![]() In synthetic data tests, we compare the resolution and stability of the traditional and adaptive methods. Both synthetic and real data are used to test the newly developed tomography algorithm. ![]() Secondly, the data in the area with dense ray sampling will be sufficiently utilized so that the model resolution can be greatly improved. First, irregular grids with different sizes and shapes can fit the ray distribution better and the traditionally ill-posed problem can become more stable owing to the different parametrizations. ![]() We present a new adaptive tomography method with irregular grids that provides a few advantages over the traditional methods. Large grid spacing is usually used to reduce the number of inversion parameters, which may not be able to solve for small-scale velocity structure. A sharp increase in velocity from 2000 m=s at approximately 50 m below the ground surface in many parts of the study area may indicate a shallower water table than expected for the region.read more read lessĪbstract: S U M M A R Y Traditional ambient noise tomography methods using regular grid nodes are often ill posed because the inversion grids do not always represent the distribution of ray paths. Lateral boundaries between high and low velocity zones agree relatively well with the location of local faults from previous studies, and low near-surface velocities are associated with faults and fumarole locations. The travel-time data were fit to a root mean square (rms) misfit of 31 ms, close to our estimated picking uncertainty. Models with 100 m horizontal and 20–50m vertical node spacing were obtained, covering an area 2000 m by 1300 m, with acceptable resolution extending to about 250 m below surface. The travel times were then used to invert for the 3D P-wave velocity structure. After removal of the sweep signal from the raw data, the first P-wave arrivals were automatically picked using a combination of methods. Sweeps were repeated up to four times at each site during four different stages of power plant operation: normal operation, shutdown, high and oscillatory injection and production, and normal operation. A large vibroseis truck was operated at 196 locations, exciting a swept-frequency signal from 5 to 80 Hz over 20 s using three vibration modes (vertical, longitudinal, and transverse), with three sweeps per mode at each site. The acquisition systems provided 15 days of continuous records, including active-source and ambient noise signals. The geophone array had about 60 m instrument spacing in the target zone, whereas DAS channel separations were about 1 m with an averaging (gauge) length of 10 m. The array was composed of 238 threecomponent, 5 Hz nodal instruments, 8700 m of distributed acoustic sensing (DAS) fiber-optic cable (FOC) installed horizontally in surface trenches, and 400 m of FOC installed vertically in a borehole. Abstract: We deployed a dense seismic array to image the shallow structure in the injection area of the Brady Hot Springs geothermal power plant in western Nevada.
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