78 Well seismic surveying and acoustic logging velocity of compression waves is a common and relatively well-established practice (Summers and Broding, 1952; Vogel, 1952). Full waveform acoustic logging or acoustic coring is based on the analysis and processing of the various wave trains (refracted waves, guided waves, reflected waves) recorded by the tool. Full wave field recordings enable the determination of the propagation velocities of the various modes and some petrophysical parameters, and the ascertainment of lithological and mechanical information (Gaudiani, 1982; Arditty, Arens and Staron, 1984; Morris, Little and Letton, 1984; Paillet and Turpening, 1984; Mari, Coppens, Gavin and Wicquart, 1992; Mari, Arens, Chapellier and Gaudiani, 1998; Mari, Gaudiani and Delay, 2011). The borehole may be an open hole, a cased hole (steel and/or PVC), or a cemented cased hole. In the latter type, acoustic logging is used to monitor the cementation and to determine the characteristic parameters of formations (velocities…). Acoustic logging has a vertical resolution of a few centimeters, and a lateral one of centimeters for interface modes, decimeters up to a meter for retracted modes, and up to ten meters for reflected modes. It provides detailed information of a borehole as a function of depth, in terms of acoustic wave velocities and rock petrophysical characteristics. The exploitation of the reflected modes can provide an image comparable to a time microseismic section which, in favorable cases, allows the tracking of layer boundaries and an estimation of their dip. Combined with density logging, it provides an acoustic impedance log, which is variable with depth, and is converted to time after correction, calibration and tying. Figure 3.1 is an example of a full waveform acoustic log. The acoustic tool (left image in figure) is a flexible tool with a small diameter and composed of a transmitter and 2 receivers. The distance between the transmitter and the first receiver is 3 m, the distance between the two receivers is 25 cm. The depth reference is ground level. Recording depth corresponds to the depth of the point located halfway between the two receivers. The right side of the figure shows an example of an acoustic section obtained by using a transmitter-receiver pair, 3 m apart. In this representation, the vertical axis represents the depth at which the sensor is located (3 m in this case), and the horizontal axis represents the listening time (3 ms). The acoustic section is composed of acoustic traces. Each acoustic trace is the acoustic recording measured by the receiver, which is 3 m from the transmitter, over a listening time of 3 ms. Different wave trains can be identified on the recording.
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