f Variable frequency monopole-dipole sonic logging for mechanical and hydrogeologic properties

Most traditional sonic logging tools operate at a single frequency. For slow rocks or large boreholes, fixed frequency tools may not excite the desired modes. Conversely, if the desired modes are excited, their interpretation may be complicated by unwanted high order modes. A variable frequency sonic tool can minimize these problems. Low frequency logging surveys can provide waveforms from which it is easy to pick shear wave arrivals in real time. Compressional head wave amplitude can be reduced by selecting a proper low survey frequency so that the first high amplitude head wave is the shear wave. Low frequency dipole logging can be used to find shear velocities for soft or unconsolidated sediments. Stoneley wave amplitude has been shown to be related to permeablity. Stoneley waves are readily interpreted from low frequency surveys when the amplitude of other modes is reduced. Permeability index can be directly generated while logging from this type of survey. A wide band tool was tested in unconsolidated sediments, shale, sandstone, and granitic boreholes. The results are compared with traditional logging tools. The tool exhibited greater versatility than conventional tools by being able to provide good shear wave and Stoneley wave data for low velocity conditions.