The article presents the results of experiments conducted to understand the effect of large overburden stress on electrical resistivity of three Indian sandstones. The intact rock specimens have been tested in high pressure triaxial setup under different confining pressures simulating rock overburden stresses. The maximum confining pressure of 12 MPa applied on rock specimens corresponds to a overburden of around 525 metres of rock. The studies included the combined effect of confining pressure and degree of saturation on electrical resistivity of these sandstones. An attempt has been made to correlate electrical resistivity with the strength properties of these rocks.
Engineers working in various fields such as mining, underground engineering, oil and gas etc. have well realised the importance of 'geophysical techniques' for subsurface explorations. These techniques possess very high potential to explore deep depths with fair degree of accuracy. Electrical resistivity is one such technique which is very much popular amongst engineers due to its suitability for variety of subsurface conditions and depths. The field electrical resistivity soundings when interpreted furnish complete information regarding strata in addition to depth of ground water table. The interpretation of field electrical resistivity soundings data to derive subsurface profiles involves converting observed apparent resistivity of strata at different depths into their true resistivity values which are then matched with those given in the standard tables leading to identification of various subsurface strata. The major drawback in this procedure is that, the resistivities of different materials available in standard tables are the resistivity values obtained in the laboratory under zero pressure conditions. However, the interpreted values of true resistivities of strata at different depths in the field corresponds to the values under overburden pressures depending upon their depths. This introduces an error in interpretation, the magnitude of which commensurates with the overburden pressures. Thus, it is necessary to apply the relevant corrections to improve the degree of accuracy of interpretation. Subsequently, this suggests an immediate need to understand the behaviour of rocks with reference to their electrical resistivity under high pressures simulating the overburdens. In view of this, in the present laboratory investigation an attempt has been made to study the effects of overburden pressures on electrical resistivity of intact rock specimens. To enable simultaneous applications of high pressures and measurement of electrical resistivity of rock specimens, special attachments to laboratory high pressure triaxial cell were designed and fabricated.
The electrical resistivity of rocks depends upon the applied pressure, among other factors. During the application of uniaxial pressure, Parkhomenko and Bondarenko (1960) observed a decrease in resistivity of crystalline rock under low pressure followed by resistivity increase at high pressures. Whereas, the effect of confining pressure on electrical resistivity of rocks reported by Brace et al (1965) indicated small increase in resistivity at low stress followed by large decrease in resistivity at high pressure. These contradicting observations clearly reveals that the influence of pressure on rock resistivity is a complex phenomenon and cannot be generalized.
