Laboratory calibration of amplitude variation with angle using an acoustic goniometer

Abstract

Summary A variety of approximations to Zoeppritz’s equations, describing the variation in reflectivity with angle of incidence from the interface between two elastic media, are employed in AVO impedance inversions. However, Zoeppritz’s equations describe the variation in reflectivity with angle of incidence from the interface between two nonporous, perfectly elastic media; and while this is probably adequate it is important to further study reflectivity from fluid-filled porous materials. Here, initial results from new method for experimentally measuring Amplitude Variations with Angle (AVA) in complex materials is described. Samples are placed precisely on an acoustic goniometer placed in a large water filled tank. Experimental difficulties are reduced by employing a specially designed transducer that produces plane wave flat over a 5 cm by 3 cm area and stable to at least 37 cm of propagation distance. Reflected signals are detected with small hydrophone. Amplitudes observed in reflectivity calibration tests on glass and copper samples agree very well with theoretical models for both with maxima seen at both the P-wave and S-wave critical angles indicating that the system will work well for more difficult studies over porous media. However, a more substantial drop in amplitude is seen at the Rayleigh angle, i.e. the angle of incidence at which the incoming ray is critically refracted with respect to the pseudo-Rayleigh interfacial wave between the water and the material. This effect is not predicted by the Zoeppritz based formulations but is well known in the acoustics community. The Rayleigh angle may be used to obtain additional information about the medium. Further, this Rayleigh angle has the potential to be produced at the boundary between two solid media such that AVO observations and inversions may be biased.