Real-time Seismics

A real-time seismic system was established not only to perform quality control, but to provide instant backup of the data. The two Geometrics Geodes were connected to laptop computers which in turn were connected to the ship’s gigabit network (which failed and became 100Mb). The laptop computers were Windows based and “exported” each seismic line as a unique directory using the Samba protocol. Hopper’s iMac mounted each directory as it became available (a manual process). One Geometrics/laptop was to record the MCS data and the other one for the two sonobuoy receivers.

The Bergen acquisition group setup in the “aft con” (science lab overlooking the fantail along with all the winch controls). Hopper & Henkart doing the processing setup down the hall in the “Future Lab” (a room which will be made into a lab someday).

The Healy’s computer system broadcasts a variety of information via the “UDP” protocol; the posmv position is broadcast on port 33101. The Geometrics laptop read port 33101 and inserted the contents along with a time tag into the normal Geometrics log file. SIOSEIS read the log file and used this fix as the position of the shot in processing. There was no check that the time of the fix was close to the time of the shot. (The fix was the last fix received at the time of the shot).

The laptop computers were not time synchronized initially, but were set with NTP (network time protocol) after the first week of shooting. Relating the time of shot to the navigation before NTP was installed is a problem, so using the navigation from the log is the best that can be done.

The Bergen group wanted to record in SEG-Y rather than the SEG-D format. The existing real-time input in SIOSEIS was using the SEG-D format, so process DISKIN was modified to keep track of the SEG-Y file disk address, then close the input file, sleep for a few seconds, then open and position the SEG-Y file before reading again.

Initially, the “latest_shot” file was plotted on demand on the iMac as the quality control. A script to plot a single trace from the entire line was developed, including converting the SIOSEIS raster file to an HP RTL file and plotting on the large format HP plotter in the computer lab. Steve Roberts had a freeware program “display” (part of ImageMagick) that displays Sun rasterfiles using the X11 standard. A plot of each “latest_shot” and a plot of the last 500 traces of a selected trace were displayed on a Bergen laptop using the “display” program from the iMac.

The shot number or trigger count was also written the Geometrics log file along with the nav. The Geometrics numbers each shot recorded with the SEG-D FFID; a file id. The FFID is supposed to increment for each shot written to disk. A subroutine was written for SIOSEIS’s process GEOM that moves the FFID from SEG-Y word 3 to word 5 and places the shot number into word 3. This routine also converted the shot long/lat into 100ths of an arcsecond and wrote them into SEG-Y x/y coordinates in words 19 and 20 (along with the scalar in short word 36). Complicating this procedure was that the Geometrics produced duplicate FFIDs occasionally. The Geometrics also does not “flush” the log write buffers after each write so there can be a large delay before the log entry is actually written to disk. This latter problem didn’t affect us too much since we never got to the real-time stack stage of processing.

Many SIOSEIS processes use parameters that are given relative to the water bottom, so the same subroutine searched and read the SeaBeam centerbeam file for the water depth and put it ine SEG-Y word 16. The routine was modified later to use the Knudsen depth file because SeaBeam was inoperable for a while. The last good depth is used whenever SeaBeam was unable to determine a depth.

Other changes to SIOSEIS while on HLY0503 were:

1) Change process stack to discard zero amplitudes from the stack average.

2) Real-time geometry (type 9 or 14) computed the DFLS (distance from last shot) incorrectly after a missing shot.

3) Parameter ENDMUTE was added to process despike to zero the remainder of the trace after a spike is detected.

4) The maximum number of semblance velocities was increased in process VELAN.

5) The maximum number of samples per traces was increase from 16384 to 32768 (signed 16 bit integer to unsigned) so that long Knudsen traces (22222 samples) can be convert to envelope using a 32K fft.

Unfortunately, the SIOPLT program for trace edited did not work on any computers on the Healy since SIOPLT was written for 8bit displays and all modern displays are 24 bit.

The distance from the GPS antenna to the mid-point for the first trace is 89m. (offset = 88m). The Posmv is ~38m from the fantail (Consolidated survey: Posmv=-49.57, aft fantail survey point = -86.74, eyeball survey point to the end of the fantail = -.8m). Eyeball fantail to guns = 5m, guns to trace 1 = 90m. (38 + .8 + 5 + 45 = 88.8)

Script 1 determines the most recent file on the Bergen Geometrics laptop. It writes every shot to a SEG-Y file on the MacIntosh. It writes every second shot to a file “latest_shot”, from the beginning each time – a circular file. Every 15^th shot is also written to a circular disk file after being filtered and amplitude balanced.

Script set 2 displays the “latest_shot” and the “near trace: on the Bergen group’s laptop computers.