Ground-Truth
Scientists have recognized the importance of directly sampling areas they are remotely sensing from the very early days of ocean explorations. The objectives of such ground-truth efforts are to obtain accurate samples or images of the seabed with minimal disturbance of the environment. The first soundings taken from small boats often acquired sediment samples from the seabed, using a blob of wax on the sounding tool to which a bit of the sediment would adhere when the sounding device touched bottom. On the HMS Challenger Expedition, the sounding devices were used to take samples of the bottom deposits at 362 stations over 140 million sq. km. of ocean floor (Bailey, 1971, p.21). Dragging dredges across the sea floor was also used to collect samples. These dredges were bags of metal netting with weights and bottom-swabs attached to the base of the bags. (after Rehbock,ed, p. 29).
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| Dredge and pulley device used aboard the H.M.S. Challenger. | The steam pinnacle, a smaller boat specially adapted for dredging in harbors and shallow water. It was equipped with a small engine & for hauling in dredging and sounding lines. The bow had a dredging platform. The dredge rope being coiled away on the bottom in the after part. | Sifting deposits aboard the H.M.S. Challenger, a first step in sample analysis. |
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| Figures above are from the H.M.S. Challenger Results Narrative, Part I (the dredge, p. 74; the steam pinnacle, p. 4; and the sifting of deposits, p. 191). Courtesy of The Royal Society. | ||
As ground-truth systems evolved from the Challenger era, samplers, corers, and dredges have generally maintained fairly simple principles, but are coupled with sophisticated mechanisms designed with lack of disturbance of the sea floor during sampling, ease and repeatability of deployments/retrievals, and security of the sample during retrieval as objectives. Currently, sediment samples are obtained by samplers that gather deposits from the top few centimeters and by corers that penetrate the sea floor for centimeters to meters and extract the material for analysis. Samplers take on many shapes and sizes. One, the simple clamshell grab sampler, has been used for decades; it is reliable, simple to use, and low cost to operate. It literally grabs the top few centimeters of soft sediment. The Van Veen grab sampler, which is part of the WHSC sampling and photography system, SEABOSS (Seabed Observation and Sampling System; please click on link in the tree to the right), scoops up to 20 centimeters deep within a sampling area of 0.1 sq. meters, depending on the sediment type. There are limitations to a Van Veen -type sampler, as it disturbs and mixes the sample, and at times washes away the uppermost layer of sediment.
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| Van Veen sediment grab sampler. |
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Samplers take on many shapes and sizes. One, the simple clamshell grab sampler, has been used for decades; it is reliable, simple to use, and low cost to operate. It literally grabs the top few centimeters of soft sediment. The Van Veen grab sampler, which is part of the WHSC sampling and photography system, SEABOSS (SEABed Observation and Sampling System; please click on link in the tree to the right), scoops up to 20 centimeters deep within a sampling area of 0.1 sq. meters, depending on the sediment type. There are limitations to a Van Veen -type sampler, as it disturbs and mixes the sample, and at times washes away the uppermost layer of sediment.
Coring is another method by which to obtain samples. An empty core barrel, or tube, penetrates the sea floor, and fills the tube with sediment. Although there are many types of coring devices; the Woods Hole Science Center maintains gravity corers and push corers. Dampened gravity corers are mounted on frames (Please navigate to the Samplers and Corers page in the tree to the right). Also, divers can manually operate push cores to sample shallow layers.
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| Diver holds a filled push-core in a setting with significant suspended sediment in the water column. | Diver pushing a corer into the sea floor. Orange caps trap the sample within corer. | Please click on the image above for a short video of a diver collecting a push-core. |
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Samples are analyzed for grain size class (e.g. silty sand), percent content of mud, clay, silt, sand, and gravel, and composition. Additional analyses may be performed, dependent on the objectives of the study.
Photography is another means by which the remote environment can be described. Light energy, emitted by the photographic or an accompanying device, propagates through the water column and is reflected back by the various targets on the sea floor. Underwater photography was perplexing for a long period, as light energy easily dissipates and distorts in sea water. During the WW1I period, the early deep-sea camera, developed by Maurice Ewing, meant sea floor photography became a viable tool for marine science. (Ewing, Worzel, and Vine, 1967, p. 13).
