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

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.

Geophysical survey locates an elusive Tlingit fort in south-east Alaska

“…Thomas M. Urban & Brinnen Carter (e.g. Urban et al 2016. These published surveys, however, are on a much smaller scale than the Sitka EM survey.…”

“…In recent years, geophysical methods have been gaining traction in Alaskan archaeology, yielding a range of new discoveries and insights spanning the full human history of the region Thomas M. Urban & Brinnen Carter (e.g. Urban et al 2016). These published surveys, however, are on a much smaller scale than the Sitka EM survey.…”

Archaeological geophysical survey of a Prehistoric Bronze Age site in Cyprus (Alambra Mouttes)—applications and limitations

“…Time slices were made using the hyperbola fitting function to estimate the velocity of the electromagnetic signal in a given volume of the medium. This velocity was then used to calculate the two-way travel time to get a depth estimate (Goodman and Piro 2013; Jacob and Urban 2015; Urban et al 2016). These depth estimates generated in the software were then verified in the excavations.…”

Archaeological Prospection with Motorised Multichannel Ground-Penetrating Radar Arrays on Snow-Covered Areas in Norway

“…The topsoil, which in agriculturally used fields corresponds to the plough layer, is rich in organic material and pore space between the solid particles, comprising the soil water [30]. In media where liquid water is present, dramatic changes of the GPR velocity can occur, complicating the imaging of buried structures, as described by Urban et al [19]. The electrical properties of water change dramatically when freezing [31], resulting in frozen soil to behave very differently than soil that is not frozen.…”

“…The relationships between snow quality, liquid water content in the snow, and GPR signal quality have been studied e.g., [14][15][16]. Crucial factors for GPR pulse propagation are velocity changes at the interfaces of frozen and wet ground layers, as well as the water content of the snow [17][18][19]. These studies have generally been conducted with single-channel GPR systems, and large-area, high-resolution, multi-channel GPR investigations for archaeological prospection purposes on snow and ice have been limited so far.…”