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Shallow, high-resolution converted-wave seismology for coal exploration

Despite the considerable successes of multi-component seismology in the petroleum industry, there has been relatively little effort devoted to shallow, high-resolution converted-wave imaging in the coal sector. By analogy to petroleum-scale applications, converted-wave imaging in the coal environment offers interesting possibilities for independent validation of mapped structures, clearer imaging in the shallow sub-surface, and detection of gas, sandstone channels and/or fracture swarms. Over the past two years Velseis Pty Ltd has conducted the first experiments in Australia to utilise shallow, high-resolution multi-component data and converted-wave technology to image coal seams. Three 2D multi-component coal-seismic datasets have been acquired in the Bowen Basin, Australia. Only minimal changes to conventional recording equipment and procedures have been required. A conventional dynamite source has been used, with a single, multi-component geophone replacing the conventional array of vertical geophones at each receiver. Converted-wave processing algorithms developed within the petroleum sector have been successfully applied to the three trial datasets. This has involved specialised approaches to S-wave receiver statics, PS normal moveout and common conversion-point binning. Considerable experimentation to fine tune processing parameters and the converted-wave processing sequence has been necessary for optimum handling of the shallow, high-resolution data. Overall the processing of the PS data has been significantly more challenging, and has required more geological input, than conventional P-wave processing. This research has demonstrated that converted-wave imagery is viable in the shallow environment. The derived PS images can extend the interpretation achieved with conventional P-wave images.