GPR study of a prehistoric archaeological site near Point Barrow, Alaska Rhett B Hermana, Anne M. Jensenb

Introduction A ground penetrating radar (GPR) study was performed on the prehistoric Thule cemetery site near Point Barrow, Alaska. The goals of this study were (a) to test this technology in this type of polar environment, and (b) to search for burials and other archaeological features in a location in imminent danger from ocean erosion. The Nuvuk site is currently eroding at an average rate measured at over 6 m/year. Prior archaeological work at the site had recovered over 80 burials with nearly 100 individuals represented, all of which were less than 1 m below surface, and detectable with small test pits. In addition, the first coastal Ipiutak occupation known north of Point Hope had been recently discovered, at a depth of nearly 2m below surface, in the erosion face. The occupation appeared to have been terminated by a large storm which overwashed the site, leaving a strandline immediately superimposed on the living surface. After that, approximately 1.5 m of sterile gravels had been deposited before the surface on which the Thule people were living formed. Both occupations are of considerable scientific interest. The matrix at the site consists of unconsolidated beach gravels, which necessitates opening large surface areas or use of shoring to test even small units to the depths of the Ipiutak deposit (approximately 8m x 8m at the surface to test 1m x 1m at 2m depth). Such excavations promote erosion, and are very costly in terms of time and labor, so a means to detect features buried at depths greater than those exposed by shovel test pits was desirable. GPR seemed a likely candidate, but it had not been used in such conditions before, and thus it was necessary to test it thoroughly prior to relying on GPR to eliminate areas from physical testing.

References Hinkel, K. M., Doolittle, J. A., Bockheim, J. G., Nelson, F. E., Paetzold, R., Kimble, J. M. and Travis, R. (2001), Detection of subsurface permafrost features with ground-penetrating radar, Barrow, Alaska. Permafrost Periglac. Process., 12: 179–190. doi: 10.1002/ppp.369 Munroe, J. S., Doolittle, J. A., Kanevskiy, M. Z., Hinkel, K. M., Nelson, F. E., Jones, B. M., Shur, Y. and Kimble, J. M. (2007), Application of ground-penetrating radar imagery for three-dimensional visualisation of near-surface structures in ice-rich permafrost, Barrow, Alaska. Permafrost Periglac. Process., 18: 309–321. doi: 10.1002/ppp.594 Arcone, S. A., and A. J. Delaney (1982), Dielectric properties of thawed active layers overlying permafrost using radar at VHF, Radio Sci., 17(3), 618–626, doi:10.1029/RS017i003p00618.

Affiliations a. Department of Physics, Radford University, Radford, VA, United States. b. UIC Science, LLC, Barrow, AK, United States.

C13A-0594

GPR Parameters

• Frequency=500MHz • Data points acquired every 2.5cm along survey line • Survey lines for burial searches spaced ½ meter apart • 8 stacks per data point • Speed of pulse in this area=12cm/nsec (multiple hyperbolae observed) • Time window set to 50nsecmaximum possible depth 2.7m

• “critical depth” 𝑧 =2𝜎

2𝜋𝑓𝜇 where 𝜎=electrical conductivity of subsurface, f=GPR

frequency, 𝜇 = 4𝜋 𝑥 10−7 𝑇 ∙ 𝑚/𝐴 The GPR time window was set so that the maximum depth to which the pulses could have traveled and returned to the surface was 2.7 meters, with that distance based on the wave speed of 12cm/nsec. This depth was set for a number of reasons—the anticipation that relevant archaeological finds would not be expected to lay deeper than that, the frequency with which data points were collected along each survey line, and the anticipated signal loss in that particular subsurface. However, after processing the data to generate the subsurface images it was concluded that features at greater depths would have been discerned by the GPR.

Previous Arctic GPR Work

Hinkel, et al (2001) studied the effectives of GPR in imaging the active layer of the permafrost in May, 1999, in the Circumpolar Active Layer Monitoring (CALM) grid located 5km east of Barrow. Using a 400MHz antenna, they found that the speed of the radar waves in the subsurface was 13cm/nsec. This was revealed by reflections from small ice wedges that stood out from the rest of the subsurface—the presence of the ice wedges was supported by a number of core samples. The final images in this paper showed details down to a depth nearly 2 meters. Munroe, et al (2007) found that the radar pulses in their surveys in the area near Barrow penetrated up to 3 meters. They used a GPR antenna operating at 400MHz, and surveyed while the ground was still frozen (April, 2006). This study again yielded a radar wave speed of 13cm/nsec in the permafrost, and imaged a number of ice wedges that allowed this determination. The authors used software to generate 3-dimensional models of subsurface features from the data. They were able to visualize a number of features at depths greater than 1 meter including a network of ice wedges. Core samples supported the structures seen in both the 2d and 3d images. Arcone, et al (1982) used GPR frequencies between 75MHz and 225MHz to investigate the dielectric properties of the thawed layers overlying the permafrost at four sites in Alaska. One of the sites was located near the coast of the North Slope in the Naval Petroleum Reserve Number 4, near Fish Creek. This group investigated the dielectric properties of the sandy matrix in this area that would affect both the penetration depth and the resolution of GPR signals. They concluded that this type of matrix not only did not hinder the radar signals, but that this material lowered the effective wavelength of the GPR signal, which actually improved the resolution of the signals. This is the same type of sandy matrix present at Point Barrow.

