and Preserves are indicated, including

Figure 1. Study location. The boundaries of the National Parks and Preserves are indicated, including from East to West, Gates of the Arctic (GAAR), Kobuk Valley (KOVA), and Cape Krusenstern from East to West, Gates of the Arctic (GAAR), Kobuk Valley (KOVA), and Cape Krusenstern (CAKR), all in the Arctic Circle, and Bering Land Bridge (BELA) which straddles the boundary of the (CAKR), all in the Arctic Circle, and Bering Land Bridge (BELA) which straddles the boundary of the Arctic Circle. Arctic Circle.

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Ground-Penetrating Radar (GPR) in Frozen and Partially Frozen Media Sub-surface velocity is the most crucial parameter to determine when applying GPR to archaeological investigations, since accurate estimation of depth and dimensions of archaeological features is contingent upon the accuracy of velocity estimates [5]. This is of great concern in Arctic settings where velocities may abruptly change with a shift from thawed to frozen media. A review of frequently used methods for estimating GPR velocity can be found in [5–7]. Case studies in this paper relied primarily on the technique of hyperbola fitting to estimate velocity. When a GPR profile is collected perpendicular to a point-scatter (i.e., an anomaly in the ground that results in a diffraction hyperbola) velocity may be estimated from the resulting hyperbolic curve on the basis of increasing travel time associated with increasing path length with regard to horizontal distance from the sub-surface anomaly [8,9]. The relationship of antenna position x, point-scatter depth d, travel time T, velocity v, and T 0 (the time at which the antenna is directly above the point-scatterer) can be described in simple terms by (Equation (1))  T=

4x2 2 + T0 v2

where T0 =

2d v

 21 (1)

(2)

A hyperbola fitting method may be used to match the spread of the observed hyperbola tails to those of calculated hyperbolas for a range of velocities and is undertaken as part of the post-processing of GPR data [10]. Accurate velocity estimates are especially important in partially frozen settings where velocities often approach extremes (Table 1). This is due to the abrupt contrast in relative electrical permittivity ε r between frozen and wet material, as ε r in most instances (within the frequency range used for GPR and normally encountered ranges of electrical conductivity) will be the primary property controlling velocity, and is itself controlled by liquid water content. The relationship in the simplest case can be described by (Equation (2)), where C is the speed of light. εr =

 2 C v

(3)

This disparity in electrical properties between thawed active layer and permafrost has previously been described in Alaskan contexts [11]. While a fully frozen medium results in a fast, though uniform velocity, the partially frozen medium often results in extreme velocity shifts between frozen and active layers (the portion of the ground which thaws seasonally). Determining the accurate depth and dimensions of an archaeological feature in such a case requires an accurate estimate of velocity in the substrate in which the feature of interest occurs (i.e., active layer or frozen layer). Of course archaeological features may cross this boundary and rest partially in a frozen layer and partially in the active layer. Another consideration of the partially frozen medium is that the active layer may be saturated with water and will therefore exhibit a very slow velocity. Since water cannot infiltrate vertically when underlain by a frozen layer, it often remains perched upon that layer. Velocities as slow as 0.05 m/ns might occur in such situations. If features in both the active layer and the frozen layer are to be assessed (or if features straddle the vertical boundary of these layers), the permafrost interface must be identified, and the two layers must be given separate velocity estimates and separate processing. Not doing so could result in distorted estimates of depths and dimensions. Further, it must be kept in mind that energy returned from within the permafrost layer has passed through the active layer twice. For this reason, estimating the velocity from a table (e.g., Table 1) can be inaccurate, as the average velocity must also account for transmission through the active layer, which is why an empirical method for determining average velocity is best. For the examples used in this paper,

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hyperbola fitting was undertaken using the spread of hyperbolic curves to estimate the average velocity (of the total ground from surface to a particular depth) for purposes of both depth determination and migration. When the estimate is correct, the majority of hyperbola tails will be eliminated in Remote Sens. 2016, 8, 1007 4 of 23 the post-migration data for the layer of interest. Even so, all methods for estimating velocity have shortcomings and limitations [5],When so when checks of depth comparing to determination and migration. the possible, estimate isdirect correct, the majority of estimates hyperbola by tails will be excavation, cores, scarps etc. to correlate known interfaces with those byfor GPR is advisable. eliminated in the post-migration data for the layer of interest. Even so, detected all methods estimating In thevelocity case studies presented here, observed[5], depths from excavation have been used as a check have shortcomings and limitations so when possible, direct checks of depth estimates by on comparing to excavation, cores, scarps etc. to correlate known interfaces with those detected by GPR velocity/depth estimates yielded from GPR results. Depth estimates were found to be consistent with is advisable. excavated cases. In the case studies presented here, observed depths from excavation have been used as a check on velocity/depth estimates yielded from GPR results. Depth estimates were found to be consistent with excavated cases. Table 1. Common ground-penetrating radar (GPR) velocities for a range of materials in order of decreasing velocity. Compiled and adapted from [6,12–14]. Table 1. Common ground-penetrating radar (GPR) velocities for a range of materials in order of decreasing velocity. Compiled and adapted from [6,12–14]. Material Velocity (m/ns)

Material Air Snow Air ColdSnow ice Temperate Cold ice ice Permafrost Temperate ice Dry gravel Permafrost Wet gravel Dry gravel Silt (wet) Wet gravel Clay (wet) Silt (wet) Fresh water Clay (wet) Fresh water

Velocity (m/ns) 0.3 0.3 0.2 0.16–0.18 0.2 0.15 0.16–0.18 0.12–0.14 0.15 0.12 0.12–0.14 0.10 0.12 0.07 0.10 0.06 0.070.033 0.06 0.033

A transmission line model, adapted from Heaviside’s telegrapher’s equations (1880) can illustrate a GPR pulse through a multi-velocity medium.from ThisHeaviside’s can be represented schematically an equivalent A transmission line model, adapted telegrapher’s equationsas(1880) can illustrate a GPR pulse 2). through a multi-velocity medium. This can be represented schematically an circuit diagram (Figure In this instance, the GPR pulse propagation is analogous to as electrical equivalent circuit diagram (Figure 2). In this instance, the GPR pulse propagation is analogous to in voltage propagation where V is voltage, I is current in amperes, R = resistance per unit length electrical voltage propagation where V is voltage, I is current in amperes, R = resistance per unit Ohm/m, L = inductance per unit length in Henry/m, C = capacitance per unit length in Farads/m, length in Ohm/m, L = inductance per unit length in Henry/m, C = capacitance per unit length in G = conductance per unit length in Seimens/m. The observed case can be more complicated due to Farads/m, G = conductance per unit length in Seimens/m. The observed case can be more complicated intermittent ice, perched water at the permafrost interface, and textural variations within the active due to intermittent ice, perched water at the permafrost interface, and textural variations within the layer,active factors which alsowhich complicate velocityvelocity estimates. layer, factors also complicate estimates.

Figure 2. Diagrammatic model and equivalent electrical circuit for GPR pulse in a partially frozen

Figure 2. Diagrammatic model and equivalent electrical circuit for GPR pulse in a partially setting. frozen setting.

