Oriented surface passive seismic location using local slopes
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A common acquisition scenario in microseismic monitoring is the deployment of large areal receiver arrays at or near the surface. This recording geometry has the advantage of providing coverage of the source's focal hemisphere as well as characterization of the arrival time moveout curve; however, the accuracy of many location techniques applied to these data sets depends on the accuracy of the depth velocity model provided prior to location. We have developed a simple oriented time-domain location technique so that full knowledge of the velocity model is not required a priori. The applicability of the technique is limited to horizontally layered models and also to models with dipping interfaces of small angles; however, this restriction is acceptable in many unconventional reservoirs. Implementation of the technique includes three steps: (1) smoothing of the observed time arrivals by fitting a hyperbolic moveout curve with a broad set of constraints, (2) updating and restricting the constraints using a local-slopes-based location workflow, and (3) estimation of the focal coordinates of passive sources using the updated constraints for the final least-squares fitting of the moveout curves. We have tested the performance of the proposed technique on several 2D examples and a 3D field data set. The results from synthetic examples suggest that, despite the assumption of the method that the arrival moveout can be modeled using a constant effective velocity, a reliable event location is achieved for layered models without considerable lateral heterogeneities. Our tests on the field data set find that the focal point coincides with a previously derived estimate of the source location. To assess the uncertainty of the proposed technique, bootstrap statistics was also used and applied to the field data set.
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