Comprehensive study of fracture patterns and densities in the Geysers geothermal211 reservoir using microearthquake shear-wave splitting tomography. (Quarterly 211 progress report 03/16/1998-06/15/1998)

摘要

We completed the process of identifying shear-wave splitting in the Geyser area.211u001eA total of 2700 observations were recorded with about 1700 observations from the 211u001e1988 data and about 1000 observations from 1994. Fast polarization direction map 211u001ein Figure 1 shows that most of the stations in the Geyser area display consistent 211u001edirection throughout the main field, between 0(degree) azimuth to 40(degree) 211u001eazimuth. Some exemptions to the consistent crack alignment (fast polarization 211u001edirection) can be seen in stations 9 and station 3, and also in stations 13 and 211u001e14 outside the field. Since the stations are in boreholes it is possible that 211u001esome of the station orientations, calculated using P-wave arrivals from located 211u001eevents, are erroneous. If we treat measurements of polarization direction as a 211u001estatistical process, same as deep of layer measurement, we can say that in the 211u001esmall area of the station we have aligned cracks. Figures 2 and 3 show results of 211u001ethe crack density inversion assuming regional crack azimuth of 20(degree). Almost 211u001e2400 raypaths were used to perform this tomographic inversion. There is weak 211u001edependency of the results on the regional crack direction, but the main areas of 211u001ehigh and low crack density are the same. The changes are mainly in the size of 211u001ethe anomalies. Since the amplitudes of those anomalies depend mainly on the 211u001edamping parameter we use in the inversion, exact regional crack direction is not 211u001ea critical parameter of the inversion. The map in figure 2 and cross-sections in 211u001eFigure 3 show two areas of high crack density at the top 1 km one at station 8 211u001eand the other between stations 6 and 5. At greater depth of 1 to 2 km those two 211u001earea converge to one high crack density anomaly between stations 3, 4, 11, and 211u001e10.