1. n. [Formation Evaluation]

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.

See: acoustic transducer,  altered zone,  borehole compensation,  delta t stretch,  depth of investigation,  first break,  head wave,  interval transit time,  invaded zone,  long-spacing sonic log,  monopole,  P-wave,  sonic log,  ultrasonic measurement

2. n. [Formation Evaluation]

The technique for recording a borehole sonic log, in the sense of measurement of any of the acoustic properties in and around the borehole. The standard sonic measurement, based on first motion detection, normally can be used only to determine formation compressional slowness. For all other sonic measurements, such as shear, flexural and Stoneley slownesses and amplitudes, it is necessary to record the full waveform using an array-sonic tool and process with a technique such as slowness-time coherence.

See: acoustic mode,  array sonic,  flexural mode,  slow formation,  slowness time coherence,  sonic log,  Stoneley wave