Qantitative Geophysical Log Interpretation for Rock Mass Characterisation

Underground » Geology

Published: April 04Project Number: C11037

Get ReportAuthor: Peter Hatherly, Renate Sliwa, Roland Turner, Terry Medhurst | CSIRO Exploration & Mining, Borehole Logging Consultancy Services, Australian Mining Consultants

Geophysical borehole logging is routinely employed as part of exploration drilling in open pit and underground mining operations. Analysis of results is often empirical or based on theoretical considerations that need not relate to the actual properties of the rocks under consideration. The objectives of this project are to develop techniques for quantitative geophysical log interpretation techniques to enable:

  • better estimation of coal and rock properties such as strength and permeability
  • better lithological interpretation and strata correlation between boreholes
  • a rock mass rating scheme for mine design purposes which is based on geophysical logging.

This study has placed the techniques for quantitative geophysical log assessment on a firm footing. We have demonstrated an approach for log assessment that can be routinely applied. Many of the mineralogical and physical rock properties that impact on the assessments have been investigated and discussed. We have also demonstrated the benefits of quantitative geophysical log assessment. The major recommendation that we therefore make is that geologists and engineers in the coal mining industry take the time to study this report and begin to put the approach that we have described into practice. The collective understanding that this experience will provide can only help fuel the drive to take the benefits of geophysical logging to greater levels.

Geophysical estimates of density, porosity and shaliness are sufficiently accurate to allow these to be combined with sonic data to provide robust estimates of rock strength. For empirical applications, schemes tailored to specific sites are best. For more general and quantitative uses we have proposed a rock mass rating system based on the geophysically derived parameters and their relationship to the relevant rock mass properties. Just as other geotechnical rating schemes (RMR, CMRR, Q-value) have been progressively refined through on-going experience, our proposed geophysical scheme needs to be tested and refined through application at as many mine sites as possible. ACARP may be in a position to coordinate such an effort. Appropriate compensation for the effects of joints and bedding planes remains the most problematic aspect of the scheme and further work in this area is required.

In general terms, the geophysical logs that we have assessed in this project have been of sufficiently high quality to enable quantitative analysis. The density, sonic and natural gamma logs represent a minimum suite for any analysis but inclusion of neutron and resistivity logs greatly assists with the identification and resolution of the anomalous situations that can often arise. The spectrometric nuclear logs offered to the Australian coal mining industry as Sirolog can also play a significant role in resolving interpretational intricacies but the current system needs calibration. If full waveform sonic logs are being used to derive conventional sonic transit times, care needs to be taken to ensure that cycle skipping does not become an issue in the automated threshold crossing analysis used to provide the transit times. For all analysis, the software environment for manipulating geophysical logs could be improved.

For the assessment of the effects of joints and bedding planes, methods for using microresistivity and full waveform sonic logs need further development. Acoustic scanner logs can also provide this information but these are currently expensive to run, require good hole conditions and involve labour intensive analysis.

The mineralogical analyses that were undertaken in this project were on carefully selected samples. They have shown that the geophysical log responses reflect the major changes in the overall mineralogy. As a result, the logs can be used to better understand both local and regional geological environments. A significant change in the mineralogy in the Moranbah/German Creek Coal Measures has been proposed. This needs to be confirmed through further log, mineralogical and sedimentalogical analysis. More than likely, other changes can be found in other parts of the Sydney and Bowen Basins. Further work of the type undertaken here needs to be conducted to identify these. If more mineralogical and petrological data become available, these should also be used to refine the geophysical log analysis techniques and, in turn, any geotechnical schemes to which those geophysical data are being applied.


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