Sonic logging

, which is a measure of a how fast elastic seismic compressional and shear waves travel through the formations.

This means that a sonic log can be used to calculate the porosity, confining stress, or pore pressure of a formation if the seismic velocity of the rock matrix,

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.

[1] 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.

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 accuracy of modern compressional and shear sonic logs obtained with wireline logging tools is well known now to be within 2% for boreholes that are less than 14 inches in diameter and within 5% for larger boreholes.

The seismic source is fired with the geophone(s) at a series of different depths, with the interval transit times being recorded.