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 . 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  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  (Figure 1 and 2) 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 or bedding planes and are internally reflected . 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 (Figure 3). 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 grows larger 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. Figure 4 demonstrates the different monopole and dipole systems.
A slow formation refers to a formation in which the compressional wave velocity measured in the borehole fluid exceeds encompassing shear wave velocities . 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 produce measurable 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. Shear waves as well as compressional and Stoneley waves can all be observed with a monopole measurement. An example of a fast formation would be 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.
Single Receiver Sonic Logs
Single receiver sonic logs were the first type of sonic logs and contained only one transmitter and one receiver. These logs are no longer used unless additional data analysis methods are present; however, old data still remains in less updated files. Figure 5 illustrates a time line of previous sonic logs that have since been outdated.
Two Receiver Sonic Logs
Soon after single receiver sonic logs, two receiver sonic logs became more commonly used in order to compensate for borehole effects. Two receiver sonic logs are still in use but require a significant amount of editing before an evaluation can be made from the logs.
Borehole Compensated Sonic Logs
Borehole compensated sonic logs  have two sets of transmitters and receivers, which measure compressional waves and average the travel time . This type of log automatically compensates for variations in the borehole diameter or size and sonde tilt, as well as, increases resolution from 2 feet to 9 inches.
Array Sonic Logs
Array sonic logs were developed to provide information about a variety of acoustic properties such as porosity, permeability, gas zones, acoustic impedance, and elastic properties of a formation. This type of sonic log can have both a monopole or dipole source. Similar to the dipole shear sonic log, the array sonic log with a monopole source measures compressional, Stoneley, and wave velocities which are used to estimated shear wave velocities. The monopole source is not able to measure shear wave velocities in slow formations. In fast formations, the monopole source is able to measure compressional, shear, and Stoneley waves. The dipole source produces a more consistent and accurate shear wave velocity for both slow and fast formations.
Sonic Logs and Seismic Ties
Once seismic data is processed and the sonic log data is acquired, they can be correlated in order to obtain a better understanding of the subsurface and particularly the lithology. This can be completed by producing a synthetic seismogram in order to accurately tie seismic to well data. Several steps must be taken in order to ensure that both sets of data are being accurately compared.
- It is important to start with sonic log data that has gone through a quality control process.
- Both data sets must be converted to the same units. Seismic is collected in time (ms) whereas sonic logs are collected in depth (m of ft.). Using check shot data, either the seismic or sonic logs can be converted.
- Once the units are converted, a wavelet extraction can take place in order to produce the synthetic seismogram.
In the seismic, reflections are indicative of changes in acoustic impedance which can indicate a change in lithology. Figure 6 demonstrates the steps and information which can be obtained from the sonic log in order to produce the synthetic seismogram.
Crain's Petrophyscial Handbook - https://www.spec2000.net/07-soniclog.htm
Schlumberger Oilfield Glossary - https://www.glossary.oilfield.slb.com/Terms/s/sonic_log.aspx
Division of Marine and Large Programs - http://mlp.ldeo.columbia.edu/research/technology/schlumberger-wireline-tools/borehole-compensated-sonic-tool-bhc/
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- “Snell's Law.” Wikipedia, Wikimedia Foundation, 21 Nov. 2019, en.wikipedia.org/wiki/Snell%27s_law.
- Burch, Don. “Sonic Logs Need Troubleshooting.” AAPG Explorer, AAPG Explorer, Mar. 2002, explorer.aapg.org/story/articleid/46752/sonic-logs-need-troubleshooting.
- “Cross-Dipole Acoustic Log.” Cross-Dipole Acoustic Log | Open Energy Information, openei.org/wiki/Cross-Dipole_Acoustic_Log.
- “Borehole Compensated Sonic Tool (BHC*).” Division of Marine and Large Programs, mlp.ldeo.columbia.edu/research/technology/schlumberger-wireline-tools/borehole-compensated-sonic-tool-bhc/.