Coal seismic surveying over near-surface basalts: Experience from Central Queensland, Australia

Abstract

Seismic reflection surveying in basalt-covered areas often fails to image underlying reflectors. To gain insights into the nature of the problem and obtain potential solutions, we have conducted experimental 2D seismic reflection and offset VSP surveys at two coal mines in the Bowen Basin of Australia. At the first mine, the basalt is relatively deep (114 m) and relatively thin (20 m). Conventional seismic acquisition and processing of a 2D seismic line provide poor results. However, upgoing reflections from layers below the basalt are clearly evident in the VSP survey and prestack depth migration is able to improve the continuity of the reflectors beneath the basalt. At the second mine, the 360 m wide basalt is at a depth of 40 m and has a thickness of about 40 m. It is fresh and unweathered and consists of multiple flows which are interlayered with unconsolidated sediments.Long-offset data acquisition combined with prestack depth migration was expected to produce satisfactory results but this is not the case. The associated VSP survey suggests that the problems at this mine are due to (1) the generation of complex downgoing and upgoing wave-fields within the basalt and (2) significant scattering of surface waves from outside the basalt at the margins of the basalt. Another problem is that the target coal seams are at about 300 m depth and the muting required to remove refraction events limited the effectiveness of the prestack depth migration. Reducing the strength of the surface waves through selection of an appropriate source and placement of shots at the base of the low-velocity zone (as had been the case at the first mine) will therefore improve the chances for a successful outcome. A Vibroseis survey subsequently undertaken at the second mine, which produced shot records with reduced surface waves, shows this to be the case.

Seismic Survey in Lesser Himalayan Thrust Belt, Western Nepal

“…The PSTM section shows that the structure of the area is relatively complex, the stratigraphic reformation is very strong; folds and small faulting blocks develop. The prestack depth migration (PSDM) can improve the imaging of steep dip structures, but at the same time, migration smiles are more obvious [46]. The accuracy of seismic imaging needs to be improved.…”

3D DAS VSP for Coal Seam Exploration: A Case Study from Queensland, Australia

“…The study area is characterised by the presence of basalts at a relatively shallow depth (70–100 m). It is generally difficult to obtain a reliable image under the basalt cover from conventional 3D surface seismic, as these rocks have very high seismic velocities (relative to sedimentary rocks at similar depths) and significantly scatter seismic energy [ 22 ]. This issue can be partially addressed by deploying receivers in boreholes, as the reflected energy travels through the highly reflective basalts only once.…”

Shallow seismic reflection survey for imaging deep-seated coal layer - Case study from Muara Enim coal

“…From America, geophysicist uses 2D-3D high-resolution seismic reflection for identifying deep-seated coal layer and geotechnical studies in the coalfield area. In Australia, Driml, Reveleigh and Bartlett (2001) ;Hearn, (2004) and Zhou et al (2014) developed a proper acquisition, instrumentation and signal analysis for shallow seismic reflection. Nowadays, In China, (Zou et al, 2013) and (Cao, Chang and Yao, 2019) developed the advanced seismic signal analysis not only for delineating deep-seated coal layer but also predict the methane content from coal.…”