SHEAR-WAVE SPLITTING CRACK DENSITY MAPS FOR THE GEYSERS AND MAMMOTH

Abstract

We have developed a method for mapping subsurface fracture density using the time differences of split shear waves from microearthquakes. In regions where a consistent direction of fracturing exists, shear waves from different source and receiver locations are systematically split into fast and slow components. The greater the fracture density, the greater the time differences between the fast and slow components for a given path length. Also, the greater the path length in the fractured rock, the greater the time difference. With a large number of spatially distributed sources and receivers, tomographic backprojection of the time differences can be used to map the distribution of fracture density.

We have applied this method to 3 geothermal fields, the Geysers, Mammoth, and Coso. Our best example to date comes from Mammoth, where we recorded over 1/4 million source-receiver pairs, from which we extracted 1480 splitting times. Inversion of these times suggests the presence of zone of higher fracture density at depths of 1.5 to 2.5 km north and east of the Casa Diablo geothermal field.