CRCMining at The University of Sydney recently completed its study into “Detailed geological characterisation from seismic data”. The project was led by researchers Peter Hatherly, Binzhong Zhou, Troy Peters and Milovan Urosevic.
The project concentrated on the potential of using acoustic impedance inversion to add to the geotechnical information already gathered from seismic surveys.
For Hatherly, the project fitted his ethos of “there are no excuses for unforeseen geological problems”
“How often do we hear about mining projects coming unstuck because of ‘unforeseen geological circumstances’? In an ideal world, the geological exploration program would determine the scale of meaningful geological variation and uncover all of the unforeseen problems. Mines could then proceed exactly according to plan,” Hatherly said.
“In my view, there should be no excuses for unforeseen geological problems. The geology can and should be properly worked out beforehand.”
With this in mind, Hatherly and his team started ACARP project C15018 with seismic reflection data from the Grasstree, North Goonyella, West Cliff and Dendrobium mines. Seismic reflection data is used to map the various geological horizons below the ground surface – including coal seam position and geological faults.
The data also shows progressive changes in the properties of the seismic waves and is analysed to reveal changes in the physical properties of the rocks. Seismic waves respond to the elastic properties (stiffness and shear) and the density of rocks.
These physical properties can be related to geotechnical properties. The geophysical process used to extract these parameters is known as seismic inversion. It was these associations that were investigated by the team.
“In underground coal mining we are interested in the properties of the roof and floor rocks and how they influence the support requirements and the caving behaviour. To perform a seismic inversion, it is necessary to have a good understanding of the geophysical properties of the subsurface from the geophysical logs obtained in exploration boreholes,” Hatherly said.
Seismic inversion recovers acoustic impedance – a multiplication of the rock density and the speed of the seismic wave (controlled by the elastic properties).
“In its own right, acoustic impedance is not a measure used by geotechnical engineers. However, through our research we established a good correlation with a geotechnical parameter we have called the Geophysical Strata Rating – GSR,” Hatherly continued.
“GSR determinations are based on the analysis of geophysical logging data and the values have similarities with the widely used Coal Mine Roof Rating. So in this project we converted acoustic impedance data into GSR values and used this to produce information on the geotechnical properties of the strata.
“From 2D seismic data, vertical sections of strata properties can be produced. From 3D seismic data, it is also possible to produce maps showing in plain view the lateral variations in strata properties.”
Hatherly said there were some limitations on the results provided by this method, including resolution issues and ambiguity of results.
With inversion results, properties are determined over 5-10m intervals, which means broad scale changes in roof properties can be detected, but not, for example, the phasing in and out of a 30cm weak band in the immediate coal seam roof.
“The ambiguity issue means there can be a number of different impedance results that all give a good solution to the inversion problem. We overcome this by exploring the range of options and choosing the result that best fits the available geological data and geophysical logging results.”
Hatherly has recommended coal companies undertake inversion as part of the seismic reflection surveys.
“After all, most of the cost in a seismic survey lies with the acquisition of the data. Processing [including inversion] is still a relatively minor component of the overall cost. Useful geotechnical insights on strata conditions between boreholes can be obtained.
“Seismic processing companies are able to offer commercial inversion services. It is a simple matter to convert impedances to GSR.”
Hatherly and his team are now working towards developing the GSR to allow for coal and other carbonaceous units. This project is also looking at demonstrating the GSR in practical mining situations.
Hatherly said good progress had been made to date.
