[1] In the process of drilling and completing a well, cement is injected through the wellbore and rises up the annulus between the steel casing and the formation.
[2] A sonic tool is typically run on wireline by a service company that detects the bond of the cement to the casing and formation via a principle based on resonance.
[citation needed] In this sense, the amplitude of the waveform received is the basic measurement that is evaluated.
The data is collected by the tool and recorded on a log which is used by the oil producing company as an indicator of zonal isolation in the well.
There are production reasons and legal reasons (governed by a petroleum regulatory body in each individual state) that dictate the well must have specific areas of isolation.