High resolution 3D seismic reflection acquisition applied to coal mining

Abstract

In recent years significant improvements in survey design, acquisition and processing technologies have advanced high resolution 3D seismic reflection imaging. However, shallow coal applications require high-resolution imagery that is more costly than conventional petroleum-scale reflection. The 3D seismic grid design fundamentally controls the effective spatial sampling and signal-to-noise ratio of the final seismic volume. Whilst orthogonal, brick and slant-line shot-receiver geometries have all been employed, the latter has been found to produce fewer imaging artefacts, at a lower shot density and line-kilometre preparation cost. For targets at depths typical of underground coal-mining operations (70 to 700 m), a CMP bin size of approximately 50 square meters provides an optimal trade-off between spatial resolution and acquisition cost. If the bin size is increased beyond this point, the definition of subtle structural features in the target coal seam is compromised. In typical coal imaging situations, dynamite charge sizes are in the range of 150g-400g which are small enough to provide necessary bandwidth, whilst still generating acceptable signal levels. Practical experience has determined that optimum source coupling is achieved by locating the charge at least 2m below base of weathering, and at least 12m below surface. To minimise the reduction in high-frequency fidelity, geophones with a natural frequency of at least 30 Hz are best.