Deep seabed rock dredging in areas with high relief

Abstract

Deep‐seabed dredging has been for more than 125 years and continues today to be the primary tool for sampling deep‐seabed, hard‐substrate environments. Much effort and innovation have been invested in this pursuit to accommodate the varying seabed types and the objectives of the investigators. Little progress has been made, however, during this period to improve the accuracy of locating the samples that are dredged and the efficiency of the technique. With the current availability of high‐resolution, Global Positioning System (GPS) navigation and accurate bathymetric maps in many deep‐seabed areas, considerable improvement in both accuracy and efficiency is possible. The purposes of this study 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 Johnston Island in the north‐central Pacific Ocean, and also to test two different dredge designs for their performance characteristics. One type was fabricated for the University of Hawaii Marine Minerals Technology Center (MMTC) using specifications provided by the U.S. Geological Survey (Menlo Park, CA). The other type was modified from this proven design by the engineers and scientists of the Bureau of Mines (USBM) Twin Cities Research Center. The basic modifications include the addition 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, averaging 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 Survey; it is not possible, however, to determine whether the superior efficiency reflected in these numbers is a function of the dredge design or because of innate differences between the Karin Seamount site and the Keli Ridge site. For dredging operations similar to those described here, the availability of good bathymétrie data and GPS navigation make it possible to determine actual dredge positions with unprecedented accuracy. The inferred dredge positions appear to be accurate to within about 200 m in water depths of 1,500 m.