Airborne electromagnetics (AEM) has previously been demonstrated effective at defining stratigraphy (coarse texture versus silts and clays, channels versus linear features), and for delineating water quality (brackish water resources and seawater intrusion), especially when integrated with borehole geophysical logs, lithology from well drilling and construction, and water quality data. Additionally, AEM surveys provide much greater spatial coverage than traditional methods in terms of time and resources, resulting in improved hydrogeologic correlation and interpretation between wells. In this project, approximately 800 linear kilometers of AEM data were collected in the Indian Wells Valley (IWV) groundwater basin to define the nature and extent of brackish groundwater resources, including the local and regional structure and stratigraphy.
The Indian Wells Valley (IWV) groundwater basin is located at the southeast termination of the
Sierra Nevada Mountains in a tectonically active area of rifting and crustal thinning associated with differential movement along the North American and Pacific plates. The regional setting has been causal to the IWV basin complexity, having formed a half graben with mountain uplift and IWV basin downdrop along the Sierra Nevada Frontal Fault, with multiple associated faults running through the basin. The IWV basin has been subject to a long series of studies, initially to characterize the geology and hydrology, and later to further delineate geothermal potential of the basin, based primarily upon data from wells, surface soils and geologic mapping, and 11 seismic sections. The AEM survey supplemented the existing data distribution, providing a more continuous spatial coverage of the basin.
Results of AEM survey of the IWV basin have shown the utility of the technology for mapping subsurface structure and stratigraphy, with multiple faults visible in the AEM data, showing evidence that internal basin structure controlled the depositional environments. A number of buried channels, alluvial fans and coarse delta deposits as well as fault controlled barriers to groundwater flow were mapped. The AEM data also provided detailed information on the extent of brackish groundwater as well as potential locations for groundwater recharge.