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2. Methods 2. Methods The instrument used for all examples described in this paper was a 250 MHz center frequency The instrument used for all examples described in this paper was a 250 MHz center frequency (band width 125–375 MHz) Noggin System by Sensors and Software Inc. Instrument, and survey (band width 125–375 MHz) Noggin System by Sensors and Software Inc. Instrument, and survey parameters are listed in Table 2. All sites used as examples in this paper required access by small parameters are listed in Table 2. All sites used as examples in this paper required access by small aircraft, including float planes, ski planes and helicopters (Figure 3). This, in turn, required light aircraft, including float planes, ski planes and helicopters (Figure 3). This, in turn, required light equipment loads, which in some cases had to be backpacked some distance to and from a suitable equipment loads, which in some cases had to be backpacked some distance to and from a suitable landing site. This situation made options such as push-cart GPR setups and multiple antenna arrays landing site. This situation made options such as push-cart GPR setups and multiple antenna arrays less desirable. reason, a mid-range antenna frequency (i.e., (i.e., one that trusted for a balance less desirable.For Forthis this reason, a mid-range antenna frequency one can thatbecan be trusted for a ofbalance both reasonable resolution and penetration) and small sled deployment setup were chosen for the of both reasonable resolution and penetration) and small sled deployment setup were chosen work (Figure for the work 3). (Figure 3). Data procedures included: included: visual visual inspection editing of data, time-zero Data processing processing procedures inspection andand editing of data, time-zero rere-picking, dewow, velocity estimation with hyperbola SEC-2 gain, background picking, dewow, velocity estimation with hyperbola matching,matching, SEC-2 gain, background subtraction subtraction (avg. ofmigration all traces);(where migration (whereenvelope indicated); envelope (wheretime-depth indicated);conversion; time-depth (avg. of all traces); indicated); (where indicated); conversion; and topographic corrections (when necessary). In some instances, a running average and topographic corrections (when necessary). In some instances, a running average (three-trace (three-trace window) was as a horizontal pass to emphasize the interface the frozen window) was applied asapplied a horizontal low passlow filter to filter emphasize the interface of the of frozen and and active layers. Data processing was undertaken with EKKOProject by Sensors and Software Inc. and active layers. Data processing was undertaken with EKKOProject by Sensors and Software Inc. and final images were produced with Voxler 4 and/or Surfer 13, both by Golden Software Inc. Final figures final images were produced with Voxler 4 and/or Surfer 13, both by Golden Software Inc. Final include time-depthtime-depth slices (plan-view) and 3-D perspective view images figures profiles include(vertical profiles cross-section), (vertical cross-section), slices (plan-view) and 3-D perspective using partially opaque data volumes and/or fence diagrams. view images using partially opaque data volumes and/or fence diagrams. Table parameters. Table 2. 2. Instrument and survey parameters.

Parameter Parameter Gridded traverse Gridded traverse Transectinterval interval Transect Traceinterval interval Trace Tx-Rx Tx-Rxoffset offset Time Timewindow window Stacks Stacks

(varied) anti-parallel, sometimes bi-directional (varied) anti-parallel, sometimes bi-directional (varied) 0.20–0.5 mm (varied) 0.20–0.5 0.05 mm 0.05 0.279 0.279 mm (varied) 80–200 (varied) 80–200 nsns 8 per station 8 per station

Figure 3. Fieldwork landing site at Bering Land Bridge. Shown: T. Urban. Photo by J. Rasic. Figure 3. Fieldwork landing site at Bering Land Bridge. Shown: T. Urban. Photo by J. Rasic.

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3. Results and Discussion 3.1. Dynamics of the Partially Frozen Environment: The Old Whaling Ridge at Cape Krusenstern National Monument 3.1.1. The Old Whaling Site In the mid-twentieth century, J. Louis Giddings and his students documented two groups of prehistoric house features dating to ca. 3000 BP. These were located on an ancient beach ridge in what is now Cape Krusenstern National Monument in northwestern Alaska (Figure 1), and they were noteworthy because they contained a unique artifact assemblage and house forms unlike any other site yet (or since) discovered in Alaska. The site also contained whale bones, and large weapon heads and butchering tools, which Giddings interpreted as evidence for active whale hunting by the inhabitants, and the basis for what he named the “Old Whaling” culture [15]. Based on the different construction techniques between the various house features, Giddings believed that the site contained evidence of both cold and warm season occupations by the same cultural group [15,16]. Other researchers have since highlighted similarities between the Old Whaling site and maritime cultures evident in the Bering Strait region, primarily in Chukotka [17,18]. Others have suggested connections to the widespread and comparatively well-known Northern Archaic tradition [19,20]. Renewed test excavations at the site in 2003 found additional buried deposits suggesting the history of occupations was more complex than previously envisioned [19,21]. 3.1.2. GPR Investigations at the Old Whaling Site, 2011–2016 The Old Whaling site lies on beach ridge 53, roughly 1.3 km from the coast. There is a 100 m strip of open space between two discrete clusters of earthen houses—Giddings’s summer and winter settlements [15,16]. A 2011 GPR survey of this open space drew attention to the freeze-thaw dynamics of the site [3] and added concern over the general threat of permafrost variations, leading to subsequent data collection over the next five years for the purpose of monitoring these dynamics in relation to global environmental change. The resulting data offer a glimpse of how this type of environment varies throughout the summer and from year to year from a GPR perspective. As average temperatures rise above freezing in May, the frozen ground begins to thaw. By the end of summer, this thawed layer (active layer) has reached its maximum thickness. In early fall, as average temperatures drop below freezing, the thawed ground begins to freeze once again. The interface between the active and frozen layers is detectable with GPR, and leads to predicable velocity changes (Figure 4). By late August, the thickness of the active layer may be double what is observed in June or July (Figure 5). Because this layer thaws and re-freezes while often exhibiting a significant water content, networks of distinct patterns form in the sub-surface that are visible in GPR data (Figure 6), a phenomenon occasionally observed on the surface of some areas underlain by permafrost [22]. Similar features have been mapped with GPR at other locations in Alaska [23,24]. Many components of these patterns, if viewed in isolation, could easily be mistaken for house perimeters on the basis of size and shape. Since these patterns appear as broad networks, they do not exhibit the detail of internal house structure (hearths, floors etc.) and most often continue unchanged for the full depth range of the active layer (while houses would vary much more with depth); these are thus readily distinguishable from most cultural features observed in GPR data.

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Figure 4. Permafrost table in July 2016 at the Old Whaling Ridge. The frozen interface results in a Figure 4. Permafrost Permafrost table table in in July July 2016 2016at atthe theOld OldWhaling Whaling Ridge.The Thefrozen frozeninterface interfaceresults resultsinina Figure sudden4.velocity increase, along with changes in diffractionRidge. tails that indicate an average velocity asudden suddenvelocity velocityincrease, increase,along alongwith withchanges changesinindiffraction diffractiontails tails that that indicate indicate an an average average velocity velocity change. Here the active layer is ca. 1 m thick before frozen ground is encountered. change. Here the m thick thick before before frozen change. Here the active active layer layer is is ca. ca. 11 m frozen ground ground is is encountered. encountered.

Figure 5. The late summer (August) permafrost table at the Old Whaling Ridge. This is the full extent Figure 5. Themonitoring late summer (August) permafrost at the Old Whaling Ridge. This the full extent of the same station shown in Figure table 5. A three-trace running average hasisbeen to Figure 5. The late summer (August) permafrost table at the Old Whaling Ridge. This applied is the full of the same monitoring station shown in that Figure 5.active A three-trace runningnearly average hasdeeper been applied to emphasize horizontal reflectors. Note the layer extends 1 m than the extent of the same monitoring station shown in Figure 5. A three-trace running average has been emphasize horizontal reflectors. Note 5.that the active layer extends nearly 1 m than the mid-summer depth shown in Figure The layer exhibits a network of deeper cracks from the applied to emphasize horizontal reflectors. Noteactive that the active layer extends nearly 1 m deeper than mid-summer depth shown in Figure 5. The active layer exhibits a network of cracks from the freeze-thaw cycle,depth whichshown are clear in plan-view data (bottom). of these the mid-summer in Figure 5. TheGPR active layer exhibitsInaisolation, network perimeters of cracks from the freeze-thaw cycle, whichbe aremistaken clear in plan-view GPR data in (bottom). In isolation, perimeters of these frost polygons can easily for cultural features the sub-surface. freeze-thaw cycle, which are clear in plan-view GPR data (bottom). In isolation, perimeters of these frost polygons can easily be mistaken for cultural features in the sub-surface. frost polygons can easily be mistaken for cultural features in the sub-surface.

