This example shows applying a datum correction to some Edgetech towed fish chirp data where the fish depth nor the wireout distance are known. The datum correction is calculated by identifying and picking different events or wave paths on the data. The first method involves identifying and picking the surface reflection (the upward pulse from the fish, reflected off the water surface, then back down to the fish). This is simply the fish depth in two way travel time. See figure 1 and figure 2. The second method requires picking the event caused by the water bottom reflection of the upgoing surface reflection and picking the water bottom reflection of the downgoing pulse. The first step in Edgetech data processing is to convert the data into SEG-Y. sioseis << eof procs diskin xstar diskoa end diskin format edgetech ipath line164.sgy end end xstar type 2 end end diskoa opath line164.segy end end end eof ***** Note ***** There's a new XSTAR parameter, TYPE, that's needed to differentiate between 1 trace and two trace XSTAR system. METHOD 1 Pick the surface reflection ------ - METHOD 2: Pick the water bottom reflection of the surface reflection ------ - A quick QC plot (created with this script) shows unrealistic geology caused by the variation in the depth of the towed fish. In an earlier Edgetech project I was able to correct the data to a datum by an automated pick of the water bottom multiple. In a different project I was able to do the datum correction using the water depth from Seabeam center depth. Unfortunately, neither of those methods worked on this dataset. The recorded data length of this dataset is not long enough to include the water bottom multiple. The corrections using the Seabeam (from an xyz file), didn't work well either because the wireout distance was changed frequently and was not recorded, thus the fish depth is not known. While looking at the above plot, I noticed a nice, geologically realistic, event at the bottom. If we assume that this event is the water bottom, relative to the sea surface, and shift the water bottom of the data to be at this time, then the plot might be correct! Unfortunately, I could not find an automatic picking algorithm that would pick the even. An automated pick would also introduce the "heave" (wobble) I noticed on the event. Picking the event "by hand" was too tedious. The script was used to produce a SIOSEIS rasterfile (plot)and header file suitable for the picking program SIOPLT. SIOPLT was executed as: sioplt -if siofil -wbt wbt -hf hdrfil and file siopltrc had: width 1400 height 800 The picks were loaded into another script which automatically picks the water bottom time after doing a 3 trace mix, computes the time shift between the water bottom time and the event picked as where the water bottom is supposed to be, then applies the shift. The data are save to disk (in IBM floating point so SU can read it) and then filtered and plotted for quality control. Notice that the water bottom has several peaks and troughs. This is caused by the filter being too narrow and or too steep slope. With experimentation I found dbdrop 12 made the water bottom into a single cycle. The end user also wanted to data compressed more, so I decimated the data by throwing out 2 of every 3 traces (this was also the reason why I did a three trace mix earlier; average 3 traces and then keep only 1 of the 3). It was also decided to get as much dynamic range as possible without apply any gains, so the plot is with gray scale. The script created the final plot.Back to SIOSEIS examples. Go to SIOSEIS introduction. Go to the list of seismic processes.