Geoelectric and Seismic Characterization of the Precambrian Granite Gravel Aquifer, Llano Uplift, Central Texas

Science Center Objects

The USGS Texas Water Science Center (TXWSC) is testing the suitability of seismic refraction tomography (SRT) and time-domain electromagnetic sounding (TDEM) geophysical methods for mapping the contact boundary between the Granite Gravel aquifer and the underlying crystalline Town Mountain Granite bedrock in southwestern Burnet County. The Granite Gravel aquifer is anticipated to show a contrast in compressional (P-wave) seismic velocity compared to the underlying crystalline granite bedrock, and may potentially show electrical contrast between the unsaturated aquifer matrix and the water-table. Because of this anticipated contrast in seismic and electrical properties, SRT and TDEM geophysical methods will be tested for their applicability with respect to distinguishing the positions of the aquifer-bedrock contact boundary and the water-table in the Granite Gravel aquifer.    

Collecting seismic refraction data using a sledge hammer and aluminum plate as the source, granite gravel aquifer, Texas

Geophysical methods

 

 

Collecting seismic refraction data using a sledge hammer and aluminum plate as the source, granite gravel aquifer, Texas

Collecting seismic refraction data using a sledge hammer and aluminum plate as the source, granite gravel aquifer, Texas.  (Public domain.)

The seismic refraction method requires an input source of seismic energy, such as striking a sledge hammer onto an aluminum plate, to impart an elastic wave into the subsurface.  The elastic wave propagates radially outward from the seismic source, and reflects and refracts when it encounters boundaries between materials of contrasting density and P-wave velocity.  The refracted wave travels parallel to the boundary and upwards towards the surface, where geophones detect the refracted energy of the wave.  The data may be processed to look at variations in the P-wave velocity in the subsurface, which can be related to the elastic properties of the subsurface materials. Tomography, in the so-called SRT method, implies that imaging will be performed to look at these variations in the subsurface. If there is a strong contrast in P-wave velocity between the aquifer and bedrock, or between the unsaturated and saturated aquifer matrix, this method may be considered for more extensive mapping of the aquifer.

Time-domain (TD) electromagnetic (EM) soundings use EM waves as a primary source of input energy to induce alternating currents in electrically-conductive geologic formations in the subsurface.  These electrical currents dissipate in time (within microseconds to milliseconds) and produce secondary EM waves that propagate back to the surface where they are detected by the TDEM instrumentation.  The effect of the primary EM waves is removed from the secondary EM waves that are received, and the data are subsequently processed to produce a vertical profile of electric resistivity beneath the center of the surveyed area.  This profile can be interpreted based upon the electrical contrasts caused by subsurface geologic layering and the presence of groundwater. The TDEM method will be used to determine if there is a distinguishable electrical contrast between either the unsaturated and saturated aquifer matrix or the aquifer and the bedrock.  If there is a distinguishable electrical contrast in one or both scenarios, the TDEM data will provide an auxiliary form of geophysical data for mapping the aquifer, as well as an interpretation aid for the SRT data.