ESTIMATION OF MECHANICAL ROCK PROPERTIES USING WIRELINE GEOPHYSICAL MEASUREMENTS

Laboratory methods to establish mechanical properties of rock are inhe rently biased towards stronger strata, which best survive the sample p reparation procedures. Geophysical techniques are not subject to such bias and may be capable of predicting mechanical properties both speed ily and cheaply. Such techniques would also be viable in circumstances where samples cannot be obtained. The findings are reported of a majo r research contract undertaken by the Universities of Newcastle upon T yne and of Leeds on behalf of the European Coal and Steel Community an d the British Coal Corporation. Some 151 samples were obtained from a borehole that had been comprehensively geophysically logged. The mecha nical properties of the samples were established in the laboratory and correlated with the geophysical responses of the strata from which th ey came. The results indicate that, for homogeneous strata at least, i t is possible to predict the unconfined compressive strength reasonabl y well from sonic travel time measurements made in the field. Predicti ons of static Young's modulus may also be made from the same data sour ce, although with a lesser degree of confidence. A multivariate regres sion equation is presented that relates down-hole sonic travel time to unconfined compressive strength, depth and the frequency of fractures in the rock mass. This equation, which has high statistical significa nce, may offer potential for the prediction of mechanical rock propert ies of fractured and layered strata.