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Description
The Lamont Shear Sonic Tool (SST) uses a transducer design generating flexural waves that significantly improve the signal-to-noise and acoustic isolation for waveform acquisition in slow-velocity environments. It has one compressional and one dipole shear transmitter and a semi-rigid receiver string with three shear receivers (separation is 61 cm) and four compressional receivers (separation is 61 cm), allowing both compressional and shear data to be recorded during a single tool deployment. In addition, a record of the azimuth of the dipole tool in the borehole is recorded with each waveform, documenting the differences when anisotropy is encountered.
Applications
The SST can measure compressional and Stonely wave velocities, and it can directly measure shear wave velocities in "soft" formations in which the shear wave velocity is equal or less than the velocity of the drilling fluid, such as in water-saturated clays. Computer software for the tool allows for conventional first-motion detection methods for determining wave-propagation velocities. Recorded waveforms can be used for post-cruise analysis to extract additional information, such as sonic wave attenuation and signal spectrums.
The ratio of compressional to shear wave velocities provides information on porosity, lithology, and the dispersion of Stonely wave, which yields useful permeability and fracture data. Compressional and shear wave ratios also can be used to assess the amount of gas hydrate in a sedimentary sequence.
Deployment
The tool connects directly to the Schlumberger cablehead and uses Schlumberger downhole weight and centralized springs. It also interfaces directly with the Schlumberger MAXIS unit to acquire depth information. In addition to downhole tool control and data acquisition, this system enables interactive processing and visualization of waveforms for quality control.
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