ANALYSIS OF ELECTROMAGNETIC AND SEISMIC GEOPHYSICAL METHODS IN INVESTIGATION OF SHALLOW SUB-SURFACE HYDROGEOLOGY IN HYPER-ARID SALINE SOILS

An electromagnetic (EM) and seismic geophysical study was initiated on behalf of Nye County, Nevada to evaluate non-invasive approaches to measure depth to shallow groundwater in hyper-arid environments with high soil salinities using a combination of geophysical methods where the installation of piezometers is impractical or prohibited. The study area is Carson Slough near Ash Meadows National Wildlife Refuge in Amargosa Valley, located ~113 km northwest of Las Vegas, Nevada. The area is underlain by valley fill, with generally shallow, albeit variable, depths to water. The methods used include electromagnetic induction using Geonics EM-38, EM-34, and EM-31-SH tools allowing for multiple antenna-receiver spacings, which permit resolution of changes in conductivity with depth. We also acquired P-wave seismic reflection (nominal 48-fold) profiles at high resolution using a short (0.305 m) geophone spacing. Both methods were deployed over four profiles where shallow piezometer control was present. Local conditions at Carson Slough present numerous challenges to water table characterization using geophysical methods due to its shallow depth, high soil and water conductivity, and high degree of horizontal and vertical heterogeneity of the shallow subsurface. In order to better constrain the effectiveness of the geophysical methods, a calibration study was conducted along the southeast shore of Utah Lake, Utah County, Utah. This area is a semi-arid environment with a shallow well-defined water table, but less saline than Carson Slough and with more homogeneous soil. This allowed for development of a straightforward modeling approach. EM results at both sites show that the water table corresponds with a drop in conductivity. This is due to high concentrations of evaporative salts in the vadose zone; however, because this method relies on a conductivity contrast between the vadose and phreatic zones, it is difficult to identify the top of water table in areas of low conductivity contrast. Interpretations of seismic data show reflections and strong head waves corresponding to water levels at piezometer control points, although this method is labor intensive when using short geophone spacing. Thus seismic reflection or refraction could be used to supplement EM data, especially in areas of low conductivity contrast.