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Natural Science Forum / Physics / Acoustics / December 2003marine seismic alternatives?
Now as long ago as ten years ago there were some protests in BC on the effect or marine seismic airguns on marine life, and now there is a fair bit more in Nova Scotia. So what is the latest on the effect of such airguns on marine mammals and perhaps even on fish and if on fish, what sort of frequency range affects the fish most? The question is what alternative can be come up with to marine seismic airgun arrays. Now on land the alternative when dynamite cannot be used is to use an array of Vibroseis trucks with each truck sending out a low volume but long duration sweep waveform and when the reflected data is cross-correlated with the sweep waveform the resulting data is close to that of dynamite. So perhaps the same thing can be done at sea with a semi-submersible (submersible to avoid too much wave action) large flat-bottomed vessel or array of four medium sized flat-bottomed vessels that would give off these low volume sweeps. The coupling with the water would not be as good as of Vibroseis trucks with the road but would be OK, and the motion of the water would be much lower frequency than the sound waves and the submersion would mean that sway from vertical would not be too high. However that might add to the cost of marine seismic data acquisition by a factor of perhaps five but of course maybe less when environmental costs are factored in. Another thing is that I think you can use lasers to induce sound in water so a weighted fiber optic cable might even be able to act as a sweep source at the bottom, not the surface of the sea. Those are just some brainstorming ideas I am throwing out for discussion and may perhaps not be practical yet but feel free to build on them if you like (but reference this post I guess) and to follow up if you have more practical/constructive ideas and/or more knowledge of recent research into alternate marine seismic sources and/or the effect of marine seismic activity on marine life. David (geophysicist but more applied math than practical) > Another thing is that I think you can use > lasers to induce sound in water so a weighted > fiber optic cable might even be able to > act as a sweep source at the bottom, not the > surface of the sea. Actually when I thought of that I was thinking of sonoluminescence which however is the production of light by sound passing through a liquid, not the reverse, but I still think the reverse, so laser stimulation of sound in water, is also possible but won't crosspost this to the optics and main physics newsgroups. I'll see if I can brainstorm some more ideas about this in the next while, perhaps involving EM to acoustic conversion in a layer so that layer can act as a near plane wave low volume sweep source though of course in such a case the coupling would be such that the EM energy would be a fair bit higher than the resultant acoustic energy and may itself have negative effect on marine life. But that is just a quick last minute idea that if I contemplated for a few minutes might decide it is not at all feasible even if I was Tesla or Jim Wait. :-) David http://www.nfld.com/~dalton (CV, abstracts, music stuff, mystic BS) > Now as long ago as ten years ago there were some protests in BC > on the effect or marine seismic airguns on marine life, and now > there is a fair bit more in Nova Scotia. About time. The planets oceans are full of life that uses audio for orientation and navigation. Simple common sense should dictate that we try to avoid making unnecessary noise in the sea. Hearing damage is considered as being one possible explanation of the beached cetacean problem. I think we should look into using something other than sound for charting the seas, the current scientific noise pollution is as defendable as "scientific whaling" (presumeably done to verify that the cooking time for whale meat is unchanged!). > David (geophysicist but more applied math than practical) Kind regards Peter Larsen  Signature************************************************************* * My site is at: http://www.muyiovatki.dk * ************************************************************* > > Now as long ago as ten years ago there were some protests in BC > > on the effect or marine seismic airguns on marine life, and now [quoted text clipped - 16 lines] > > Peter Larsen During the 60's, Conoco were experimenting with marine vibrator as an energy source. I have worked with some of the data they collected in Malaysia and it is quite good. You might want to contact ConocoPhillips to see if they have published anything in that regard. >Now as long ago as ten years ago there were some protests in BC >on the effect or marine seismic airguns on marine life, and now >there is a fair bit more in Nova Scotia. So what is the >latest on the effect of such airguns on marine mammals >and perhaps even on fish and if on fish, what sort of >frequency range affects the fish most? >The question is what alternative can be come up with >to marine seismic airgun arrays. >Now on land the alternative when dynamite cannot be >used is to use an array of Vibroseis trucks with >each truck sending out a low volume but long duration >sweep waveform and when the reflected data is >cross-correlated with the sweep waveform the >resulting data is close to that of dynamite. >So perhaps the same thing can be done at sea >with a semi-submersible (submersible to avoid [quoted text clipped - 11 lines] >of