Mapping Deep Channel Deposits in the Ogallala Aquifer Using Electrical Resistivity Surveys

The Ogallala formation was deposited as an extensive braided stream system that drained the eastern slope of the Rocky Mountains. The thickness of the sand and gravel deposits is highly variable with the thickest deposits filling stream valleys eroded into the underlying land surface. The thickness varies from about 0 to over 500 feet in a few buried channels.

The aquifer has been extensively pumped for decades and significant dewatering has occurred. In portions of the aquifer the saturated thickness is becoming a limiting factor controlling the production of a well and the sustainability of the supply. While the mining is not sustainable, wells in deeper portions of the aquifer with coarser sands will provide higher yields for longer periods as the thinner parts of the aquifer are dewatered but the deeper parts of the aquifer still contain saturated permeable material. While it is not possible to predict the thickness of the Ogallala based on surface topography, there are geophysical methods that can be used to map the thickness of permeable sand and gravel units above the underlying units.

A high resolution electrical resistivity survey conducted in west Texas for an industrial client. The client had an extensive well field to provide cooling water for an industrial facility but regional declines in water levels continues to reduce the capacity of most of the wells. Seven miles of resistivity lines were used to map the deeper portions of the aquifer. Subsequent test borings confirmed the channels, which varied from about 180 to about 270 feet deep, with the coarsest formation in the deepest part of the channels. A horizontal well was constructed in one of the deeper channels. The additional submergence provided by siting the well in the deeper channels and coarsest sands helped increase the production from the horizontal well.