Fine Scale Mapping Near the Deep Sea Floor

Over a period of fifteen years our group in the Marine Physical Laboratory has developed a system for mapping various quantities associated with the deep sea floor, and has used it extensively in both the Pacific and Atlantic Oceans. Although some elements of the system have been in successful use for many years, new capabilities have been added in more recent time. The basic concept is to use instruments towed close to the sea floor, thus allowing fine spatial resolution of the parameters, (or in some instances giving the possibility of measuring aspects which could not even be sensed from the sea surface). Position determination is a key element. This is achieved by use of acoustic transponders and related computational procedures which can yield accuracy of the order of a few meters. The present instrument suite includes routine use of precision echo sounder, 4 kHz bottom penetration sounder, side looking sonar, proton magnetometer, photography, television, and temperature instrumentation. In use on an intermittent basis are newer capabilities including conductivity measurement, near bottom plankton sampling, filter system to sample suspended fine particulate material, controlled water sampling, and an instrument to measure the optical properties of the water. All of the subsystems are powered and controlled from the ship, using the coaxial electrical core to the towing cable. The signals from the various sensors (hydrophones, magnetometer, thermometer, television, etc.) are telemetered up the wire using various methods geared to the requirements of the particular unit. A major recent advance has been the move toward quantitative use of the sonar systems. The 4kHz unit produces actual acoustic reflectivity information, giving the added capability in some circumstances of calculating sound absorption as well. Most recently the side-looking sonar has been adapted to produce quantitative measurements of backscattered energy.