Richard T. Coates

Finite-difference modeling of faults and fractures

For the purposes of seismic propagation, a slip fault may be regarded as a surface across which the displacement caused by a seismic wave is discontinuous while the stress traction remains continuous. The simplest assumption is that this slip and the stress traction are linearly related. Such a linear slip interface condition is easily modeled when the fault is parallel to the finite-difference grid, but is more difficult to do for arbitrary nonplanar fault surfaces. To handle such situations we introduce equivalent medium theory to model material behavior in the cells of the finite-difference grid intersected by the fault. Virtually identical results were obtained from modeling the fault by (1) an explicit slip interface condition (fault parallel to the grid) and (2) using the equivalent medium theory when the finite-difference grid was rotated relative to the fault and receiver array. No additional computation time is needed except for the preprocessing required to find the relevant cells and their associated moduli. The formulation is sufficiently general to include faults in and between arbitrary anisotropic materials with slip properties that vary as a function of position.

Acoustic imaging of reservoir structure from a horizontal well

Recent advances in well‐placement technology now allow a borehole to be steered with considerable precision inside the reservoir. With modern completion technology, the subsequent flow of hydrocarbons within the reservoir can be controlled to a greater extent than ever before. These developments have further emphasized the need for an improved ability to image structures such as caprock, stringers, faults, fractures, and pore‐fluid contacts, on the reservoir scale.

Wave equation receiver deghosting

Current solutions to receiver deghosting generally involve making complementary measurements of the wavefield or, alternatively, involve estimation of data not recorded due to ghost interference. Both solutions offer challenges in practice in that marine multimeasurement streamers are commercially available only on a limited basis and existing single-measurement deghosting methods must estimate unrecorded frequencies near the ghost notches. Here, we develop a new wave equation-based approach for single measurement deghosting that does not rely on such estimation procedures.

Extending the seismic bandwidth — A simultaneous uncorrelated VSP and surface seismic field test study

A controlled simultaneous VSP and surface seismic field test was conducted to evaluate the evolution of the vibroseis chirp wavelet. We verify that the partitioning of seismic energy is independent of sweep rate for different sweep designs. We demonstrate under what conditions the seismic bandwidth can be extended using the semblanceweighted deterministic deconvolution process in the uncorrelated domain for zero-offset VSP, walk-away VSP, and simultaneous borehole and surface seismic data. The deterministic deconvolution process requires knowledge of the chirp wavelet in order to transfer the harmonic energy from the seismic noise domain into the seismic signal domain. Its performance depends on knowledge of the chirp signature and ambient noise. As expected, the ambient noise conditions in the well were extremely low as compared to the higher noise levels recorded in the surface seismic data. Under low ambient noise conditions, the semblance-weighted deconvolution can extend the recoverable bandwidth beyond the predefined pilot bandwidth for both the VSP and surface seismic data independently. The high level of surface ambient noise conditions in our field test limited the effectiveness of the VSP operators to extend the recoverable bandwidth of the surface seismic data.

Interferometric Deconvolution of VSP Data

In this paper, we present recent research involving interferometric deconvolution of downhole recordings which used drill noise as a seismic source to create reverse vertical seismic profiles. We refer to these profiles as drill noise vertical seismic profile data. The data was collected at the site of the San Andreas Fault Observatory at Depth which is located about 2 kilometers from the surface trace of the San Andreas Fault near Parkfield, California.