Monday, September 13, 2010

Arriving in Honolulu soon

Well, it's hard to believe, but this nearly-never-ending cruise will be finally over in less than 9 hours. I went to bed early this evening, but couldn't sleep (because of being too excited?), so I decided to get up in the middle of the night. It's actually in the morning in the East Coast time, so maybe it's a good idea. I need to adjust my time zone anyway, so that I can get back to my normal teaching duty more easily.

We took a group photo the other day. Most of people shown in the picture are from the science party (only one from the Langseth crew), so this is just half the population on this research vessel (actually I noticed a few members of the science party are missing here; they were probably sleeping). We usually work in different places at different shifts, doing our own duties, and we rarely get together like this. So this picture is great because it vividly testifies that a research cruise like ours is supported by so many hard-working people.

Though this cruise is going to end, we'll try to keep this blog active to post anything interesting we find during post-cruise data analysis or whatever we think it appropriate to post. We'll revisit Shatsky Rise in the spring of 2002, and you'll see posting from seas again.

Friday, September 10, 2010

The End is Nigh

Only a few more days remain in the long transit from Shatsky Rise to Honolulu. As we approach our MGL1004’s final destination, I have started to think about how I will enjoy becoming submersed in all the simply luxuries of civilization again. For example; being able to make a phone call whenever I chose and being able to chose what I want for dinner from the super market, just to name a few. It is easy to take such luxuries for granted while shore side, but life at sea helps develop a little greater sense of appreciation.

Life at sea has had its advantages though. As Jun mentioned, the scenery is gorgeous. Looking out upon a vast ocean gives you a new sense of space. Being able to see for miles in all directions, with no buildings, mountains, or trees to obstruct your view. While at sea I have found myself in the galley for hours, engaged in interesting conversations with people who have had many interesting experiences from sailing. This has helped me find a greater appreciation for the entertainment value of a good old fashion conversation. Finally, being at sea for so long and isolated, to some extent, from the rest of the world you learn something about yourself. How you handle certain situations that most people never experience in a lifetime.

Overall, my experience aboard the R/V Marcus G. Langseth, as a watchstander for the Shatsky Rise cruise, has been a positive one. I am very glad that I signed on and will take away a great deal of experience and appreciation for life at sea and seismic data acquisition. I am appreciative to all who have made this experience possible!

Thursday, September 9, 2010

The Sheltering Sky

I asked watchstanders to post a blog on a daily basis, but as you've seen, they seem to be exhausted and are not posting as frequently as I'd like to see... It may be understandable because this cruise turned out to be unusually long; it's 60 days total, and this is by far the longest cruise I've been on (my previous record was 44 days). One of our graduate students, Duayne, told me this morning that he couldn't look at computer screen any more because he was sort of burned out. I found it interesting because I never get tired of working with computer...

As I wrote in my previous post, this cruise was a great success overall, and this is mainly brought by the professionalism of the Langseth crew, the Lamont science tech group, and the WHOI OBS team. Will and I did all the planning, but the actual implementation of the plan was done so gracefully by them, and there was literally nothing left for us to do (watchstanders worked hard during their watch under the supervision of the tech group, but the PIs just had to look at monitors). The chief science officer, Robert Steinhaus, originally came from the industry of exploration geophysics, and we all benefited from the high industry standard he brought to the vessel. Though I never worked in the oil industry, I somehow felt I was having a virtual experience of being a client from ExxonMobil.

We also had a good fortune of having nice weather throughout the cruise. One of major concerns we had before the cruise is that this survey area can sometimes be hit by wayward typhoons, but we didn't have any during this cruise. Actually, we had more than just nice weather. We had spectacularly calm seas on several occasions. The picture shown was taken in the morning of August 26, and you don't normally expect this kind of sea in the middle of the Pacific. That was simply gorgeous. Just imagine yourself sailing in the vast ocean filled with these colors... Now approaching the end of the cruise, I've started to regret not spending more time outside the lab. I was busy working on computer, and because we had so many days at seas, I thought I could see these things as many times as I want. Scientific problems in front of me seemed more important and urgent, but maybe I should've enjoyed the life at seas more. This reminds me of the following quote from one of my favorite movies, "The Sheltering Sky":

"Because we don't know when we will die, we get to think of life as an inexhaustible well, yet everything happens only a certain number of times, and a very small number, really. How many more times will you remember a certain afternoon of your childhood, some afternoon that's so deeply a part of your being that you can't even conceive of your life without it? Perhaps four or five times more, perhaps not even that. How many more times will you watch the full moon rise? Perhaps twenty. And yet it all seems limitless."

