sea90E - an oceanographic expedition to the Indian Ocean



	Dredging Operations onboard the R/V Revelle by Amy Eisin We are using a technique called dredging in order to collect rock samples from the seafloor at sites along the Ninetyeast Ridge that we think are interesting. The dredge onboard the R/V Revelle is called a basket dredge because it is essentially a large “basket” consisting of a net of metal chains suspended from a metal frame. The dredge is large enough to hold a person, so it is capable of collecting a large volume of rock samples, over 1000 pounds of rock! The dredge is suspended from a strong metal cable that runs from the ship to the A-frame, over a pulley and out into the water. The weight of the empty dredge is over 400 pounds. Here, Will gets ready to attach the wire cable to the chain harness that suspends the dredge. Once the pinger is attached, the winch operator lets out enough cable for the dredge to be 200m (656 ft) from the seafloor. Then, control of the winch is handed over to the Resident Technician (“ResTech”) in the computer lab where the cable is let out via remote control until the dredge reaches the seafloor. The captain then drives the ship slowly to drag the dredge along the seafloor as rocks get gathered into the basket. In the picture below, Ron, our dredge expert, explains the dredge depths to Masako, the lead geophysicist. Typically, we drag the dredge for 2 miles along the seafloor, while the tension and depth are continuously monitored. Based on the tension of the dredge we can estimate how many pounds of rocks we may have gathered. This process is key to successfully collecting rock samples. During several dredges, the basket became caught on some rocks and only by carefully working with the cable length and ship position were we able to free the basket again. Once, it overturned on the ocean floor and the chain wrapped around the teeth of the bucket. Sometimes, it does happen! (but not often) After we have traveled far enough, the ResTech uses the remote control to pull the dredge off the seafloor. The depth of one of our dredges was 4124 m (13,530 ft), which is probably our deepest dredge ever! When the dredge is 200m above the seafloor, control is handed back over to the winch operator who pulls the dredge all the way up and back on the deck of the ship. After the dredge basket is secured on deck, the rocks we collected are taken out of the basket and sorted by type. They are then weighed, washed, and described, and some even get sawed in half so we can see what they look like on the inside. We have already done 24 dredges and have more to go! Some dredging operations are successful with lots of rocks...and some are not (sometimes the basket comes back empty). Nevertheless, each rock is considered a treasure from the seafloor that can teach us many new things about the Ninetyeast Ridge. updated July 21, 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