2 research outputs found
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Variability of relative site response at Los Alamos, NM
To estimate the range of seismic response at low strain of sites within Los Alamos National Laboratory, ground motion recordings were obtained at 13 sites from nuclear tests carried out in Nevada. The sites are distributed within a 10 X 10 km area. The ground motions recorded at each site were conceptually modelled as the result of source, path, and site contributions. Because almost all of the paths are in common, the variations seen for each source can be attributed to site response. The sites were monitored in various combinations with seven nuclear tests; each site recorded only a few of the tests. Because horizontal ground motion is more important for structural engineering and was larger than the vertical, we focused on horizontal site response. The range of relative site response seen is about a factor of 5 to 6 at 1.5 Hz. Topography has a strong effect on response, with sites in canyons being a factor of 3 to 4 lower than nearby sites on mesas. Increased depth to seismic basement beneath some stations also correlates with higher relative site response. Relative site response does not obviously correlate with variation of seismic velocities in the near surface (e.g. upper few meters). 5 refs., 4 figs
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Imaging of reservoirs and fracture systems using microearthquakes induced by hydraulic injections
Predicting the future performance of a geothermal reservoir and planning a strategy for increasing productivity from the reservoir require an intimate knowledge of the fracture system through which geothermal fluids permeate. Microearthquakes often accompany hydraulic fracturing as well as normal production activities in geothermal fields. The waveforms from the se microearthquakes provide valuable information that can be used to infer the three-dimensional structure of the fracture system in the reservoir. The locations of the microearthquakes can be used to infer the presence of large fractures along which shear slip has occurred. Tomographic imaging using arrival times of the seismic waves, provides a three-dimensional image of the P and S wave velocity structure of the reservoir. These velocities yield information about the presence of microfractures in the rock. Waveform stacking methods can be used to both corroborate seismic velocities and image seismic scatters in the reservoir. The most prominent seismic scatters are likely to be fluid-filled fractures. Thus, seismic data provide information about a fractures over a large scale range which can be of use in reservoir engineering. 32 refs., 4 figs