We have recently reported on the use of magnetic sensors to detect mammoth foot prints at WHSA11 and the limitations reported in that work are overcome by the use of GPR in the work reported here. First, the magnetometer is less reliable in detecting smaller tracks such as those made by humans. Human tracks are only detected if the base of the true track (plantar contact surface) is deeply impressed relative to the surface (>200 mm). Track depth and therefore fill-volume appears critical. Mammoth tracks are always detected, however. The instrument is also subject to intermittent external electromagnetic noise from nearby military activity which is specific to WHSA but none the less important. Lastly, magnetic data are not well-suited to providing 3D data, which are especially valuable in instances where tracks comprise a palimpsest of overwritten track-making events7. It does, however, offer a method of broader prospection that could be applied where there are surface-evident tracks, or for general reconnaissance of suspected track locations, particularly for larger fauna. For example, shore normal survey transects at playas and former lake beds should in theory identify tracks by analogy with the types of distribution described in recent field observations21,22.

GPR surveying, on the other hand, was able to resolve 96% of the human tracks present and all of the larger vertebrates. Results suggest, however, that with refined survey design all human tracks could be detected simply as a matter of scaling. The tracks are detectable due to the infill which exhibits higher amplitude GPR reflections than the surrounding substrate. This contrast is likely due to stronger electrical properties within the tracks than in the surrounding sediment matrix and the higher electrical permittivity probably reflects a difference in water content due to textural contrasts. In this case, the GPR response suggests that the substrate filling the prints holds more moisture than the surrounding sediment even under dry conditions, something that is evident when the tracks are excavated.

However, it is the subsurface amplitude variations below the mammoth tracks that we consider to be of particular significance. These are manifest as areas of higher amplitude that likely have a different explanation from that postulated above for the prints themselves. With the sub-track anomalies, it is not newly introduced sediments that explain the reflections, but we theorize this is caused by compression of the existing sediments which alters the electrical permittivity. The similarity between the plantar pressure records of extant elephants and the areas of anomalous high amplitude immediately beneath the area the footprint is striking, and suggests GPR is detecting a plantar pressure record below the mammoth track due, we suggest, to compression of sub-track sediment. This proposed compression pattern was not visible with excavation, and appears at least at the tested location to be only discernible with GPR.

Conventional biomechanical inference from footprints often relies on a pressure to depth substitution in which deeper areas reflect higher plantar pressures. This has been found to only hold however for shallow tracks in the case of human footprints23, but is something that is rarely questioned in ichnological studies for larger or extinct trackmakers. Other experimental studies that have examined this relationship in modern human footprints have also found the relationship to be tenuous24,25. There are many reasons why strain may not be solely accommodated by footprint volume (i.e. depth), and moreover taphonomic processes can rapidly modify observed footprint topology obscuring any relationship26. For example, the topology of the manus track (T 1) in Location-2 (Figs 2b, 6c) does not reflect the sub-surface pressure anomalies (Fig. 6b). Our results suggest that irrespective of variation in track depth, a pressure record is encoded via sub-track sediment properties (likely compression), and in some cases this is independent of the track and its topology.

Taken to its logical extent, potentially thousands of plantar pressure records are waiting therefore to be collected at sites like WHSA and elsewhere in North America1,7 and Namibia26,27 without the need for tracks to be excavated. The potential here to enhance our understanding of the biomechanics of extinct animals may yield important information for developing more sophisticated biomechanical models from and for extant elephants and by analogy from anatomically similar dinosaurs such as sauropods28. It may also improve the quality of geotechnical models applied to both elephants and mammoth tracks since it would allow estimated plantar pressures to be used rather than as now uniform indenters29.

At sites such as White Sands, a new data archive in the form of a rare and unique ichnological record is there to be unlocked. Accessing this record in all conditions using non-destructive geophysical methods has significant implications for the effectiveness of research, conservation and resource management at White Sands and beyond. Knowing where unexcavated tracks are located is a key step in management, monitoring and prioritization for resource conservation, especially since excavation leads ultimately to loss. With higher resolution surveys possible, we anticipate in the future that excavation may not always be necessary on an extensive scale for study of such tracks. Unpublished experimentation with different antenna frequencies has shown that the most consistent results are obtained with a 250 Hz antenna, at least at WHSA where the substrate exhibits high electrical conductivity, and that enhanced resolution can be obtained by increasing the grid resolution and combining perpendicular lines. Our unpublished tests with higher antenna frequencies (500 MHz and 1 GHz) show some promise in obtaining finer detail on smaller tracks in some cases, such as those of humans, which will be the subject of a future paper. Irrespective of this, the power of the approach lies not just in imaging buried track topology but in the additional information obtained on sub-track compression. So beyond the immediate and obvious benefits of locating and imaging the tracks themselves, GPR offers ancillary information on pressure and momentum due to detectable effects on sediments below and around the tracks. Resolvable and consistent trends in the GPR data suggest that each footprint has an associated sub-structure caused by compaction of surrounding sediment. Initial findings suggest that this is related both to compression from the weight of the track-maker, along with shearing forces from the momentum of the trackmaker. Therefore, information about the size and direction of the trackmaker are likely exhibited in the broader GPR patterns, offering a previously uninvestigated avenue into the biomechanics of extinct species such as the mammoth. This has implications for the study of fossil tracks well beyond White Sands, including the possibility that under suitable conditions these sub-track compaction contrasts could be retained after lithification and therefore be present below the tracks of dinosaurs or other extinct vertebrates.