Seismic Monitoring for High-Precision Delineation of Fault Geometry and Stress; Case Study for the West Texas Subscription Array

Seismicity potentially induced through wastewater disposal, hydraulic fracture completion, or other industrial operations, has been a cause for increasing public concern over the last decade. Monitoring for this activity has focussed on the problems of location and characterization, often to a relatively rough degree of precision. Regulations typically spell out responses for operators should an event exceed a magnitude threshold within a specified distance of their facilities. While this type of monitoring is critical for ensuring injections be conducted effectively while minimizing potential damage from shaking and public alarm, it often leaves many unanswered questions in terms of the underlying processes.

Understanding these questions entails that we demand more out of the seismic networks, essentially upgrading the data products to a “next generation” level.  The data from the network needs to be used to provide a detailed understanding of critical geological structures and geomechanics of the study area. This goal is facilitated through both a densification of hardware and a higher order of event processing. High-precision locations delivered through relative relocation methodologies delineate slipping fault structures, often resolving previously unknown features. Moment tensor inversion processing also helps reveal the orientations of faults and provides information on stress in the region. The resolution of these structures provides critical insight into understanding how a field is reacting.

We illustrate the application of this “next-generation” seismicity monitoring system to the Delaware Basin in West Texas, where we have deployed a network of 25 broadband seismometers complementing monitoring from TexNet and other networks. Despite being an exceptionally challenging recording environment, by aggregating all of these data we obtain a high-resolution catalog of earthquake hypocenters delineating a number of fault features. Inverting the stresses from the moment tensors of the highest-quality events shows a dominantly normal stress regime and tangibly resolves a rotation of axes transitioning across the basin. We illustrate both the logistical and processing requirements necessary for timely delivery of results highlighting the dynamics of seismicity in an active study area.