SIOSEIS was used aboard the R/V Ewing during the EW0008 cruise to Blake Ridge (September 2, 2000 to October 17, 2000; Newark, N.J) Step 1: Reformat SEG-D 3490 to SEG-Y DLT script includes assigning the streamer geometry (trace ranges and CMP numbers) as well as extracting the GPS clock, LDEO shot number, and streamer depth. The script also writes every 20th shot to a circular disk file of QC plotting. This script was run immediately after each 3490 tape was ejected from the Syntron acquisition system. Each tape was read another time after the copy/reformat was completed (see step 2 below). This script had to be changed each time the streamer was deployed since the tow leader and streamer bird locations changed each time the streamer was deployed. During EW0008 a new feature was added to SIOSEIS process GEOM so that the shot to shot distance can be determined on each shot using the realtime GPS locations that are in the SEG-D and SEG-Y headers. EW0008 leased a realtime differential GPS system (diagram) from John E. Chance (written description). While in Port Newark, this system performed within our 1 meter specification. Step 1a: QC "plot.shot" script was run occassionally to check on the data quality. Generally, this plot was used to check on noisy channels. In one case this plot showed that alternate shots were dead before the watch recognized the problem. Step 1b: The frequency analysis script was run in the early part of the cruise to show the effect of different tow depths of the streamer and guns as well as to reassure ourselves that a ,002 seconds (2 mil) sample interval was sufficient. e.g. freq_tr.2mil /export/home/EW0008/Realtimestack/latest.shot.segy 301 This frequency plot shows that the data is dominated by steamer noise, which can be filtered out using a (sioseis script). The frequency plot of the filtered trace shows the predominant frequency to be around 40Hz. Step 2: The brute stack script (version 1) and (version 2) has many unique features. The most important objective was to provide a hardcopy plot for geologic and geophysical purposes as well as another quality control step. This script generates a plot file which is constantly being read by program atlantek that writes to the Atlantek thermal plotter. The brute stack plot thus contain data that were collected around one hour previously. All traces are read by SEGDIN, assigned streamer geometry and gathered according to cmp so that every 50th gather could be written to a disk file for velocity analysis. New parameter LIMITS was used in process TREDIT to exclude all ranges except for the 1000 meters closest to the ship because the velocities were not known well enough. Before starting the velocity analysis, a quick plot was made of a few cmp gathers. The script that generated the plot has a couple of interesting twists. New plot parameter RECSP YES was used so we didn't have to worry about the number of traces in each gather. Process header was used to convert the Hydrosweep water bottom time from seconds to milliseconds because process plot can only annotate integers. This vpick script runs on Matlab version 5.3.0 which uses function ginput rather than getline. These velocities were checked by plotting each moved-out cmp gather individually using a C-Shell script. These picks weren't satisfactory when used in the stack. There was only one machine with Matlab on the ship and the license was for a single user and other users were using Matlab. So, we used the constant velocity stack method provided by SIOSEIS (scripts). The stack script was short and simple, with only 5 velocities necessary. NMO velocity interpolation parameter vintpl 3 should be used for smooth velocity interpolation. The mute parameters were selected to mute out the far half of the streamer down through the BSR because of NMO stretch. The velocities generally increase, except for the BSR. e.g. fno 309701 vtp 1510 3.65 1527 3.9 1535 4.15 1515 4.2 1555 4.25 1570 4.5 1590 4.85 1650 6 end where the BSR is .55 seconds after the water bottom. The stacked data were then FK migrated with a velocity of 1520m/s, reversed in order (since the line runs from west to east), the water bottom picked, the muted to water bottom, filtered, and an exponential gain applied from the water bottom. The script uses a brand new parameter in process gains; TADD. e.g. gains subwb yes tadd 3. type 3 alpha 4 end end subtracts the water bottom time of each trace and then adds 3 seconds. Thus, each trace gets exactly the same gain function, as if each trace started at 3 seconds. A comparision of realtime and post-processed navigation was performed. The LDEO post-processed ts.n file was converted to SIOSEIS navigation file using PERL script ts2sio. This comparison was done on a relatively calm day. Many factors should be considered when deciding which method to use for cmp binning: The realtime nav block is a predicted location. The realtime location has more ship pitch, roll, yaw in it. The guns and streamer are somewhat decoupled from ship pitch, roll, yaw. ******************************************************************* ************** RECOMMENDATION *********************** ******************************************************************* TS.N FILES SHOULD BE USED FOR POST-PROCESSING NAVIGATION AND BINNING.Return to SIOSEIS examples.
Go to the list of seismic processes. Go to SIOSEIS introduction.