So, watchstanders, how many times did you see a spellbinding sunrise or sunset during this cruise? I hope you saw many.

Wednesday, September 8, 2010

Mission complete

As of September 3, 2010, our survey of Shatsky Rise was completed, and we started transit to Honolulu. Because of two medical diversions, we couldn't finish everything we planned, but given the science days we ended up with, what we were able to achieve can be called a great success. The following is the executive summary of the cruise report, which we are currently trying to write up before the end of the cruise:

"R/V Marcus G. Langseth MGL1004 formed the major data acquisition phase of the NSF-funded project, "Geophysical Constraints on Mechanisms of Ocean Plateau Formation from Shatsky Rise, Northwest Pacific" (OCE-0926611). Deciphering the origins of large oceanic plateaus is a critical element for understanding mantle dynamics and its relation to terrestrial magmatism, and Shatsky Rise was chosen as a high-priority target because it provides a unique tectonic setting to distinguish between various models proposed for the formation of oceanic plateaus. The purpose of this survey was to provide critical missing information on (1) the thickness, velocity structure, and composition of the Shatsky Rise crust, and (2) the history of magmatic emplacement and later tectonic development of the Rise. This was planned to be achieved by acquiring seismic data along two refraction lines over the Tamu Massif, which represents the early, most voluminous phase of the Rise construction, and over 3,000 km of reflection lines covering both the Tamu and Ori Massifs, the latter of which corresponds to the intermediate phase of the plateau evolution.

The cruise was unfortunately hampered by two medical diversions, which took ~16 days in total, and even with a seven-day extension provided by NSF, the survey had to be scaled down to focus on the southern part, leaving the northern part to be completed in another cruise tentatively scheduled for spring 2012. The southern part includes all of refraction lines (yellow lines in the map) and around 1800 km of reflection transects (red lines), all on the Tamu Massif. The work remaining to be done includes the rest of reflection transects (dotted red lines), which extend from the northern flank of the Tamu Massif to the center of the Ori Massif.

The Langseth fired over 47,000 shots from its 36-gun tuned airgun source into an array of seismic receivers: the Langseth's 6-km-long multichannel streamer and 28 Woods Hole Oceanographic Institution ocean-bottom seismometers (OBSs). As far as the southern part of the survey is concerned, the operational goals of the experiment were achieved in full. All of 28 OBSs deployed (shown as circles) were recovered successfully, and all instruments returned high-quality data. Multichannel seismic (MCS) profiling was also conducted with no major issues, yielding high-quality reflection data. Migrated brute stacks of all MCS lines were produced during the cruise, exhibiting intriguing intrabasement reflectors as well as revealing the true lateral extent of Shatsky Rise. OBS data show spectacular wide-angle refraction and reflection arrivals with the source-receiver distance often exceeding 200 km. The data collected during this experiment are sufficient to accurately determine the entire crustal structure of the Tamu Massif and will provide key information on the early magmatic construction of Shatsky Rise. By combining with future seismic data from the northern part of the survey, this information will provide an important tectonic framework for synthesizing existing geological, geophysical, and geochemical data and for resolving the formation mechanism of this large igneous province."

Monday, September 6, 2010

Back to Hawaii & other things on my mind

It's about a week to the end of the cruise. We picked up all the seismometers and are on our way back to Hawaii. Life on the langseth is a little less hectic now. I had a lot of fun retrieving the seismometers and speaking with Jimmy Elsenbeck on very interesting features of the device that make them withstand large pressures deep down at the bottom of the sea. Impressive devices they are. Sitting down there at the bottom of the sea, at about 5 km for the deepest deployment, the seismometers can withstand pressures as large as 7,000 lbs/sq in. To make that possible they have to be made of thick hard borosilicate glass, yet they float to the surface when remotely activated, despite their being denser than water. We can thank the principle of floatation for that. Just hollow out the insides, get enough air in ( I spare us the math) and we can define a dimension for neutral buoyancy. That's convenient.

I feel that the trip was auspicious despite the two unscheduled transits to Japan due to medical emergencies. For one, I got to participate in the first seismometer deployment. I also picked up the last seismometer. Good times I say. There is now no need to do long shifts. We are cruising at a steady speed of 11 knots towards Hawaii and should be on land in about 6 days. I hear I may get a little land disorientation. Just a little, though.

