Ismael Vera Rodriguez

Resolution of Seismic-Moment Tensor Inversions from a Single Array of Receivers

Moment tensor inversion techniques are widely used in global and regional seismic applications. When the ray-path trajectories are confined in a single plane (e.g., in isotropic media using a vertical array of receivers or an array that deviates from the vertical in the direction of wave propagation) only five out of six elements of the moment tensor are resolvable. This study investigates the resolvability of the complete seismic moment tensor for single-well monitoring geometries. By analyzing the resolution matrix, we demonstrate that a correct representation of the five resolvable elements of the moment tensor is only possible in a local reference system. For a vertical array of receivers, a suitable choice of condition number can assist the acquisition design. For a non-vertical array, our numerical modeling experi- ments suggest that the required distance and orientation of receivers for a full moment tensor inversion can be satisfied in a deviated well. In this case, information embedded in the condition number is valuable for determining the required distribution of receivers along the well.

Continuous hypocenter and source mechanism inversion via a Green's function-based matching pursuit algorithm

The hydraulic fracturing of rock formations bearing oil and gas is a process that aims to improve well productivity. The breaking of rocks releases seismic energy that is recorded by stations usually located in nearby wells. By processing these recordings, the hypocenters of induced microseismic events are retrieved and interpreted to estimate the fractured volume. Several techniques have been developed to determine hypocenters; the most common are based in the inversion of P- and S-wave time picks from multiple stations (Pujol, 2004). There are also procedures that involve, for example, the back propagation in time of the recorded wavefields (Gajewski and Tessmer, 2005). Besides the hypocenter location of microseismic events, there is additional information embedded in the seismic recordings: the source mechanism, which is expressed via the seismic moment tensor (SMT). Not as common as the estimation of the hypocenter location, the SMT of induced microseismic events is also estimated from monitoring reco...

Microseismic Record De-noising Using a Sparse Time-frequency Transform

This work presents a noise attenuation technique based upon applying a sparsity constraint to a time-frequency transform. It is demonstrated that the solution obtained from applying the sparsity constraint rather than the more common minimum norm constraint produces a superior noise attenuated signal. The sparsity constrained transformation is achieved by finding a sparse representation of the input data in terms of a dictionary of complex Ricker wavelets. The utilization of a complex wavelet dictionary possesses the advantage that signals with arbitrary phase can be represented with enhanced sparsity. Examples with synthetic and real microseismicity data illustrate the capacity of the technique to attenuate ambient noise in microseismic records with low signal-to-noise ratio.