Splitting.html

SHEAR-WAVE SPLITTING

Applications to the determination of fracture patterns in Geothermal Reservoirs

Shear-wave splitting due to the alignment of vertical cracks in the crust has been widely observed in a variety of tectonic settings and data gathering experiments, from earthquake recordings to controlled-source seismic data. It has also been recognized that the polarization of the fast split shear wave is usually parallel to the local strike of cracks (or direction of the maximum horizontal stress), and that the time delay between fast and slow shear waves is closely related to the intensity of crack-induced anisotropy in the medium (Crampin, 1987; Crampin and Lovell, 1991; Crampin, 1993).

The Coso, CA Geothermal Field

The Coso geothermal area is a very active seismic zone, averaging 20 microearthquakes per day, nearly half of which are associated with the geothermal field injection/production activity (Malin, 1994). Since 1992 the Duke University seismology group and the Wave Propagation Laboratory at UNC-Chapel Hill have collaborated in the seismic surveying of the Coso reservoir using a number of techniques, including the detailed analyses of split shear waves.

Although straightforward, the analysis of shear-wave splitting for the purposes of seismic imaging must be based on a large set of shear-wave seismogram data. The behavior of shear-wave splitting above small earthquakes is usually very complicated, because of the complexity of the source signal, subsurface geology structure, and surface topography. One major restriction to the analysis of shear-wave splitting described here is that in order to obtain a correct interpretation of split shear-waves, the recording site needs to be within a specific shear-wave window. The shear-wave window beneath a recording site is defined by a critical angle, ic = arcsin(b/a), where a and b are the P-wave and S-wave velocities, respectively. For angles of incidence greater than ic ( measured from the vertical-down direction), shear waves strongly interact with any free surface or interface, and thus all similarities with the incoming waveform are irretrievably lost. The critical angle defining the shear-wave window is about 35o in a half space with a Poisson's ratio of 0.25. Fortunately, ray curvature due to low-velocity surface layers usually allows the effective window to be enlarged to angles of incidence of 45 or 50 degrees.

Crack densities. Tomographic Results

Six consecutive horizontal levels at depths ranging from 0.5 to 5.5 km show the distribution of crack densities under the Coso gethermal area. Yellow triangles are down-hole, three component seismic sensors. The elliptic outline is Sugar Loaf mtn. area

Besides crack orientation, crack density is another important parameter to characterize subsurface crack patterns. We estimated that the crack density ranges between 0.010 and 0.035 throughout the Coso volume as shown in the above figure, changing with different depths and locations. The three-dimensional crack density distribution is obtained through a three dimensional tomographic inversion scheme (Shalev, Lou, Rial and Malin 1995) for which a total of 450 data points (time delays) were used in a target area of 21 (km) X 21 (km) X 6 (km). The figure above shows the horizontal sections of crack density distribution at six different depths from 0.5km - 5.5 km. We note that the relatively large crack density (0.035) areas concentrate on two northeast -trending blocks with at depths between 1.5 - 3.5 km, which are in fact the most active geothermal production areas. The tomographic inversion results are however severely limited due to the uneven distribution of microearthquake source locations, as well as the low number of data points within the split shear-wave recording window. A rather large distance (2 km) between grid nodes was used to image the crack density distribution of the target area.

Selected References

Crampin, S., 1987, Geological and industrial implications of extensive-dilatancy anisotropy: Nature, 328, 491-496.

Crampin, S., 1993, A review of the effects of crack geometry on wave propagation through aligned cracks: Can. J. Expl. Geophys., 29, 03-17.

Crampin, S., and Lovell, J., 1991, A decade of shear-wave splitting in the Earth's crust: what does it mean? what use can we make of it? and what should we do next?: Geophys. J. Int., 107, 387-407.

Hudson, J.A., 1981, Wave speeds and attenuation of elastic waves in material containing cracks: Geophys J. R. astr. Soc., 64, 133-150.

Lou, M. and J.A. Rial,1994, Characterization of the crack geometry at the Coso, California geothermal reservoir by analyzing shear-wave splitting from microearthquakes, in Proceedings of the 19th Workshop on Geothermal Reservoir Engineering, pp. 1-20 ; Stanford University.

Lou, M. and J.A. Rial, 1994, Locating an active fault zone in the Coso geothermal field by analyzing seismic guided waves from microearthquake data, in Proceedings of the 20th Workshop on Geothermal Reservoir Engineering, pp. 115-122; Stanford University.

Lou, M. and J.A. Rial, 1995, Modeling Elastic Wave Propagation in Inhomogeneous Anisotropic Media by the Pseudospectral Method, Geophys. Jour. Int; (120), 60-72.

Lou, M. and J.A. Rial (1995): Application of the wavelet transform in detecting multievents in microearthquake data; Geophys. Res. Lett (22) No. 16, pp. 2199-2202.

Lou, M. and J.A. Rial (1995): Characterization of geothermal reservoir crack patterns using shear-wave splitting, Geophysics, (submited).

Malin, P., 1994, The seismology of extensional hydrothermal system: Geothermal Resources Council, TRANSACTIONS, 18, 17-22.

Shalev, E., Lou, M., Rial, J.A., and Malin, P., 1995, Crack density tomographic inversion in the Coso geothermal field, in preparation.

Research Activities and Projects

Computer Simulations of Wave Phenomena
Earthquake Response of Sedimentary Basins
Signal Analysis of Geologic and Climatologic Time Series
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