Abstract
We have performed several sensitivity studies to assess the ability of the Full Wave Inversion method to detect, delineate and characterize faults in a crystalline geothermal reservoir from OVSP data. The distant goal is to apply the method to the Soultz-sous-Forêts site (France). Our approach consists of performing synthetic Full Wave 2D Inversion experiments using offset vertical seismic and comparing the estimated fields provided by the inversion, i.e., the estimated underground images, to the initial reference model including the fault target. We first tuned the inversion algorithmic parameters in order to adapt the FWI software, originally dedicated to a sedimentary context, to a crystalline context. In a second step, we studied the sensitivity of the FWI fault imaging results as a function of the acquisition geometry parameters, namely, the number of shots, the intershot distance, the maximum offset and also the antenna length and well deviation. From this study, we suggest rules to design the acquisition geometry in order to improve the fault detection, delineation and characterization. In a third step, we studied the sensitivity of the FWI fault imaging results as a function of the fault or the fault zone characteristics, namely, the fault dip, thickness and the contrast of physical parameters between the fault materials and the surrounding fresh rocks. We have shown that a fault with high dip, between 60 and 90° as thin as 10 m (i.e. lower than a tenth of the seismic wavelength of 120 m for Vp and 70 m for Vs) can be imaged by FWI, even in the presence of additive gaussian noise. In summary, for a crystalline geological context, and dealing with acceptable S/N ratio data, the FWI show a high potential for accurately detecting, delineating and characterizing the fault zones.
Highlights
For any deep georesources exploration project, finding the location of underground resources is an important step
In a second part (Section 4), we have studied the effect of the acquisition geometry on the inversion results, including the intershot distance, number of shots used in the same run, the maximum offset, etc
Reiser et al [10]obtained accurate fault imaging for faults, especially those with a shallow dip, and demonstrated that the Kirchhoff migration cannot provide accurate fault imaging for high fault dips: the faults the granites using the full wave inversion (FWI) technique? Even if the thickness is a geometrical parameter of the fault, its estimation is related to the physical parameter contrasts and the frequency content of the seismic signal; this is why we have considered this parameter separately
Summary
For any deep georesources exploration project, finding the location of underground resources is an important step. The surface of rock-to-hot fluid interaction is increasing with the interconnected fractures network. This improves heat exchange by driving the deep hot fluid to shallower and exploitable depths. The presence of an adequate fault network in geothermal fields is crucial. This was observed during the drilling and different hydraulic tests performed in Soultz-sous-Forêts geothermal boreholes (e.g., [2] and included references) that the geothermal brine inflows to the wells at depths where faults are imaged and clearly identified
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