perhaps five but of course maybe less when >environmental costs are factored in. Now if famous elder geophysicist Klaus Helbig had still been visiting Newfoundland (he likes it here and will be back in 2004) and at the desk next to me still I would have floated such ideas off of him and he would have said impractical, probably, but at least I am trying to start discussion. Two more ideas I had last night were on: 1. Directivity Now simplistically for a 2-D seismic survey you might think you need mostly in-plane energy and almost vertical energy. But in practice though seismic data is processed to simulate vertical incidence it is most commonly recorded on a wide spread of hydrophones around the sourcs and also in areas of complex geology for a 2-D survey there can be out-of-plane reflections. Moreover for the now-common 3-D marine survey there can be a wide 3-D aperture needed around the source. Still, even if that is a wide cone it is still less than the full halfspace and would not travel as far sideways directly though of course there would be multiple reflections in the water layer. So ideally a marine seismic source could resrict energy to that wide cone. However an airgun injects a very high pressure bubble into the water and is not very directional (I think) and I doubt it is possible to fit it with an acoustic analogue to a parabolic reflector. So maybe some research should be done into more directional sources. 2. migration tracking Modern marine biologists and fisher elders do a fair amount of tracking of marine mammal migration patterns and fish stock migration patterns so it should be possible to predict such migrations to some extent and schedule surveys so as to not interfere with such migration patterns. This would just be one added factor along with weather, sometimes ice, and ship availability, to weigh into consideration. OK, those are my points for today and if any more come to me tonight I will follow up again tomorrow briefly. David <snip> > The question is what alternative can be come up with > to marine seismic airgun arrays. [quoted text clipped - 21 lines] > of perhaps five but of course maybe less when > environmental costs are factored in. <snip> David: The sea bottom is usually floored with such thick unconsolidated materials. If you had a sunken barge bouncing up and down, low-rider-like, on the sea floor, you might get a fair amount of information, but a thumper truck would likely punch a hole through the floor of the barge, unless it was steel of a significant thickness, in which case the signal gets diffused somewhat, no? And thumping on a floating barge would send shivers along my spine; that had better be an unmanned ship, due to the effect such activities would have on the spine and belly of the ship. Maybe you should count on using say, five ships in succession until your seismic program was completed. "Let me see if I have this aright, matey: you want to put a giant hammer aboard my ship and bang it up and down until the sea boils around it? I may be a poor, unemployable former cod fisherman, but I'm not ready to die yet. Off with you now, and ask Lloyd's of London what they think of yer scheme there bye." Solely in jest, Daryl Krupa > The sea bottom is usually floored with such thick unconsolidated > materials. [quoted text clipped - 8 lines] > Maybe you should count on using say, five ships in succession until > your seismic program was completed. No, I want the entire bottom of the barge to vibrate, not to have a thumper truck inside a barge. I'm not sure if the submersion is needed at all but in any case I did not mean submersion as far as the sea floor but just a bit below surface so a flat-bottomed barge (well to submerge it would need walls of some kind though) would be more stable but again such submersion may not be needed and indeed a flat bottom might not be needed but the outer hull of a double hulled ship at least in its part below water could sweep vibrate and act as an acoustic antenna/source. The large surface area of such a hull (or barge flat bottom) would mean that the vibration would not have to be as intense as on the small surface area of a Vibroseis (thumper) truck base plate. Also surely engineers reading can build on my comments and design something practical and stable (a giant phallic vibrator sub :-) ). David > No, I want the entire bottom of the barge to vibrate, not to > have a thumper truck inside a barge. How would that save the whales? >> No, I want the entire bottom of the barge to vibrate, not to >> have a thumper truck inside a barge. > > How would that save the whales? You mustn't have read my initial post. Vibroseis works with a long low volume sweep not an extremely high volume short explosion (or in the case of airguns injection of a very high pressure bubble of air very quickly). So that low volume long sweep would be much easier on the whales ears than the very high volume airgun explosion. But it remains to be seen if such a vibrating barge bottom or hull can be designed and even if so it will take a while and in the short term adjusting marine seismic schedules if possible to account for marine life migration patterns should be considered. I know, the whales might prefer a total ban on marine seismic, but I went for the compromise. But you know, the oil industry might be still using dynamite in populated areas if they could but only switched to Vibroseis because they had to at first at least but since may have found the Vibroseis setup has some advantages when there are roadways for