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

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Article 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 Cornell Tree Ring Laboratory, Department of Classics and Cornell Institute of Archaeology and Material Studies, Cornell University, Ithaca, NY 14853, USA; [email protected] 2 U.S. National Park Service, Gates of the Arctic National Park and Preserve, Yukon-Charley Rivers National Preserve, Fairbanks, AK 99709, USA; [email protected] 3 Histoire de l’Art et Archéologie, Paris 1 Pantheon Sorbonne University, Paris 75005, France; [email protected] 4 Department of Anthropology and Circumpolar Laboratory, Brown University, Providence, RI 02912, USA; [email protected] 5 Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA; [email protected] 6 U.S. National Park Service, Alaska Regional Ofﬁce, Anchorage, AK 99501, USA; [email protected] 7 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 www.mdpi.com/journal/remotesensing Remote Sens. 2016, 8, 1007 2 of 23 of Giddings and Anderson. Multiple geophysical methods, including ground-penetrating radar (GPR), haveRemote now Sens. been 2016 successfully, 8, 1007 used for archaeological investigations in Alaska [2–4]; however,2 of 23 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 extraordinarygeophysical investigations. potential of theHere method we describe for site some reconnaissance recent applications in a varietyof ground-penetrating of environmental radar settings, in whilehigher also describinglatitude regions some of common Alaska, pitfallshighlighting of GPR the inextraordinary such places. potential of the method for site Wereconnaissance draw on examplesin a variety fromof envi multipleronmental locations settings, while managed also describing by the U.S. some National common pitfalls Park Serviceof (FigureGPR1). in At such Cape places. Krusenstern National Monument, GPR revealed complex patterned ground created by the freeze-thawWe draw cycleon examples at the Old from Whaling multiple beach locations ridge managed [3], while by multi-year the U.S. National GPR data Park collected Service at this location(Figure illustrates 1). At temporalCape Krusenster variationsn National in the permafrostMonument, table.GPR reveal In Kobuked complex Valley patterned National Park,ground a GPR investigationcreated by of the an freeze-thaw early contact cycle period at the Inupiat Old Whal settlementing beach revealed ridge [3], an while earlier, multi-year deeper GPR occupation data 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 portionsBering in theLand saturated Bridge National active layerPreserve, and a deeperdeep Bi portionsrnirk period frozen house in complex permafrost. was investigated Also at Bering with Land BridgeGPR, National with shallower Preserve, portions the skeletal in the remainssaturated of active a late layer Pleistocene and deeper mammoth portions embeddedfrozen in permafrost. in the bottom of a thermokarstAlso at Bering lake Land were Bridge successfully National Preserve, mapped th frome skeletal the surface remains of of the a late frozen Pleistocene lake. Finally, mammoth in Gates of theembedded Arctic National in the bottom Park andof a Preservethermokarst an lake alpine we icere successfully patch was mapped from with the GPR, surface showing of the clear beddingfrozen and lake. suspended Finally, in Gates anomalies. of the Arctic In similar National ice Park patch and settingsPreserve an elsewhere alpine ice inpatch the was region, mapped ancient hunterswith targeted GPR, showing caribou clear and bedding occasionally and suspended lost weaponry anomalies. and other In similar tools ice that patch have settings long been elsewhere preserved in frozenin the conditions. region, ancient With hunters such targeted features caribou now melting, and occasionally artifacts lost of theseweaponry hunters and areother being tools revealed,that 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,how GPR may and be contribute integrated tointo the such management investigations, of and invaluable contribute cultural to the management resources, particularly of invaluable those that facecultural the resources, threat of environmentalparticularly those change. 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. Remote Sens. 2016, 8, 1007 3 of 23

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 T0 (the time at which the antenna is directly above the point-scatterer) can be described in simple terms by (Equation (1))

1 2 2 4x 2 T = + T0 (1) v2

where 2d T0 = (2) v 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 (Table1). This is due to the abrupt contrast in relative electrical permittivity εr between frozen and wet material, as εr 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.

