87 3. Seismic tomography The diffraction tomography applied to the single-shot geophone data has a target zone, which encloses the reservoir area near the receiver borehole (161), and is defined as follows: • from 40 to 75 m away from the emitter borehole (162) to the receiver borehole (161); • depth interval 475-625 m. Since the tomography technique produces reliable estimates of changes in elastic parameters only when the source and receiver coverage is satisfactory, then the upper part of the elastic images (above 560 m) should not be interpreted due to insufficient coverage (Figure 3.21). In the lower right portion of the images (below 560 m), source and receiver coverage is very good (maximum coverage), so a confidence region can be defined here in the target zone. The target zone for the single-shot hydrophone data, which encloses the reservoir area near the receiver borehole (164), is defined as follows: • from 50 and 86 m away from the emitter borehole (162) to the receiver borehole (164); • depth interval 500-650 m. In spite of the different nature of both geophone and hydrophone data (particle velocity and pressure), in coupling with the formation (clamped geophone versus hydrophone string), and spatially (holes 161 and 164), a good correlation between the images is observed. Furthermore, the three reservoir levels, R3=575 m, R2=583 m, and R1=600 m, can be identified. In both boreholes, the density tomograms were assessed in a practical manner, by carrying out a comparison between the density images with a pseudo-density log. The density logs were convolved with a characteristic signal matching the frequency bandwidth of the density tomogram. Therefore, the results could be compared directly to identify similarities and differences to aid in interpretation. In borehole 164, an FDC log was used, which verifies a good correlation between the two independent sets of density information. In borehole 161, a density log was generated from a combination of gamma-ray, neutron-porosity, and the sonic logs from this borehole, and the density log of borehole 164. The tomography of hydrophone data produced a better and cleaner density tomogram than the geophone data image. Comparison of the images from both data sets revealed a vertical resolution of the geophone data image that is lower than that of the hydrophone data. This also fits with differences in signal bandwidth, 150 Hz versus 350 Hz. However, the fit at the Lower Hauterivian level, around R1= 600 m, seems reasonable. Since the thicknesses of these reservoirs are of the order ≈ 3 m, the very limit of seismic resolution, it is difficult and delicate to attempt any detailed interpretation within each reservoir level, especially with only one-shot record.
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