Underground » Geology
This project involves a comprehensive investigation into the use of geophysical logging for rock strength estimation.
Full waveform sonic logging
Aspects of seismic wave propagation are reviewed to provide a basis for understanding full waveform sonic logs and the behaviour of the P-wave, S-wave and Stoneley wave motions exhibited in them. In clastic sediments, there are empirical relationships between P- and S-wave velocities and the porosity, the quartz content and the clay content. These allow variations in the ratio of the P- to S-wave velocities to be interpreted in terms of lithology, with the ratio getting larger for clay rich rock types. Stoneley waves travel within the borehole and are sensitive to changes in the shear modulus and permeability of the formation in the borehole wall. When the permeability is higher on account of either fracturing or the inherent permeability, the Stoneley wave is strongly attenuated. It is possible to analyse the Stoneley wave to obtain the shear modulus and the permeability.
Full waveform sonic logs provide a very good visual indication of rock conditions. Variations in travel times and the amplitudes for the P-, S- and Stoneley waves can be readily assessed and interpreted in geotechnical terms. Variations in rock strength, permeability and fracturing can all be inferred.
Estimation of rock strength from geophysical logging
The conventional approach to estimating UCS from sonic transit times is reviewed and assessed in light of previous work and the study of full waveform sonic logging. Sonic transit times relate to the elastic properties of a rock and provide a general correlation with rock strength, but there are reasons for there not to be a direct relationship between transit time and strength. An improved method of estimating rock strength is developed via the estimation of the proportions of quartz and clay plus the porosity. Multivariate analysis of these derived constituents against rock strength data from a number of mines shows a better correlation than from transit time data alone. This technique is demonstrated on an independent logging data set and the results compare favourably with the available strength data and UCS estimates based on transit times.
Automatic geotechnical classifications from geophysical logs
CMTE's computer program LogTrans, for the automated interpretation of geophysical logs has been successfully used to automatically interpret geophysical logging data to show the main geotechnical units affecting longwall caving. It is possible to automatically distinguish strong sandstones from coal seams and weaker laminated units. The automated interpretation can be undertaken using conventional logging suites (sonic, gamma and density logs) or wave parameters from a full waveform sonic log.
Full waveform sonic logging should be routinely practiced as part of geophysical logging. The logs can form a basis for quantitative analysis and useful geotechnical insights can be obtained from a qualitative assessment of Stoneley wave behaviour and variations in the Vp/Vs ratio. Routine means of determining formation and fracture permeability from a Stoneley wave analysis still need to be developed.
The full waveform sonic tools used in geotechnical holes should preferably have source to receiver separations between about 0.6 m and 1.5 m with a minimum of two receivers so that accurate velocity information can be obtained. At these separations, the various seismic waveforms are sufficiently separated in time to allow their recognition and analysis.
Continued development of the constituent based UCS relationship is recommended for a larger sample of rock types. Relationships between the constituents and the cohesion and friction angle should also be investigated. Additional methods for quantitative assessment of rock constituents from geophysical logs should be developed for general quantitative log analysis.
An investigation into the use of measurement-while-drilling data (drill penetration rates, torques, weight on bit, etc.) still remains to be done. The insights into rock properties and the interpretation of logs of geophysical measurements or drilling parameters gained from this current project would be very useful in such an investigation.