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Figure 6. Semi-subterranean house diagram. Figure 6. Semi-subterranean house diagram.

3.2. Semi-Subterranean Houses in Partially Frozen Settings: Bering Land Bridge National Preserve and 3.2. Semi-Subterranean Houses in Partially Frozen Settings: Bering Land Bridge National Preserve and Kobuk Kobuk Valley National Park Valley National Park Houses throughout most of the occupational history of Alaska were semi-subterranean Houses throughout most of the occupational history of Alaska were semi-subterranean structures structures of wood and earth (sometimes bone). Though such structures varied in size and of wood and earth (sometimes bone). Though such structures varied in size and complexity, complexity, they generally included a wood-framed depression, often with an entry tunnel and they generally included a wood-framed depression, often with an entry tunnel and sometimes a sometimes a central hearth. A general diagram of such a house is shown in Figure 6. The central hearth. A general diagram of such a house is shown in Figure 6. The archaeological remains of archaeological remains of such a structure may, at various times of year, be fully or partially frozen, such a structure may, at various times of year, be fully or partially frozen, often straddling thawed often straddling thawed and frozen layers. In this section, we give examples from two sites where and frozen layers. In this section, we give examples from two sites where house features occur both in house features occur both in thawed and frozen ground. thawed and frozen ground. 3.2.1. A A Birnirk Birnirk House House Complex Complex at at Cape Cape Espenberg, Espenberg, Bering Bering Land Land Bridge Bridge National National Preserve Preserve 3.2.1. Over the the past pastseven sevenyears, years,aateam teamofofresearchers researchershas hasbeen been excavating and studying remains Over excavating and studying thethe remains of a series of semi-subterranean house features that document the cultural transition from Birnirk to aofseries of semi-subterranean house features that document the cultural transition from Birnirk to Thule Thule of the Northern Maritime tradition at Cape Espenberg, Bering Land Bridge National of the Northern Maritime tradition at Cape Espenberg, Bering Land Bridge National Preserve [25,26]. Preserve [25,26]. Cape of Espenberg of formations a series ofthat beach formations that have Cape Espenberg consists a series of consists beach ridge haveridge accumulated horizontally for accumulated horizontally for 5000 years [27], preserving within them a 4500-year-old archaeological 5000 years [27], preserving within them a 4500-year-old archaeological record of human adaptation record of human adaptation to environmental change [27–31]. One characteristic thatthe delineates the to environmental change [27–31]. One characteristic that delineates the Birnirk from later Thule Birnirk is from the later Thule is the apparent variety andsome complexity ofinclude house designs, some of culture the apparent varietyculture and complexity of house designs, of which many connected which include many connected rooms, while others are reminiscent of the later Thule forms rooms, while others are reminiscent of the later Thule forms [31–33]. Typical Thule houses [31–33]. tend to Typical tenda long to have one main room entrance and side often[32] a kitchen have oneThule main houses room with entrance tunnel andwith oftenaalong kitchen alcovetunnel off to one (p. 81). alcove to one [32] (p. 81). A used recent study byofDarwent et al. [27] used attributes of surface A recentoff study by side Darwent et al. [27] attributes surface depressions to assess whether house depressions to assess whether house shape and design at Cape Espenberg showed evidence for shape and design at Cape Espenberg showed evidence for change through time. The results indicate change through time. The results indicate designs evolved from complex multi-room structures designs evolved from complex multi-room structures towards simple one-room houses with straight towardsExcavations simple one-room houses and withMason’s straight2011 tunnels. by among Hoffecker and Mason’s 2011 tunnels. by Hoffecker teamExcavations [25] found that six house depressions team [25] found that among six house depressions excavated on three beach ridges, house floors excavated on three beach ridges, house floors could be located beneath apparent depressions in all couldbut be located apparent in all cases butexcept one. Infor thealatter, nobuild clear tunnel house feature cases one. Inbeneath the latter, no cleardepressions house feature was found deeply which was found except for a deeply build tunnel which suggests potential reuse of structural wood to suggests potential reuse of structural wood to build new houses. This raised the question: can build GPR new houses. This raised the question: can GPR be used to locate deeply buried house features with sufficient resolution to demarcate the design and dimensions?

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be used to locate deeply buried house features with sufficient resolution to demarcate the design Remote Sens. 2016, 8, 1007 9 of 23 and dimensions? To question we weinitiated initiateda astudy study Cape Espenberg using to collect To answer answer this this question at at Cape Espenberg using GPRGPR to collect data data from from a series of depressions thought to contain buried house ruins. The goal was to non-invasively a series of depressions thought to contain buried house ruins. The goal was to non-invasively document document house housedesign, design,floor floorconfiguration, configuration,and andtunnel tunneldirections. directions. Processing Processing and andinterpreting interpretingthe the data datawas wascomplicated complicated by bythe thefact factthat thatportions portionsof ofthe thesame samefeature-complex feature-complexstraddled straddledthe theboundary boundary of of the theactive active and and frozen frozenlayers layers (Figure (Figure 7), 7), thus thusinvolving involving two two different different GPR GPR velocities velocities as as shown shown in in Table 1. To deal with this situation, separate velocity estimates were undertaken for these two layers Table 1. To deal with this situation, separate velocity estimates were undertaken for these two layers and and data data processing processing adjusted adjusted accordingly accordingly (Figure (Figure 8). 8). The The results results show show that that not not only only are arestructural structural elements detectable but cultural deposits extend as deep as 3 m and indicate multiple elements detectable but cultural deposits extend as deep as 3 m and indicate multiple structures, structures, possibly cache pitspits present at many levels. This likely indicates multiple rebuilding events possiblyfloors, floors,with with cache present at many levels. This likely indicates multiple rebuilding (Figures 9 and 10). The GPR results also show that many of these features are located below the events (Figures 9 and 10). The GPR results also show that many of these features are located active below layer, exceeding 1 m deep (Figure 7) but still resolvable with GPR (Figures 9 and 10). While such frozen the active layer, exceeding 1 m deep (Figure 7) but still resolvable with GPR (Figures 9 and 10). While material bodes well for preservation of organic artifacts, it isartifacts, a limiting factor for timely such frozen material bodes well for preservation of organic it is a limiting factorexcavation for timely where field seasons are short. These results suggest that the design history of the buried excavation where field seasons are short. These results suggest that the design history of thehouses, buried particularly on the Birnirk beach ridge, appears much more complex than indicated from the surface. houses, particularly on the Birnirk beach ridge, appears much more complex than indicated from the Interestingly, however, however, results show the appearance of a room that one that to was excavated surface. Interestingly, results show the appearance of a connects room thattoconnects one that was at about the same depth (Figures 10 and 11). The layout of these house structures resemble, perhaps not excavated at about the same depth (Figures 10 and 11). The layout of these house structures resemble, superficially, sketch maps the of house features the Birnirk site nearatBarrow [33] (p.site 38) near and perhaps notthe superficially, sketch maps recorded of houseatfeatures recorded the Birnirk Jabbertown at Point Hope [34] (p. 171). This provides further support for the hypothesis that Birnirk Barrow [33] (p. 38) and Jabbertown at Point Hope [34] (p. 171). This provides further support for people were present at Cape Espenberg and that their settlements included multi-roomed the hypothesis that Birnirk people were present atsome Cape of Espenberg and that some of their settlements house designs. Planned excavations will provide an opportunity to not only confirm these included multi-roomed house designs. Planned excavations will provide an opportunityhypotheses to not only but to refine thehypotheses analytical method as well. confirm these but to refine the analytical method as well.

