Open Cut » Geology
Compressional-wave (P-wave) seismic reflection is a valuable tool used throughout the coal industry, for exploration (mainly 2D reflection) and mine planning (3D). Shear-wave (S-wave) reflection presents interesting theoretical potential, but is more difficult to implement. Previous ACARP projects (C10020, C13029) have successfully demonstrated the 2D implementation of converted-wave (PS-wave) reflection at the coal scale. Those projects demonstrate that a more complete geological interpretation can be obtained by integrated interpretation of the P-wave and PS-wave information.
A full 3D implementation of PS reflection is highly desirable, but presents additional challenges, including the anticipated problem of azimuthal variation in the transmission properties of PS-waves. This ACARP project (C17029) examines the feasibility and potential of integrated 3D P+PS reflection at the coal scale. The project includes the first known 3D field trial of this technology in the Australian coal industry.
The PS reflection path is asymmetrical, and the horizontal reflection point varies with target depth and rock properties. Hence, ray-path modelling has been used to design the optimal layout of sources and receivers for the C17029 field trial. In addition, far-offset PS reflections from coal seams suffer greater phase distortion than standard P reflections. These phase variations can be effectively modelled using reflectivity and finite-difference modelling. Based on the expected target seam depth (70-120m) from prior 2D work, phase distortion needs to be monitored on the longest offset PS reflections. Following comprehensive survey modelling, a small, high-fold 3D grid was successfully recorded using an Envirovibe source and 3C receivers. A 20-person crew took approximately 3 days to record two full passes over the grid (using different Vibroseis sweep parameters). Total receiver line length was 9.75 km (15m intervals) and total source line length was 18.5 km (30m intervals). The 3D volume was designed to have extremely high fold (maximum > 500), to allow subdivision of the data for azimuth-limited analysis. The field trial proceeded very efficiently. As expected the raw horizontal-component data (from which the PS images are obtained) were relatively noisy compared to the standard P-wave recordings.
The processing of the 3D-PS volume has required considerable effort, and the implementation of various 3D extensions to our existing 2D-PS software. A critical aspect is the identification of spurious time errors (statics) caused by near-surface geological variations. These effects are most obvious where the reflected S-wave reaches the surface near the receiver. Comparison of the P and PS data demonstrates that failure to account for such near-surface anomalies can result in serious misinterpretation of true geology at depth. Four different statics algorithms have been trialled, and our preference is for a solution based on the analysis of PPS refraction data. This is an S-wave extension of the popular refraction-statics algorithm used in standard processing.
We have demonstrated that a viable PS-wave volume can be obtained, and have extracted a set of target attributes from this volume. We have shown that an enhanced structural interpretation is obtained by integrating the P and PS volumes. The high fold of the volume has permitted a detailed investigation of apparent azimuthal variations in wave behaviour. Although we are still wary of static contamination, we believe that the PS volume exhibits significant evidence for azimuthal anisotropy. There is some evidence that the variation of the S-wave velocity with azimuth may relate to geology. These effects seem more pronounced in the vicinity of the faulted zone.
This project has defined a number of core technical problems which demand more robust solutions, and we have identified specific avenues for ongoing research to address these problems. Nevertheless, the project has clearly indicated the significant potential of integrated 3D P+PS seismic reflection as an emerging tool for the coal industry.