Imaging the Blue Mountain Geothermal Site Using Seismic Interferometry

Abstract

In this study, we demonstrate the power of seismic interferometry for use at geothermal sites. First, we applied “virtual earthquake” techniques to seismic deployments at the Blue Mountain, Nevada geothermal site. We obtained nearly 50,000 virtual seismograms from active source intereferometry (ASI) of the records from historical seismic surveys of the site. These allow sharp imagery of seismic velocities and attenuation along seven 2D profiles. Next we applied ambient noise correlation (ANC) to the existing seismic monitoring array deployed by AltaRock Energy, together with the short term array that was deployed contemporaneously by Sandia National Laboratory. The modern networks provide hundreds of additional paths crossing the site. This mix of instruments and networks maximizes the resolution and extends the subsurface image in 3D to depths of several kilometers. Synthetic seismograms calculated through the 3D model capture the complexity of the direct and scattered waves that are observed in microearthquake data. Microseismicity is a natural response to fluid injection at geothermal sites. We have identified over 1100 local events within the geothermal region at Blue Mountain, since the deployment of the permanent array in late 2015. The rate of seismicity spikes during operational changes, particularly during maintenance periods when injection is switched off. We located this seismicity by back-propagating through the 3D model. The spikes in seismicity appears to follow a repeating pattern: early in the sequence, microearthquakes occur near the junction of two major faults, and shift progressively deeper over the course of several days.