Return to Melville 2001 Knudsen example
From mogk@sdsioa.ucsd.edu Fri May 3 14:28:54 2002 From: "Seth Mogk" mogk@sdsioa.ucsd.edu To: henkart@sioseis.ucsd.edu It turns out that the initial data buffering in the Knudsen, for both display and recording, is based on the range & phase settings. The upshot of this is that it must be on screen to be recorded by the SEGY module--however, (here's the hitch that bit Eli) being on screen does not assure it will be recorded. This is because SEGY records the digitized carrier while the screen needs only envelope; under some combinations of parameters the carrier can overflow its buffer (though the envelope has not) and you end up recording a truncated version of the screen rather than the whole thing. In practice, if you use a screen window of 500 m. or less, you will not have a problem; if you use a 1000 m. window you'll be ok if you use the longer pulse lengths (e.g. 24 msec); and if you use a 2000 m. or greater window you will get bit no matter what. Donna Burnell (the KEL programmer) said she has included a pop-up warning window in recent software releases to warn you when this is about to happen. Also of note, there is a selection under the Display menu called Carrier--selecting this will show what will be recorded by the SEGY module. Check with Donna if you need to know details about a given software version: mailto:donna@knudsenengineering.com. Or to get the latest (we have EchoControl v1.1.5.0, distributed in Package D42-02386 V3.05, Feb. 12, 2002). Seth From mogk@sdsioa.ucsd.edu Fri May 3 15:51:54 2002 From: "Seth Mogk" mogk@sdsioa.ucsd.edu To: henkart@sioseis.ucsd.edu Another thing worth mentioning about this is what happens in Autophase. I asked, if all that's put into buffer is for the screen window, how does it figure out where the bottom is? Answer is, it goes into a Search mode that is different than when the bottom is acquired--enough so that it won't record pings in SEGY that are handled by this Search mode. Under normally decent sea conditions this isn't a problem: the bottom is acquired and when phase changes are needed Autophase does it, no search mode invoked. But under noisy conditions, bubble entrainment, waves banging the hull and all that, Autophase will probably be going into search mode a lot. So under those conditions you're better off in Manual phase operation, and spinning up the watchstander to keep an eye on it for phase changes. Seth -----Original Message----- From: henkart@sioseis.ucsd.edu [mailto:henkart@sioseis.ucsd.edu] Sent: Friday, May 03, 2002 11:11 To: smogk@ucsd.ucsd.edu Subject: Remind me of the Knuhsen SEG-Y work around Seth, Goldfinger from OSU is going collect Knudesen data soon. Remind me/us of the work around to prevent the problem Eli Silver had. I never heard back from Knudsen about fixing the problem - have you? Paul