Laboratory measurement of elastic waves in Basalt rock

This paper reports the laboratory measurement of compression and shear waves in Basalt rock. Compression or primary waves were produced using a standard ultrasonic pulse velocity tester. However, it is well accepted that the production of pure shear waves in rock is difficult because of which suitable assumptions are made in the testing. Since bender elements have traditionally been used to produce pure shear at very small strains in soft materials, their use has been extended in this study to Basalt rock. The fabrication and setup of bender element tests is first discussed. The transducers were accommodated in pre-drilled slots at the two flat ends of the samples, the effect of which was carefully investigated using experimental and numerical studies. For 10-12 mm long piezoceramic plates, the shear wave velocity was found to reduce by about 5% because of the slots. The shear wave velocity was estimated using the first arrival method after removing the near field and the crosstalk effects from the output signal. The shear wave velocity ranged between 1.15 and 3.31 km/s and was nearly equal to one-half of the primary wave velocity. The results also show that the ratio of the shear wave velocity and the compression wave velocity was independent of the density and porosity of the Basalt rock. Because most of the rock was unweathered and compact and, its cavities filled with secondary minerals such as zeolites, calcites and silicates, it is unlikely that the high overburden pressures would affect the observed wave velocities.