In the coal industry, the present energy situation and economic climate have increased the need for rapidly acquired, cost effective, reliable assessment of the strengths of rock which may be encountered in shafts, drifts and access tunnels, this need having hitherto been met by conventional sampling and testing procedures. This paper describes a new technique based on the processing of down-hole geophysical logs to provide a continuous, accurate record of these properties speedily and at relatively low cost.

The basis of the technique is to include a Neutron-Neutron log in the standard logging package and process it with the caliper log either to generate a Hydrogen Index log or simply a caliper compensated Neutron-Neutron log. The derived log is then split into selected lithology-dependent sub-groups and modified according to a predetermined set of calibration constants. These calibration constants have been determined by the authors for the Coal Measures of Staffordshire but can be readily re-determined, if necessary, to suit other coalfields or prospects as required.

The application of the technique has the potential to improve all forms of engineering site investigation where both stability and ‘extractability’ of rock are under consideration, often removing the need to acquire expensive rock core samples, and saving considerable labour costs. In the U.K. coal mining industry fracture logs synthesized using this technique are at present being supplied, so that their value, in the planning and location of shafts and tunnels, in the design of shaft insets and tunnel supports, in the selection of tunnelling machines and forecasting drivage rates and in predicting roof, floor and overburden characteristics may be assessed. It is believed that they will have applications in many other mining or site investigation environments.

The technique described in this paper is the subject of a patent filed by N.C.B. Patents Ltd.

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