General dislocation model based microseismic focal mechanism inversion

Through microseismic focal mechanism study, we can obtain the fracture directionality, scale and stress state of hydraulic fracturing areas, which are extremely important for the unconventional oil and gas development. For microseismic events, double-couple (DC) or moment tensor (MT) models are usually used to describe the source. However, DC model doesn't consider the tensile movement, MT model can't clearly show the fracturing scale. So we need to use a new model which is more suitable for microearthquakes.

In this study, we use the "Shear & Tensile" general dislocation point model to describe the microseismic source. This model takes account of tensile movement as well as shear slip and can directly reflect the fracturing scale along each direction. Based on this model, we develop an approach to calculate microseismic focal mechanism using amplitude spectra fitting and simulated annealing technique in frequency domain. The new method takes account of full waveform information including phase and polarities of first P wave arrivals and can provide dislocation length along each direction as well as common source parameters (strike, dip and rake angles) in the study area. The synthetic tests on surface and borehole network and applications to real data show that our method is robust and efficient.