The location of subsurface seismic events is important for various geophysical applications. For example, it can be used for underground intrusion detection, monitoring precursors of natural hazards (e.g., sinkholes, rock-falls, landslides, etc.), tracking the structural integrity of mines, locating trapped personnel, and microseismic reservoir monitoring. For a useful analysis, it is imperative that located events be as close as possible to their true spatial position. Although a variety of location methods have been developed, most assume that the correct velocity model is known and are very sensitive to errors in its estimation. Velocity is often estimated at recording borehole locations and calibrated if possible. However, calibration requires manual picking that may induce errors. We use moveout-corrected gathers, whose flatness serves as an objective function, for picking-free velocity model inversion using known source locations. These gathers can be constructed using any implementation of seismic wave propagation. To limit computational costs, we use an eikonal traveltimes solver. The inverse problem is solved using a global optimization approach with an adequate borehole-driven model parameterization. The method performs well in constructing an effective velocity model for location with or without knowledge of the source origin time. We show its application on reservoir-scale synthetic examples as well as a 3D shallow subsurface field one and show the significant final improvement in location accuracy. A single known source is used for calibration of the velocity model. The updated model is not necessarily correct, but it can be effectively used to improve location.
- passive seismic
- velocity analysis