Geothermal reservoir characterization: Application of microseismicity and seismic wave properties at Olkaria, Kenya rift

Abstract

A seismic network comprising 18 stations was established to study the seismicity associated with the Olkaria geothermal field. During the period of network recording, 4765 local earthquakes originating within the study area (ts-tp < 3 s) as well as 162 earthquakes at regional distances (5 s < ts-tp < 40 s) were recorded. Also, 60 rock blasts from a nearby quarry and 25 calibration shots were recorded. P and S waves from 2613 well-located microearthquakes, 45 quarry blasts, and 25 calibration shots were used in this study. Average velocities for the upper crust are estimated to be 6.4 ± 0.04 and 3.74 ± 0.03 km/s for P and S waves, respectively. For the four uppermost layers the thicknesses and average velocities are estimated to be as follows: for layer 1, 0.6 km. Vp = 2.8 km/s, and Vs = 1.65 km/s; for layer 2, 1.5 km, Vp = 3.9 km/s, and Vs = 2.28 km/s; for layer 3, 4.0 km, Vp = 4.7 km/s, and Vs = 2.66 km/s; for layer 4, >6 km, Vp = 6.4 km/s, and Vs = 3.74 km/s. The magnitude threshold is Mc = 1.4 and the observed b value is 1.05. Vp/Vs ratios were determined for the six individual fields composing the study area and were found to be low (1.58) in the east production field (EPF) and high (1.82) in the Olkaria central field (OCF). Hypocenter analysis show that the main geothermal heat source for the area is at a depth of at least 6 km. Poisson's ratio (σ) is low (0.16) in the EPF and Olkaria west field (OWF) but high (0.26) in the central (OCF and Olkaria south field (OSF)) N-S zone parallel to the Ololbutot fault zone. The low σ corresponds to a decrease in P wave velocity in areas with low reservoir pressure, high heat flow, and steam/gas saturation. Evaluation of σ and Vp/Vs with regard to directly measured reservoir parameters such as pressure, temperature, and fluid chemistry give consistent and comparable results. This shows that σ and Vp/Vs values are indicators of reservoir characteristics and tools for monitoring natural or induced changes. Copyright 2000 by the American Geophysical Union.