Variable Frequency Monopole-Dipole Sonic Logging For Mechanical And Hydrogeologic Properties

Most traditional sonic logging tools operate at a single frequency. Usually this frequency<br>is a relatively high frequency, which in turn excites many unwanted modes that can make<br>waveform interpretation difficult (Paillet and Cheng, 1991). If the survey frequency is too low,<br>fixed frequency tools may not excite the desired modes in fast rocks or small boreholes. A<br>variable frequency sonic tool can minimize these problems.<br>Low frequency tools can provide waveforms from which it is easy to pick shear wave<br>arrivals in real time. By selecting a proper survey frequency, compressional head wave<br>amplitude can be reduced, and the first high amplitude head wave is the shear wave. Low<br>frequency dipole logging can be used to find shear velocities for soft formations (Chen, 1988).<br>Mechanical properties are readily calculated from compressional and shear velocities.<br>Many authors relate Stoneley wave amplitude (or Stoneley index) to permeability (Tang,<br>et al., 1996). Stoneley waves are easily interpreted from low frequency surveys when the<br>amplitude of other modes are reduced. Stoneley index can be directly generated while logging<br>from this type of survey.<br>A prototype variable frequency tool was tested in soft shale, mudstone, and concrete<br>boreholes. By varying the source frequencies, the modal content of the received waveforms was<br>controlled. The tool can be configured so that the first high amplitude arrival is a shear wave or a<br>Stoneley wave. We conclude that a variable tool exhibits greater versatility than conventional<br>tools by being able to provide good shear wave and Stoneley wave data in real time.