access (plus traffic noise when cross-correlated with the sweep sort of goes away but would be present in dynamite data). Hmmm, maybe I should have just studied more of Sheriff and Geldart second edition as I was supposed to last week (but a different chapter) or floated my ideas by Chuck Hurich, Jeremy Hall, Jim Wright and even my supervisor Michael Slawinski (plus gotten Klaus Helbig's e-mail from him) before posting them here. But I am always in a rush and more ready to stick my neck out and play the brainstormer/fool a bit on newsgroups than off. Also of course local Newfoundland groups PEGN, NATI, NOIA, NEIA, and MUN (including Genesis centre) might be interested but perhaps not and I think the Genesis people want you to work up your idea whereas I like to crank (in more ways than one) them out and get on to the next one, just in my spare time, and if some catch on can always do some brainstorming/idea percolation consulting eventually. David <snip description of something out of "Flesh Gordon"> David: Have you considered posting your idea to sci.geo.oceanography? Also, would you not get a better signal using a parabolic dish, or a megaphone trumpet? (The latter, and the vibrator analogue, might fit in with a ship named "Rudy Vallee".) Then again, why not just cut out a portion of the hull and replace it with a diaphragm? And hit it with repetitive piston action? That should get you onto the evening news ... Daryl Krupa > David: > Have you considered posting your idea to > sci.geo.oceanography? I posted a pointer there, "whale hearing and airguns" referring to this thread, so I imagine if any were really interested they would have looked here. Hmmm, I have Jon Lien's e-mail address from a few years ago somewhere (Michael Rochester gave it to me) and if it is still the same may e-mail him in the new year with all of my past whale-related ideas and any new ones that I think of while compiling them. He heads a local group that rescues a lot of whales from fishing gear, here in Newfoundland. David <snip> > Hmmm, I have Jon Lien's e-mail address from a > few years ago somewhere (Michael Rochester gave [quoted text clipped - 4 lines] > heads a local group that rescues a lot of > whales from fishing gear, here in Newfoundland. David: Ah, you're at Memorial. Have you happened to meet Hiscott or Aksu? Daryl Krupa > Ah, you're at Memorial. Yeah, I put my off-campus e-mail in the header even when posting from campus though so that more SPAM goes there and also so I deal with newsgroup related e-mail just on Saturdays off-campus. > Have you happened to meet Hiscott or Aksu? I know the name Hiscott and have seen him around (but not at the beer mixers, I think) but haven't met him. I know Aksu fairly well since he is the graduate chair. However I know geophysicists Slawinski, Rochester, Wright, Hurich, Hall, Leitch, Miller, Quinlan, and Hodych better since they are all in my corridor, and Hiscott and Aksu are up two floors. And I guess the best known geologist here is Hank Williams who is retired but still has a professor emeritus office and I see him around a good bit; but haven't heard him play fiddle yet. But there's an end-of-exam clean-out-the-fridge Murray Club (UBC has Dawson, we have Murray) mixer this coming Wednesday, right :-) . Anyway it is a good Earth Sciences department and anyone interested in coming for a grad degree I guess could look at http://www.esd.mun.ca or ask me for info on the department and the city if you like. Other departments are listed on http://www.mun.ca . David >>The question is what alternative can be come up with >>to marine seismic airgun arrays. [quoted text clipped - 3 lines] >>sweep waveform and when the reflected data is >>cross-correlated with the sweep waveform the Is this not what we call MLSA? If so then where is the trade-off of time vs intensity falling short of satisfying your needs? Give us some numbers, please. Angelo Campanella (actually I, David Dalton, wrote the next bit and Daryl was quoting it) >>>The question is what alternative can be come up with >>>to marine seismic airgun arrays. [quoted text clipped - 5 lines] > > Is this not what we call MLSA? What does MLSA stand for (of course if it is a geophysical term I will look out of touch but I haven't been to an SEG conference since 1990 in San Francisco)? > If so then where is the trade-off of time > vs intensity falling short of satisfying your needs? Give us some > numbers, please. Well, the supposed problem is that short pulse but high intensity airgun pulses may be too hard on some marine organisms though that is still subject to research, so I suggested maybe a string of towed vibrating submersibles, whether flat-bottomed of not, towed behind a big ship perhaps, sending out a long low amplitude sweep similar to Vibroseis truck sweeps. So I guess the only problem is in designing the appropriate technology, and perhaps those with more practical experience in the field will comment on the numbers further. But ideally each towed submersible vibrating (as a whole, like a personal massage unit in the tub) unit maybe would be about the size of a life raft or something, so probably engineering students could design it as a course assignment or something, given needed time and intensity and directionality specs I guess. Some engineers of sci.engr may be following this thread and if there is any environmental group pressure on government(s) and government(s) pressure on oil companies and contractors there probably already is some behind the scenes consideration of alternatives