Microseismic Location Using Differential Backazimuth Angles

Abstract

The most common procedure for locating borehole microseismic events is solution of a minimization problem based on observed travel time differences compared to calculated travel time differences derived from a given velocity model and oriented geophones. However, both the velocity model and geophone corrections often contain errors that will incorrectly locate microseismic events. An alternate method characterizes events using only their differential backazimuth information, based only on the difference in backazimuth angles between multiple microseismic events observed at common monitor wells. Possible locations are then found by using the grid search and gradient descent search algorithm finding the minimum residual between measured and calculated differential backazimuth angles. Microseismic events can be located in the horizontal plane with only differential backazimuth information. The gradient descent search is significantly faster than exhaustive search and can be applied to real array geometries. Tests from the array geometry near Allison, Texas targeting the Granite Wash Formation indicate this method can locate events even when array geometry is very poor. Owing to the non-ideal array geometry, required initial guesses must be within 50 ft to ensure convergence to the global minimum. The backazimuth method can be implemented to locate microseismic events where the velocity model is poor. By first locating events using the backazimuth approach, the velocity model can be constrained and calibrated for subsequent locations.