Improved Methods for Mapping Permeability and Heat Sources in Geothermal Areas Using Microearthquake Data


Bruce R. Julian, Gillian R. Foulger

Key Words:

microearthquake, seismology


Stanford Geothermal Workshop







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Geothermal microearthquakes, and the seismic waves they generate, provide a rich source of information about physical processes associated with Enhanced Geothermal Systems (EGS) experiments and other geothermal operations. With support from the Dept. of Energy, we are developing several software packages to enhance the utility of microearthquake data in geothermal operations and EGS experiments. Two of these are:

1. Enhanced relative hypocenter location techniques. Relative relocations produce three-dimensional images showing in fine detail the geometry of microearthquake clusters, and thus failure zones. These images are valuable for diagnosing failure mechanisms and for identifying promising drilling targets, but they suffer from a fundamental weakness: they are relatively insensitive to absolute locations and generally yield cluster locations with excellent structural detail but poorly located as a whole.

We are refining the hypocenter-location program hypocc to use both absolute and differential arrival times, in order to constrain the absolute locations of clusters in addition to the relative locations of the earthquakes in them. hypocc can use three-dimensional ray tracing to take full advantage of detailed information about local structure when it is available, and can also use simpler (e.g., one-dimensional) models for less well-constrained cases.

2. Local-earthquake tomography produces three-dimensional images of the seismic wave-speed structure that can delineate geothermal reservoirs and detect and measure temporal changes caused by geothermal operations. However, because waves from local earthquakes travel upward to surface or shallow-borehole seismometers, they provide no information about the deeper portions of geothermal reservoirs or the regions below them. An additional problem is that some geothermal areas are only weakly seismogenic, and provide few data useful for tomographic inversion.

We are extending the local-earthquake tomography program tomo4d [Julian and Foulger, 2009] to use data from regional earthquakes (out to distances of a few hundred kilometers) as well as from local earthquakes. Waves from regional earthquakes pass beneath geothermal areas, and using them can provide more accurate and complete models, including the deeper portions of reservoirs and the underlying heat sources.

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