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SUMMARY Knowledge of attenuation structure is important for understanding subsurface material properties. We have developed a double-difference seismic attenuation (DDQ) tomography method for high-resolution imaging of 3-D attenuation structure. Our method includes two main elements, the inversion of event-pair differential $${t^*}$$ ($$d{t^*}$$) data and 3-D attenuation tomography with the $$d{t^*}$$ data. We developed a new spectral ratio method that jointly inverts spectral ratio data from pairs of events observed at a common set of stations to determine the $$d{t^*}$$ data. The spectral ratio method cancels out instrument and site response terms, resulting in more accurate $$d{t^*}$$ data compared to absolute $${t^*}$$ from traditional methods using individual spectra. Synthetic tests show that the inversion of $$d{t^*}$$ data using our spectral ratio method is robust to the choice of source model and a moderate degree of noise. We modified an existing velocity tomography code so that it can invert $$d{t^*}$$ data for 3-D attenuation structure. We applied the new method to The Geyser geothermal field, California, which has vapour-dominated reservoirs and a long history of water injection. A new Qp model at The Geysers is determined using P-wave data of earthquakes in 2011, using our updated earthquake locations and Vp model. By taking advantage of more accurate $$d{t^*}$$ data and the cancellation of model uncertainties along the common paths outside of the source region, the DDQ tomography method achieves higher resolution, especially in the earthquake source regions, compared to the standard tomography method using $${t^*}$$ data. This is validated by both the real and synthetic data tests. Our Qp and Vp models show consistent variations in a normal temperature reservoir that can be explained by variations in fracturing, permeability and fluid saturation and/or steam pressure. A prominent low-Qp and Vp zone associated with very active seismicity is imaged within a high temperature reservoir at depths below 2 km. This anomalous zone is likely partially saturated with injected fluids.
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Aquifer Diffusivity Estimation Through Joint Inversion of the Amplitude Ratios and Time Lags of Dominant Frequencies of Fluctuating Head
Abstract This note introduces the estimation of aquifer diffusivity (D) through simultaneous inversion of the attenuation and lag of multiple head fluctuation frequencies due to a dynamic source or boundary, that is, a river. Spectral analysis, with optimized moving time window length and step size, was used to extract the dominant constituents and their attenuation through space; the cross‐power spectral density method was used to determine time lags. The Jacob‐Ferris analytical model was then used for inverting forD. Unlike most similar applications to date, here we propose using all frequencies with robust signal‐to‐noise ratios (five total in our test cases) and both the amplitude attenuation and time lag in the inversion. The method was implemented using observations from wells in the banks of the fluctuating Meghna River in Bangladesh that is connected with a semi‐confined sandy alluvial aquifer. The estimatedDusing the technique provides estimates that are very similar to those from pumping tests. The estimates are more accurate compared to previous implementation of the Jacob‐Ferris model on the same data that used only a dominant frequency's amplitude attenuation or time lag. The workflow and codes for the analysis are provided for straightforward implementation of the robust and cost‐effective method.
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- PAR ID:
- 10367931
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 57
- Issue:
- 6
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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