System and method for determining the distribution and orientation of natural fractures

In oil and gas production, a process known as hydraulic fracturing is often used. Hydraulic fracturing involves pumping a fluid under pressure down a well bore into a fluid reservoir. When the pressurized fluid enters the reservoir, it produces localized failures of rock within the earth known as fractures. These fractures generate elastic waves known as microseisms that travel outward from the source of the fractures in a spherical wavefront. These microseisms can be measured with sensors located near the well bore, and their source determined. The microseismic wavefront is composed of compressional and shear waves. The amplitudes of the compressional and shear waves can be detected and measured and the ratio of the shear wave amplitude to the compressional wave amplitude which is known as the S/P ratio, can be determined. The orientation of the natural fractures in the earth are determined by comparing the S/P ratio of the waves generated by hydraulic fracturing with predicted S/P ratios for theoretical failure mechanisms and orientations. The predicted S/P ratios are calculated for theoretical failure mechanisms and orientations on the basis of the measured wave source. Forward modeling techniques are used to match the actual and predicted S/P ratios to determine the failure mechanism and the orientation of the natural fracture which generated the microseisms during the hydraulic fracturing.