High‐Accuracy Inversion of Airborne Transient Electromagnetic Data for Hydrogeologic Framework Mapping Related to Climate Change

Abstract

Addressing problems related to water quality and quantity (e.g. irrigation, contaminant transport and vulnerability to climate change), requires detailed information about the hydrogeological framework over large areas. Airborne transient electromagnetic (TEM) is a geophysical technique capable of constraining subsurface resistivity to several hundred meters depth; the resulting sections can be interpreted in terms of the hydrogeologic framework which constrains groundwater flow. The aim of this research is to define the hydrostratigraphy of the High Plains Aquifer over a range of different geological settings, in Nebraska, USA. A challenge is to accurately characterize the airborne TEM systems in order to recover subsurface resistivity models consistent with independent ground truth information, especially in areas where targets exceed 100m depth. Several airborne TEM systems are being evaluated for their applicability to these problems. Processed data are inverted in 1D with lateral constraints to obtain a pseudo‐2D resistivity section. Recently developed model assessment techniques are applied and extended to investigate regions of the model that are most sensitive to the data. These tools provide estimates of the depth of investigation (DOI), the top of investigation (TOI) and determine the uncertainty of the resulting model parameters. The DOI, TOI and start model selection problems are solved using a sensitivity‐matrix analysis method. DOI and TOI information is important because it is possible that the measured data do not have sensitivity to certain depth intervals within the target aquifer. One result is a map showing the configuration of the top‐ and base‐of‐aquifer surfaces, in relation to DOI and TOI surfaces derived from the TEM data, together with estimates from model parameter uncertainty. Results are verified with ground‐based TEM soundings and lithologic/geophysical bore hole logs.