Figure 7. A sample GPR profile from Cape Espenberg. The permafrost interface is clear at 20–23 ns. Figure 7. A sample GPR profile from Cape Espenberg. The permafrost interface is clear at 20–23 ns. The active layer exhibits an average velocity of 0.82 m/ns for a depth estimate to the frozen interface The active layer exhibits an average velocity of 0.82 m/ns for a depth estimate to the frozen interface of of ca. 1 m. This velocity, however, yields inaccurate depth estimates at depths greater than 1 m ca. 1 m. This velocity, however, yields inaccurate depth estimates at depths greater than 1 m because because the frozen ground exhibits an average velocity of 0.12 m/ns. This data set was collected within the frozen ground exhibits an average velocity of 0.12 m/ns. This data set was collected within several several days of the Cape Krusenstern 2016 expedition data and shows the frozen interface (and days of the Cape Krusenstern 2016 expedition data and shows the frozen interface (and velocities) are velocities) are comparable to those observed at the Old Whaling Ridge. comparable to those observed at the Old Whaling Ridge.

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Figure Profileprocessed processedas asaatwo-velocity two-velocity medium. medium. The ofof both thethe frozen andand Figure 8. 8. Profile Theaverage averagevelocities velocities both frozen Figure 8. Profile as a two-velocity medium. The average of both theprofile frozen and active layer from processed the same profile shown in Figure 7 were used to velocities produce the above with active layer from the same profile shown in Figure 77 were used to to produce producethe theabove aboveprofile profile with active layer from the same profile shown in Figure were used with accurate depth estimates for both layers. Depths in the frozen layer are tens of centimeters more than accurate depth estimates for both layers. Depths in the frozen layer are tens of centimeters more than accurate depth estimates for both layers.alone. Depths the frozen layer of centimeters more than estimated with the active layer velocity Theinprofile above hasare alsotens migrated using the velocity estimated with the active layer velocity alone. The profile above has also migrated using the velocity estimated with the Note activethat layer alone. The profile above alsoreduced migrated the velocity of the frozen layer. thevelocity majority of diffraction tails havehas been or using eliminated when of the frozen layer. Note that the majority of diffraction tails have been beenreduced reducedororeliminated eliminated when of the frozen layer. Note that the majority of diffraction tails have when compared to the previous figure. compared to to thethe previous compared previousfigure. figure.

Figure 9. (a) Portion of a Birnirk house complex in the active layer (upper meter) with layout and Figure 9. (a)consistent Portion of a Birnirk house complex in the active layercolors (upper meter)higher with layout and dimensions nearby excavated examples. darker indicate Figure 9. (a) Portion ofwith a Birnirk house complex in theThe active layer (upper meter) withamplitude layout and dimensions consistent with nearby excavated examples. The darker colors indicate higher amplitude reflections consistent from buried wood, boneexcavated and otherexamples. material asThe seendarker in thecolors nearbyindicate excavation; (b) Portion dimensions with nearby higher amplitude reflections from buried wood,inbone and other material as seen below in the nearby excavation; (b) Portion of a Birnirk house complex the frozen layer (extending one meter) with layout and of reflections from house buried wood, bone andfrozen other material as seen inbelow the nearby excavation; (b) Portion of a Birnirk in the (extending oneportion meter) layout and dimensions consistentcomplex with nearby excavated layer examples. This is a deeper ofwith the same house a Birnirk houseconsistent complex with in thenearby frozenexcavated layer (extending below one meter) with layout and dimensions dimensions examples. This is a deeper portion of the same house complex shown in (a); (c) Annotated interpretations 1 and 2 indicated the proposed outline of consistent nearby(a); excavated examples. This is a deeper portion of the same house complex shown complexwith shown (c) 1 and 2 The indicated the proposed of structures interredinfrom (a), Annotated along withinterpretations likely tunnel locations. orientation and scaleoutline of these in (a); (c) Annotated interpretations 1 and 2 indicated the proposed outline of structures interred from structures from (a), along with excavation likely tunnel locations. andexcavated scale of these features areinterred comparable to the nearby (Figures 10 The and orientation 11), and the and (a),features along with likely tunneltolocations. Theexcavation orientation(Figures and scale of these features are comparable are comparable the nearby 10 and 11), and the excavated and to unexcavated structures are likely connected; (d) Annotations indicate a likely cluster of timbers, three, theunexcavated nearby excavation (Figures 10 and 11), and the excavated and unexcavated structures are likely structures arefour. likelyOther connected; (d)anomalies Annotations indicatethroughout a likely cluster timbers, three, and external storage pits, discrete scattered the of survey area are connected; (d) Annotations indicate a likely cluster of timbers, three, and external storage pits, four. and external storage Other discrete anomalies scattered throughout theissurvey area are likely caused by debrispits, suchfour. as wood and whale bone external to the structures. This also consistent Other discrete anomalies scattered throughout area arestructures. likely caused byalso debris such as likely caused byexcavation. debris such as wood and whale the bonesurvey external to the This is consistent with the nearby wood external to the structures. This is also consistent with the nearby excavation. withand thewhale nearbybone excavation.

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Figure 10. Feature 12 at Cape Espenberg KTZ304 site. Excavated area in 2011 to the west of the Figure 10. Feature 12 at Cape Espenberg KTZ304 site. Excavated area in 2011 to the west of the surveyed area shown in Figure 9. Depth excavated is 0.9 to 1.2 m below surface (map by Sylvie Elies surveyed area shown in Figure 9. Depth excavated is 0.9 to 1.2 m below surface (map by Sylvie Elies and Claire AlixAlix andand photo byby Claire wasencountered encountered at depths consistent and Claire photo ClaireAlix). Alix).Frozen Frozen material material was at depths consistent withwith the GPR estimates. The layout and complexity of deposits was also consistent with the GPR results the GPR estimates. The layout and complexity of deposits was also consistent with the GPR results shown in Figures 9 and 10,10, which eastof ofthe theFeature Feature excavation shown in Figures 9 and whichisisimmediately immediately east 1212 excavation unit.unit.

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Figure Figure 11. 11. The The relationship relationship of of the the excavated excavated structure structure to to the theunexcavated unexcavatedfeatures features detected detectedwith withGPR. GPR. The comparative scale and alignment of the known and inferred features is obvious. The GPR The comparative scale and alignment of the known and inferred features is obvious. The GPR image image (right) 3-D iso-surface iso-surface showing showing high high amplitude amplitude anomalies anomalies spanning spanning aa depth depth of of ca. ca. 0.5–1.2 0.5–1.2 m. m. (right) is is aa 3-D

