The multichannel seismic (MCS) system was a Sercel Seal, provided and operated by Exploration Electronics Limited of Great Yarmouth, U.K. The system is described in two parts, the deck and out board equipment, primarily the streamer, and the laboratory based data acquisition system.
Streamer
The streamer used was a 2.4 km, 192-channel fluid filled digital streamer. The streamer is arranged into 16 active line sections (ALS), each 150 m long and containing 12 channels. Each channel is composed of 16 hydrophones in a 12.5 m long group.
A total of nine depth control birds were evenly spaced along the streamer: bird 1 at 3.55 m in front of the first active line section (9.8 m in front of the centre of channel 1); bird 2 in the centre of channel 24; bird 3 in the centre of channel 48 etc. to bird 9 in the centre of channel 192 (Table 3). The streamer was nominally towed at 10 m depth, although ship speed, sea conditions and the amount of weight attached to the streamer caused it to often be at a shallower depth.
A radar reflector was fitted to the tail buoy to provide feather angles from the ship’s radar system.
Figure 9: Seismic source and multichannel streamer geometry. The streamer has 192 channels at 12.5 m spacing, for a total length active of 2387.5. The first source-reciever offset was 100 m during the SUMD profiles, and 110 m for all subsequent profiles.
Depth control birds were located 9.8 m in front of the centre of channel 1, on channel 24, then every 24 channels to place the ninth bird on channel 192 (Table 3).
~40 m from stern to
Bird number Channel centre centre of which they are fixed.
Figure 10: Schematic diagram for the front of the multichannel streamer, as far as the first active section.
Figure 11: Schematic diagram for the end of the multichannel streamer, from the last active section to the tail buoy.
Data acquisition system
Data were acquired from the streamer using a Sercel SealXL 5.1 system (Figure 12). The SealXL system acquires the data, provides real-time quality control displays, paper hard copy and digital recording to tape and disk. The digital records are in SEG-D format, with 16 s record length at 2 ms sample
channel history display (Figure 13). Every 150th shot gather and the single channel record (channel 5) are printed on a pair of thermal printers.
Each shot gather is recorded immediately after acquisition to a separate file on the Seal system, two network attached storage (NAS) drives and to an FTP server. The IBM 3590 tape drives, both primary and spare, failed to record data reliably and were not used.
Figure 12: The multichannel seismic acquisition system. The system comprises of a number of workstations and streamer interface boxes (below the bench and the silver box behind the monitors), four main display screens (Figure 13), an IBM 3590 tape drive (grey box to the right of the screens, beneath the fan) and a pair of thermal plotters to plot shots gathers and a single channel record (cream coloured boxes at extreme right).
Figure 13: The multichannel seismic (MCS) acquisition system control screens (left and centre left), shot gather display for quality control (centre right), and the bird controller display (right).
Ship-board MCS processing
Two seismic processing systems were used. The MCS lines acquired for the University of Southampton (designated SUMD and SUME) were processed using ProMAX, those acquired for the University of Texas (designated SUMUT) were processed using Focus. In both cases, all the individual SEG-D shot gathers for an entire line were combined into a single file using the Unix cat command, and this file was imported into the respective software package. Both processing schemes use 6.25 m CMP bins to give an average fold of ~25.
The ProMAX processing scheme:
1. Desample: 4 ms
2. Apply geometry using the 2D Marine Geometry Spreadsheet:
a. Export all the shot times from the SEG-D data and calculate the ship position at each time from the ship’s 1-second navigation data
b. Determine the average line azimuth and then project, from each ship position, the location of the source and each receiver channel behind the ship (line azimuth-180°)
c. Calculate the midpoints for every source-receiver pair and bin them into 6.25 m bins along the line azimuth
3. Bandpass filter: 3-5-60-120 Hz (minimum phase Ormsby) 4. Amplitude recovery: time2
5. Predictive deconvolution: minimum phase with 250 ms operator and 128 ms gap length
6. NMO correction: velocity analyses every 800 CMPs 7. Stack: median
The data were then plotted using Seismic Unix with the following enhancements:
1. Automatic gain control (AGC): 5-second Gaussian weighted 2. Top mute: 100 ms above the seafloor
The Focus processing scheme:
1. Define geometry
a. Export shot times from focus and calculate ship’s position at each time from 1-second navigation data
b. Calculate the average shot spacing
c. Define station locations every 1.25 m along a line
d. Call PATTERN to define receiver locations relative to the sources
e. Call SOURCE
i. Define the first shot location such that the far offset receiver is at station 1
ii. Set the shot spacing to even number of stations 2. Desample: 4ms
3. Bandpass filter: 3-5-60-120 Hz (minimum phase) 4. Amplitude recovery: time2
5. Multichannel predictive deconvolution: minimum phase with 11 trace window, 250 ms operator and 128 ms gap length
6. NMO correction: velocity analyses every 500 CMPs
7. Top mute: picked during interactive velocity analysis, ~0.5s above the seafloor
8. Time-frequency noise suppression: TFCLEAN module, scales groups of three traces to the median amplitude within the 3-25 Hz frequency band
9. Stack: median
The data were then plotted using the Focus module PSPLOT, which output a PostScript file. Plots were made with an Automatic gain control (AGC) window of 1 second.