Active high‐resolution compressional wave attenuation tomography at Newberry Volcano, Central Cascade Range

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Abstract
In this study, we use the seismic data from a previous study at Newberry to compute its three‐dimensional compressional wave attenuation structure. The data consist of eight explosions recorded on an array of about 120 seismograph stations. The shots are located on two concentric circles centered on the caldera with radii of 39 and 85 km. Attenuation is estimated using spectral ratios to compute δt* at each station for each shot. The data are inverted using a least squares technique. Our results indicate that regions of inferred boiling water (i.e., two‐phase pore fluid) are correlated spatially with regions of high attenuation not corresponding to low velocities. Two regions are of particular interest. One is underneath the Newberry II drill hole, in the central part of the caldera, where results from a flow test suggest that two‐phase fluid conditions existed in the bottom of the hole before the flow test. The other region is beneath the west flank of the volcano, which has been interpreted as an attractive geothermal prospect because of a shallow conductive anomaly. Our results are consistent with the suggestion made in an earlier paper by us that two‐phase geothermal reservoirs can be located within complex three‐dimensional structures by using jointly interpreted compressional wave velocity and attenuation images.