69 2. Well seismic surveying Figures 2.13 to 2.15 illustrate the processing sequence applied to well data obtained with the source located at Jerry Point (JP). The processing phases are as follows: • Frequency filtering and amplitude recovery. Seismic data were filtered in the 5 to 80 Hz bandpass and compensated for the spherical divergence effect by application of a gain law. Each VSP trace was then normalized to the direct arrival to compensate for transmission losses. The result of this pre-processing is shown in Figure 2.13 (bottom left). The horizontal axis of the VSP section represents the cable length deployed along the well. • Picking of first arrival times and wave separation. The VSP section was flattened on the first arrival time picks. A 7-term median filter was applied to flattened data to extract the downgoing waves (Figure 2.13 bottom right). The downgoing wave section was subtracted from the initial data. The residual section was then corrected by the first arrival times to restore each VSP trace to its initial time. The residual VSP section (Figure 2.14 top left) mainly shows the upgoing waves. • Deconvolution. A Wiener-Levinson deconvolution (Mari, 2015) was applied to the downgoing waves (Figure 2.14 top right) and to the upgoing waves (Figure 2.14 bottom left). The operator, calculated on the downgoing field for the purpose of transforming the downgoing wavelet into a zero-phase signal, was applied to the upgoing and downgoing fields. A different operator was calculated for each VSP depth. • Generation of the seismic image. The seismic image is obtained from the section representing the deconvolved upgoing waves. The operation involves 4 steps: 1. Calculation of the velocity model 2. NMO corrections and conversion of upgoing waves in two-way times (Figure 2.14 bottom right) 3. Calculation of equal-abscissa lines for mirror points. Figure 2.15 (left) shows the distribution of equal-abscissa lines (iso-X lines) on the section displayed in Figure 2.14 (bottom-right) after application of a gain law. The distance chosen between two iso-X lines was 25 m 4. Migration using the method presented by Wyatt (1981). The migrated VSP section is shown in Figure 2.15 (right). The horizontal axis represents the horizontal distance between the well (0 m) and the different mirror points. The distance between two mirror points is 25 m. The lateral range of investigation of the VSP section is of the order of 1,000 m. The section is displayed in normal polarity according to the SEG convention. Under normal polarity, an upgoing compression wave reflected by a marker associated to an increase in acoustic impedance is represented by a negative amplitude value (trough)
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