C 2 ε = (3) r v

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., Table1) 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, Remote Sens. 2016, 8, 1007 4 of 23 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 shortcomingsdetermination and limitationsand migration. [5], When so when the possible,estimate is direct correct, checks the majority of depth of estimateshyperbola bytails comparing will be to excavation,eliminated cores, in the scarps post-migration etc. to correlate data for known the layer interfaces of interest. with Even those so, detectedall methods by for GPR estimating is advisable. In thevelocity case studies have shortcomings presented here,and limitations observed [5], depths so when from possible, excavation direct checks have beenof depth used estimates as a check by on velocity/depthcomparing to estimates excavation, yielded cores,from scarps GPR etc. to results. correlate Depth known estimates interfaces were with found those todetected be consistent by GPR with excavatedis advisable. 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 Tableconsistent 1. Common with excavated ground-penetrating cases. 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 andMaterial adapted from [6,12–14]. Velocity (m/ns) AirMaterial Velocity (m/ns) 0.3 SnowAir 0.3 0.2 ColdSnow ice 0.2 0.16–0.18 TemperateCold iceice 0.16–0.18 0.15 PermafrostTemperate ice 0.15 0.12–0.14 Dry gravel 0.12 Permafrost 0.12–0.14 Wet gravel 0.10 Dry gravel 0.12 Silt (wet) 0.07 ClayWet (wet) gravel 0.10 0.06 FreshSilt water (wet) 0.07 0.033 Clay (wet) 0.06 Fresh water 0.033 A transmission line model, adapted from Heaviside’s telegrapher’s equations (1880) can illustrate a GPR pulseA transmission through a multi-velocity line model, adapted medium. from This He canaviside’s be represented telegrapher’s schematically equations as(1880) an equivalent can circuitillustrate diagram a GPR (Figure pulse2 through). In this a multi-velocity instance, the medi GPRum. pulse This can propagation be represen isted analogous schematically to electricalas an voltageequivalent propagation circuit wherediagramV (Figureis voltage, 2). In Ithisis currentinstance, in the amperes, GPR pulseR =propagation resistance is per analogous unit length to in Ohm/m,electricalL = inductancevoltage propagationper unit where length V inis Henry/m,voltage, I is Ccurrent= capacitance in amperes, per R unit = resistance length inper Farads/m, unit G = conductancelength in Ohm/m, perunit L = inductance length in Seimens/m. per unit length The in observedHenry/m, caseC = capacitance can be more per complicated unit length in 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 layer, which factors also which complicate also complicate velocity velocity estimates. estimates.

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

Remote Sens. 2016, 8, 1007 5 of 23

Remote Sens. 2016, 8, 1007 5 of 23 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 Table2. 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 (Figure3). 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 lessless desirable. desirable. For For this this reason, reason, a mid-range a mid-range antenna antenn frequencya frequency (i.e., (i.e., one thatone canthat be can trusted be trusted for a balance for a ofbalance both reasonable of both reasonable resolution resolution and penetration) and penetration) and small and sled small deployment sled deployment setup weresetup chosen were chosen for the workfor the (Figure work3 (Figure). 3). DataData processingprocessing procedures in included:cluded: visual visual inspection inspection and and editing editing of data, of data, time-zero time-zero re- re-picking,picking, dewow, dewow, velocity velocity estima estimationtion with hyperbola with hyperbola matching, matching, SEC-2 gain, SEC-2 background gain, backgroundsubtraction subtraction(avg. of all traces);(avg. of migration all traces); (wheremigration indicated); (where envelope indicated); (where envelope indicated); (where time-depth indicated); conversion; time-depth conversion;and topographic and topographic corrections (when corrections necessary). (when In necessary). some instances, In some a running instances, average a running (three-trace average (three-tracewindow) was window) applied was as applied a horizontal as a horizontal low pass lowfilter pass to filteremphasize to emphasize the interface the interface of the offrozen the frozen and andactive active layers. layers. Data Data processing processing was was undertaken undertaken wi withth EKKOProject EKKOProject by by Sensors Sensors and and Software Software Inc. Inc. and and finalfinal images images were were produced produced with with Voxler Voxler 4 and/or 4 and/or Surfer Surfer 13, both13, both by Golden by Golden Software Software Inc. Final Inc. figuresFinal includefigures profilesinclude (vertical profiles cross-section),(vertical cross-section), time-depth time-depth slices (plan-view) slices (plan-view) and 3-D perspective and 3-D perspective view images usingview partiallyimages using opaque partially data volumesopaque data and/or volumes fence and/or diagrams. fence diagrams.

Table 2.2. InstrumentInstrument and survey parameters. parameters.

ParameterParameter Gridded traverse (varied) anti-parallel, sometimes bi-directional Gridded traverse (varied) anti-parallel, sometimes bi-directional Transect interval interval (varied) (varied) 0.20–0.5 0.20–0.5 m m TraceTrace interval interval 0.05 0.05 m m Tx-RxTx-Rx offset offset 0.279 0.279 m m TimeTime window window (varied) (varied) 80–200 80–200 ns ns Stacks 8 per station Stacks 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.

Remote Sens. 2016, 8, 1007 6 of 23

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 (Figure1), 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 (Figure4). By late August, the thickness of the active layer may be double what is observed in June or July (Figure5). 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 (Figure6), 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. Remote Sens. 2016, 8, 1007 7 of 23 Remote Sens. 2016, 8, 1007 7 of 23 Remote Sens. 2016, 8, 1007 7 of 23

Figure 4. Permafrost table in July 2016 at the Old Whaling Ridge. The frozen interface results in a Figure 4. Permafrost table in July 2016 at the Old Whaling Ridge. The frozen interface results in Figuresudden 4. velocity Permafrost increase, table alongin July with 2016 changes at the Old in diffrWhalactioning Ridge. tails that The indicatefrozen interface an average results velocity in a a sudden velocity increase, along with changes in diffraction tails that indicate an average velocity suddenchange. velocityHere the increase,active layer along is ca. with 1 m changes thick before in diffr frozenaction ground tails isthat encountered. indicate an average velocity change. Here the active layer is ca. 1 m thic thickk before frozen ground is encountered.

