104 Well seismic surveying and acoustic logging Acoustics is the most natural link between geophysics and logging. Although functioning in very different frequency bands (a few tens of kHz for acoustic logging and a few tens to a few hundred Hz for Very High Resolution (VHR) seismic surveys), acoustic and seismic well measurements are governed by the same laws of wave propagation. This chapter shows: • the principle of depth conversion of surface seismic methods using VSP data (Vertical Seismic Profile, Chapter 2); • the influence of cementation on acoustic measurements; • the principle of the calibration of formation velocity measurements obtained by acoustic logging with those provided by VSP; • the principle of time conversion of acoustic data (acoustic logging, Chapter 3) and the calculation of synthetic seismograms; The data used in this chapter are near surface data derived from a 3D VHR seismic survey, a vertical seismic profile, and a full waveform acoustic log. 4.2 3D VHR seismic survey and VSP The acquisition device is composed of a receiving device and a transmitting device. The receiving device, stationary during acquisition, is composed of 2 parallel lines of 24 geophones, spaced 2 m apart. The distance between the 2 geophone lines is 4 m. The device consists of 11 source lines, oriented perpendicular to the geophone lines (in-line direction) with 11 shots fired per line. The distance between 2 shots on a line is 2 m. The distance between 2 source lines is 4 m. The direction of the source lines is cross-line. There is no overlap between transmitting and receiving devices. The distance between the 2 devices is 4 m. The source used is a weight drop (Chapter 2, Figure 2.5). Seismic data processing provided a 3D block of 13 in-line sections spaced 1 m apart (Mari and Herold, 2015). Each section is composed of 44 Common Mid-Point gathers (CMP) 1 m apart. The listening time has been limited to 250 ms, the time sampling step is 0.5 ms. Figure 4.1 shows the seismic acquisition device (left) and an example of in-line and cross-line seismic sections derived from the 3D block (right). The two sections presented (section 6 in the in-line direction, and section 23 in the cross-line direction) intersect in the middle of the 3D block. They have been filtered in the 15-150 Hz bandwidth, which provides an excellent signalto-noise ratio. The CMP point located at the intersection of the in-line seismic section 3 and cross-line 6 is located about twenty meters from a borehole in which a VSP has been recorded. VSP data is used to tie surface seismic data. The VSP in this case is a near surface VSP, recorded in a steel cased vertical well.
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