Biologically inspired ultrasonic measurements of porous media, with applications in geophysics.

Abstract

Biologically inspired ultrasound has been investigated for measuring properties of materials in an underwater environment. Broadband transducers have been deployed which operate in the 40–200-kHz frequency range, with similar frequencies to those used by some echolocating mammals. Signals have been designed, which optimize the available bandwidth of the transducer, and analysis procedures have been developed to extract the desired information from acquired data. Measurements on single-layer targets, comprising plastic, metal, and glass, in solid and porous forms have established the performance of the system on well-controlled synthetic samples. The target thicknesses ranged from 0.5 to 40 mm, thus spanning the wavelength range of the signals used. Adequate penetration of the ultrasound into the samples, at the frequencies used, was demonstrated. Material and thickness discrimination was possible using frequency domain results, and modeling of the traces was performed in order to extract velocity and attenuation information. Scanning electron microscopy measurements on the porous targets revealed structural information that informs the interpretation of the ultrasonic results. The work on synthetic targets forms a basis to inform experimentation on natural geological materials exhibiting a wide range of structural characteristics. Initial results for sandstone samples are presented.