Figure 5. The late summer (August) permafrost table at the Old Whaling Ridge. This is the full extent Figure 5. The late summer (August) permafrost table at the Old Whaling Ridge. This is the full extent Figureof the same 5. The monitoring late summer station (August) shown in permafrost Figure 5. A table three-trace at the Old running Whaling average Ridge. has been This applied is the full to of the same monitoring station shown in Figure 5. A three-trace running average has been applied to extentemphasize of the horizontal same monitoring reflectors. station Note shown that the in Figureactive 5layer. A three-trace extends nearly running 1 m average deeper has than been the emphasize horizontal reflectors. Note that the active layer extends nearly 1 m deeper than the appliedmid-summer to emphasize depth shown horizontal in Figure reﬂectors. 5. The Note active that thelayer active exhibits layer extendsa network nearly of cracks 1 m deeper from than the mid-summer depth shown in Figure 5. The active layer exhibits a network of cracks from the thefreeze-thaw mid-summer cycle, depth which shown are clear in Figurein plan-view5. The GPR active data layer (bottom exhibits). In a isolation, network ofperimeters cracks from of these the freeze-thaw cycle, which are clear in plan-view GPR data (bottom). In isolation, perimeters of these freeze-thawfrost polygons cycle, can whicheasily be are mistaken clear in plan-view for cultural GPR features data (inbottom the sub-surface.). 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.

Remote Sens. 2016, 8, 1007 8 of 23 Remote Sens. 2016, 8, 1007 8 of 23

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 Figure6. 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 Birnirk House Complex at Cape Espenberg, BeringBering LandLand BridgeBridge NationalNational PreservePreserve Over thethe pastpast sevenseven years,years, a a team team of of researchers researchers has has been been excavating excavating and and studying studying the the remains remains of aof series a series of semi-subterraneanof semi-subterranean house house features features that documentthat document the cultural the cultural transition transition from Birnirkfrom Birnirk to Thule to ofThule the Northernof the Northern Maritime Maritime tradition attradition Cape Espenberg, at Cape BeringEspenberg, Land BridgeBering National Land Bridge Preserve National [25,26]. CapePreserve Espenberg [25,26]. consists Cape ofEspenberg a series of consists beach ridge of formationsa series of that beach have ridge accumulated formations horizontally that have for 5000accumulated years [27 horizontally], preserving for within 5000 themyears a[27], 4500-year-old preserving archaeologicalwithin them a 4500-year-old record of human archaeological adaptation torecord environmental of human adaptation change [27 –to31 environmental]. One characteristic change that [27–31]. delineates One characteristic the Birnirk from that the delineates later Thule the cultureBirnirk isfrom the apparentthe later Thule variety culture and complexity is the apparent of house variety designs, and some complexity of which of include house designs, many connected some of rooms,which include while others many areconnected reminiscent rooms, of while the later others Thule are formsreminiscent [31–33 ].of Typicalthe later Thule Thule houses forms [31–33]. tend to haveTypical one Thule main houses room with tend a longto have entrance one main tunnel room and with often a along kitchen entrance alcove tunnel off to oneand sideoften [32 a ]kitchen (p. 81). Aalcove recent off study to one by side Darwent [32] (p. et al.81). [27 A] usedrecent attributes study by of Darwent surface depressionset al. [27] us toed assess attributes whether of surface house shapedepressions and design to assess at Cape whether Espenberg house showed shape evidenceand design for at change Cape throughEspenberg time. showed The results evidence indicate for designschange evolvedthrough fromtime. complex The results multi-room indicate structures designs evolved towards simplefrom complex one-room multi-room houses with structures straight tunnels.towards Excavationssimple one-room by Hoffecker houses andwith Mason’s straight 2011 tunnels. team Excavations [25] found that by amongHoffecker six houseand Mason’s depressions 2011 excavatedteam [25] found on three that beach among ridges, six house ﬂoorsdepressions could beexcavated located beneathon three apparentbeach ridges, depressions house floors in all casescould butbe located one. In beneath the latter, apparent no clear depressions house feature in all was cases found but exceptone. In forthe alatter, deeply no build clear tunnelhouse feature which suggestswas found potential except for reuse a deeply of structural build tunnel wood which to build suggests new houses. potential This reuse raised of structural the question: wood can to 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?

