Comprehensive study of fracture patterns and densities in the Geysers geothermal211 reservoir using microearthquake shear-wave splitting tomography (Quarterly 211 progress report 06/16/1998-09/15/1998)

摘要

We completed the process of locating events and identifying shear-wave splitting211u001ein the mammoth area. A total of 2250 split shear wave observations were recorded 211u001ein the four month period that our network was in place. Fast polarization 211u001edirection map in Figure 1 shows that most of the stations in the mammoth area 211u001edisplay consistent direction throughout the main field, between 300(degree) 211u001eazimuth to 0(degree) azimuth. Some exemptions to the consistent crack alignment 211u001e(fast polarization direction) can be seen in station M19, and some stations 211u001edisplay inconsistent trend as can be observed in stations M25, M18, and M07. It 211u001eis possible that station M19 was misaligned during installment. Figure 2 shows 211u001ethe cumulative rose diagram for all observations with a clear preferred 211u001edirection. Figure 3 also shows that most of the observations of fast split shear 211u001ewave are in the same direction and that those observation are distributed 211u001ethroughout the target area. If we treat measurements of polarization direction as 211u001ea statistical process, same as deep of layer measurement, we can say that in the 211u001esmall area of the station we have aligned cracks. Figures 4 and 5 show results of 211u001ethe crack density inversion assuming regional crack azimuth of 340(degree). 211u001eAlmost 2000 raypaths were used to perform this tomographic inversion. There is 211u001eweak dependency of the results on the regional crack direction, but the main 211u001eareas of high and low crack density are the same. The changes are mainly in the 211u001esize of the anomalies. Since the amplitudes of those anomalies depend mainly on 211u001ethe damping parameter we use in the inversion, exact regional crack direction is 211u001enot a critical parameter of the inversion. The map in figure 4 and cross-sections 211u001ein Figure 5 show two areas of high crack density: one northeast of the Casa 211u001eDiablo area at depth of 1 to 3 km, and one near the Mammoth airport and station 9 211u001eat depth of 2 to 3 km.