Geologic Sequestration Training and Research (GSTR) -
Recovery Act: Double-Difference Tomography for Sequestration MVA
Performer: Virginia Polytechnic Institute and State University
Project No: FE0002108
NETL partnered with Virginia Polytechnic Institute and State University (VT) to develop the framework necessary for data collection and processing requirements that enabled the use of double-difference seismic tomography as a tool for imaging changing conditions underground due to CO2 geologic storage (Figure 1). Seismic tomography is an imaging technique that uses seismic waves to generate three dimensional (3-D) images of the inside of the Earth. This and similar techniques are used to characterize site geology and track injected CO2 plumes over time. Double-difference seismic tomography uses the absolute and differential arrival times to solve for the velocity distribution and the source locations simultaneously; thus improving the spatial resolution of the produced 3-D image.
The laboratory-based project used synthetic data to optimize receiver locations and the mathematical parameters used in the inversion process. Data collection and processing requirements were quantified by comparing &"synthetic” seismic velocity models to &"calculated” seismic velocity models. If the source locations and receiver array are optimized, then the calculated model will closely match the synthetic model. The synthetic models were generated using curved-ray travel-time software developed by Dr. Westman of VT. They simulated approximately 125 different combinations of plume migration, source locations, and receiver array locations. The source arrays were microseismic emissions associated with the plume migration. The receiver arrays were geophone arrays of various configurations. The synthetic data was used with the double-difference tomography code to calculate velocity models for each of the 125 different combinations of plume migration, source locations, and receiver array locations. The output of the double-difference tomography was calculated velocity models that were then compared to the synthetic velocity models through cross-validation. A microseismic dataset was selected, with input from NETL, to be analyzed using the double-difference tomography over several time periods to observe the time-dependant change within the subsurface due to CO2 geologic storage.