3.2.2. A Village Site at Kobuk Valley National Park with an Earlier Site below the Permafrost Table 3.2.2. A Village Site at Kobuk Valley National Park with an Earlier Site below the Permafrost Table The Kobuk River valley (see Figure 1) has been the site of numerous woodland Eskimo The Kobuk River valley (see Figure 1) has been the site of numerous woodland Eskimo settlements settlements spanning thousands of years [34–39]. Compared to Bering Land Bridge and Cape spanning thousands of years [34–39]. Compared to Bering Land Bridge and Cape Krusenstern, the Krusenstern, the village site of Igliqtiqsiugvigruaq in Kobuk Valley National Park has the added village site of Igliqtiqsiugvigruaq in Kobuk Valley National Park has the added challenge of being a challenge of being a forested site, where trees and vegetation limit the application of GPR [2]. The forested site, where trees and vegetation limit the application of GPR [2]. The site is an early contact site is an early contact (19th-century) Inupiat village with evidence of limited Euro-American contact (19th-century) Inupiat village with evidence of limited Euro-American contact dating to around the dating to around the time of the early 19th-century Euro-American expeditions in the region e.g., time of the early 19th-century Euro-American expeditions in the region e.g., [40,41]. Geophysical [40,41]. Geophysical investigations at the site involved three methods: GPR, magnetic gradiometry, investigations at the site involved three methods: GPR, magnetic gradiometry, and electromagnetic and electromagnetic induction (EM). Insights derived from this work included not only induction (EM). Insights derived from this work included not only understandings of the visible understandings of the visible house pits and other village features [2], but also the discovery of a house pits and other village features [2], but also the discovery of a deeper, earlier settlement beneath deeper, earlier settlement beneath the thawed active layer. The initial GPR work completed at the site the thawed active layer. The initial GPR work completed at the site in 2011 focused on areas in in 2011 focused on areas in between the visible house depressions as these were assumed to be devoid between the visible house depressions as these were assumed to be devoid of structures. The sole of structures. The sole exception was a small strip south of House I, the ongoing excavation of which exception was a small strip south of House I, the ongoing excavation of which had suggested a had suggested a possible tunnel extending from the house perimeter. Broader surveys undertaken in possible tunnel extending from the house perimeter. Broader surveys undertaken in 2013, in search 2013, in search of additional tunnels, revealed (in addition to shallower features likely contemporary of additional tunnels, revealed (in addition to shallower features likely contemporary to the known to the known village) evidence of a deeper occupation throughout the site, within the frozen layer village) evidence of a deeper occupation throughout the site, within the frozen layer beneath the beneath the known village (Figures 12 and 13). Since the substrate in this case exhibited less water known village (Figures 12 and 13). Since the substrate in this case exhibited less water content at the content at the time of data collection than the previous cases shown for Bering Land Bridge or Cape time of data collection than the previous cases shown for Bering Land Bridge or Cape Krusenstern, Krusenstern, a much smaller disparity between the velocities (i.e., dielectric properties) of active and a much smaller disparity between the velocities (i.e., dielectric properties) of active and frozen layers frozen layers was evident. The permafrost depth encountered during the excavation of House I was was evident. The permafrost depth encountered during the excavation of House I was consistent with consistent with GPR depth estimates from the surrounding area. GPR depth estimates from the surrounding area. In addition to the deeper house detected with GPR in the vicinity of House I, many other deeper In addition to the deeper house detected with GPR in the vicinity of House I, many other deeper structures were detected throughout this sprawling village site, suggesting that the area had been a structures were detected throughout this sprawling village site, suggesting that the area had been a site of significant occupations prior to the 19th-century village. Though these deeper features remain site of significant occupations prior to the 19th-century village. Though these deeper features remain untested archaeologically, optically stimulated luminescence (OSL) dating of sediments just beneath

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untested archaeologically, optically stimulated luminescence (OSL) dating of sediments just beneath the floor of House I suggests that the deeper structures are likely at least several centuries older than the floor of House I suggests that the deeper structures are likely at least several centuries older than the village. the village.

Figure 12. GPR survey area around House I depression. The outline of a deeper house appeared in Figure 12. GPR survey area around House I depression. The outline of a deeper house appeared in the the GPR survey from a depth of 1.4–2.0 m, shown above as a series of depth-slices at 0.1 m increments. GPR survey from a depth of 1.4–2.0 m, shown above as a series of depth-slices at 0.1 m increments. The projected perimeter of the deeper house is indicated in yellow in the final slice. GPR coverage The projected perimeter of the deeper house is indicated in yellow in the final slice. GPR coverage was was limited the presence of at trees this forested limited by thebypresence of trees thisatforested site. site.

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Figure 13. House I. The surface depression designated House I was excavated to the house floor and

Figure 13. House I. The surface depression designated House I was excavated to the house floor dated to the 19th century. Permafrost encountered in the deepest portions of the excavation was andconsistent dated to with the 19th century. Permafrost encountered in the deepest portions of the excavation was an interface identified in the GPR survey around the depression. The GPR survey also consistent with an interface identified in the GPR survey around the depression. The GPR survey showed an additional occupation in the frozen layer. Similar features were detected in other areas of also showed an additional occupation in the frozen layer. Similar features were detected in other areas the village site at comparable depths. The darker colors represent higher amplitude reflections. Our of the village site at comparable depths. The darker colors represent higher amplitude reflections. excavated cases at this site indicate that wood framing timbers are the cause of these anomalies. Our excavated cases at this site indicate that wood framing timbers are the cause of these anomalies. 3.3. Fully Frozen Settings: Bering Land Bridge and Gates of the Arctic

3.3. Fully Settings: Bering Land and Gates of the Arctic In Frozen the final two case studies, weBridge present GPR results from fully frozen media. These results are more experimental andstudies, speculative than results from the known humanmedia. occupations In the final two case we present GPR results from fully frozen Thesealready results are presented. These, we believe, demonstrate the incredible potential for using GPR in frozen more experimental and speculative than results from the known human occupations already presented. archaeological contexts other than known sites of human occupation, while recognizing that these These, we believe, demonstrate the incredible potential for using GPR in frozen archaeological contexts two case studies provide less definitive results than the houses mapped with GPR at Bering Land other than known sites of human occupation, while recognizing that these two case studies provide Bridge and Kobuk Valley.

less definitive results than the houses mapped with GPR at Bering Land Bridge and Kobuk Valley. 3.3.1. The Bering Land Bridge Mammoth

3.3.1. The Bering Land Bridge Mammoth

In 2015, a GPR survey was conducted on a frozen thermokarst lake in Bering Land Bridge In 2015,Preserve a GPR (see survey was on a frozen thermokarst lake in BeringofLand Bridge National Figure 1) conducted in order to assess the distribution of skeletal remains a woolly

National Preserve (see Figure 1) in order to assess the distribution of skeletal remains of a woolly mammoth (Mammuthus primigenius), and more generally to test the feasibility of using GPR to detect

