Abstract: Acoustic Microscopy of Geological Materials

N. E. Pingitore, C. L. Gillespie, K. C. Miller, K. M. Love

The acoustic microscope probes a sample with high-frequency sound waves, rather than light, to produce an image. The microscope exploits a confocal transducer source/receiver shaped into a hemispherical lens and coupled to the solid specimen through a fluid, normally water. Pulsed acoustic waves in the MHz to GHz range are focused on the surface or the interior of the solid, and the travel time and strength of the reflected wave are recorded back at the lens. Focus is achieved by vertical motion of the lens; source pulsing provides a time window for reception of the reflected signal. By rastering the lens over the specimen and storing and processing the accumulated data, a pixel-by-pixel image is created (2 to 10 minutes for a typical "exposure").

Acoustic microscopy can provide detailed images of surface topography and tomographic images of such subsurface reflecting features as voids, layers, and inclusions. Different grain types in a specimen can be mapped on the basis of their acoustic impedances. Rayleigh waves generated by the surface of the specimen can interfere with reflected waves to highlight cracks and grain orientations.

Current acoustic microscopy projects with potential application to petroleum development include rapid imaging and quantification of porosity in core slab surfaces, "point counting" grains in thin section or slabs by impedance recognition and automatic area calculation, imaging of porosity in 3-D by tomography, and measurement of impedance of individual component grains for comparison with bulk rock impedance.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994