A technique for recording the formation compressional slowness based on the transit time between transmitter and receiver. In the most basic wireline sonic measurement, an acoustic transducer emits a sonic signal, of between about 10 and 30 kHz, which is detected at two receivers farther up the hole. The time between emission and reception is measured for each receiver, and subtracted to give the traveltime in the interval between the two receivers. If the receivers are two feet apart, then this time is divided by two to give the interval transit time, or slowness, of the formation. This type of measurement is also known as first motion detection.

This technique works because the first arrival at the receiver is a wave that has traveled from the transmitter to the borehole wall, where it has generated a compressional wave in the formation. Some of this wave is critically refracted up the borehole wall, generating head waves in the borehole fluid as it progresses. Some of these strike the receiver, arriving in most cases well before any signal traveling directly through the mud. Furthermore, if the logging tool is parallel to the borehole wall, the traveltime in the mud is cancelled by taking the difference between the traveltime to the two receivers. Problems of irregular hole or a tilted tool are avoided by using borehole compensation.

The depth of investigation depends on the slowness, the transmitter-to-receiver spacing and the presence or absence of an altered zone. It is generally within the invaded zone, and of the order of several inches.