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paleontological remains in such contexts (Figure 14). Several skeletal components from a single mammoth (Mammuthus primigenius), and more generally to test the feasibility of using GPR to detect mammoth were located in 2012 on, and embedded within, the muddy bed of a shallow thermokarst paleontological remains in such contexts (Figure 14). Several skeletal components from a single lake. The skeletal materials were submerged and only identified due to a period of mammoth were located in 2012 on, shallowly and embedded within, the muddy bed of a shallow thermokarst unusually lake materials levels. They very to accessdue and due lake. Thelow skeletal wereremained, shallowly however, submerged anddifficult only identified to inventory a period of low lakeand levels. They buried remained, however, difficult to and inventory due from to to unusually their submerged partially context. Thevery site appears to access have multiple elements their submerged buried context. The site appears to have multiple elements from a single individual and and partially may contain a complete or largely complete skeleton, which is rare for athe singleCollagen individual and may contain abones complete or largely complete skeleton, which rareafor the region. from two mammoth collected from the site (KTZ-00345), a toothis and vertebra, region. Collagen from two mammoth bones collected from the site (KTZ-00345), a tooth and were radiocarbon dated to 12,330 ± 50 BP (Beta-329841, 12,720–12,140 Cal BC) and 12,430 ± 50aBP vertebra, were radiocarbon dated to 12,330 ± 50 BP (Beta-329841, 12,720–12,140 Cal BC) and 12,430 ± 50 BP (Beta-331336, 12,940–12,220 Cal BC), respectively. A total of seven faunal components were identified (Beta-331336, 12,940–12,220 Cal BC), respectively. A total of seven faunal components were identified in in 2012 with osteological examination indicating that these were likely from an individual mammoth 2012than withaosteological indicating these were likelyattempt from anto individual mammoth rather co-mingledexamination deposit. GPR was usedthat in an experimental detect and more fully rather than a co-mingled deposit. GPR was used in an experimental attempt to detect and more fully map the distribution of additional skeletal material in this poor visibility setting, and further assess map the distribution of additional skeletal material in this poor visibility setting, and further assess the completeness of the skeleton and the degree to which faunal components had been dispersed. the completeness of the skeleton and the degree to which faunal components had been dispersed. GPS coordinates recorded for several skeletal components identified in 2012 were used for the later GPS coordinates recorded for several skeletal components identified in 2012 were used for the later GPR investigation and allowed us to establish a geo-referenced grid around the coordinates of the GPR investigation and allowed us to establish a geo-referenced grid around the coordinates of the previously investigated mammoth bones. The GPR data were collected from the frozen surface of the previously investigated mammoth bones. The GPR data were collected from the frozen surface of the lake, which provided platform for forthe theGPR GPRsurvey. survey. lake, which providedananexcellent excellentflat flatand andunobstructed unobstructed platform

Figure 14. Top: Mammoth vertebra (left) and humerus (right) located in 2012. NPS photo, J. Rasic. Figure 14. Top: Mammoth vertebra (left) and humerus (right) located in 2012. NPS photo, J. Rasic. (Shown: Louise Farquharson); Bottom: View of the mammoth bone scatter survey area from the air. (Shown: Louise Farquharson); Bottom: View of the mammoth bone scatter survey area from the air. Photo by J. Rasic 2015. The landing track of the plane as well as the grid pattern of the completed GPR Photo by J.are Rasic 2015. The landing track of the plane as well as the grid pattern of the completed GPR survey visible. survey are visible.

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Multiple Multiple anomalies anomalies consistent consistent with with aa scatter scatter of of mammoth mammoth skeletal skeletal remains remains are are evident evident on on the the lake floor (Figure 15). Several of these anomalies can be co-located with GPS-recorded locations lake floor (Figure 15). Several of these anomalies can be co-located with GPS-recorded locations of of specific bones identified during the 2012 site reconnaissance. Immediately west of these marked specific bones identified during the 2012 site reconnaissance. Immediately west of these marked locations locations are are several several anomaly anomaly clusters, clusters, which which suggest suggest additional additional skeletal skeletal material, material, including including linear, linear, curved anomalies that match the size and shape of tusks—skeletal elements expected to occur but not curved anomalies that match the size and shape of tusks—skeletal elements expected to occur but identified during the 2012 field investigations. Other discrete or clustered anomalies seen between not identified during the 2012 field investigations. Other discrete or clustered anomalies seen the surveythe surface lake bottom represent additionaladditional skeletal remains, aqueous between surveyand surface and lakemay bottom may represent skeletal clusters remains,ofclusters of vegetation, or trapped gasses, all of which could result in abrupt changes in electrical properties aqueous vegetation, or trapped gasses, all of which could result in abrupt changes in electrical thus generating GPR anomalies. Some of these anomalies seem to occur with broader properties thus generating GPR anomalies. Some of these anomalies seemintoassociation occur in association with morphological trends that could in turn act to trap accumulated debris. It is unlikely given the location broader morphological trends that could in turn act to trap accumulated debris. It is unlikely given of lake that anylake of the GPR could be related tobe logs or rocks since notthese present thethe location of the that anyanomalies of the GPR anomalies could related to logs orthese rocksare since are in the area. All of the anomalies noted in Figure 15 are at the identified depth of the lake bottom, not present in the area. All of the anomalies noted in Figure 15 are at the identified depth of the lake on a fairly shallow Features detecteddetected with GPR beneath this interface appear to be natural bottom, onflat, a fairly flat, shelf. shallow shelf. Features with GPR beneath this interface appear to be cryogenic features (Figure 16). The GPR manifestation of these deeper features is comparable to natural cryogenic features (Figure 16). The GPR manifestation of these deeper features is comparable ice-wedge polygon features described by Munroe et al. [24]. to ice-wedge polygon features described by Munroe et al. [24].

Figure 15. Interpretation. (1) The location of four known skeletal components (ulna, humerus, rib, Figure 15. Interpretation. (1) The location of four known skeletal components (ulna, humerus, rib, vertebra) clearly coincides with a cluster of GPR anomalies of various sizes; (2) The known location vertebra) clearly coincides with a cluster of GPR anomalies of various sizes; (2) The known location of of two additional vertebrae also coincides with a cluster of GPR anomalies; (3) Several anomalies west two additional vertebrae also coincides with a cluster of GPR anomalies; (3) Several anomalies west of of the main cluster on known skeletal components are suggestive in form of additional bones; (4) A the main cluster on known skeletal components are suggestive in form of additional bones; (4) A cluster cluster of anomalies on a plane just above the lake bottom exhibits several velocity/polarization of anomalies on a plane just above the lake bottom exhibits several velocity/polarization oddities that oddities that may be related to frozen vegetation and/or trapped gas pockets along with additional may be related to frozen vegetation and/or trapped gas pockets along with additional bones; (5) Also bones; (5) Also shown on a plane just above the lake bottom, and perhaps most suggestive, is a pair shown on a plane just above the lake bottom, and perhaps most suggestive, is a pair of anomalies that of anomalies that exhibit forms consistent with tusks. All of these anomalies seem to be settled on (or exhibit forms consistent with tusks. All of these anomalies seem to be settled on (or embedded in) the embedded in) the lake bottom on a generally flat area located in between a shallow drop-off in the lake bottom on a generally flat area located in between a shallow drop-off in the east (about a 25 cm east (about a 25 cm drop) and a sloping drop in the west of somewhat greater magnitude. drop) and a sloping drop in the west of somewhat greater magnitude.

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Figure 16. Just beneath the shallow lake bottom, the GPR data reveal patterns characteristic of ice Figure 16. Just beneath the shallow lake bottom, the GPR data reveal patterns characteristic of ice wedge polygons. The interface of the silty lake bottom, upon which the mammoth bones rest is wedge polygons. The interface of the silty lake bottom, upon which the mammoth bones rest is indicated in the profile (top). A deeper interface seen in the profiles may be due to a textural transition indicated in the profile A deeper interface seen in the profiles may be due to a textural transition in the substrate, from(top). finer-grain sediment to coarse gravel. in the substrate, from finer-grain sediment to coarse gravel.

