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

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