95 3. Acoustic logging Figure 3.10g shows the weathering zone extension log obtained in the clay-sandstone formation. This example shows that the investigation depth of the refracted mode is a few tens of centimeters. In relation to the well wall, the weathering zone locally reaches thicknesses of 20 cm. Increases in the weathering zone in porous zones (890 - 899 m, 911 - 916 m) can be correlated with the presence of weakly consolidated formations. In clay zones (899 - 911 m), weathering zone increases correlate with shaliness. This example shows that full waveform acoustic logging provides not only formation slowness but also imaging in the well vicinity with decimetric-to-metric scale lateral investigation for refracted modes. We show that the analysis of reflected and refracted modes makes it possible to extend the investigative power of acoustic logging. 3.5.2 Reflection acoustic imaging Processing the reflected modes provides very high resolution acoustic sections (a few tens of centimeters) providing an image with an investigation depth of several meters from the well axis. In full waveform acoustic logging, the most easily accessible document is the constant offset acoustic section. Figures 3.11 and 3.12 show the results of experimental work carried out in a quarry (Mari, Gavin, Coppens, 1994). Figure 3.11 (top) shows the geological cross section produced from the well information (a vertical well, labeled R1, and a highly deviated well). The deviated well was drilled in the 80 m thick white oolite layer. The abscissa origin is the wellhead of the deviated well. For the ordinates, the geological markers are referenced in terms of depth relative to the quarry top. At the abscissa of the vertical well, the deviated well is 40 m deep. The constant offset acoustic section (Figure 3.11, bottom) shows two types of events: isochronous events with a very high apparent velocity and oblique events. Events with high apparent velocity are refracted arrivals, interface modes (pseudo-Rayleigh and Stoneley waves) and arrivals reflected on acoustic markers parallel to the drain. Oblique events are arrivals reflected on pseudo-vertical fractures or on acoustic markers dipping with respect to the drain. At the 55 m abscissa, the various wave trains are strongly attenuated, indicating the presence of open fractures. Figure 3.12 shows the acoustic section after the filtering of events with a very high apparent velocity. Oblique events, combined with reflections on acoustic reflectors, are clearly visible. The full waveform recordings provided by the multi-transmitter and multi-receiver tools used for acoustic logging make it possible to carry out a micro-seismic survey of the well, based on the analysis of the reflected or diffracted modes on acoustic impedance discontinuities within formations or at formation boundaries.
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