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 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. Some suggest that the fact that regular- and long-spaced log measurements often conflict means these logs are not accurate. That is actually not true. Quite often there is drilling induced damage or chemical alteration around the borehole that causes the near-borehole formation to be up to 15% slower than the deeper formation. This "gradient" in slowness can be as large as 2–3 feet. The long-spaced measurements (7.5–13.5 ft) always measures the deeper, unaltered formation velocity and should always be used instead of the shorter offset logs. Discrepancies between seismic data and sonic log data are due to upscaling and anisotropy considerations, which can be handled by using Backus Averaging on sonic log data. Some suggest that 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. However, this is usually prone to leading one to the wrong conclusions because the more compliant formations that are prone to washouts or diameter enlargements also inherently have "slower" velocities. 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(s) 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. Use in mineral exploration Sonic logs are also used in