CATS: Controlled Archeology Test Site The objective of this project is to image and locate buried objects 1 to 2 meters deep. Three Geometrics Strataviews were used to record several different seismic experiments with several different seismic sources. photographs of the data acquistion The Reflection Survey --- ---------- ------ The three Geometrics had a total of 108 recording channels which in turn had three different type of Mark Product geophones; 48 40Hz. phones, 48 12Hz. phones, and 12 horizontal phones. The 48 trace Stratview (the unit was rented from Geometrics, so the log book calls it the "Geometrics") was set with recording parameters of -10 mil delay, 3200 samples per secord sample rate (the highest since the vibrator would sweep from 20Hz. to 1600Hz.). It was also set to record 4096 samples per trace ( from -10mils to .127 seconds) The first round of data collection was a combined reflection, refraction, surface wave experiment. The first 48 channels, with the 40Hz. geophones, were a conventional split spread reflection survey. The phones were spaced 5cm apart (yes, .05m) with a 5cm offset from the shot to phones 24 and 25. i.e. phones 1-24, shot, 25-48. Each shot location had three "shots" of different sources. A sledge hammer was used to drive a 1in. diameter pipe into the ground. Typically, 4 sledge pounders were stacked and recorded. A small hand hammer was then used to create a higher frequency source. Nine small hammer taps were stacked. A small vibrator was then attached to the pipe and driven by a non-linear sweep of frequencies. The shot location and geophones were then advanced 20cm in order to obtain 6 CMP (Common Mid Point) coverage. After examining a few shots with all the geophones and source types for quality control, it was decided to concentrate on the small hammer reflection data first. I thought the bookkeeping and data management would be simpler if all the small hammer SEG2 shots were in a single SEG-Y file and the shots were renumbered.
Files 1102, 1106, 1110, 1114, 1118, 1122, 1126, 1130, 1134, 1138 had errors while reading. Upon examination (hex dump), the Geometrics appears to have dropped a single byte on several traces of each shot. SIOSEIS was changed to recover from the error. Since there are only 38 shots, I decided to plot every shot on the laser printer. I plotted the first shot by itself and tried various parameters until I found what I wanted. AGC is needed to examine the whole trace. I also decided to plot all shots and all traces with the same scalar, but this wouldn't really show the amplitude variation from shot to shot or trace to trace because of the agc. The Geometrics was set to start recording slightly before the actual trigger, so the plot parameter stime 0. is used. The time of the first sample is -.020 seconds Yikes, there are bad traces scattered throughout the line. Some traces are bad probably due to a bad connection between the geophone on the cable. All the shots with read errors have multiple bad traces.
Before applying the geometry and doing CMP gathers, I'll kill the bad traces by either weighting the whole trace to zero using PROCESS WEIGHT or by muting the back end of the trace using PROCESS SMUTE.
The above C shell script is executed by using the first shot number at the first argument and the last shot number to plot as the second argument. e.g. plt 29 29 plots just shot 29. I plotted each shot on the screen, one at a time, to check that I got all the bad traces. Good thing too, because I'd missed a few.
Note that xloadimage is terminated by typing the letter q in the plot window. Also note that the pause between plots is caused by xloadimage being in the foreground. Continuous plots (without a pause between shots) may be obtained by doing xloadimage in background (by placing the ampersand sign (&) on the xloadimage command line. Each SEG2 shot is a separate file, so I callected them all together into one SEG-Y file using this script. The next step shown in script is to: 1) edit the data (kill bad traces, invert traces with reverse polarity and surgically mute noise bursts). 2) Assign the shooting and geophone geometry as well as assigning the topographic corrections. 3) Do a CMP gather (collect traces according to common mid points) The final script is short, but took a great deal of analysis to derive. 1) Only the traces near the shot were retained. Traces further than 40cm from the shot don't appear to have reflections and have some other mode (surface waves or shear waves or ???); so ranges 45cm and greater are eliminated from the stack by weighting them to zero. 2) Filter was applied before NMO because the low frequencies didn't move-out correctly. I never determined why. 3) The datum or topographic corrections are applied via process SHIFT. 4) A 100x800 Hz bandpass filter was applied. 5) The plot is done in reverse direction since the first shot was on the southe end of the line. The plot annotation is by meters with the top of the mound being zero.final plot
New (2001) Analysis --- ------ -------- The recording unit clipped some of the traces on some of the shots, so it was decided to mute the clipped data first thing. Script is the sioplt script used to pick the mutes. The tsets were then used to create a new set of cmp gathers using (script). The cmp gathers can be plotted with script. The cmp gathers were moved out and stacked with script. The stack was fk migrated with script. The final plot was generated with script. The plot of the filtered agced shot directly over the mound was generated by script. Tomography Survey ---------- ------ The tomography survey geophone layout has "Geometrics" channel 2-48, LeRoy channel 1-32, Alistair channels 1-34. 1) Create a SEGY file of all 72 shots
2) Create a file with the "closest" trace to the vibrator. Note: There is confusion about the polarity between the two series of Geometrics. After step 1 above, traces 1-47 are from the rental Geometrics and traces 48-105 are from LeRoy and Alistair. The polarity of the pilot traces of all "closest" traces of the Geometrics are reversed in this script.
3) Pick and print the time of the largest positive amplitude after correlating with the trace closest to the vibrator. SIOSEIS does not permit specifying a different pilot trace on every shot when the pilot is in a different file, so each shot is correlated in a new sioseis job. The first sioseis job created the output file and subsequent jobs positioned after the last shot in the output file. The special lprint value of 4 was created in process WBT to print the picks and a perl script was run to write the pick in the "Cornuelle" format and to only print if the time of the pick was greater than .05 seconds.
GPR (Ground Penetrating Radar) Survey --- ------ Larry Conyers of the University of Denver collected and processed a bunch of GPR data. I will try some seismic processing techniques on these data. The digital data were recorded in the GSSI DZT format. GPR uses nanosecond sampling and SEG-Y can do no better than micro- seconds, so all processing units have been scaled by 10**3. Individual GPR traces are displayed here. A typical GPR line is displayed here GPR line 59, meters 26 to 33 corresponds to the location of the seismic reflection line. A simple plot doesn't reveal anything useful. (plot script). A common GPR processing technique is to use a "background" filter, which subtracting the average trace from all the traces. SIOSEIS found the average trace through process STACK and process UADD then subtracted it (script). The plot does help! A plot with a bandpass filter of 500 2000. A plot with a bandpass filter of 500 1000. A plot with the background filter and 500x1000 bandpass. A plot and script for filter and migration. A plot with background filter, filter and migration. A plot with filter, migration and topography corrections. A plot with background filter, filter, migration and topography corrections. The units used in GPR processing in SIOSEIS are confusing: 1) DZT2SEGY uses a sample interval in microseconds rather than nanoseconds since the SEG-Y standard (and SIOSEIS) expect it that way. DZT2SEGY sets the sample interval to 59, where it should be .059. 2) The elevations in SEG-Y are also integers, but they need to be in millimeters, so the elevations are multiplied by 1000. Thus GEOM parameters datume 600 datumv 200000 are in millimeters and mm/sec. 3) The FK migration was done using a sample interval of .001 so the program wouldn't bomb. The migration velocity was 2500. The distance between traces in migration was set to 16 rather than the .016m actually recorded.Go to the list of seismic processes. Go to SIOSEIS introduction.