The velocity is calculated by measuring the travel time from the piezoelectric transmitter to the receiver, normally with the units microsecond per foot. To compensate for the variations in the drilling mud thickness, there are actually two receivers, one near and one far. This is because the travel time within the drilling mud will be common for both, so the travel time within the formation is given by: where = travel time to far receiver; = travel time to near receiver. If it is necessary to compensate for tool tilt and variations in the borehole width then both up-down and down-up arrays can be used and an average can be calculated. Overall this gives a sonic log that can be made up of 1 or 2 pulse generators and 2 or 4 detectors, all located in single unit called a “sonde”, which is lowered down the well. An additional way in which the sonic log tool can be altered is increasing or decreasing the separation between the source and receivers. This gives deeper penetration and overcomes the problem of low velocity zones posed by borehole wall damage.
Cycle skipping
The returning signal is a wavetrain and not a sharp pulse, so the detectors are only activated at a certain signal threshold. Sometimes, both detectors won’t be activated by the same peak and the next peak wave will activate one of them instead. This type of error is called cycle skipping and is easily identified because the time difference is equal to the time interval between successive pulse cycles.
Calculating porosity
Many relationships between travel time and porosity have been proposed, the most commonly accepted is the Wyllie time-average equation. The equation basically holds that the total travel time recorded on the log is the sum of the time the sonic wave spends travelling the solid part of the rock, called the rock matrix and the time spent travelling through the fluids in the pores. This equation is empirical and makes no allowance for the structure of the rock matrix or the connectivity of the pore spaces so extra corrections can often be added to it. The Wyllie time-average equation is: where = seismic velocity of the formation; = seismic velocity of the pore fluid; = seismic velocity of the rock matrix; = porosity.
Accuracy
The accuracy of sonic logs is rather poor, evident by the fact that regular- and long-spaced log measurements often conflict, and this should be taken into account when there are disagreements between seismic data and sonic log data. Other considerations include that the resolution of sonic logs is on a scale of inches whereasseismic reflection data resolution is on the scale of meters, and the two methods use significantly different frequency ranges so travel times will vary due to dispersion. In order to investigate how the varying size of a borehole has affected a sonic log, the results can be plotted against those of a caliper log.
Calibrated sonic log
To improve the tie between well data and seismic data a "check-shot" survey is often used to generate a calibrated sonic log. A geophone, or array of geophones is lowered down the borehole, with a seismic source located at the surface. The seismic source is fired with the geophone at a series of different depths, with the interval transit times being recorded. This is often done during the acquisition of a vertical seismic profile.
