sea90E - an oceanographic expedition to the Indian Ocean



	Anatomy of a Seamount Survey Figure 1 shows the map we made of the seamount at site NER2. Before our survey the predicted bathymetry from satellite measurements did not show much, just a somewhat teardrop shaped seamount with the pointy end on the east. Our multibeam sonar bathymetry maps shows lots of detail. Those lines all over the plot are the ship tracks. Looks like a plate of spaghetti, right? So were we just wandering around? Was somebody drunk behind the wheel? (definitely not – this is a dry ship!) So what’s that all about anyway? The next few maps break it down for you. In the first map, (above) we see that the Revelle came down from the north. We wanted to do a seismic profiling survey first, because that shows the sediments and tells us where to find outcropping rock for dredging. At point 1, we pointed the bow into the seas (southeast) to give us the best course for deploying the seismic air guns and streamer. By the end of that bit of southeast track, all the gear was in the water. Whoa! But soon after, we discovered that neither airgun was operating properly. What to do? Time for a quick plan B. Lee Ellett, our seismic technician, said that the airguns needed to be taken apart and this would take 12 hours or so. We decided to loop the seamount, collecting multibeam sonar bathymetry data to look for good dredge sites for later. While Lee, Brandi, and Meghan stayed up all night taking the air guns apart and refitting the parts and o-rings, we sailed around the seamount, ending up at point 2. It was the next morning and the techs had the guns ready to go. Time for seismic survey, take 2. (Figure 2) This time the air guns worked OK, so we went on with the seismic survey. First we made a north-south pass over the middle. Wow! Look at that big volcanic cone at the north end of the line. We were lucky and passed right over the middle. And it has what appears to be a 1 km steep, bare slope on the north side. We need to dredge there! Near the south end of the first line, we turned west to go over the western boundary and to set up some east-west lines. At point three we began a long, west-east line that stretches over to point 4. That line goes way out to the east because we wanted to find out why the volcano is asymmetric. At point 4, we had to turn back, so we steamed south a few miles and headed back to the seamount summit. Looking at the initial seismic lines, we picked a few points where we wanted to make crossing lines. When the seismic lines cross at nearly perpendicular angles, we can do a better job of recognizing the three dimensional structure. At point 5, we went north to cross a place where the sediment section was thick. This also crosses the low spot on the east side. The line between points 5 and 6 crosses the basement (igneous rock) high on the north. At point 7, we turned south to cross both the valley incising the west side of the summit and the basement high on the southwest side. At point 8 we looped back to the north to cross the valley complex in the middle and go across the north high again; points 9 to 10. At point 10 we put the finishing touches on the seismic survey with another north-south line to intersect the other lines and finish by imaging the south side of the volcano. At point 11, we headed up into the seas again so the techs could bring in the seismic gear. With the seismic gear all put away, the geologists have been itching to dredge for two whole days. Now it’s time for action. Up until now, all of the dredges have been done with the stern of the ship facing the wind because Captain Desjardins does not know whether he can keep the bow of the ship pointed into the wind without a bow thruster. The winds have been blowing at 20 knots or more and under those conditions, the Revelle is just a big blue and white kite. If you have ever paddled a canoe solo, sitting in the rear of the canoe, and tried to paddle into a stiff wind, then you know the problem the captain faces. It is very difficult to keep the canoe (or the Revelle) pointed into the wind when you are pushing from the rear. Before dredging, the captain conducts a test to see how well the ship will hold position into the wind. The Revelle hangs tight and we decide to give into-the-wind dredging a try. We needed to do into-the-wind dredging because the best exposures are on the north and west side of the seamount and moving up slope would take us into the wind. Our first attempt is at the big volcanic cone on the north, but first we have to get there. We steam west to collect multibeam bathymetry data over the trough at the south side of the volcano and over a part of the southern flank that we did not image before. At point 13, we turn north to run a bathymetry swath through a gap in the previous surveying, up to point 14. And at point 15, we stop to dredge. We do two dredges on the north side of the big volcanic cone, but both come up empty. Drat! We decide that we might have better luck over the escarpment at the base of the seamount’s west side, so we move down to point 16 for another dredge. This one comes back with rocks! Now it’s time to move on and we want to connect this seamount with a ridge-like seamount to the south using seismic profiling. We steam to point 17 to deploy the seismic gear – pointed into the wind again – and that is done by point 18. From there we sail south toward NER3, air guns firing, collecting multibeam bathymetry and seismic profiles as well as all of the other geophysical data. updated July 12, 2007 from the Indian Ocean
			High.Seas.HIGHTECH/ analyze!ocean@voyages, by lots of us... How to Become a Captain, by the Captain The Search for the Indo-Pacific Humpback Dolphin, by Leslie Nemazi. Rocks Rock! (or ) The Fresh Cut Surface of Beautiful Ocean Rock, by Evelyn Mervine Exploring the Bridge of the RV Roger Revelle Dredging Operations onboard the R/V Revelle, by Amy Eisin Geochemistry of Volcanic Rocks, by Fred Frey Cool Expedition Discoveries Already!, by Will Sager Anatomy of a Seamount Survey, by Will Sager M agnetometers (“Maggie”) and Attracting Sharks, by Rory Wilson Acoustics: Substitute for Superman Vision?, by Will Sager What is Bathymetry?, by Will Sager Hotspots and Ninetyeast Ridge: What’s a hotspot?, by Will Sager