Borehole receiver orientation using a 3D velocity model

The orientation of three-component borehole geophones used for recording during a microseismic monitoring experiment is estimated. The standard technology for deploying multi-component geophones in a deep borehole is wireline-based, in which the azimuthal rotation of the geophone string cannot be controlled. Each receiver can have a different rotation angle that is compensated by the particle motion analysis of the direct P-wave arrivals, picked from a walk-around VSP carried out in the proximity of the well. Knowing the orientation of borehole receivers is critical, as inaccuracies lead to systematic errors in determining the hypocentral coordinates of microseismic events. Additional errors arise from over-simplifications of P- and S-wave velocity Earth models. In this paper, we propose a tomographic approach for improving the orientation estimates of borehole receivers based on hodogram analysis. The initial velocity model built from well logs and upholes is refined by 3D tomographic inversion of walk-around VSP data and some string shots fired in a nearby borehole. Taking into account ray bending, the estimated errors due to local velocity anomalies can be reduced. This makes our estimates of fracture orientation and microseismic hypocentres more reliable when borehole receivers are used in passive seismic surveys.