Characterisation of Vesicular Basalts of Mumbai Using Piezoceramic Bender Elements

Abstract

This study reports the laboratory measurement of primary and shear wave in basalt rock. Compression or primary waves were produced using a standard ultrasonic pulse velocity tester. Tests on shear were measurement, however, were not simple. It is well accepted that the production of pure shear waves in rock is difficult because of which suitable assumptions were made in the testing. Since bender elements have traditionally been used in soft materials to produce pure shear at very small strains, in this study, their use was extended in basalt rock. Tests were conducted on rock samples by accommodating the transducers in slots, pre-drilled at the two flat ends of the samples. Shear wave was estimated using the first arrival method after removing the near field and the crosstalk effects from the output signal. This velocity ranged between 0.94 and 3.31 km/s and was nearly equal to one-half of primary wave velocity. The study of wave propagation through jointed rock was investigated in the second part of the study since it bears importance in the engineering field. In these tests, selected rock cores were sliced to produce joint at five different angles. Kaolin slurry was used as equivalent gouge material to lubricate the joints. Experiments were conducted using ultrasonic and bender element to study the influence of orientation of jointed rock on primary and shear wave velocity. The results show that propagation of shear wave was significantly influenced by the joint’s orientation vis-a-vis the propagation of primary wave. Shear wave velocity significantly reduced when the joints were horizontal, that is, normal to the direction of propagation. Surprisingly, the reduction in shear wave velocity was notably more in dry joint than lubricated joint. Clearly, simplified assumptions to produce a dry frictional joint in the rock, presents a formidable experimental challenge.