ABSTRACT:

In this paper we measure the reflected and transmitted waveforms from a liquid-anisotropic solid interface for two different materials. The first one, considered low attenuating, is a quartz crystal with trigonal symmetry; the second one, considered highly attenuating is a Phenolic CE material expected to be weakly orthorhombic. Reflectivity of the quartz crystal shows that the Schoch shift, observed in isotropic materials, is duplicated. This duplication of the Schoch shift depends on the azimuth and, can be used to identify principal planes of symmetry. Effects of tilting are studied with the Phenolic material for tilts of 0°, 30° and 90°. Our results show that, although difficult, tilting variations can be estimated mean azimuthal analysis of the reflectivity with the angle of incidence. However, the most important feature is variation of the critical angle, which increases with tilting. From the transmissivity tests, we also observed that the velocities of the two quasi-S waves are more affected by the tilt than the velocities of the quasi-P.

1. INTRODUCTION

It is common to find anisotropy on the properties of a real Earth material, such as fractures, thin layering, or lattice preferred orientation of minerals. Behavior of the wave propagation in anisotropic materials is a constant issue for seismic exploration. This is because in the case of the general anisotropic solid the scattered waves include a quasi P-wave (qP), with approximately longitudinal particle polarization, and two quasi Swaves, normally referred to as qS1 and qS2, with approximately transverse particle motion. That makes the seismic response to be more complex, and also makes the interpretation of the different events more difficult. The importance of having physical models for modeling the reflectivity and transmissivity of anisotropic materials has been pointed out by several authors [1-5]. In consequence an important number of laboratory experiments have been carried out using different materials, as plexiglass for representing vertical and horizontal transverse isotropy (VTI and HTI) [2,3], and Phenolic CE for representing orthorhombic symmetry [4-6]. The effect of tilting on the travel times has also been studied [5]. The results, in general, show that the use of ultrasonic transducers can successfully help to better understand the complexity of wave propagation in anisotropic media. In this work we present a set of measurements for the reflectivity and the transmissivity of a liquid-solid interface. The experimental setup we used has shown to have the required sensitivity for comparing results to theoretical estimations, as the Zoeppritz's equations for isotropic materials [7, 8]. An effect that is not represented by those equations, but that can be reproduced with high quality ultrasonic measurements is the Schoch shift [9]. This is an effect that happens near and at the Rayleigh angle, just past the S critical angle. In that range the energy of the incident beam is partitioned into two components which are out of phase producing interference, which observed as a drop in the recorded amplitudes. Our setup also allows us to record waveforms for different azimuthal angles.

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