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Geophysical Strata Rating

Underground » Geology

Published: July 08Project Number: C15019

Get ReportAuthor: Peter Hatherly, Terry Medhurst, Stuart MacGregor | CRCMining

The Geophysical Strata Rating (GSR) is a means of empirically estimating the quality of rock masses through an analysis of geophysical logging data. It is based mainly on sonic log data and it recognises that in clastic sedimentary rocks, sonic velocity varies with both the rock type and the defects present in the form of bedding planes and fractures. Different rock types have different porosities and clay contents. The GSR provides scores based on compositional information obtained from other geophysical logs and combines these with scores based on the sonic velocity. A defect score is then added according to the variability of the clay content and the initial GSR.

In this project, our earlier formulation for the GSR has been extended through the development of the defect score. We have analysed geophysical logs from numerous mine sites and compared the GSR values with other ratings. GSR values are provided as a continuous log for each depth point in the borehole. Blocking of units is not required. In rocks of reasonable quality, GSR values are similar to those that are obtained with the Coal Mine Roof Rating (CMRR). In poor quality rocks, particularly those with high clay contents, relatively high porosities and low sonic velocities, GSR values are lower than the CMRR.

The calculations are undertaken using Visual Basic macros written for Excel spreadsheets. Analysis does not require specialist geophysical knowledge and the time taken to analyse the geophysical logs from a typical coal exploration borehole is about one hour.

There are a number of reasons to incorporate GSR analysis into conventional coal mine geotechnical studies. Chief amongst these is the objective nature of geophysical logs and their analysis. Most exploration boreholes are geophysically logged and as a consequence, there is an enormous data base of logs available from which objective geotechnical information can be obtained - for the entire length of the boreholes. The GSR analysis also provides useful geological information that can be used to understand the nature of the rock units present and the reasons for their geotechnical properties. Borehole correlations are also assisted by a GSR analysis.

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