Aquifer characterization using towed time-domain electromagnetics in a variably saturated, data-sparse region

Abstract

Parowan Valley, Utah is an agricultural area experiencing significant subsidence in recent decades due to extensive groundwater extraction. The subsidence occurs primarily due to consolidation in fine-grained units as groundwater heads decrease due to pumping. Efforts to predict future subsidence would be facilitated by an accurate understanding of the distribution of fine-grained materials in the subsurface. An analysis of drillers' logs across Parowan Valley from previous research indicates that significant fine-grained units are present, but the variable quality of these logs, as well as the limited spatial distribution of borings, makes the accurate determination of the location and extent of fine-grained units challenging. To overcome the limitations of drillers' logs analysis, ground-based and borehole geophysical data were acquired. The ground-based data were collected over large sections of the valley using a towed Time-Domain Electromagnetic (tTEM) system that measures electrical resistivity over different depth intervals. These data were used to characterize the distribution of fine- and coarse-grained sediments within the tTEM depth of investigation of ∼60 m. Borehole gamma data were acquired in three boreholes near the tTEM traverses to compare with resistivity data and drillers' logs for further validation of tTEM resistivity data. In this study, we developed a methodology for rock physics transforms in regions with sparse geological information and variable saturation, as well as a scheme for using a variety of methods depending on the availability of lithology information, enabling us to produce robust rock physics transforms in an area with complex geological conditions.