Building 3D models of rock strength using geophysical logs

In Australian coal mining, geophysical logs are run in most exploration boreholes. The conventional suite of logs includes the natural gamma, density and sonic logs. A qualitative examination of these logs allows the basic geological section to be inferred. For many years, geotechnical engineers and geologists have also used sonic data to empirically estimate UCS.

Through a series of projects funded by the Australian Coal Industry Research Association (ACARP), we have developed a rock mass rating scheme for coal measure rocks which draws upon the compositional data contained within the geophysical logs as well as the sonic data. The scheme is known as the Geophysical Strata Rating (GSR) and our approach has some similarity to the method proposed by Barton for estimating Q-value from seismic velocity corrected for porosity and depth. As with other rating schemes, it also considers the properties of the intact rock mass as well as defects. These aspects of the GSR will be explained.

GSR data are obtained for the full length of each borehole. Data in this form can be used as input for the construction of 3D geotechnical models. For coal mining, we can extract information on key horizons and units so as to assist with decisions relating to bolting, caving and roadway stability. In this talk I will also provide examples of the use of the GSR in the construction of 3D geotechnical models.

Speaker biography

Peter Hatherly is a geophysicist with 35 years experience in research and consulting for coal mining. Over that time he has had involvement with many geophysical techniques but mostly with geophysical logging, seismic reflection methods and microseismic monitoring. The applications cover diverse areas such as mapping coal seam structures, geotechnical characterisation of rock masses, monitoring of the stability of rock masses and estimation of greenhouse gas emissions. In seeking to solve practical mining problems, he has worked closely with geologists, geotechnical engineers and other geophysicists. Peter currently offers geophysical services through his own company, Coalbed Geoscience. He is also employed part-time at the University of Sydney where he is contributing to a large mine automation project funded by Rio Tinto. Previous appointments include CRC Mining Professor of Mining Geophysics at the University of Sydney and Senior Principal Research Scientist at CSIRO. Peter holds a PhD from Macquarie University and has received a number of awards for his contributions to geophysics and coal mining.