| Popis: |
There is a common misconception in microseismic monitoring that shear arrivals are more difficult to detect at the surface than compressional arrivals. This is despite the fact that, for common fracture types, the energy released in the form of shear waves is greater than compressional energy. To reconcile such claims, it has been assumed that the near surface has higher attenuation for S-waves than P-waves. In this paper we demonstrate through modeling and field data acquired in 2011 and 2012, that S-waves are not only detectable at the surface, but in our experience have larger amplitudes than P-waves. To unequivocally observe this phenomenon the full wavefield must be sampled with 3-component instruments for microseismic data acquisition. We conclude that: 1) field results support our theoretical and numerical predictions of the larger S-wave content in wavefields generated by common fracture mechanisms, 2) the frequency content of the constituent components scales roughly with velocity, and 3) the S-wave attenuation would have to be unrealistically different than the P-wave attenuation to make the S arrival undetectable when a P-arrival is recorded. Lastly, given a sufficiently accurate velocity model, the Swaves can be used to accurately locate microseismic events, including events that are undetectable using P-waves alone. |
