Seismic Velocity Analysis in the Olkaria Geothermal Field

Abstract

4352 local events originating within the study area [ts-tp<3 sec], 162 earthquakes at regional distances [5 sec<ts-tp<40 sec], 60 blasts from a nearby road construction quarry and 15 calibration shots are used to analyze for the hypocenter distribution and velocity variations.

The hypocenter intensity show that the brittle-ductile transition zone is at a depth of 4-5 km. Vp/Vs ratios varies from 1.58 within the East Production Field to 1.82 in the Olkaria Central Field. Poissonís ratios follow the same trend and vary from 0.15 to 0.30. The variation of s and Vp/Vs ratio is consistent with measured reservoir fluid chemistry, pressure and temperature thus suggesting seismic parameters can be associated
with reservoir properties. The low s and Vp/Vs values are interpreted to be due to high temperature and steam/gas saturation. The result shows that measurement of s and Vp/Vs changes in a geothermal field are useful indicators and tools for monitoring natural or induced reservoir
phase changes.