AN INSTRUMENTATION BASED ON THE PIEZOPOLYMER P(VDF-TRFE) FOR THE ANALYSIS OF THE ELASTICAL PARAMETERS OF ROCKS UNDER STRESS

At great depths, the rocks of the sea bottom are exposed to very high pressures that can modify their elastic parameters. The classic labora tory techniques on bore hole rock samples are carried out at ambient p ressure, so we have developed an innovative instrumentation to study t he rock samples under stress. This was made possible by using a film o f a piezopolymer, P(VDF-TrFE), which can easily generate and detect ac oustic waves under high pressures. Two films of P(VDF-TrFE) are stuck on both sides of the rock sample with grease, and placed under a stres s machine. One of the P(VDF-TrFE) plates, stimulated with a high volta ge pulse, generates an acoustic wave through the rock sample, where th e second plate is used as the detector. The mechanical waves generated are of longitudinal and transversal types. Their velocities are conne cted to the elastic constants of the rocks. With our instrument we gen erated and detected acoustic waves in different materials such as marb le, granite, and sandstone. We investigated the influence of pressure on the output level to point out that the relative sensitivity was not affected by pressure, up to 80 MPa. We also measured the wave velocit ies in some rocks at various pressures, to confirm that this velocity increases with pressure. (C) 1998 Elsevier Science B.V.