A comprehensive study of fracture patterns and densities in the Geysers geothermal reservoir using microearthquake shear-wave splitting tomography [Quarterly progress report 06/16/1998 - 09/15/1998]

摘要

We completed the process of locating events and identifying shear-wave splitting in the mammoth area. A total of 2250 split shear wave observations were recorded in the four month period that our network was in place. Fast polarization direction map in Figure 1 shows that most of the stations in the mammoth area display consistent direction throughout the main field, between 300{degree} azimuth to 0{degree} azimuth. Some exemptions to the consistent crack alignment (fast polarization direction) can be seen in station M19, and some stations display inconsistent trend as can be observed in stations M25, M18, and M07. It is possible that station M19 was misaligned during installment. Figure 2 shows the cumulative rose diagram for all observations with a clear preferred direction. Figure 3 also shows that most of the observations of fast split shear wave are in the same direction and that those observation are distributed throughout the target area. If we treat measurements of polarization direction as a statistical process, same as deep of layer measurement, we can say that in the small area of the station we have aligned cracks. Figures 4 and 5 show results of the crack density inversion assuming regional crack azimuth of 340{degree}. Almost 2000 raypaths were used to perform this tomographic inversion. There is weak dependency of the results on the regional crack direction, but the main areas of high and low crack density are the same. The changes are mainly in the size of the anomalies. Since the amplitudes of those anomalies depend mainly on the damping parameter we use in the inversion, exact regional crack direction is not a critical parameter of the inversion. The map in figure 4 and cross-sections in Figure 5 show two areas of high crack density: one northeast of the Casa Diablo area at depth of 1 to 3 km, and one near the Mammoth airport and station 9 at depth of 2 to 3 km.