An airborne transient electromagnetic (TEM) survey was completed in the Upper San Pedro Basin in southeastern Arizona to map resistivity distributions within the alluvial aquifer. This investigation evaluated the utility of ID vertical resistivity models of the TEM data to infer lithologic distributions in an alluvial aquifer. Comparisons of the resistivity values and layers in the ID resistivity models of airborne TEM data to ID resistivity models of ground TEM data, borehole resistivity logs, and lithologic descriptions in drill logs indicated that the airborne TEM identified thick conductive fine-grained sediments that result in semiconfined groundwater conditions. One-dimensional models of ground-based TEM surveys and subsurface lithology at three sites were used to determine starting models and constraints to invert airborne TEM data using a constrained Marquardt-style underparameterized method. A maximum structural resolution of six layers underlain by a half-space was determined from the resistivity structure of the ID models of the ground TEM data. The ID resistivity models of the airborne TEM data compared well with the control data to depths of approximately 100 m in areas of thick conductive silt and clay and to depths of 200 m in areas of resistive sand and gravel. Comparison of a 3D interpolation of the ID resistivity models to drill logs indicated resistive (mean of 65 ohm-m) coarse-grained sediments along basin margins and conductive (mean of 8 ohm-m) fine-grained sediments at the basin center. Extents of hydrologically significant thick silt and clay were well mapped by the I D resistivity models of airborne TEM data. Areas of uncertain lithology remain below conductive fine-grained sediments where the ID resistivity structure is not resolved: in areas where multiple lithologies have similar resistivity values and in areas of high salinity.
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