Abstract

We studied the characteristic of acoustic wave propagation in a fluid-filled borehole using as a laboratory model a concrete cylinder 2 ft high and 2 ft in diameter with a 1/4-inch diameter borehole along its axis. The model represents sonic logging in the field reduced by a factor of 40. We recorded the full wave train consisting of a refracted compressional P wave, a refracted shear S wave, and guided waves including a number of normal modes and a Stoneley wave. Exploiting the dispersive properties of a modal wave and the source-receiver frequency characteristics, we were able to isolate the S-wave, which contains much valuable information about the formation rock, but which has not been widely used since it is difficult to extract from the full wave train. The observed Stoneley wave had a very high amplitude at low frequency and showed little dispersion. Stoneley-wave velocity is closely related to S-wave velocity and formation density, and can be measured very accurately because the Stoneley wave generally has high amplitude and low attenuation. It can therefore be used indirectly to obtain the S-wave velocity, even when the S-wave cannot be measured directly. In general, the observed characteristics of each component wave agreed with our theoretical calculations but their relative amplitudes did not. We believe these discrepancies were caused, in part, by the fact that rock attenuation and the latitudinal angular dependence of the source radiation were not taken into account in the theoretical calculations.

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