Passive Seismic Monitoring - understanding and reducing event location uncertainties
Résumé
Passive seismic monitoring is now a standard tool to follow the evolution of hydrocarbon, geothermal as well as CO2 geological storage reservoirs. Despite the fact that large volumes of data have already been acquired in one of the contexts just mentioned, our understanding of the relation of the microseismicity to the hydraulic fracture geometry is still very poor. Very precise location of seismicity is the first step to better understand and delineate hydraulic fracture geometry. Among many factors that contribute to microseismic location errors, the largest contribution is due to the lack of knowledge of the wave-propagation medium. Precise location of hypocenters requires a very accurate P and S velocity model, the calibration of this model is a difficult task but cannot be neglected. In addition, this required calibration/inversion of the velocities is a totally non-linear problem; taking into account this difficulty we developed a non-linear (Monte Carlo) tomographic inversion algorithm. This paper presents our non-linear calibration algorithm that updates the P and S velocity model in the context of Passive Seismic Monitoring. Furthermore the uncertainties associated to the velocity model are computed.