Virtual Seismometers in Geothermal Systems: Using Microquakes to Illuminate the Subsurface

Abstract

A natural response to fluid injection at enhanced geothermal sites is the creation of microseismicity. Thousands of microquakes are often associated with an injection well. When processed using novel geophysical techniques, these microquakes effectively illuminate the subsurface, and can be used to monitor plume growth and identify otherwise hidden structures. The virtual seismometer method (VSM) is a new technique of seismic interferometry that provides precise estimates of the GF between earthquakes. It is very sensitive to the source parameters (location, mechanism and magnitude) and to the Earth structure in the source region. Using VSM, we are able to focus sharply on the cloud of microseismicity. We can monitor the evolution of seismicity over time, measure changes in the style of faulting and sort microseisms by location and magnitude. Our ultimate intent is to use it to image structures within the microseismic cloud in an attempt to identify previously un-observed fault zones. In simple terms VSM involves correlating the waveforms from a pair of events recorded at an individual station and then stacking the results over all stations to obtain the final result. In the far-field, when most of the stations in a network fall along a line between the two events, the result is an estimate of the GF between the two, modified by the source terms. In this geometry each earthquake is effectively a virtual seismometer recording all the others. When applied to microquakes, this alignment is often not met, and we need to address the effects of the geometry between the two microquakes relative to each seismometer. Nonetheless, the technique is quite robust, and highly sensitive to the microseismic cloud.