Distribution of the Vp/Vs Ratio within the Basel 1 Geothermal Reservoir from Microseismic Data

We estimate the spatial distribution of the Vp/Vs ratio within the Basel 1 geothermal reservoir from arrival time measurements of micro seismic data. High-precision P and S arrival time differences are calculated by an automatic, iterative, correlation-based procedure for pairs of 2149 multiplet events. The Vp/Vs values in the stimulated volume of the Basel reservoir range from about 1.45 to 1.90 at a depth interval between 3.8km and 4.8km. The average value of 1.70 is close to the characteristic value of the reservoir, 1.72, which was independently determined from sonic log measurements. Also, the depth distribution of Vp/Vs correlates well with the low-pass filtered sonic log measurements in the vicinity of the borehole. Vp/Vs values are less than 1.70 at the top of the seismicity cloud at <3.9km depth, close to average at 4.0-4.4km depth, and exceed the value of 1.70 at larger depth (4.4-4.6km), consistent with the sonic log data. This supports the validity of our results and suggests that using micro seismic data to determine the Vp/Vs ratio of a reservoir may be a competitive alternative or complement to prevailing tools such as sonic log or AVO inversion.