A sonic log is an acoustic log that emits sound waves which start at the source, travel through the formation, and return back to the receiver (Schlumberger). The travel time from the source to the receiver is called slowness and as a result sonic logs are sometimes referred to as sonic slowness logs.
A sonic log produces data which illustrates P-wave travel time versus depth (Schlumberger) and is recorded as microseconds per foot (ms/ft). This data provides information about how fast acoustic waves travel through rock.Wave propagation which produces the P-waves in sonic logs follow properties according to Snell’s Law (CPH) and demonstrates how waves travel through different interfaces or rock layers in the subsurface.
Waves will propagate until attenuation, which can be a result of several situations. Some degree of absorption will affect waves, turning the mechanical energy into heat. Waves can also be attenuated by coming in contact with fracture of bedding planes and are internally reflected (CPH). Another form of attenuation occurs when a foreign substance, usually gas, enters the mud column and decreases the sonic signal. This type of attenuation, referred to as cycle skipping, can produce low quality logs. All of these occurrences must be accommodated for when relating to seismic. Another characteristic to accommodate and correct for when analyzing sonic log data is tool stretch. As the wireline is lowered into the borehole, the weight of the line will cause some stretch that increases with increasing depth. This must be accommodated for especially when comparing sonic log data to core data.
Dipole Shear Sonic Logs
The most modern type of sonic logs are the dipole shear sonic logs. These logs measure values for compressional, shear, and Stoneley slowness through both monopole and dipole sources. A monopole source emits radially while a dipole energy source emits energy in one direction. When determining whether to use a monopole or a dipole measurement, the type of formation that the acoustic waves will be traveling through is most important. IMAGE HERE of how dipole shear sonic logs work
A slow formation refers to a formation in which the compressional wave velocity measured in the borehole fluid exceeds encompassing shear wave velocities (Schlumberger). This monopole measurement results in compressional and Stoneley arrivals but no detected shear waves. Therefore, a dipole measurement is better suited for a slow formation because it can generate flexural or bender waves which generate measureable shear waves. An example of a slow formation is a high porosity gas sand layer.
A fast formation has higher shear wave velocities than compressional wave velocities, and shear waves as well as compressional and Stoneley waves can all be observed with a monopole measurement. An example of a fast formation would a low porosity carbonate layer. Although both monopole and dipole measurements have advantages for different types of rock layers, modern sonic logs contain both types of measurements as to most accurately measure the acoustic properties of the subsurface.