Open AccessMeasuring anisotropy in rocks using laser‐generated ultrasound
Author(s) -
Young R. Paul,
Hutchins David A.
Publication year - 1987
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1987.tb05242.x
Subject(s) - anisotropy , optics , piezoelectricity , laser , birefringence , transducer , materials science , shear waves , ultrasonic sensor , isotropy , acoustics , shear (geology) , physics , composite material
Summary. Measurements using standard contacting piezoelectric transducers and non‐contacting laser sources and detectors, have been investigated for the study of ultrasonic anisotropy in rocks. An ultrasonic polariscope has been constructed in order to obtain reproducible travel‐time and amplitude measurements. Three case studies are described to demonstrate the apparatus, namely isotropic halite, anisotropic calcite and transversely anisotropic mudstone. A novel technique has been developed in order to construct pseudo‐particle motion diagrams, to highlight shear‐wave birefringence in rock samples using 2.25 MHz transducers. A pulsed laser has been used to generate compressional and shear waves for comparison with piezoelectric transducer results. The pulses generated by laser irradiation have many advantages for the study of velocity and attenuation anisotropy because of their known characteristics, broad bandwidth and high level of reproducibility. The use of a non‐contacting laser source and detector eliminates the need for elaborate coupling agents, stress bonding or immersion techniques. Point‐source and line‐focusing of the laser beam provides an indirect method of studying shear‐wave polarization phenomena. Results from rotation of the line‐focused laser beam and rotation of piezoelectric shear‐wave transducers with respect to anisotropy, are compared for both velocity and amplitude phenomena in an anisotropic rock sample.