Gillian R. Foulger has 40 years experience in microearthquake (MEQ) studies in geothermal areas. She worked in Iceland for five years, pioneering the development of MEQ-based methods for prospecting and monitoring geothermal reservoirs. She applied the techniques developed to the Hengill and Krafla geothermal areas, Iceland (Foulger, 1988a;b; Foulger et al., 1989; Arnott & Foulger, 1994a;b).

Gillian subsequently added further MEQ-based techniques to her portfolio, including three- and four-dimensional Vp, Vs and Vp/Vs tomography, moment tensor analysis, and high-resolution relative MEQ locations. She applied these techniques to The Geysers, Long Valley and Coso geothermal areas (Calfornia), and geothermal and hydrocarbon fields in Indonesia and Papua New Guinea.

Interpretive work emphasizes cross-disciplinary integration of all geophysical and geological results available.

Gillian has authored more than 50 geothermal papers published in peer-reviewed journals and given more than 70 related conference presentations.

Example of a recent project

The Coso producing geothermal area, California

The Coso geothermal area is utilized to generate ~250 MW of electricity. Steam removal induces intense microearthquake activity suitable for advanced analysis techniques that are powerful to monitor the reservoir and optimize production strategies. Gillian used the ~80,000-earthquake dataset currently available for time-dependent seismic tomography, moment-tensor analysis, and high-resolution relative MEQ locations.

These studies yielded a 3-dimensional image of the entire reservoir, and show changes in reservoir properties with time caused by production. Fracture enhancement is achieved using hydraulic stimulation to improve steam recoverability. A unique proof-of-concept of the moment-tensor technique to image stimulated fractures and their mode of failure was recently achieved.

This project was conducted in collaboration with the U.S. Geological Survey, the U.S. Navy, the Department of Energy and the University of Utah.

Examples of recent significant related publications:

Gillian's wider expertise embraces general geophysical surveying, including gravity, magnetics, active seismics and GPS. She has also conducted pure research into the physics of earthquake sources, GPS surveying, the structure of the Earth’s mantle and the origin and existence of hot spots and mantle plumes.



Example of time-dependent three-dimensional tomography


Anomalies in Vp/Vs at The Geysers in 1991 (left panels), along with changes during the time periods specified (other panels), from Gunasekera et al. (2003).



Example of moment tensors

Theoretical focal hemisphere plots for combined shear and tensile crack sources, and examples from the Nesjavellir geothermal field, Iceland, from Miller et al. (1998).



Example of high-resolution MEQ locations

Example of MEQs from Long Valley Caldera, California, showing the improvement in location accuracy achieved by using a three-dimensional crustal model, and the high-resolution relative location technique.



Example of Integrated Interpretation

High-resolution relatively located MEQs line up to dissect the dilational field of the focal sphere plot, indicating that the seismically active lineation must be an opening crack and not a shear fault. Example from Long Valley Caldera, California, from Foulger et al. (2004).


last update 10th September, 2018