Remote Sens. 2016, 8, 1007 9 of 23 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? ToTo answeranswer this question question we we initiated initiated a astudy study at atCape Cape Espenberg Espenberg using using GPR GPR to collect to collect data datafrom froma series a series of depressions of depressions thought thought to tocontain contain buried buried house house ruins. ruins. The The goal goal was toto non-invasivelynon-invasively documentdocument househouse design,design, floorfloor configuration,configuration, andand tunneltunnel directions.directions. ProcessingProcessing andand interpretinginterpreting thethe datadata waswas complicatedcomplicated byby thethe factfact thatthat portionsportions ofof thethe samesame feature-complexfeature-complex straddledstraddled thethe boundaryboundary ofof thethe activeactive andand frozenfrozen layerslayers (Figure(Figure7 ),7), thus thus involving involving two two different different GPR GPR velocities velocities as as shown shown in in TableTable1 .1. To To deal deal with with this this situation, situation, separate separate velocity velocity estimates estimates were were undertaken undertaken for for these these two two layers layers andand datadata processingprocessing adjustedadjusted accordinglyaccordingly (Figure(Figure8 ).8). The The results results show show that that not not only only are are structural structural elementselements detectabledetectable butbut culturalcultural depositsdeposits extendextend asas deepdeep asas 33 mm andand indicateindicate multiplemultiple structures,structures, possiblypossibly floors, floors, with with cache cache pits pits present present at many at many levels. levels. This likelyThis likely indicates indicates multiple multiple rebuilding rebuilding events (Figuresevents (Figures9 and 10 ).9 andThe 10). GPR The results GPR also results show also that sh manyow that of thesemany features of these are features located are below located the activebelow layer,the active exceeding layer, 1exceeding m deep (Figure 1 m deep7) but (Figure still resolvable 7) but still with resolvable GPR (Figures with GPR9 and (Figures 10). While 9 and such 10). frozen While materialsuch frozen bodes material well for bodes preservation well for preservation of organic artifacts, of organic it is artifacts, a limiting it is factor a limiting for timely factor excavation for timely whereexcavation field where seasons field are seasons short. Theseare short. results These suggest results that suggest the design that the history design of history the buried of the houses, buried particularlyhouses, particularly on the Birnirk on the beach Birnirk ridge, beach appears ridge, muchappears more much complex more complex than indicated than indicated from the from surface. the Interestingly,surface. Interestingly, however, however, results show results the appearanceshow the appearance of a room thatof a connectsroom that to connects one that to was one excavated that was atexcavated about the at same about depth the same (Figures depth 10 and(Figures 11).The 10 and layout 11). of The these layout house of structuresthese house resemble, structures perhaps resemble, not superficially,perhaps not the superficially, sketch maps the of house sketch features maps recordedof house at features the Birnirk recorded site near at Barrow the Birnirk [33] (p. site 38) near and JabbertownBarrow [33] at (p. Point 38) Hopeand Jabbertown [34] (p. 171). at ThisPoint provides Hope [34] further (p. 171). support This for provides the hypothesis further thatsupport Birnirk for peoplethe hypothesis were present that Birnirk at Cape people Espenberg were present and that at someCape ofEspenberg their settlements and that some included of their multi-roomed settlements houseincluded designs. multi-roomed Planned excavations house designs. will Planned provide anexcavations opportunity will to provide not only an confirm opportunity these hypothesesto not only butconfirm to refine these the hypotheses analytical methodbut to refine as well. 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 8. Profile processed as a two-velocity medium. The average velocities of both the frozen and FigureFigure 8. Profile8. Profile processed processed as as a a two-velocity two-velocity medium.medium. Th Thee average velocities velocities of of both both the the frozen frozen and and activeactive layer layer from from the the same same profileprofile shownshown in Figure 7 werewere used to to produce produce the the above above profile profile with with accurateactive layer depth from estimates the same for profileboth la yers.shown Depths in Figure in the 7 frozenwere used layer to ar producee tens of centimetersthe above profile more thanwith accurate depth estimates for both layers. Depths in the frozen layer are tens of centimeters more than estimatedaccurate depth with theestimates active layerfor both velo lacityyers. alone. Depths The in profile the frozen above layer has ar alsoe tens migrated of centimeters using the more velocity than estimated with the active layer velocity alone. The profile above has also migrated using the velocity ofestimated the frozen with layer. the Noteactive that layer the velo majoritycity alone. of diffr Theaction profile tails above have has been also reduced migrated or usingeliminated the velocity when of the frozen layer. Note that the majority of diffraction tails have been reduced or eliminated when comparedof the frozen to the layer. previous Note thatfigure. the majority of diffraction tails have been reduced or eliminated when comparedcompared to to the the previous previous figure. figure.

