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GPR detects Pleistocene “Ghost Tracks”

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n a recently published research article1, T. M. Urban et al. document the use of ground penetrating radar (GPR) to successfully detect footprints in White Sands National Monument, New Mexico, USA.

These are no ordinary footprints however, but so called “ghost tracks” from the Pleistocene Epoch. These Ice Age footprints were made by mammoths, giant sloths, and humans more than 11,000 years ago. Unlike fresh footprints on a beach, which appear in negative relief, these footprints were filled-in with new sediment over the years making them flush with the ground surface or, in some cases, just beneath a layer of sand. This means that the prints are often not visible, but they may become temporarily visible under certain moisture conditions, only to disappear again when those conditions change – hence the colloquial term used for them, “ghost tracks”. The good news is that these footprints, although not visible to the human eye, can be found with geophysical methods, including GPR, allowing the researchers to locate and study them at any time. Additionally, because excavating the footprints is time-consuming and destructive, large numbers of footprints can be quickly documented with GPR while leaving them intact.

GPR archaeology survey
Figure 1
Depth slice map from 2 to 4 ns shows mammoth, giant sloth and human footprints detected with a NOGGIN® GPR system using a center frequency of 250 MHz.

 

The team’s study describes their attempt at imaging footprints using a NOGGIN® 250 MHz SmartTow™ system, which was a success (Figure 1). Since this initial work, however, the researchers have collected several other GPR data sets using NOGGIN® and pulseEKKO® GPR systems with 250, 500 and 1000 MHz center frequency antennas (Figure 2), which will be described in future research articles.

GPR scanning underground foot prints
Figure 2
Collecting GPR data at White Sands National Park with a NOGGIN® 250 SmartTow™ system (left) and a pulseEKKO® 500 SmartTow™ system (right). A foam pad was used to protect the footprints while the work was conducted.

 

It was found that mammoth footprints result in substantial compression of sediment that shows up especially well in 3D representations of the GPR data (Figure 3); not surprising since these would have been the largest land animal around at that time. Foam mats were used to cover the ground surface to protect the delicate prints from damage. Close grid-line spacing was required to detect the much smaller human footprints. GPR data processing included routine procedures such as Dewow, gain, background subtraction, migration, and envelope, as described in the original paper1.

3D GPR underground map
Figure 3
3D representation of mammoth footprint obtained using a NOGGIN® 250. Mammoths would have been the largest land animals in the area, and their footprints lead to substantial compression of the sediment underneath.

 

The team, a collaborative, interdisciplinary group including researchers from Cornell University (USA), Bournemouth University (UK), and the U.S. National Park Service, has also previously successfully imaged mammoth footprints with a magnetometer2, though they highlight some of the advantages of GPR in their new article. In particular, the magnetometer is less sensitive than GPR when it comes to smaller footprints, such as those of humans. The GPR also provides depth information, which is useful when assessing the footprints; especially in cases where two or more track- making events are superimposed, or in cases where sediment compression beneath the footprint is useful to understand, such as with mammoths (Figure 4).

GPR Archaeology data
Figure 4
Depth slice of mammoth footprint (center) and giant sloth footprints (top) detected with GPR. Data collected with a 250 MHz center frequency NOGGIN® system.

 

This unusual application of GPR has captured significant public attention, with the findings being covered in more than 120 news outlets worldwide. The Ice Age footprints of White Sands had also received widespread press coverage in 2018 when the same team published a study suggesting that footprints showed Pleistocene humans harassing and stalking a giant sloth3.

Story courtesy of Thomas M Urban from Cornell University.

  1. 3-D radar imaging unlocks the untapped behavioral and biomechanical archive of Pleistocene ghost tracks
  2. Use of magnetometry for detecting and documenting multi-species Pleistocene megafauna tracks at White Sands National Monument, New Mexico, U.S.A
  3. Footprints preserve terminal Pleistocene hunt? Human-Sloth interactions in North America
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