USING MICROGRAVITY AND PASSIVE SEISMIC METHODS JOINTLY TO EXPLORE THE BRAZOS RIVER ALLUVIUM AQUIFER

This study seeks to delineate depth to bedrock, aquifer heterogeneities (clay vs. sand vs. gravel), and saturated thickness in the northern segment of the Brazos River Alluvium Aquifer using microgravity and passive seismic methods. The primary goal is to find productive areas in the alluvium that can be tapped by water wells (e.g. gravel-filled buried nickpoints or buried channels). The evaluation of both methods in a central Texas unconfined aquifer setting is an additional objective of the project. Both methods, if proven to be viable, would be improvements over expensive and highly labor-intensive exploration techniques such as drilling boreholes, seismic refraction, and electrical resistivity surveys. The Brazos River Alluvium Aquifer is continuing to garner more attention as an alternative water source in the Waco, TX area as the more heavily used Trinity aquifer, a widespread deep confined aquifer, is experiencing a continual decline in water levels of around 10 ft/year. Previous studies have succeeded in characterizing the Brazos River Alluvium Aquifer in a broad sense with data from both boreholes and water wells. However, few geophysical investigations of the aquifer in the study area (Bosque, Hill, McLennan, and Falls counties) have been published. So far, preliminary data from two gravity and passive seismic surveys suggest that depth to bedrock will be able to be estimated with ~10% error; however, delineating aquifer heterogeneities may only be possible if large changes in gravel/sand thickness occur within the survey bounds. Estimating saturated thickness by incorporating scattered water level information from local wells is also predicted to be successful with ~10% error. The study is still in its early phase with secondary data collection occurring during summer 2019. Specific instruments used are the Scintrex CG-6 Autograv and the Tromino 3G+ Zero.