Seismic Profiles Reveal Gas Pathways at Storage Field

Combined P-wave and S-wave seismic profiles over a gas storage facility in north-central Illinois are being used to locate shallow gas in storage facility. Although several faults had been documented in the Mt. Simon Formation during development of the storage facility, they were not thought to extend to the shallower bedrock. Our detailed reflection profiles showed that the faults do extend within the bedrock to the upper layers and provide the migration pathway for gas to move upwards from the storage facility. Seismic S-waves are not sensitive to the presence of gas so they were used to image the stratigraphy. Even with our very small S-wave source (a 1-kg hammer horizontally striking a weighted steel plate), the bedrock layers could be imaged down to 400 to 500 feet. We attribute this success to the use of innovative S-wave reflection data acquisition and processing techniques developed at the ISGS. Seismic P-waves are extremely sensitive to the presence of even a few percent (by volume) of gas in pore space, and we used our P-wave land-streamer technology to detect and map the gas zones. By combining the results of the two methods, both the gas zones and the gas migration paths can be clearly mapped in three dimensions. This study has been conducted in collaboration with Illinois State University and will be the basis of a thesis for a master’s degree candidate in geology. The techniques used here should be broadly applicable for mapping the movement of gas in the shallow Earth. (Contact: T. Larson)