The philosophy and technique of ocean bottom imaging and map making is changing as new undersea remote sensing technology provides more versatile equipment with variable resolution. Integrated towed systems can now simultaneously provide quantified acoustic backscatter images, swath bathymetry and multi-spectral echo soundings from a towed vehicle over a wide range of altitude, swath width, and water depth. This data base allows geological facies mapping and hazard evaluation to be made with a single system and a minimum of ship time in the survey area. System resolution is no longer simply maximized; it is care full selected and traded off with survey data on a mission by mission basis to optimize survey efficiency and minimize cost of the off shore operation.
Graphs are presented which show how mapping system resolution varies as a function of system bandwidth, tow speed, and the geometric relationship between the tow is hand the swath of bottom to be imaged and contoured. This information is necessary for designing survey strategy and selecting the optimum mapping system for the job.
The Hawaii Institute of Geophysics and Lamont-Doherty Geological Observatory have been using Sea MARC instrumentation systems successfully for several years. Both systems have worked in water depths from 50 to 5000 meters and together have covered over 100,000 square kilometers of ocean bottom with cruises funded by government agencies, industrial firms and foreign governments.
Variables with multi-sensor remote characterization of the ocean bottom is a cost effective way to increase our understanding of geologic processes. A tremendous increase in information gathered for a given amount of ship?s time can be realized with this new technology, both for scientific and commercial purpose.
Ever since mankind emerged from the sea, learned to walk on two legs and how to swim again, trying to 'see' the bottom of the ocean has been one of our most frustrating activities. Furthermore, we have been spoiled by the efficiency of our human visual system in air; not only does it have angular resolution better than one minute of arc but it is intimately connected to the most sophisticated image processing and tracking system we know of our brain. Beneath the sea surface the propagation medium is not so accommodating. With the exception of a 'window' in the green light region of the spectrum, seawater is essentially opaque to electromagnetic energy, and most of the sophisticate imaging equipment (either optical or radar) which has been developed for use from aircraft can not see more than a few meters beneath the sea even in clear water. All submarine optical systems, whether based on conventional cameras or laser scanners with range gated receivers, are also fatally dependent on water clarity. Since only acoustic energy propagates relatively well through water, sonar based imaging systems are used to extend our vision underwater in the way that optics thermal and radar imaging systems have been used to extend our vision on land and in space.
