Time-lapse geophysical surveys of analogues for residential school unmarked burials

The search for unmarked graves at former Indian Residential School (IRS) sites in Canada can be assisted by geophysical surveys, yet these surveys face several challenges. Many of these graves are over a century old and become increasingly difficult to distinguish from the surrounding soil, because as decomposition advances and the disturbed backfill re-equilibrates with the host soil, the dielectric, conductive, and density contrasts that produce detectable anomalies progressively diminish. This study addresses these challenges through a geophysical monitoring program at the Sage Test Facility near Brandon, Manitoba, established and led with the Sioux Valley Dakota Nation under Indigenous protocols, where nine test units were constructed to approximate documented IRS burial styles. These test units ranged from 0.5 to 2.0 m in depth and contained wrapped and unwrapped faunal remains, empty wooden boxes, superabsorbent polymer bundles, or soil only, and each excavation was backfilled with its own excavated soil, homogenized across depths to interrupt the natural stratigraphy. Repeated, time-lapse ground-penetrating radar (GPR), electromagnetic induction (EMI), and Light Detection and Ranging (LiDAR) surveys were conducted over 624 days and were integrated to evaluate how target observability and soil moisture around the test units changed over time. The results demonstrate that soil moisture variations, both natural seasonal changes and an induced water addition over the test units relative to the surrounding undisturbed ground, play a critical role in the discernibility of burial features. For example, shallow targets were detected in up to 75% of cases during the coldest surveys (temperature quartile T1) but in as few as 12.5% during the warmest, driest surveys (T4), and detectability declined markedly following heavy rainfall. The combined GPR, EMI, and LiDAR framework supports the refinement of geophysical workflows for subtle target detection while honoring the Indigenous-led protocols under which the facility was established. This research advances burial detection by demonstrating the benefits of surveying during periods of high soil moisture or frozen ground, of refilling surface depressions to improve GPR coupling, of optimizing GPR frequency and survey-mode selection, and of integrating EMI and LiDAR surveys. These refinements contribute meaningfully to community-led searches, to the identification of unmarked graves, and to honoring the children who disappeared from IRS sites.