Life on the Langseth also has a new dimension to it. We have ping pong tournaments. Every one participates, and its fun. Unfortunately, I didn't make it to the finals on any of the games - singles or doubles. Some people are just way better than I am. I guess I'll focus on my soccer skills. Jun, was my favorite though. But one of the WHOI guys won the singles. I guess his eye hand coordination from picking up all those seismometers came in handy.

I look into my screen and I feel it can't be long now. We'll get to Hawaii soon. Not that I haven't enjoyed the trip, but I think I'm about done with the beautiful blue seas, the amazing rush of device deployment, and the tireless hours in the deep bowels of the cruise ship, Langseth. It's time to go home, feel the solid hard ground under my feet, touch the green grass, and take in the whole experience, again and again.

Friday, September 3, 2010

Marine Multi-channel Seismic Processing (part-3)

(6) NMO & further multiples removal
When the velocity model is ready to go, we can apply NMO to get the data further ready for stack (ProMAX module: Nromal Moveout Correction). But right here, we may have something more to do. That is to further remove multiples. Remember we have applied deconvolution to suppress when pre-process. But it does not often come up the result as good as we want. So at this point, we have the velocity model in hand, we have chance to remove multiples in further extent after NMO. Typically, we would like to employ inside mute, F-K filter or Radon filter to deal with marine seismic multiples. Read the materials about these two kinds of filters, select the proper paramters for the executive modules, then test and compare to find out the best result. Please be patient again, paramter test will need some time.

(7) Stack
When the data are done in CDP domain after applying NMO and filters for multiple removal, they are ready to stack. There is not much work to do except choosing the proper parameters for stack and wait for it done (ProMAX module: CDP/Enmble Stack). But before stack, remember to employ Bandpass Filter to remove the noise generated by the previous processing step, and maybe employ Automitical Gain Control to enhance the deeper reflectors (TBD). Because the multiple removal filter(s) would suppress the primaries signal at the same time. And then, we have to wait for the flow to execute and complete, which will take a long time when we have like 60000 CDPs to stack.

(8) Time migration
Comparing the stacked seismic section plot and the formal near-trace plot at the very beginning, we can easily find that the stacked one has much stronger and clearer signal on the plot, showing more geological details. However, we could still see some diffrations on the stacked plot, which are due to steep and dip change on topography or layering. So it is time to do the migration to correct them. We use Memory F-K Migration with the smoothed velocity model (relative to the velocity model we built up for stack) to make it happen. On the seismic image after migration, we can tell the difference apparently in the dip and steep areas in the seismic section. At this point, most of our work on marine multi-channel seismic data processing are done.

(9) SEG-Y Output & Print
Pick the one with the best effect after stack and migration, and output as a standard SEG-Y file. Then use GMT (command: pssegy) to make a postscript plot of the seismic section and print it out on a big piece of paper in moderate scale with proper vertical exaggeration. Now let the scienctist to tell the geological story when they are pointing at that big paper.

Wednesday, September 1, 2010

OBS Recovery!

Hello! Remember those seismometers we sent to the bottom of the ocean a little over a month ago? Well we are currently in the process of resurrecting them from the abyss and hopefully they are pregnant with scientifically illuminating data seismic data.

The OBS recovery process is fairly straightforward, consisting of a few steps. First we must navigate to the location above the instrument. Once we are on site, the OBS guys begin their dialog with the instrument via acoustics and send the instrument the release command. The OBS is anchored to the seafloor by a steel plate that is attached by a metal cable. When the release command is sent, an electric current is sent through the metal cable and it dissolves. The process that causes the cable to dissolve is known as electrolysis, which is a chemical reaction that occurs with the assistance of electrical current. After the OBS is liberated from its seafloor anchor, it begins its 70 meter per minute accent to the surface. Some of the OBSs are being recovered from depths just over 3,000 meters. From such depths, it takes the OBS around 45 minutes to float to the surface!

Once on the surface, we must gain a visual report of it, then navigate to its location. It is sometimes quite difficult to maneuver a ship as large as the R/V Langseth around in order to collect a relatively small OBS. Once we are next to the OBS, we hook it and haul it on board using the A-frame winch. The salt water is the washed off of the OBS and it is secured for transit to the next OBS site. We have been averaging about 3 hours per OBS recovery and it will take us roughly 2.5 days to recover them all.

The OBSs are then taken back to Woods Hole Oceanographic Institute in Massachusetts, where the data is extracted from the instrument, QCd, and processed.