3.3.2. An Alpine Ice Field in Gates of the Arctic National Park and Preserve 3.3.2. An Alpine Ice Field in Gates of the Arctic National Park and Preserve Globally significant archaeological and paleoecological discoveries have been made in the last two decades in the archaeological surprising context alpine snow patches and icehave fields, which canin contain Globally significant andofpaleoecological discoveries been made the last perishable objects of the sort rarely in most archaeological contexts. twowell-preserved decades in the surprising context of alpine snowpreserved patches and ice fields, which can contain Discoveries include an abundance spear in andmost dartsarchaeological complete withcontexts. intact well-preserved perishable objects of oftheweaponry—arrow, sort rarely preserved wooden shafts, fletchingof and fiber lashings—asspear well and as skin clothing andwith barkintact containers. Discoveries includefeather an abundance weaponry—arrow, darts complete wooden Hunters understood that animals such as caribou congregated on snow-patches during certain shafts, feather fletching and fiber lashings—as well as skin clothing and bark containers. Hunters seasons for relief and temperature regulation and they targeted game at these locations, understood thatinsect animals such as caribou congregated on snow-patches during certain seasons for insect occasionally losing tools and equipment in these natural freezers [42] (p. 313), [43]. High latitude relief and temperature regulation and they targeted game at these locations, occasionally losingand tools mountainous settings are prerequisite for these rare contexts, but not all regions seem to exhibit them and equipment in these natural freezers [42] (p. 313), [43]. High latitude and mountainous settings in equal abundance. Particularly rich areas include British Columbia [44], southern Yukon Territory [43] are prerequisite for these rare contexts, but not all regions seem to exhibit them in equal abundance. Particularly rich areas include British Columbia [44], southern Yukon Territory [43] and Northwest

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Territories [45,46] in Canada, areas of interior Alaska [47,48] and the Rocky Mountains in the western United States [49], and portions of Scandinavia and the European Alps [50]. The significance of ice patch finds stems from their ability to provide unique insights into prehistoric cultural developments Remote Sens. 2016, 8, 1007 18 contexts. of 23 and technology since these perishable items are rare in the vast majority of archaeological These same deposits often also contain rich assemblages of well-preserved plant and animal remains and Northwest Territories [45,46] in Canada, areas of interior Alaska [47,48] and the Rocky Mountains in that can shed light on long-term environmental change [51]. There is an urgency to learn about these the western United States [49], and portions of Scandinavia and the European Alps [50]. The unique frozen archives because, broad areas of the globe, glaciers and alpine patches are significance of ice patch finds across stems from their ability to provide unique insights into ice prehistoric melting at a rate not seen for thousands of years [52]. cultural developments andhundreds technologyorsince these perishable items are rare in the vast majority of Despite substantial research in manyoften areasalso now spanning almost two decades, this resource archaeological contexts. These efforts same deposits contain rich assemblages of well-preserved animal remainsor that can shed light oninlong-term environmental change [51]. There is anthese is stillplant not and well-inventoried well-understood most areas. Great effort is required to find urgency to learn about these unique frozen archives because, across broad areas of the globe, glaciers small features in what is almost always rugged and remote terrain, and that must occur within narrow and windows alpine ice patches are melting at a rate seen for hundreds orwithin thousands of years seasonal of maximum melting. Thenot central Brooks Range Gates of the[52]. Arctic National Despite substantial research efforts in many areas now spanning almost two decades, this very Park and Preserve (GAAR) is one area with good potential for snow patch finds, but within which resource is still not well-inventoried or well-understood in most areas. Great effort is required to find few have been documented. Reconnaissance surveys, ethnographic accounts, and reports from local these small features in what is almost always rugged and remote terrain, and that must occur within residents have highlighted several dozen snow patch locations with potential to contain archaeological narrow seasonal windows of maximum melting. The central Brooks Range within Gates of the Arctic and paleoecological materials examination these locations ongoing. National Park and Preserve and (GAAR) is one areaof with good potentialisfor snow patch finds, but within In 2016 our field team examined a low sloping ice field in GAAR (see Figure 1), atand an elevation of which very few have been documented. Reconnaissance surveys, ethnographic accounts, reports ca. 2150 m above mean sea level (AMSL), to test the feasibility of using GPR in ice and snow patch from local residents have highlighted several dozen snow patch locations with potential to contain archaeological paleoecological materials and of these is ongoing. investigation. Thisand particular area was selected asexamination one that would belocations accessible by both caribou and In 2016 our field team examinedarchaeological a low sloping icepotential. field in GAAR Figure 1), at was an elevation people, and therefore have reasonable This (see alpine ice-field accessed by of ca. 2150 m above mean sea level (AMSL), to test the feasibility of using GPR in ice and snow patch helicopter, and nearly 1 km of GPR profiles were collected (Figure 17). All profiles clearly showed investigation. This particular area was selected as one that would be accessible by both caribou and the type of reflections from internal bedding and diffractions from embedded targets that would be people, and therefore have reasonable archaeological potential. This alpine ice-field was accessed by crucial for archaeological and paleo-environmental investigations of such features (Figures 18 and 19). helicopter, and nearly 1 km of GPR profiles were collected (Figure 17). All profiles clearly showed Our findings show that GPR, when paired with coring and surface investigations at the melt-zone the type of reflections from internal bedding and diffractions from embedded targets that would be of such features, could prove useful in identifyinginvestigations areas whereofarchaeological deposits are likely crucial for archaeological and paleo-environmental such features (Figures 18 and 19). to occurOur and may be subject to future loss due to melting. Though only in the trial stage, we believe findings show that GPR, when paired with coring and surface investigations at the melt-zone of such that GPR could offer a major research resource management for these threatened features, could prove usefulcontribution in identifyingto areas whereand archaeological deposits are likely to occur and subject to futuremeasurement loss due to melting. Though onlydepth in the trial we believe that GPR could sites.may Thebe high-resolution of the ice patch andstage, cross-section can provide a useful offer a major contribution to research and resource management for these threatened sites. The highmeans to monitor changing conditions of ice patches and track melting, though it should be noted resolution measurement of the ice patch can depth cross-section a useful means to that ice thickness and volume estimation beand subject to error can [53].provide Although layers of caribou monitor changing conditions of ice patches and track melting, though it should be noted that ice dung—typically a key indicator of the presence of cultural materials—were not present in this ice thickness and volume estimation can be subject to error [53]. Although layers of caribou dung— patch, GPR methods would be well suited to detecting dung layers and mapping or quantifying typically a key indicator of the presence of cultural materials—were not present in this ice patch, GPR their methods frequency, depth, and areal extent, which could help to identify the most productive ice patches would be well suited to detecting dung layers and mapping or quantifying their frequency, for regular monitoring. Thewhich example demonstrates the feasibility of deploying GPR in depth, and areal extent, couldshown help tohere identify the most productive ice patches for regular such monitoring. cases. The example shown here demonstrates the feasibility of deploying GPR in such cases.

Figure 17. T. Urban collecting GPR data on ice patch. Photo by J. Rasic 2016.

Figure 17. T. Urban collecting GPR data on ice patch. Photo by J. Rasic 2016.

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Figure 18. An example of GPR data from an alpine ice patch shown in standard seismic color scheme. Figure 18. An example example of of GPR GPR data data from from an an alpine alpine ice ice patch shown shown in in standard standard seismic seismic color color scheme. scheme. Figure 18. An The bedrock is easily distinguished from the ice, while patch embedded anomalies generate clear diffraction The bedrock is distinguished from the while embedded embedded anomalies anomalies generate generate clear diffraction The bedrock(e.g., is easily easily distinguished from the ice, ice, while clearthroughout diffraction hyperbolas 1). These These wereused used todetermine determine avelocity velocityof of0.17 0.17m/ns. m/ns. Bedding is clear hyperbolas (e.g., 1). were to a Bedding is clear throughout hyperbolas 1). Thesethat werethe used to determine a velocitythat of 0.17 m/ns. Bedding is clear throughout the ice ice (e.g., (e.g.,(e.g., 2) indicating indicating types of ancient ancient surfaces surfaces might contain archaeological material the 2) that the types of that might contain archaeological material the ice (e.g., 2) indicating that the types of ancient surfaces that might contain archaeological material would be readily detectable with GPR. Such interfaces may also contain a range of paleowould bebereadily detectable withwith GPR. GPR. Such interfaces may alsomay contain a range of paleo-environmental would readily detectable Such interfaces also contain a range of paleoenvironmental material relevant to archaeology. material relevant to archaeology. environmental material relevant to archaeology.

