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.