Hypocenter location errors of microseismic events located by using a 3-C VSP downhole geophone array at Violet Grove

Zuolin Chen, Robert R. Stewart

Detection of passive microseismic events in oil reservoirs is often accomplished by using an array of subsurface receivers. The location of these events is often determined by a technique which combines arrival times and back-azimuth. However, the accuracy of hypocenter location is difficult to estimate due to the straight line geometry. Also, due to the close distances of events and the effect of nonlinearity on travel times, the error distribution can not be evaluated by the conventional generalized inverse method. Hence, we developed an alternate method to estimate the error distribution, subject to some preconditions, by numerical experiments. We estimate that vertical and radial errors are about 10% of the distance of the array from the events. More geophones in the array or wider geophone spacing decreases the location errors. The results may provide some guidelines for the future design of VSP monitoring arrays.