f A Comparison of Helicopter-Borne Electromagnetic Systems for Hydrogeologic Studies

The increased application of airborne electromagnetic (AEM) surveys to hydrogeological studies is driving a demand for data that can consistently be inverted for accurate subsurface resistivity structure from the near surface to depths of several hundred meters. We present an evaluation of four commercial AEM systems over two test blocks in Western Nebraska, USA. The selected test blocks are representative of shallow and deep alluvial aquifer systems, and have a conductive base-of-aquifer. The aquifer units show significant lithologic heterogeneity, and include both modern and ancient river systems. We compare the various data sets to one another, and inverse resistivity models to borehole lithology and to ground geophysical models. We find distinct differences among the AEM systems as regards the spatial resolution of models, the depth of investigation, and the ability to recover near-surface resistivity variations. We further identify systematic biases in some data sets which we attribute to incomplete calibration or compensation procedures.