Figure 9. (a) Portion of a Birnirk house complex in the active layer (upper meter) with layout and Figure 9. (a) Portion of a Birnirk house complex in the active layer (upper meter) with layout and Figuredimensions 9. (a) Portionconsistent of with a Birnirk nearby house excavated complex examples. in the The active darker layer colors (upper indicate meter) higher with amplitude layout and dimensions consistent with nearby excavated examples. The darker colors indicate higher amplitude dimensionsreflections consistentfrom buried with wood, nearby bone excavated and other examples.material as The seen darker in the colorsnearby indicate excavation; higher (b) amplitudePortion reflections from buried wood, bone and other material as seen in the nearby excavation; (b) Portion reﬂectionsof a Birnirk from house buried complex wood, bonein the and frozen other layer material (extending as seen inbelow the nearby one meter) excavation; with layout (b) Portion and of dimensionsof a Birnirk consistent house complex with nearby in the excavated frozen layerexamples. (extending This is belowa deeper one portion meter) of with the samelayout house and a Birnirk house complex in the frozen layer (extending below one meter) with layout and dimensions complexdimensions shown consistent in (a); with (c) Annotatednearby excavated interpretations examples. 1 Thisand is2 indicateda deeper portionthe proposed of the same outline house of consistent with nearby excavated examples. This is a deeper portion of the same house complex shown structurescomplex showninterred in from (a); ((ca)), Annotatedalong with interpretations likely tunnel locations. 1 and 2 Theindicated orientation the proposed and scale outline of these of in (a); (c) Annotated interpretations 1 and 2 indicated the proposed outline of structures interred from featuresstructures are interred comparable from (toa), thealong nearby with excavationlikely tunnel (F igureslocations. 10 Theand orientation11), and the and excavated scale of theseand (a), along with likely tunnel locations. The orientation and scale of these features are comparable to unexcavatedfeatures are structurescomparable are tolikely the connected;nearby excavation (d) Annotations (Figures indicate 10 and a likely 11), clusterand the of excavatedtimbers, three, and the nearby excavation (Figures 10 and 11), and the excavated and unexcavated structures are likely andunexcavated external storagestructures pits, are four. likely Other connected; discrete (d )anomalies Annotations scattered indicate throughout a likely cluster the ofsurvey timbers, area three, are connected; (d) Annotations indicate a likely cluster of timbers, three, and external storage pits, four. likelyand external caused bystorage debris pits, such four. as wood Other and discrete whale boneanomalies external scattered to the structures. throughout This the is survey also consistent area are Other discrete anomalies scattered throughout the survey area are likely caused by debris such as withlikely the caused nearby by excavation. debris such as wood and whale bone external to the structures. This is also consistent woodwith and the whalenearby bone excavation. external to the structures. This is also consistent with the nearby 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 Figure9. Depth excavated is 0.9 to 1.2 m below surface (map by Sylvie Elies and Claireand Claire Alix Alix and and photo photo by by Claire Claire Alix). Alix). Frozen Frozen material was was encountered encountered at depthsat depths consistent consistent with with the GPRthe GPR estimates. estimates. The The layout layout and and complexity complexity of of depositsdeposits waswas also also consistent consistent with with the the GPR GPR results results shownshown in Figures in Figures 9 and9 and 10, 10 which, which is is immediatel immediatelyy easteast of of the the Feature Feature 12 12 excavation excavation unit. unit.

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Figure 11. TheThe relationship relationship of of the the excavated excavated structure structure to to the the unexcavated unexcavated features features detected detected with with GPR. GPR. The comparative scale and alignment of the known and inferred features is obvious. The GPR image (right) is a 3-D iso-surface showing high amplitudeamplitude anomalies spanning aa depthdepth ofof ca.ca. 0.5–1.2 m.

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 Figure1) 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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Remote Sens. 2016, 8, 1007 13 of 23 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 limitedwas limited by the by presence the presence of trees of attrees this at forested this forested 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 19than interface century. identi Permafrostfied in the encountered GPR survey inaround the deepest the depression. portions The of GPR the excavation survey also was consistentshowed withan additional an interface occupation identified in the in thefrozen GPR layer. survey Similar around features the depression.were detected The in other GPR surveyareas of also showedthe village an additional site at comparable occupation depths. in the The frozen darker layer. colors Similarrepresent features higher wereamplitude detected reflections. in other Our areas of theexcavated village cases site at this comparable site indicate depths. that wood The framing darker timbers colors representare the caus highere of these amplitude anomalies. reflections. 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. FullyIn Frozenthe final Settings: two case Bering studies, Land we Bridge present and GPR Gates results of the from Arctic fully frozen media. These results are moreIn the experimental final two case and studies, speculative we present than results GPR resultsfrom the from known fully frozenhuman media.occupations These already results are morepresented. experimental These, and we speculative believe, demonstrate than results fromthe incredible the known potential human occupationsfor using GPR already in frozen 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 NationalIn 2015, Preserve a GPR (see survey Figure was 1) conductedin order to assess on a frozenthe distribution thermokarst of skeletal lake inremains Bering of Landa woolly Bridge National Preserve (see Figure1) 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 Remote Sens. 2016, 8, 1007 15 of 23 Remote Sens. 2016, 8, 1007 15 of 23 paleontologicalmammoth (Mammuthus remains inprimigenius such contexts), and more (Figure generally 14). Severalto test the skeletal feasibility components of using GPR from to detect a single mammothpaleontological were located remains in in 2012 such on, contexts and embedded (Figure within,14). Several the muddyskeletal bedcomponents of a shallow from thermokarst a single lake.mammoth The skeletal were located materials in 2012 were on, shallowlyand embedded submerged within, the and muddy only identifiedbed of a shallow due tothermokarst a period of unusuallylake. The low skeletal lake materials levels. They were remained, shallowly however,submerged very and difficult only identified to access due and to inventorya period of due tounusually their submerged low lake and levels. partially They buriedremained, context. however, The very site appearsdifficult to to access have multipleand inventory elements due fromto a singletheir submerged individual and and partially may contain buried a completecontext. The or largelysite appears complete to have skeleton, multiple which elements is rare from for a the region.single Collagen individual from and two may mammoth contain a bones complete collected or largely from thecomplete site (KTZ-00345), skeleton, which a tooth is andrare a for vertebra, the wereregion. radiocarbon Collagen datedfrom totwo 12,330 mammoth± 50 BPbones (Beta-329841, collected from 12,720–12,140 the site (KTZ-00345), Cal BC) and a 12,430tooth ±and50 a BP (Beta-331336,vertebra, were 12,940–12,220 radiocarbon dated Cal BC), to 12,330 respectively. ± 50 BP (Beta-329841, A total of seven 12,720–12,140 faunal components Cal BC) and 12,430 were identified± 50 BP in(Beta-331336, 2012 with osteological 12,940–12,220 examination Cal BC), respectively. indicating A that total these of seven were faunal likely components from an individual were identified mammoth in rather2012 than with a osteological co-mingled examination deposit. GPR indicating was used that in anthese experimental were likely attemptfrom an toindividual detect and mammoth 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 previouslypreviously investigated investigated mammoth mammoth bones. bones. TheThe GPRGPR data were were collected collected from from the the frozen frozen surface surface of ofthe the lake,lake, which which provided provided an an excellent excellent flat flat and and unobstructedunobstructed platform for for the the GPR GPR survey. survey.

