Importance of Accurate Earth Models and Network Geometry for Earthquake Location in CCS and Wastewater Projects

For more than two decades induced seismicity has been an important topic due to the increased number of Oil&Gas well operations. Recently, induced seismicity has become an important issue related to wastewater and CCS projects. One of the most challenging problems is the strong dependence of hypocenter location, especially depth, on the velocity model used for the subsurface and the seismic network geometry.

We have developed an approach using 3D models based on sonic log, checkshot data and tomography results for the Midland basin, West Texas, to investigate the sensitivity of earthquake depth to network geometry and the velocity structure. In a subarea of the basin where the network is dense, we find that including stations at distances greater than ∼12 km shows a strong effect of the basement velocity on the locations. In order to avoid the streaks of earthquake location and large time residuals we have incorporated an approach based on simplified wide angle refraction seismology to optimize the sub-basement velocity to create an improved reference 3D velocity model. We used this 3D reference velocity model to relocate the seismicity in the Midland basin area and identify the bias on hypocentral depth compared to 1D velocity models.