A towed multi-channel FM acoustic reflection profiler has been developed for locating and generating images of buried objects. One significant application of this sonar is buried pipeline surveying. The multi-channel reflection profiler uses 16 line arrays mounted in a towed vehicle to determined the position and burial depth of an 18 inch steel pipe filled with concrete buried under 1.5 meters of sand. This sonar will allow a survey vessel to continuously track a buried pipeline providing a continuous record of pipe burial depth and position.
Locating buried objects under the seafloor is a difficult problem because sediments have surface and volume scattering coefficients that are three orders of magnitude greater than that for water and, consequently, generate acoustic scattering noise that is usually greater than the reflection amplitudes of buried targets. Single channel profilers are practically unusable for finding single objects in sandy seabeds because they have to be towed within a couple of meters of the seabed, can only effectively cover a narrow swath (e.g., less than 2 meters), andprovide little information about object shape and size. Recent experiments using a single channel FM profiler for finding ordnance buried less than a meter in sand show that the beamwidth must be less than 30 degrees and the vehicle tow height must be about a meter above the seafloor in order to reliably detect the ordnance shells.
Buried pipeline surveying is not as difficult as finding buried objects that have a finite length, because towed vehicle positioning is not as critical; the towed sonar vehicle will always pass directly over the pipeline during a crossing. Usually a sonar with a wide along-track beamwidth and a narrow across- track beamwidth is used to find buried pipelines. Survey vessels usually measure the burial depth of pipelines by performing 90 degree crossings of the pipe at selected locations along the pipeline. As the sonar approaches the pipe during each crossing, the two way time travel of the pipeline echo decreases until the sonar is directly over the pipeline. Then the time travel increases as the slant range from the sonar to the pipeline increases. On a paper chart the pipeline echo forms a hyperbola which has a peak at the location where the profiler was directly above the pipeline. Because the crossings must be approximately perpendicular to the pipeline to keep the normal incidence component of the pipeline in the main lobe of the sonar, a pipeline survey can be costly in terms of time and can not provide a continuous record of pipeline burial depth.
Some sediments which have high scattering coefficients due to gas bubbles, shell or other in homogeneities prevent the use of conventional pipeline tracking sonars. In these environments, EM techniques are commonly used to measure pipeline burial depth under the seabed.
