Acoustical data represent a valuable source of information in rock mechanics, in particular in cases where direct measurements of rock mechanical parameters are impossible. By using proper interpretation techniques, acoustic measurements may reveal information about stress state, static elastic properties and strength, as well as large scale inhomogeneities as fractures and joints.
Rock or soil mechanical parameters are required for a wide range of engineering applications, from establishing slope or foundation stability on the earth surface to predicting borehole instabilities at several kilometers depths in drilling or production of petroleum. Such parameters, like static elastic coefficients and rock strength, are often unavailable by direct measurements. Acoustic waves, being by nature mechanical disturbances, clearly may give information about rock mechanical features. Surface seismic recordings give wave velocities at depth, and when a borehole has been drilled, VSPs (Vertical Seismic Profiles) and sonic logs contribute more detailed analyses. However, the acoustic waves provide only indirect information, so there is a need for interpretation methods by which the rock mechanical parameters can be obtained. Current methods of mechanical properties logging do not take proper account of the difference between static and dynamic rock mechanical behaviour. Further, rock strength is often deduced from the elastic moduli. These assumptions clearly have severe limitations: In particular, in weak rocks, the difference between static and dynamic moduli may be several hundred %. This paper will discuss the basis of these assumptions, and review different theoretical and empirical models that have been put forward in order to solve the mechanical properties logging problem. The discussion will be accompanied by experimental results, where acoustic measurements have been performed during failure tests in various rock materials. An interesting feature of the acoustic behaviour, is the development of stress-induced acoustic anisotropy. This anisotropy is related to the generation of an oriented distribution of microcracks, which again relates to the failure mechanism itself. In the following, we will discuss how acoustics can be applied in estimates of earth stresses (Section 2), elastic moduli (Section 3), and rock strength (Section 4) in " intact" rock. In Section 5 we will briefly discuss how acoustic waves also may be applied to explore fractured/jointed rock masses, which is of practical importance for optimization of oil & gas production in low permeability reservoirs, for preventing leakage into tunnels or mines or from waste storage reservoirs.
Experimental experience has repeatedly proved that acoustic wave velocities are stress dependant. Besides being a manifestation of non-linearity in rock mechanical behaviour, this also offers a potential of indirect stress monitoring through acoustic measurements. The effect of an increasing hydrostatic stress is to increase p- and s-wave velocities. As a general trend, the effect is more pronounced at low than at high stress levels. This is exemplified in Fig. 1, showing the pressure dependence of acoustic wave velocities in a relatively weak sandstone. Pore pressure is generally thought to affect the sound velocities in accord with the effective stress principle.
