BIRPS MOBIL LINE 1 (CCSS DATASET 2)
DATA PROCESSING AND MODELLING
The following approach has been used for processing the coincident reflection and refraction dataset. The wide-angle recordings are available for offsets greater than 40 km from the two CSSP stations 54 and 47.
Therefore we made use of the results of Page 7 velocity analyses of the COP data along this line, available at 3 km intervals, to construct the 2-0 velocity-depth model V(x,z) for the sedimentary section down to the basement of 6.0 km/s velocity. This approach involved conversion of two-way travel times into corresponding depths followed by layer velocity identification and smoothing of various shallow layers along the COP line.
Keeping this sedimentary section down to the basement unchanged, the wide-angle reflection data have been modelled to construct the deeper crustal section along the line. This crusta! depth section has finally been converted into a two-way time section in an attempt to compare with the poststack processed COP reflection time section.
-139-Modelling of wide-angle reflection data
The wide-angle record sections from the two stations, 54 and 47, consistently reveal five prominent phases correlatable in the later arrivals. Due to poor reproduction quality of the copies of the original crust, however with smaller velocity contrasts. The phase designated as PM P which is well recorded out to 160-180 km offset with high 'diffraction-like' event on these record sections is however interpreted as a reflection from a steep boundary (about 17° dip) at subcrustal depths successively refined by iterative 2-D ray tracing and synthetic seismogram modelling of wide-angle reflection data discussed above, using the SE IS 81 package (Cerveny and Psencik, 1981). The computed representative locations along this line.
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Figure 2: Travel times plot of the wide-angle reflection data correlated from the CSSP station 54 record section. Computed travel time curves for various phases are also shown by continuous curves.
The broken curve for the POP phase is computed for a 2 km shallower depth of the westerly dipping subcrustal reflector shown in Figure 3, indicating that it may be more steeply dipping on the western end.
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Figure 5: Compressed variable area seismogram section recorded, from airgun shots along MOBIL LINE 1, at the CSSP station 54 in the distance range 55-75 km showing the consistent fit of the P1 P, PCP, P2P, PMP and the PDP phases computed for the structural model shown in Figure 3.
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--An interesting feature that may be seen from the model given in these (between the COPs 5600-5200). The synthetic seismogram sections computed for the two common land seismic stations 54 and 47 are shown sedimentary sequences above the basement. The velocity increase of 6.22 to 6.5 km/s at the PCP reflector, the Pn velocity 7.9 km/s and a sharp Moho at 30 km depth, as inferred by Bott et al. (1985}, are also consistent with the present findings on the crusta! structure of this region. We have however recognized the P1 P and P2P reflectors also indicating a further vertical differentiation of the upper and the lower crust.
Poststack processing of near-vertical reflection data
Poststack processing of near-vertical reflection data on the MOBIL LINE 1 has been carried out on the recently established seismic data processing system at the National Geophysical Research Institute, Hyderabad. The hardware configuration of this system consists of a COC CYBER 180/850A host computer running the dual operating systems NOS
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149-and NOS/VE, a MAP IV array processor, a versatec plotter, large capacity disk storage units, high speed tape drive units and printers. The seismic data processing software package GEOMASTER of the Compagnie Generale de Geophysique, Masse, France, is running with its VOS control system under the NOS operating system. The poststack processing sequence for the MOBIL LINE 1 consisted of attenuating steeply dipping coherent noise by frequency-wavenumber (f-k) dip filtering, suppressing the multiples by predictive deconvolution, time variant band pass frequency filtering to readjust the spectrum, finally the amplitude equalization and display of the final stack section. The only important processing step that has not been carried out in the present processing exercise is that of migration.
This is because while processing deep crustal seismic data the main objective is to obtain the final section that is best suitable for interpretation rather than only achieving the highest S/N or resolution. We believe that the final stack section that has been obtained by us meets this requirement and thus is suitable for structural interpretation.
The plot of the raw stack section with only a time variant frequency filtering (1 0-20/45-60 Hz, 0-500 ms., 5-10/40-55 Hz, 1500-2500 ms., 5-10/35-45 Hz, 3500-4500 ms., 5-8/30-40 Hz, 5500-6500 ms., 5-7/25-32 Hz, 12000-16000 ms) and amplitude equalization is displayed in three parts of the section in Figures 10 a, b, and c. The application of f-k filtering by rejecting signals with dips exceeding +1- 16 ms/trace gives adequate improvement by attenuating the steeper coherent noise and hardly affecting the real reflections. In fact, after f-k filtering, the lateral continuity of various reflections has significantly improved as can be seen from the plot displayed in three parts of the section in Figures 11 a, b, and c. lt is further found that predictive deconvolution when applied a second time after stack gives a substantial improvement by suppressing the multiples periodicity significantly.
Figures 10 (a,b,c) and 11 (a,b,c), see next pages.
Figures 1 O(a,b,c): Raw stack section (Two way time versus COP locations) plotted with a time variant frequency filtering and amplitude equalization.
Figures 11 (a,b,c): Same as Figure 10, after application of the f-k filtering (rejecting slopes exceeding + 16 ms/trace).
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The plot of the final stack, with application of predictive deconvolution (operator length 700 ms, gap 50 ms) on the shallow (600-7000 ms) and deep (5000-1 0500 ms) design windows, is displayed in three parts of the section in Figures 12 a, b and c. In Figure 12, the two-way time section obtained by converting the 2-D crusta! depth section inferred from the wide-angle reflection data, has been superimposed mostly in those parts of the section where a coincident subsurface coverage has been obtained have laterally variable reflectivity appearing as bands of discontinuous reflector segments along this line. The possible explanation for this laterally variable reflectivity is that the actual physical contrasts responsible for these reflections may not be sharp throughout but are likely to be more gradational where the reflectors are not obviously visible in the COP reflection section.