DEEP SEABED ROCK DREDGING IN AREAS WITH HIGH RELIEF

Deep-seabed dredging has been for more than 125 years and continues to day to be the primary tool for sampling deep-seabed, hard-substrate en vironments. Much effort and innovation have been invested in this purs uit to accommodate the varying seabed types and the objectives of the investigators. Little progress has been made, however, during this per iod to improve the accuracy of locating the samples that are dredged a nd the efficiency of the technique. With the current availability of h igh-resolution, Global Positioning System (GPS) navigation and accurat e bathymetric maps in many deep-seabed areas, considerable improvement in both accuracy and efficiency is possible. The purposes of this stu dy are to examine the techniques used and results obtained using these relatively new tools during a recent expedition that dredged several seamounts at water depths of 1,300 to 2,500 m in the vicinity of Johns ton Island in the north-central Pacific Ocean, and also to test two di fferent dredge designs for their performance characteristics. One type was fabricated for the University of Hawaii Marine Minerals Technolog y Center (MMTC) using specifications provided by the U.S. Geological S urvey (Menlo Park, CA). The other type was modified from this proven d esign by the engineers and scientists of the Bureau of Mines (USBM) Tw in Cities Research Center. The basic modifications include the additio n of drag-bit teeth to the collection ring and the addition of weights across the center of the ring. Overall, the MMTC dredges returned 545 kg in seven successful sampler recoveries, averaging 78 kg per haul. The USBM design returned 815 kg in five successful recoveries, averagi ng 163 kg per haul. According to the ship's scientific crew, the USBM dredge performed better than the one designed by the U.S. Geological S urvey; it is not possible, however to determine whether the superior e fficiency reflected in these numbers is a function of the dredge desig n or because of innate differences between the Karin Seamount site and the Keli Ridge site. For dredging operations similar to those describ ed here, the availability of good bathymetric data and GPS navigation make it possible to determine actual dredge positions with unprecedent ed accuracy. The inferred dredge positions appear to be accurate to wi thin about 200 m in water depths of 1,500 m.