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| Suite of early deep-sea photography equipment and actual photographs from the sea floor. Top row from left: Components of the first deep-sea camera made by Ewing, Worzel, and Vine at Lehigh University in 1939; a 16-mm camera that could take single shots or motion pictures was positioned behind a glass window in the pressure vessel powered by a 12-volt battery; camera 2-B being assembled by Vine, Wilson, and Ewing including the new additions of a pendulum, compass, and bottom sampler; one of the light sources designed from pressure-proof lamp socket for flashbulbs, made from an automobile carburetor sediment bulb lapped onto an aluminum base. Bottom row from left: the first bottom photograph taken in the deep sea, this print was hand-dodged to equalize the 15-sec exposure from the R/V Atlantis cruise 94, March 1940, Station C-13; image from R/V Atlantis cruise 100, Stations C-46-2 and C-48. Note dramatic improvement in image resolution and quality over a three-month period of development. Hersey, John Brackett, Deep Sea Photography, pp. 14, Fig. 1-1b; 17, Fig. 1-4b; 19, Fig. 1-5c; 20, Fig. 1-6a-c. c 1968. Reproduced with permission of The Johns Hopkins University Press. | ||
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Innovations continued at a rapid rate. The need to maximize the variety and amount of data collected with each deployment was paramount, so systems added more components. The U. S. Navy Electronics Laboratory (NEL) developed a compact system that collected stereoscopic bottom photographs, sediment and water samplers, and a current meter (Shipek, 1967, p. 89).
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| New deep-sea oceanographic system developed by the Navy Electronics Lab to obtain oriented images of the micro-roughness of the sea floor, circa 1960. Hersey, John Brackett, Deep Sea Photography, p. 90, Fig. 7-1c; 1968. Reproduced with permission of The Johns Hopkins University Press. |
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Details of a deep-sea stereoscopic camera developed in 1960 are depicted below. This camera could be used for close-up work, about 28 cm from the sea floor. The stereo pairs of photographs obtained provided a depth-perspective of the sea floor, used additionally to complement bathymetric contour studies of certain areas (Owen, 1967, p.95-96). These concepts of maximizing deployments and accuracy of the data, both with regard to resolution, location, and ambient physical conditions, have continued to spur developments, such as the SEABOSS systems described below.
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| Combination photo-drawing depicting camera assembly at moment of exposure on the ocean bottom. All components were enclosed in the frame, except the tripping device, 1960. Hersey, John Brackett, Deep Sea Photography, p. 98, Fig. 8-4. Reproduced with permission of The Johns Hopkins University Press. |
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| Close-up view of a video camera on SEABOSS |
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In present-day investigations, digital devices are used to obtain high definition images. The WHSC uses both photographic and video devices to image the seabed. Please visit http://woodshole.er.usgs.gov/operations/ia/photographs.html to view a collection of images that depict the sea floor used in our ground-truth efforts. The WHSC developed the SEABOSS to combine ground-truth methods. It has two video cameras, a still camera, a pressure-depth sensor, a modified Van Veen sediment grab sampler, and light sources for the video and still cameras. The SEABOSS is powered through a cable from the host ship.
The SEABOSS is used in a variety of environments and water depths, allowing real-time selection of sampling locations, by a scientist who monitors the seabed from a ship-based video monitor. A second-generation SEABOSS is undergoing sea trials at present. Additionally, a Mini SEABOSS was developed for more restricted, small-boat, shallow survey environments.
References
Bailey, Herbert S., 1971, The Voyage of the Challenger, in J. Robert Moore, ed., Oceanography, W.H. Freeman and Company, San Francisco, p.
Ewing, M., J.L. Worzel, and A.C. Vine, 1967, Early development of ocean-bottom photography at Woods Hole Oceanographic Institution and Lamont Geological Observatory, p. 13-41, in J.B. Hersey, ed., Deep-Sea Photography, The Johns Hopkins Press: Baltimore, 1967, 310 pp.
Owen, David M., 1967, A multi-shot stereoscopic camera for close-up ocean-bottom photography, p. 89-94, in J.B. Hersey, ed., Deep-Sea Photography, The Johns Hopkins Press: Baltimore, 1967, 310 pp.
Rehbock, P.F., 1993, At Sea With the Scientifics: The Challenger Letters of Joseph Matkin, University of Hawaii Press, 415 pp.
Shipek, C.J., 1967, Deep-sea photography in support of underwater acoustic research, p. 95-105, in J.B. Hersey, ed., Deep-Sea Photography, The Johns Hopkins Press: Baltimore, 1967, 310 pp.
Thomson, Sir C. Wyville and John Murray, Report on the Scientific Results of the Voyage of the H.M.S. Challenger During the Years 1873-76, Narrative, vol. I, First Part, 1885, London:H.M.S.O.
