| Abstract: | Seismic wave attenuation in porous rocks consists of intrinsic or anelastic attenuation (the lost energy is converted into heat due to interaction between the waves and the rocks) and the extrinsic or geometric attenuation (the energy is lost due to beam spreading, transmission loss and scattering). The first is of great importance because it can give additional information on the petrophysical properties of rocks (permeability, degree of saturation, type of saturant, etc.). The most difficult problem in attenuation measurements is estimating or eliminating extrinsic attenuation, so that the intrinsic attenuation can be obtained. To date, in laboratory attenuation measurements using wave propagation, several methods have been used. The difficulties vary with the method. The coupling effect and the geometric divergence or beam spreading are the major problems. Papadakis’ diffraction corrections have been used extensively by Winkler and Plona in their modified pulse-echo high-pressure attenuation measurements. These corrections are computed for homogeneous liquid media and their failure to fit data for solid material implies that these corrections must be used with caution, especially for high Q values. Three new methods for laboratory ultrasonic attenuation measurements are presented. The first is the ‘ultrasonic lens’ method for attenuation measurements at atmospheric pressure, in which an ultrasonic lens placed between transmitter and sample transforms the initially oblique incident beam into normal incidence so that the geometric divergence is eliminated. The second method is the ‘panoramic receiver’, in which the beam spreading can be eliminated by integrating the ultrasonic energy over a large area. The third method is called 'self-spectral ratio’ and is applicable for all pressure conditions. Attenuation is estimated by comparing two signals recorded on the same rock but with two slightly different thicknesses under the same pressure conditions. Hence the extrinsic attenuation for both thicknesses is approximately the same. A comparison between the self-spectral ratio method and that of Winkler and Plona demonstrates a very good agreement for a broad band of frequencies. Hence the Winkler-Plona technique and Papadakis’ diffraction corrections can be accepted as reliable in any future work. |
