Wave Propagation in a Borehole

Abstract

In full waveform acoustic logging, a pressure source generates various types of elastic waves propagating down the borehole. These waves, because their wavelengths are of the order of the borehole radius, are guided waves. Depending on the velocity of propagation, these guided waves are either normal modes, like the Stoneley wave, where all the energy is contained around the borehole, or leaky modes, like the P leaky mode, where some of the energy is converted into body waves and radiates away from the borehole into the formation. Both the normal and leaky modes are significantly affected by the P and especially the S wave velocity of the formation. Other physical parameters affecting their propagation are attenuation, permeability and anisotropy. With proper processing techniques in combination with inversion algorithms, we can obtain estimates of the S wave velocity even in soft marine sediments where no refracted S wave arrival exists on the full waveform logs.