| Title | Fracture Density Maps from Shear Wave Splitting Observations |
|---|---|
| Authors | E. Shalev & P.E. Malin |
| Year | 2006 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | |
| Abstract | Shear wave splitting observations from seismic events recorded by a local seismic network can be used for mapping subsurface fracture density. This type of data collection and analysis is one part of our Joint Geophysical Imaging (JGI) approach to geothermal exploration. Shear waves from different source and receiver locations are systematically split into fast and slow components in regions where a consistent direction of fractures exists. The greater the fracture density, the greater the time differences between the fast and slow components for a fixed path length. With a large number of spatially distributed sources and receivers, tomographic inversion of the time differences can be used to map the distribution of fracture density. The method utilizes the fast shear wave polarization azimuth and the time differences between fast and slow split shear waves. These observations are obtained by using a hybrid system of computer codes for signal identification and measurements, where an observer marks the potential splitting location, and an automated computer algorithm computes the polarization angle and the fast/slow time delay. This technique was initially used at Long Valley to successfully target new production and reinjection drilling. It can be used in a fracture dominated geothermal areas with sizable local seismic activities and a local seismic network. Our current research into data collection for JGI studies involves the use of borehole ìelectro-seismicî networks, two of which are under construction, one in Hawaii and one in Iceland. The networks consist of borehole based seismometers, electrodes, and coils. Examples of data and data analysis from both networks will be presented. |