Understanding glacial deposits in a bedrock valley with seismic data and well descriptions, Allen County, Indiana

Seismic exploration methods provide a non-invasive method of imaging the subsurface. This method is valuable in many fields outside of geophysics and the petroleum industry, such as engineering, geology, archeology and environmental work. This work utilizes seismic data as a tool to understand the geomorphology at a field site in Allen County, Indiana. The field site is the location of an abandoned municipal water field. The site is within a large bedrock valley filled with thick deposits of diamicton and silty sandy gravels; however, the aquifer was unable to produce enough water for municipal usage. Several vertical seismic profiles (VSP) surveys and a P-wave reflection profile were conducted in order to better understand the glacial deposits. Well descriptions described by A. Fleming provide ground truth for interpreting the seismic data. The reflection data was processed using standard seismic processing techniques. The P- and S-wave layer velocities were calculated using a least squares inversion based on the VSP data. The P-wave velocities ranged from ∼700–2500 m/s while the S-wave velocities ranged from ∼350–500 m/s. Based on the results of the inversion, synthetic VSP and reflection data were produced using wave number integration approximation. The reflectors were correlated to specific strata using the synthetics and approximated arrival times. The deeper reflection events arrived earlier than the shallow reflection event, which resulted in an inverted profile. This inverted profile is the result of the velocity structure and the source-receiver spacing. Recommended future works include conducting and processing more seismic profiles over the boreholes using 3-component geophones, analyzing and inverting recorded surface waves, and collecting more complex borehole surveys such as walkaway VSP and crosshole tomography.