Dual-Mode Ultrasonic Apparatus for Measuring Compressional and Shear Wave Velocities of Rock Samples

An ultrasonic apparatus and accUiate experimental techniques have been developed for measuring consecutively the bulk velocity of plane compressional and shear waves in samples of porous solid material. The apparatus was developed primarily to study the elastic behavior of rock samples recovered from well bores in the earth as deep as 7800 meters. Appropriate conditions of triaxial st(ess in the range 0-25 000 lbf/in
²
(172 × 10
⁶
N/m
²
), fluid saturation, and temperature are imposed on the sample to simulate the surroundings of the parent rock in situ. In the apparatus, two electromechanical transducer cells operating in the frequency range 0.25-5 MHz are placed in direct contact with each end of the sample. Phase-coherent tone-burst pulses are used. The cells contain two sets of piezoelectric ceramic disks. One set generates and detects compressional waves, and the other set serves as the soUice and detector for shear waves, which form by mode conversion at an oil-aluminum interface. The mechanical ·design of the apparatus is based upon sound principles of geometrical acoustics. This method has substantial advantages over existing ones for testing rocks because it has greater accuracy and a shorter testing time. Transit times through nondispersive rocks can be measUied with a resolution of 10
^-8
second. The time formerly required for making separate measurements of compressional and shear wave velocities has been reduced by one half. Velocities and elastic moduli are reported for aluminum, Solenhofen limestone, and Boise sandstone samples under stress.