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 Photosurvey by J.are Rasic visible. 2015. The landing track of the plane as well as the grid pattern of the completed GPR survey are visible. Remote Sens. 2016, 8, 1007 16 of 23 Remote Sens. 2016, 8, 1007 16 of 23

MultipleMultiple anomaliesanomalies consistentconsistent withwith aa scatterscatter ofof mammothmammoth skeletalskeletal remainsremains areare evidentevident onon thethe lakelake floorfloor (Figure(Figure 1515).). SeveralSeveral ofof thesethese anomaliesanomalies cancan bebe co-locatedco-located withwith GPS-recordedGPS-recorded locationslocations ofof specificspecific bonesbones identifiedidentified duringduring thethe 20122012 sitesite reconnaissance.reconnaissance. ImmediatelyImmediately westwest ofof thesethese markedmarked locationslocations areare severalseveral anomalyanomaly clusters,clusters, whichwhich suggestsuggest additionaladditional skeletalskeletal material,material, includingincluding linear,linear, curvedcurved anomaliesanomalies that that match match the the size size and and shape shape of tusks—skeletalof tusks—skeletal elements elements expected expected to occur to occur but notbut identifiednot identified during during the 2012 the field2012 investigations. field investigations. Other discreteOther discrete or clustered or clustered anomalies anomalies seen between seen thebetween survey the surface survey and surface lake bottomand lake may bottom represent may repr additionalesent additional skeletal remains, skeletal clustersremains, of clusters aqueous of vegetation,aqueous vegetation, or trapped or gasses, trapped all gasses, of which all couldof which result could in abrupt result changesin abrupt in changes electrical in properties electrical thusproperties generating thus generating GPR anomalies. GPR anomalies. Some of these Some anomalies of these anomalies seem to occur seem in to association occur in association with broader with morphologicalbroader morphological trends that trends could that in turn could act in to turn trap accumulatedact to trap accumulated debris. It is unlikelydebris. It given is unlikely the location given ofthe the location lake that of the any lake of the that GPR any anomalies of the GPR could anomalie be relateds could to be logs related or rocks to logs since or these rocks are since not these present are innot the present area. in All the of area. the anomalies All of the notedanomalies in Figure noted 15 in areFigure at the 15 are identified at the identified depth of thedepth lake of bottom,the lake onbottom, a fairly on flat, a fairly shallow flat, shelf.shallow Features shelf. Features detected detected with GPR with beneath GPR beneath this interface this interface appear to appear be natural to be cryogenicnatural cryogenic features features (Figure (Figure16). The 16). GPR The manifestationGPR manifestation of these of these deeper deeper features features is comparable is comparable to ice-wedgeto ice-wedge polygon polygon features features described described by Munroeby Munroe et al. et [al.24 ].[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 indicatedin the substrate, in the profile from ( topfiner-grai). A deepern sediment interface to coarse seen ingravel. the profiles may be due to a textural transition 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 lastGlobally two decades significant in the archaeologicalsurprising context and of paleoecological alpine snow patches discoveries and ice have fields, been which made can incontain the last twowell-preserved decades in the perishable surprising objects context of the of alpinesort rare snowly preserved patches andin most ice fields,archaeological which can contexts. contain well-preservedDiscoveries include perishable an abundance objects of of the weaponry—a sort rarelyrrow, preserved spear inand most darts archaeological complete with contexts.intact Discoverieswooden shafts, include feather an abundance fletching ofand weaponry—arrow, fiber lashings—as spear well andas skin darts clothing complete and with bark intact containers. wooden shafts,Hunters feather understood fletching that and animals fiber lashings—as such as caribou well ascongregated skin clothing on andsnow-patches bark containers. during certain Hunters understoodseasons for that insect animals relief such and as cariboutemperature congregated regulation snow-patchesand they targeted during game certain at these seasons locations, for insect reliefoccasionally and temperature losing tools regulation and equipm andent they in targeted these natural game freezers at these [42] locations, (p. 313), occasionally[43]. High latitude losing and tools andmountainous equipment settings in these are natural prerequisite freezers for [42 these] (p. rare 313), contexts, [43]. High but not latitude all regions and mountainousseem to exhibit settings them 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 Remote Sens. 2016, 8, 1007 18 of 23

