Increasingly mining companies want greater definition of subtle features, driving research into new seismic technologies that can enhance geological characterisation of the sub-surface. One such novel technique is converted-wave (or PS-wave) seismic reflection, the subject of a recently completed ACARP* research project: “Investigation of converted wave seismic reflection for improved resolution of coal structures.”

The research was conducted by Brisbane-based company, Velseis, a major provider of 2D and 3D seismic surveys to the coal industry. The company undertook the study to examine if it was possible to adapt converted wave techniques being used in the petroleum industry for use in coal.

To date, conventional seismic techniques (as used in coal) were concerned only with compressional (P) wave data, generated by a conventional seismic source such as dynamite. However, geophysicists have known for years that a significant fraction of the energy can be reflected back toward the surface as a shear (S) wave. Conventional seismic reflection ignores P-to-S mode conversion, and records only the vertical component of ground motion.

This means that additional information contained in the complementary S-wave data is not currently being exploited. Recent advances in acquisition hardware and processing technology now make converted-wave exploration viable.

To date, however, there has been no significant practical assessment of this new technology in the coal environment.

According to Velseis, over the past five years, converted-wave technology has been embraced by the petroleum seismic industry, with several striking successes including improved structural imaging, lithological classification, and fracture characterisation.

Velseis researcher, Dr Natasha Hendrick - research & development geophysicist, said converted-wave technology could be carried out in the same way as conventional seismic acquisition. The only difference is that the conventional vertical geophone is replaced with a multi-component geophone, capable of recording all incoming energy in a true vector sense. This provides the capability to discriminate between arriving P and PS waves.

A greater challenge is the processing of the data. The volume of data collected is threefold and PS-wave processing is more complicated.

Overall, the cost of a combined P-wave / PS-wave dynamite survey is expected to be about 30-50% more than the cost of P-wave alone. Early indications are that this cost increment will be justifiable in terms of recovering enhanced sub-surface information, Dr. Hendrick said.

The overall objective of the just-completed research was to demonstrate that converted-wave seismic technology could be used in the coal environment, and this has been achieved.

The ACARP monitor, Andrew Willson, described the research as an excellent piece of work: “As far as we could tell, this was the first time this technique has been used in coal and the results are very encouraging,” he said.

“Two datasets were acquired. The first - over a shallow, thick coal seam in the Rangal Measures - delivered a significant improvement in resolution.”

In addition, the results show that the PS image validates structure seen on the standard P wave image, providing greater certainty in terms of what lies beneath the surface.

“Perhaps the most exciting result from Trial #1 is that the PS image shows an apparent improvement in vertical resolution when compared to the P-wave image. This significant result supports previous theoretical predictions that PS resolution advantages would be more likely in the shallow environment than at the petroleum scale,” Dr Hendrick said.

From the second dataset, taken over contrasting multi-seam geology, has emerged an interesting 'lithological' opportunity, she added.

Comparing the P-wave travel time to the S- wave travel time has yielded a subsurface map of Poisson's Ratio. This is believed to relate to lateral variations in lithology (e.g. sand vs. shale), and could prove significant for mine planning.

Already Velseis has conducted its first commercial seismic survey using this technique at a near surface (

In pointing ahead to ongoing research the final report outlined areas including: fine-tuning processing algorithms to improve PS image resolution, and demonstrating the practical implications of integrated P and PS interpretation, particularly in lithological-style interpretation.

Such research will stimulate additional methodological advances, and ultimately lead to the use of converted-wave seismology as a standard coal-imaging tool.

* ACARP is the Australian Coal Association Research Program, funded and managed by the Australian coal mining industry.