Figure 19. An example of GPR data from an alpine ice patch after attribute analysis (instantaneous Figure 19. An An exampleand of GPR GPR data from from an alpine alpine ice ice patch after attribute analysis (instantaneous amplitude envelope) topographic corrections. This offers a more intuitive image with truer Figure 19. example of data an patch after attribute analysis (instantaneous amplitude envelope) and topographic corrections. This offers a more intuitive image with truer truer amplitude topographic corrections. This dimensions.envelope) This is theand same profile as shown in Figure 18.offers a more intuitive image with dimensions. This is the same profile as shown in Figure 18. dimensions. This is the same profile as shown in Figure 18.

4. Conclusions 4. Conclusions 4. Conclusions Ground-penetrating radar (GPR) has been shown to be useful for addressing archaeological (and Ground-penetrating radar has shown to be for addressing archaeological (and closely related paleoecological, geological and paleontological) questions in a range of settings in Ground-penetrating radar(GPR) (GPR) hasbeen been shown to useful be useful for addressing archaeological closely relatedincluding paleoecological, geological and paleontological) questions in a range of settings in Arctic Alaska, both partially frozen and frozen contexts. A number of straightforward (and closely related paleoecological, geological andfully paleontological) questions in a range of settings in Arctic Alaska, including both partially frozen and fully frozen contexts. A number of straightforward lessonsAlaska, can beincluding taken from thispartially work tofrozen guide and future research resource management efforts: Arctic both fully frozenand contexts. A number of straightforward lessons can be taken from this work to guide future research and resource management efforts: lessons can be taken from this work to guide future research and resource management efforts: 1. Seasonal changes in the depth and character of the active layer influence the results of GPR 1. Seasonal changes in the depth and character ofitthe active the results of GPR surveys. areas with substantial soil moisture, may makelayer senseinfluence to surveythe when active 1. SeasonalIn changes in the depth and character of the active layer influence results of layer GPR surveys. Inreaches areas with substantial soil moisture, may make sense to survey when active layer thickness seasonal maximum. The itground besense waterlogged, and surveys. In areas its with substantial soil moisture, it maymay make to surveyhowever, when active thickness reaches its seasonal maximum. The ground may be waterlogged, however, and penetration can be ensured by using a moderate antenna frequency as used in the examples layer thickness reaches its seasonal maximum. The ground may be waterlogged, however, penetration be ensured by usingdue a moderate antenna as allow used in the examples shown here.can Shortened wavelengths the high water frequency content may spatial and penetration can be ensured by using atomoderate antenna frequency as used excellent in the examples shown here. Shortened due to the high water content may allow excellent spatial and temporal resolutionwavelengths even with lower frequencies. shown here. Shortened wavelengths due antenna to the high water content may allow excellent spatial temporal resolution even with lower antenna 2. and Accurate velocity determinations are critical forfrequencies. estimating both depth and dimensions of and temporal resolution even with lower antenna frequencies. 2. Accurate velocity determinations are critical for estimating both depth and dimensions of archaeological features and are therefore the most parameters to be assessed. In partially 2. Accurate velocity determinations are critical forcrucial estimating both depth and dimensions of archaeological features and are therefore the most crucial parameters to be assessed. In partially frozen substrates, great should be to consider the to multiple velocities likely archaeological features andcare are therefore the taken most crucial parameters be assessed. In partially frozen substrates, great care should to consider the multiple likelya represented in the medium, which maybein taken some cases exhibit extreme shifts, velocities such as when represented in the medium,awhich may in some cases exhibit extreme shifts, such as when a slow, wet layer is overlying fast, frozen layer. slow, wet layer is overlying a fast, frozen layer.

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frozen substrates, great care should be taken to consider the multiple velocities likely represented in the medium, which may in some cases exhibit extreme shifts, such as when a slow, wet layer is overlying a fast, frozen layer. Patterns generated by freeze-thaw processes can be mistaken for cultural features such as house remains. Care should therefore be taken to understand how natural variations within a given survey setting are manifest in GPR data. In fully frozen contexts, attenuation is limited and wavelengths are correspondingly increased with velocity. Surveying with higher antenna frequencies than those used for the examples in this paper may be warranted in order to maximize resolution. It must be kept in mind that not all surfaces encountered (e.g., snow) are amenable to the highest antenna frequencies due to difficulty of surface traverse and impracticability of the very closely spaced traverses that might be required to optimize horizontal resolution.

The central goal of cultural resource management is to learn about and protect significant heritage resources, a task that GPR and other geophysical methods aid tremendously in doing. These tools allow archaeologists and other specialists to effectively gather useful information about what lies below the ground, yielding data of direct value in portraying cultural features, documenting geologic contexts, and understanding site-formation processes. Geophysical data, including GPR, also plays an important role in designing smart sampling approaches that allow archaeologists to be judicious and focused with destructive means of data collection like excavation. Furthermore, geophysical techniques allow archaeologists to both target features of interest such as houses and hearths, and also to avoid impacting features such as burials, which no one wants to uncover without careful consideration, consultation, respect and appropriate means. This means that under the auspices of resource management, GPR may be included in aspects of tribal consultation. Though no examples were included in the study, our team has used GPR to locate tribal cemeteries in several regions of Alaska, often with indigenous stakeholders participating in the fieldwork. With the Kobuk Valley case study in the paper, local Inupiat community members participated in various aspects of the project, including the geophysical surveying. We believe that the potential of GPR for frozen and partially frozen archaeological contexts has yet to be fully exploited and that this is a research and resource management area in need of further development, particularly with high latitude and high altitude archaeological resources facing threats from global environmental change. While GPR results are sometimes ambiguous, when used in tandem with other field methods such as excavation, auguring, and additional geophysical techniques, uncertainty can be reduced. In the coming years, GPR is likely to play an increasingly important role in the archaeology of frozen environments. Acknowledgments: Work presented here from GAAR, BELA, and CAKR was funded by NPS National Center for Preservation Technology and Training (NCPTT) grant NPS-P15AP00094, as well as Cooperate Ecosystem Study Unit (CESU) agreements NPS-P14AC01303 and NPS-P16AC0052, awarded to Cornell University. Work in Kobuk Valley was funded by NSF awards 0908162 and 0908462 to Douglas A. and Wanni W. Anderson, and initial work at CAKR was funded by NSF award 1048194 to Christopher Wolff. Work at Cape Espenberg was funded by NSF awards ARC-0755725 to John F. Hoffecker and Owen K. Mason and award ARC-1523160 to Claire Alix and Owen K. Mason. We also thank Chris Ciancibelli and Tracy Asicksik for assistance with mapping and graphics. Author Contributions: Thomas Urban collected and processed the GPR data presented in this paper and organized the initial draft, the overall concept of which is attributed to an NCPTT award developed by Manning, Rasic, and Urban. Fieldwork for the GAAR ice field survey and BELA (mammoth) survey was conducted by Urban and Rasic and the associated sections of the paper were authored initially by Rasic and Urban. Fieldwork for BELA (Cape Espenberg) was conducted by Urban and Tremayne under the direction of Alix and Mason. The BELA (Cape Espenberg) section was co-authored by Tremayne, Alix, Mason, and Urban. Fieldwork undertaken at CAKR included Urban, Wolff (2011), and Tremayne (2016), and the section on CAKR was authored by Wolff and Urban. Fieldwork at KOVA was conducted by Urban under the direction of Anderson, and the KOVA section was co-authored by Anderson and Urban. All co-authors made substantial edits to the overall text. Conflicts of Interest: The authors declare no conflict of interest.

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