84 Well seismic surveying and acoustic logging 3.2.2.3 Acquisition and visualization parameters The acoustic recording can be visualized in the form of constant offset acoustic sections, the offset being the distance between the transmitter and a receiver. Each constant offset section is a two-dimensional record (time: vertical axis; depth or length: horizontal axis, or vice versa). The sampling interval in depth must be chosen to avoid the spatial aliasing phenomenon on the constant offset sections for subsequent processing of the acoustic data. In practice, the sampling interval in depth is chosen to be equal to a fraction of the distance separating the tool’s two receivers, which enables the refracted arrivals to simulate direct-inverse shots and to make compensated velocity measurements. Acquisition gains must be chosen to avoid saturation, especially for guided modes. However, if acquisition is done to obtain a P-wave velocity log, one can choose gains to amplify the compression waves so as to facilitate the picking of the first arrival times (threshold), even if it means saturating the guided modes and having to make a second “run” to acquire guided modes in preserved amplitude. 3.2.2.4 Acoustic logging in deviated well If the well is deviated, the reflected waves at the boundaries of the layers crossed by the borehole are recorded by the acoustic tool. These waves can be exploited and processed to provide detailed micro-seismic analysis in the vicinity of the well. 3.2.2.5 Security Site security must be ensured by the Head of Mission, in accordance with the Quality System of the service provider. Access to the measurement area must be secured. 3.2.2.6 Quality control During the ascent of the acoustic probe, the operator controls the quality of the recordings on the various receivers of the acoustic probe, particularly the noise level. It may be beneficial to reduce the speed of ascent in noisy areas, but the sampling interval in depth must remain constant. 3.2.2.7 Production For an acoustic log, the average logging speed is between 4 and 6 m/min. This type of logging is rarely performed on its own. It is typically part of a set of measurements obtained over several “runs”.
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