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 and technologyRemote Sens. 2016 since, 8, 1007 these perishable items are rare in the vast majority of archaeological18 contexts.of 23 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 uniquesignificance frozen archives of ice patch because, finds acrossstems from broad their areas ability of theto provide globe, glaciersunique insights and alpine into iceprehistoric patches are meltingcultural at a ratedevelopments not seen forand hundreds technology or since thousands these perishable of years [items52]. are rare in the vast majority of Despitearchaeological substantial contexts. research These effortssame deposits in many often areas also now contain spanning rich assemblages almost two decades,of well-preserved this resource is stillplant not and well-inventoried animal remains or that well-understood can shed light on in long-term most areas. environmental Great effort change is required [51]. There to findis an these smallurgency features to in learn what about is almost these unique always frozen rugged archives and remote because, terrain, across broad and that areas must of the occur globe, within glaciers narrow seasonaland windowsalpine ice patches of maximum are melting melting. at a rate The not central seen for Brooks hundreds Range or withinthousands Gates of years of the [52]. Arctic National Park andDespite Preserve substantial (GAAR) isresearch one area efforts with goodin many potential areas now for snow spanning patch almost finds, buttwo withindecades, which this very 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 paleoecologicalNational Park and materials Preserve and(GAAR) examination is one area ofwith these good locations potential is for ongoing. snow patch finds, but within Inwhich 2016 very our few field have team been examined documented. a low Reconnaiss sloping iceance field surveys, in GAAR ethnographic (see Figure accounts,1), at and an elevationreports of ca. 2150from m local above residents mean have sea levelhighlighted (AMSL), several to test doze then snow feasibility patch oflocations using with GPR potential in ice and to contain snow patch investigation.archaeological This and particular paleoecological area was materials selected and as examination one that would of these be locations accessible is ongoing. by both caribou and people, andIn 2016 therefore our field have team reasonable examined archaeologicala low sloping ice potential. field in GAAR This (see alpine Figure ice-field 1), at wasan elevation accessed by helicopter,of ca. 2150 and m nearly above mean 1 km sea of GPRlevel (AMSL), profiles to were test collectedthe feasibility (Figure of using 17). GPR All in profiles ice and clearlysnow patch showed the typeinvestigation. of reflections This particular from internal area was bedding selected and as diffractionsone that would from be embeddedaccessible by targets both caribou that would and 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 suchcrucial features, for archaeological could prove and useful paleo-environmental in identifying in areasvestigations where of archaeological such features (Figures deposits 18 and are likely19). to occurOur and findings may beshow subject that GPR, to future when paired loss duewith tocoring melting. and surface Though investigations only in theat the trial melt-zone stage, of we such believe that GPRfeatures, could could offer prove a major useful contribution in identifying to areas research where and archaeological resource management deposits are likely for theseto occur threatened and sites.may The be high-resolution subject to future measurement loss due to melting. of the Though ice patch only depth in the trial and stage, cross-section we believe can that provide GPR could a useful meansoffer to a monitor major contribution changing to conditions research and of iceresource patches management and track for melting, these threatened though sites. it should The high- be noted that iceresolution thickness measurement and volume of the estimation ice patch candepth be an subjectd cross-section to error can [53 ].provide Although a useful layers means of caribouto dung—typicallymonitor changing a key conditions indicator of of ice the patches presence and oftrack cultural melting, materials—were though it should not be presentnoted that in ice 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 theirmethods frequency, would depth, be well and suited areal to extent, detecting which dung could layers help and to mapping identify or the quantifying most productive their frequency, ice patches for regulardepth, monitoring.and areal extent, The which example could shown help to here iden demonstratestify the most productive the feasibility ice patches of deploying for regular GPR in suchmonitoring. 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.