Vertical slices over numerous strand lines

Line 01

Line 03

Line 05

Line 08

Line 11

Line 13

Line 15

Line 18

To Northwest To Southeast

GPR planviews at depths of (left-to-right) 0.65m, 0.70m, and 0.75m, compared to field map of the same grid. The feature at (x,y)=(13.0m, 12.2m) proved to be the deepest burial ever found at the Nuvuk site.

Disturbance from old (2007)

excavation

Deep burial (new find)

GPR planviews of selected depths in Grid E, plus field map GPR Imaging of the Nuvuk Site

The image to the left shows the locations of the GPR surveys conducted July 7-15, 2011. Five neighboring GPR grids were surveyed, indicated as Grids A-E. Grids A-D were 30mx60m, while Grid E was 20mx80m. All five grids had GPR lines spaced 0.5 meter apart. In addition, a 21-line grid was laid out parallel to the shoreline of the Beaufort Sea, whose shoreline at the time was only ~10m to the north of the grid. The grid was located to maximize its potential to cross a number of features exposed by the erosion of the bluffs, and excavated by Dr. Jensen’s team. The erosion of the shoreline is revealed by the field map which shows the locations of the Beaufort Sea bluffs from 2001-2012. A number of Ipiutak features are marked. These features were excavated as they were exposed by erosion. A number of GPR anomalies are marked. These have not been excavated but are interpreted as probable Ipiutak features given their location, depth and size. The GPR imaged the subsurface to a depth of 2.7 meters at a frequency of 500MHz. Meter-deep test pits were placed at 2-meter intervals in the survey area in a grid pattern since the efficacy of the technology had yet to be shown. It was anticipated that there might be few or no remaining burials in this location since the number of burials had been declining with distance from the center of the larger site. The results of the test pits and the GPR were in agreement for the vast majority of the survey area in that both indicated the lack of burials. The image to the right shows planview slices at depths of 0.65m, 0.70m, and 0.75m (left-to-right) in Grid E. The anomaly indicated by the arrows at (x,y)=(13.0m, 12.2m) was flagged for excavation. It was surprising when the GPR detected this anomaly since this area was so far from the other burials at the site. Nothing was seen after digging at this location to the depth of the standard shovel testing. However, the excavators continued digging due to the GPR hit. In the end, the excavators found that this anomaly was a burial, and it turned out to be the deepest burial in the whole site. It proved to be a very well-preserved individual, with fairly intact garments. The feature indicated by the arrows at (x,y)=(6m, 34m) is an old excavation from 2007. This feature is due to the disturbed soil from that excavation.

Imaging of Buried Strandlines

The image to the left shows a number of vertical cross sections from the 21-line grid in shown in the Google Earth image above. The northernmost line is Line00, and the southernmost line is Line20. The lines were 2.0m apart. The lines at the left are Lines 01, 03, 05, 08, 11, 13, 15 and 18. Line01 is closest to the exposed bluffs; Line18 is furthest from the bluffs. A number of reflections are indicated by the yellow arrows in the topmost and bottommost images. These are consistent with buried strandlines. One of these features is followed through all of the images with arrows (slightly faded). Other features may be followed from slice to slice in the same manner. These features correspond in depth and orientation to two partial Ipiutak features which had been exposed and previously recorded in the erosion face in two separate field seasons. It was not possible to test this area to the depth indicated by the GPR. However, subsequent coastal erosion has exposed additional strandline debris at the depth and location predicted by the GPR data. The fiducials “F1” were placed in the data whenever the GPR unit crossed locations that would have been perpendicular to the Ipiutak feature exposed on the nearby Beaufort Sea bluffs. See the image at the top for a map of these features.

“We started investigating the last GPR hit and came down on a jumble of wood. The excavators were not optimistic, but I kept pushing to go a little further. Eventually, this appeared:” -Anne Jensen, “Out of Ice and Time: Arctic archaeology as seen from Barrow, Alaska”

From http://iceandtime.wordpress.com/

The outlines of five co-located GPR survey grids (Grids A-E, line spacing 0.5m), along with the 21 lines in the “strand line” survey, (line spacing 2.0m; all lines shown). The superimposed map shows a number of features including GPR anomalies in the 21-line grid, excavated Ipiutak features, and the erosion of the Beaufort Sea bluffs from 2001-2012. Background image courtesy of Google Earth, using imagery from March 22, 2006.

10 m

Grid A

Grid B

Grid C

Grid D

Grid E

July 2011 GPR survey grids with progression of sea

bluff erosion