Combined Use of a Deep Monitoring Array and Shallow Borehole Arrays for Moment Tensor Inversion

This study explores the feasibility of deriving the spatio-temporal evolution of the seismic moment tensor in a multi-stage Fayetteville shale stimulation and quantifies the benefits provided by a monitoring configuration where a deep borehole seismic array is supplemented with seismic arrays located in shallow boreholes. For the moment tensor inversion, the focal sphere of the source space is sampled in a Bayesian approach that evaluates the probability of a possible source with respect to the observed body wave arrival polarity information. The body-wave arrivals were picked manually and assigned relative error weightings regarding the associated confidence of the polarity pick. Although the shallow boreholes enhanced focal sphere coverage, it was only possible to determine source mechanisms for events with a moment magnitude of -1 or greater. Solutions with the greatest certainty comprised a combination of tensile and double-couple components. There was little evidence of a consistent evolution in source parameters in time and space and the spatial event density was lower in regions of relatively high horizontal compression.