Introduction to this special section: Borehole geophysics and sonic logging
Sonic logs are most frequently used to tie surface and borehole seismic amplitude measurements to P- and S-wave velocities encountered along wellbore trajectories. Other common uses of sonic logs include the estimation of dynamic elastic properties for geomechanical analysis, such as wellbore stability studies and the design and planning of hydrofracturing operations. Modern sonic logs are acquired with multiple transmitters and an array of closely spaced receivers (anywhere between eight and 13 receivers) in the form of time waveforms. Sonic transmitters can come in the form of monopole, dipole, and quadrupole actuators. By design, sonic transmitters are immersed in a fluid — the borehole mud — whereby formation shear waves can be detected and quantified only by the elastic coupling that exists between wave motion taking place in the borehole fluid and in the surrounding rock formations. Such an elastic coupling gives rise to markedly frequency-dispersive behavior of the detected waves that presents some technical challenges when one interprets their speeds.