Shear Wave Analysis

Single component data recordings delivered by conventional 2D/3D seismic recording systems ignore a large portion of the information radiated by microseismic events and are likely responsible for past failures of surveys acquired at the surface.

The S-waves triggered by microseismic events are often the strongest signals recorded in microseismic data. They are normally observed on the horizontal components of a 3-C receiver and are weak or completely absent on the vertical component.

Assuming a Vp/Vs ratio of 2, the energy of an S-wave phase (SH or SV) radiated by a shear event is 8 times larger than the associated P-wave (Aki and Richards 2002), but a single component system probably won’t record it.

Strong shear wave arrivals recorded by a line in a surface array over a shale gas play in North America.  Dashed lines are drawn just above the P and S arrivals.

A standard requirement in borehole microseismic and VSP applications, three-component (3-C) recording is essential in both borehole and surface microseismic surveys because only 3-C data embodies the full three-dimensional wavefield.

3-C instruments not only capture all wave modes, but also facilitate polarization analysis to distinguish easily between the different modes. Classification of the recorded signals into vertically polarized P-waves and horizontally polarized S-waves is crucial to avoid imaging false positives from misinterpreted wave modes.

The results of any geoscience data analysis are only as good as the quality and integrity of the recorded data set. Capturing the most complete wavefield in time, space and frequency is a good strategy to maximize the value of microseismic monitoring data.

Shear wave radiation pattern for a microseismic faulting event demonstrating the non-uniform, directional nature of shear wave energy. Most of the energy generated by microseismic faulting events is this shear wave energy.