going on. David > What does MLSA stand for (of course if it is a geophysical > term I will look out of touch but I haven't been to > an SEG conference since 1990 in San Francisco)? "Multiple Length Sequence (analysis?)" Basically instead of one big boom, the sound (or vibration) is generated at a continuous low level, being modulated in a very special and phase coherent way. (A series of pulses or aqaure waves cued in a specail and detern=ministic fashion, which "sequency", lasting a minute or more before it needs to be repeated, can be repeated indefinitely, the result being averaged ("integrated") for more precision or resolution. The received signal is alayzed with a demodulator driven by exactly the same signal so that a long coherent result occurs. In the case of measuring auditorium reverberation time, instead of one big BANG!, the emitted signal is a hushed steady random noise that may last up to a few minutes. It can be done while folks are millinmg around and talking. The result occurs with persons barely noticing. I think the marine animals would be rather pleased. Angelo Campanella > "Multiple Length Sequence (analysis?)" > [quoted text clipped - 4 lines] > and talking. The result occurs with persons barely noticing. I think the > marine animals would be rather pleased. Seems like a good idea but just watch that the path conditions are constant during the measurement. It's great if you can manage with a 10 minute test but if not, the time escalates very rapidly. Theory says you get 3 dB more signal to noise ratio from doubling the duration, so from 10 minutes, to get another 6 dB you have to go to 40 minutes. I would guess that this might be a problem with long ocean paths due to currents moving water at different temperatures around. If you're actually trying to track things that change it's hopeless. Tony Woolf >> "Multiple Length Sequence (analysis?)" >> [quoted text clipped - 17 lines] > temperatures around. If you're actually trying to track things that > change it's hopeless. Well, in this case you are trying to measure what's in the solid rock under the ocean layer, and that doesn't move around, so it shouldn't be a problem, unlike long path ocean acoustics. The density and velocity variations between rock layers can be much greater than the time-variant small variations in the ocean layer. David > Well, in this case you are trying to measure what's in > the solid rock under the ocean layer, and that doesn't > move around, so it shouldn't be a problem, unlike > long path ocean acoustics. The density and velocity > variations between rock layers can be much greater than > the time-variant small variations in the ocean layer. I suspect that the marine animal damage from geological exploration would be caused by charges set off near or on the bottom. Such charges will deliver a tremendous wallop to the water that would affect animals for miles around. In that case, it makes sense to me to set a mechanical shaker on the bottom, or on a firm pedestal driven into the bottom, to deliver the continuous MLSA signal. Angelo Campanella >> Well, in this case you are trying to measure what's in >> the solid rock under the ocean layer, and that doesn't [quoted text clipped - 7 lines] > will deliver a tremendous wallop to the water that would affect animals > for miles around. Well, in the early days of marine seismic exploration I think they used dynamite blasts just above the water but have switched to an array of airguns (injecting extremely high pressure bubbles) with slightly varying signatures such that they constructively interfere to give a combined wavelet similar to that of a dynamite blast. So, no, it is done at or near surface but there still is a pretty intense blast to the water. > In that case, it makes sense to me to set a mechanical shaker on the > bottom, or on a firm pedestal driven into the bottom, to deliver the > continuous MLSA signal. But in seismic work while it is possible to get some results from a single source with an array of receivers around it, in general the source is moved around as well to improve coverage even further. Now perhaps a fast crawling bottom MLSA source can be developed (but the bottom is sometimes a bit irregular), though the advantage would be that the coupling would be directly with the sea floor and thus the loss from the reflection coefficient between sea and sea floor would not occur. But the surface or near surface Vibroseis-like or MLSA source, though it would have that reflection coefficient problem, which airguns also have, might be cheaper in the near term but again may take a couple of years to develop a prototype and a few years beyond that to phase in so for now at least there should be scheduling of airgun use to minimize interaction with marine organism migration patterns. Good night, David There is something called a marine vibrator. Its manufacted by IVI of Tulsa. Works very similar to land vibrators. Best used in shallow water ecologically sensitive areas. Can build up signal by remaining on station whereas airgun must be constantly moving. Anyway I have to pull off almost all newsgroups including these for a while except perhaps for Saturdays. My Ph.D. candidacy exam will probably be rescheduled for late January and I need to get a proposal draft in this week and last night wasted quite a few hours on newsgroups. But I will still reply to e-mail to the address in the header on Saturdays. Bye for a while, David http://www.nfld.com/~dalton |
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