Dynamic cold regions terrain effects on time-domain electromagnetic induction data

Abstract

Abstract Subsurface geophysical characterization efforts at a US Army legacy munitions waste site were negatively impacted by dynamic cold regions surface processes including increased snow depth, surface soil erosion and deposition, and seasonal water levels. One concern identified in the characterization efforts showed the time-domain (TD) electromagnetic induction (EMI) response from the buried munitions was diminished due to an increased snow pack. It is well known that increasing the distance between the ground surface and the time-domain electromagnetics (TDEMI) sensor will result in a decreased response, all other factors remaining the same. However, in cold regions or other locations where seasonally dynamic changes in surface elevations occur, the depth to target may also change, potentially resulting in false negatives when site based regulatory thresholds are used for detection criteria. Here, we present measured and synthetic data illustrating the impact of dynamic cold regions environments on subsurface EMI geophysical surveys. We model the TDEMI response from a single target at varying depths and relate this to snow pack thickness in increments of 0.10\u202fm. Results suggest for a target at an original depth of 0.86\u202fm, 0.10\u202fm of snow accumulation, or sensor height change, will result in a false negative for this size munition at this site. Next, a model from the synthetic data to correct for the diminished response due to increased sensor offset is developed, and effectiveness of the model as applied to potentially missed targets is demonstrated. The synthetic data were plotted against the modeled data and exhibited a close correlation. Additionally, we examine the effects of dynamic surface grade on the TDEMI results, demonstrating a significant difference between soil and snow surface grade impacts on target detection and locating. Finally, an approach to correct the TDEMI response based on the change in surface elevation resulting from increased snow depth, surface soil erosion and deposition, or water levels is proposed.