Oceanographic Research, Side Scan Sonar, Sub-bottom Profiling, Hydrographic Surveys, ROV's, Navigation Services

It is often necessary to find a target lost at sea or to pinpoint that object even if its location is supposedly known. A side scan sonar system (see specifications) can be a very useful tool to accomplish such a task. The system works by pulling a sonar “towfish” behind a vessel at a pre-determined altitude over the sea floor. This towfish contains a sideward/downward looking transducer on each side that transmits and receives high frequency sound waves. When various acoustical reflectors (i.e. the sea floor or a vessel’s hull sitting on that floor) reflect a transmitted pulse of sound back to the transducer, the sound is received and processed, resulting in a finished product which resembles a black and white photograph of the sea floor (called a sonograph). Depending on the size of the target one wishes to find, large areas of sea floor can be scanned in short order. Typically a 300 meter wide swath (perpendicular to the vessel track) can be covered at a tow speed of 5 knots, resulting in roughly 20 square miles of coverage in a 24 hour period. This type of coverage can be very effective in finding almost any size object, up to sunken vessels or parts thereof.

A newly developed system utilized in collecting and processing side scan sonar data is the Chesapeake Technology SonarWiz Map Real-time Data Acquisition and Mosaic Production Package. It is a full function package that accepts data from the side scan sonar system and then digitizes and processes these signals in real-time. The sonar imagery is then combined with navigational data from the onboard vessel positioning system to geo-encode all images that are displayed. This combined data set is constantly stored to large capacity optical disks allowing for complete data archiving and playback to assist in post processing of the data.

When a target is detected during the side scan sonar survey, the operator clicks on a cursor placed over the target on his sonar video screen. This procedure creates a target file which saves the target image along with a data file that describes the length, width, height and description of the target as well as its exact geographic position. As the survey progresses, a database is created containing all targets encountered in the area. This database can then easily be transferred to other personnel for further examination and/or an expedient return to the particular site for inspection, salvage, etc..

A sub-bottom profiler can also be incorporated in addition to side scan sonar to provide a two dimensional view of objects and sediments located below the sea floor. As the research vessel proceeds down the transect, a digital sub-bottom profiler could be incorporated to identify any anomalies the vessel passes over. A sub-bottom profiler operates by transmitting acoustical energy from a vehicle towed behind the research vessel and listening for its return, much like a ship’s sounder. Increased energy output and quiet detection circuitry result in a plot of what would appear as a cross-sectional area of the line directly under the towed vehicle.

A relatively new technology in sub-bottom profiling is called “swept FM chirp”, which in essence describes the transmission and reception by the profiler of a band of acoustic energy in the .5-15 kHz range, rather than the use of a single frequency source. This technology dramatically increases both profile penetration and resolution. I believe that we should be able to penetrate the maximum target depth of 12 feet at a resolution of 10 centimeters or better, which should easily allow for identification of the desired anomalies.

Through the integrated navigation system, lateral offsets corresponding to the sub-bottom “towfish” layback can be input to more accurately calculate towfish position. This corrected positioning data is then sent to the shipboard sub-bottom recorder, allowing for record annotation at given distance intervals.

Should anomalies of possible interest be detected while following the prescribed transects, they will be noted as events and recorded for further scrutiny. Once the survey grid has been completed, post-processing of the data begins. Inspection of the individual records will reveal all detected anomalies, whose position will be interpolated from record annotations to determine their precise location. These locations will then be plotted on a survey area chart as a final product of the survey.

A third device to aid in the detection of submerged metallic targets is the cesium magnetometer. Like the proceeding two devices, a sensor mounted towfish is pulled down the survey line. When the magnetic field produced by a metallic object excites this sensor, a graphic and digital display reveal the strength of that field. These numbers are directly related to the mass of the detected anomaly, therefore designating its size. Once a target is located by side scan sonar or sub-bottom profiler, this tool can prove invaluable in the classification of that target.

While the survey is being carried out, depths are obtained through the use of a survey grade precision fathometer. The device is interfaced to the integrated navigation system and depth data is recorded by this system along with vessel and towfish positions. Once the data is post processed, two and three dimensional seafloor maps can be generated to assist in mission planning when re-visiting detected anomalies with divers or ROV’s. Real-time depth displays driven by this data also enable the surveyors to properly adjust the altitudes of the various towfish as the vessel proceeds down the line.