Remote Sens. 2016, 8, 1007 19 of 23 Remote Sens. 2016, 8, 1007 19 of 23 Remote Sens. 2016, 8, 1007 19 of 23

Figure 18. An example of GPR data from an alpine ice patch shown in standard seismic color scheme. Figure 18. An example of GPR data from an alpine ice patch shown in standard seismic color scheme. FigureThe bedrock 18. An is example easily distinguishe of GPR datad from thean alpineice, while ice patchembedded shown anom in standardalies generate seismic clear color diffraction scheme. The bedrock is easily distinguished from the ice, while embedded anomalies generate clear diffraction Thehyperbolas bedrock (e.g., is easily 1). These distinguishe were usedd from to determine the ice, while a velocity embedded of 0.17 anom m/ns.alies Bedding generate is clear clear throughout diffraction hyperbolas (e.g.,(e.g., 1).1). These were usedused toto determinedetermine aa velocityvelocity ofof 0.170.17 m/ns.m/ns. Bedding Bedding is clear throughout thethe iceice (e.g.,(e.g., 2)2) indicatingindicating thatthat thethe typestypes ofof ancientancient surfacessurfaces thatthat mightmight containcontain archaeological materialmaterial thewould ice (e.g.,be readily2) indicating detectable that the with types GPR. of ancient Such surfacesinterfaces that may might also contai containn archaeological a range of material paleo- would bebe readily readily detectable detectable with with GPR. GPR. Such interfacesSuch interfaces may also may contain also a rangecontain of paleo-environmentala range of paleo- materialenvironmental relevant material to archaeology. relevant to archaeology. environmental material relevant to archaeology.

Figure 19. An example of GPR data from an alpine ice patch after attribute analysis (instantaneous Figureamplitude 19.19. Anenvelope) example and of GPRtopographic data fromfrom corrections. anan alpinealpine iceiceTh patchispatch offers afterafter a more attributeattribute intuitive analysisanalysis image (instantaneous(instantaneous with truer amplitudedimensions. envelope) This is the and same topographictopographic profile as shown corrections. in Figure Th Thisis 18. offers offers a a more intuitive image with truer dimensions. This is the same proﬁleprofile as shown inin FigureFigure 1818.. 4. Conclusions 4. Conclusions Ground-penetrating radar (GPR) has been shown to be useful for addressing archaeological (and closelyGround-penetrating related paleoecological, radar radar (GPR) (GPR)geological has has been and been shown paleontological) shown to be to useful be useful questions for addressing for addressing in a rangearchaeological archaeological of settings (and in closely related paleoecological, geological and paleontological) questions in a range of settings in (andArctic closely Alaska, related including paleoecological, both partially geological frozen and and fully paleontological) frozen contexts. questions A number in a rangeof straightforward of settings in Arctic Alaska, including both partially frozen and fully frozen contexts. A number of straightforward Arcticlessons Alaska, can be includingtaken from both this partially work to frozenguide andfuture fully research frozen and contexts. resource A number management of straightforward efforts: 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 of the active layer influence the results of GPR 1. surveys.Seasonal In changes areas with in the substantial depth and soil character moisture, of it the may active make layer sense inﬂuence to survey the when results active of layer GPR surveys. In areas with substantial soil moisture, it may make sense to survey when active layer thicknesssurveys. Inreaches areas its with seasonal substantial maximum. soil moisture, The ground it may may make be sensewaterlogged, to survey however, when active and thickness reaches its seasonal maximum. The ground may be waterlogged, however, and penetrationlayer thickness can reachesbe ensured its seasonalby using maximum.a moderate Theantenna ground frequency may be as waterlogged, used in the examples however, penetration can be ensured by using a moderate antenna frequency as used in the examples shownand penetration here. Shortened can be ensuredwavelengths by using due ato moderate the high antennawater content frequency may as allow used excellent in the examples spatial shown here. Shortened wavelengths due to the high water content may allow excellent spatial andshown temporal here. Shortened resolution wavelengthseven with lower due antenna to the high frequencies. water content may allow excellent spatial and temporal resolution even with lower antenna frequencies. 2. Accurateand temporal velocity resolution determinations even with lowerare critical antenna for frequencies. estimating both depth and dimensions of 2. Accurate velocity determinations are critical for estimating both depth and dimensions of 2. archaeologicalAccurate velocity features determinations and are therefore are critical the most for crucial estimating parameters both depthto be assessed. and dimensions In partially of archaeological features and are therefore the most crucial parameters to be assessed. In partially frozenarchaeological substrates, features great and care are thereforeshould be the taken most crucialto consider parameters the tomultiple be assessed. velocities In partially likely frozenrepresented substrates, in the medium,great care which should may be in takensome casesto consider exhibit extremethe multiple shifts, velocitiessuch as when likely a representedslow, wet layer in theis overlying medium, a which fast, frozen may inlayer. some cases exhibit extreme shifts, such as when a slow, wet layer is overlying a fast, frozen layer.

Remote Sens. 2016, 8, 1007 20 of 23

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. 3. 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. 4. 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. Remote Sens. 2016, 8, 1007 21 of 23

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