Groundwater Exploration Using High-Resolution Seismic and Resistivity Methods: Gulf Coast, Texas
Ross, Abigail 1993-
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In some clastic environments, groundwater can be linked directly to fluvial sand deposits and then imaged using near-surface seismic and electrical resistivity surveying. Two sites at the University of Houston La Marque Geophysical Observatory (LGO) are characterized to develop seismic and resistivity signatures for water-saturated sands as a means of locating fresh groundwater. A new multiphysics methodology for sand-channel imaging and identification is presented to provide local information about undiscovered aquifers available for efficacious drilling. Joint analysis of seismic imaging, surface resistivity, and downhole measurements provide lithologic and saturation characteristics with links to porosity and the volumetric extent of subsurface anomalies. At the LGO, Rayleigh-wave surveying and inversion (MASW) are interpreted for zones of increasing shear velocity, an indicator for increased rigidity and water-saturated sand channels. A velocity transition zone of 250 to 350 m/s is found to represent clay to sand interface transitions. The velocity layer ties with peaks in resistivity logs ranging consistently from 20 to 23 Ωm. P-wave and SH-wave seismic surveys identify reflectivity signatures matching interface changes and producing an impedance signature for use deeper where MASW cannot image. In the detailed study area, measured apparent resistivity is inverted to true resistivity as a function of distance and depth using an iterative L2 -based minimization of calculated apparent resistivity versus inverted true resistivity. P-wave refraction analysis indicates a water table to support our interpreted areas of interest at 7 m to 9 m depth where top layer velocities abruptly change from 400 m/s to 1650 m/s. The intersection of resistivity highs and shear-wave velocity indicators are drilled at three locations for confirmation and indicate that the groundwater is fresh, with conductivity averaging 775 μS/cm. Borehole logging of these wells confirms the exploration results, with gamma-ray and resistivity logs agreeing with the electric and seismic results. In addition, drill cuttings match lithology predicted by MASW, where dense clays at 5 m matched low shear-wave velocities of 150 to 200 m/s, and freshwater saturation in the field matched lithology characteristics. Anomalies and drilling results are in agreement with a water-saturated sand.