Virtual Seismometers for Induced Seismicity Monitoring

Abstract:

Induced seismicity is associated with subsurface fluid injection, and puts at risk efforts to develop geologic carbon sequestration and enhanced geothermal systems. We are developing methods to monitor the microseismically active zone so that we can identify faults at risk of slipping.

We are using the Virtual Seismometer Method (VSM), which is an interferometric technique that is very sensitive to the source parameters (location, mechanism and magnitude) and to the earth structure in the source region. Given an ideal geometry, that is, when two quakes are roughly in line with a recording station, the correlation of their waveforms provide a precise estimate of the Green's function between them, modified by their source mechanisms. When measuring microseismicity, this geometry is rarely ideal and we need to account for variations in the geometry as well. VSM enables us to virtually place seismometers inside a micro events cloud, where we can focus on properties directly between induced micro events, and doing so, monitor the evolution of the seismicity and precisely image potential fault zones. Here, we show that the cross-correlated signals recorded at the surface are a combination of the strain field between two sources times a moment tensor. Based on this relationship, we demonstrate how we can use this measured cross-correlated signal to invert for focal mechanism.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.