127 3. Borehole geophysical methods as a constraint to transform the seismic section into pseudo-porosity section. The results are shown within the 30–190 m depth interval in Figure 3.8, with high-porosity layers appearing in red (Mari et al., 2021). The pseudo porosity section, associated with a flowmeter recorded in the slotted PVC part of borehole B2, informs on preferential areas where flows occur. Levels of in-flows or out-flows, indicated by blue arrows on the flowmeter, clearly show two flow loops, completely independent over depth (83–143 m and 159–181 m depth intervals). With full waveform data, provided by multi-source and multi-receiver logging tools, recorded in deviated or horizontal boreholes, it is possible to conduct a well micro seismic survey based on the analysis of modes reflected and diffracted on acoustic impedance discontinuities within formations or at formation boundaries. Processing of reflected modes leads to depth migrated acoustic sections with very high resolution (a few tens of centimeters) providing an image with a depth of investigation of several meters from the well trajectory (Hirabayashi et al., 2024). Hirabayashi et al. (2024) shows an example of depth migrated acoustic section obtained in a highly deviated geothermal well with a sonic tool consisting of 13 receiver stations spaced at 0.1524-m intervals, each with eight azimuthal receivers. The minimum distance between source and receivers is 3.795 m, and a dipole chirp source was used during data acquisition. Figure 3.9a shows depth migration images above and below the actual borehole trajectory indicated by the black curve. Parallel reflectors dipping down to the right by about 3º are consistently observed. Figure 3.9b shows a zoomed image for the black box shown in Figure 3.9a. Stratigraphic structures (of approximately 10º dip) are observed within a potential geothermal reservoir, with the reflector at ∼1634 m vertical depth corresponding to the base of an oolite dune. Full waveform acoustic logging has a very good vertical resolution (a few decimeters). Its lateral investigation with respect to the borehole is of a few centimeters for interface dispersive modes, a few tens of centimeters for refracted modes and a few meters for reflected modes (less than 20 meters).
RkJQdWJsaXNoZXIy NjA3NzQ=