In April of 2011, FFI led a sea trial near Larvik, Norway on FFIs research vessel the H.U. Sverdrup II with participation by representatives from Canada, United States, and France. One objective of the sea trial was to acquire a data set suitable for examining incoherent and coherent change detection and automated target recognition (ATR) algorithms applied to Synthetic Aperture Sonar (SAS) imagery. The end goal is to produce an automated tool for detecting recently placed objects on the seafloor. To test these algorithms two areas were chosen, one with a comparatively benign seafloor and one with a boulder strewn complex seafloor. Each area was surveyed before and after deployment of objects. The survey time intervals varied from two days to eight days. In this paper we present the trial and show examples of SAS images and change detection of the images.
When resurveying a seafloor area of interest during change detection operations, an automated method to match found bottom objects with objects detected in a previous survey allows the surveyor to quickly sort new objects from old. The change detection system developed at the Naval Research Laboratory contains modules for automatic object detection, feature matching using shadow outlining, scene matching using control-point matching, and visualization capabilities. This system was developed for sidescan sonar surveys using instrumentation such as the high-frequency Marine Sonic Technology sidescan sonar. In this paper, the authors describe modifications to the sidescan-based system required to perform change detection using Synthetic Aperture Sonar (SAS) bottom imagery.
Historic oceanographic sound speed profiles have traditionally been grouped by area and time period, usually one degree square area and monthly time. After grading the profiles, mean profiles and standard deviations are calculated from the accepted profiles and in the acoustics community they are then used to predict the expected acoustic response of the region. Here the historic profiles in NOAA's World Ocean Database 2005 (WOD2005) will be divided into the same area and time periods, but in subsets with a sufficient number of profiles, fuzzy clustering will be employed on acoustically relevant oceanographic parameters (mixed layer depth, surface temperature, sound speed gradient, etc) to divide the population into multiple clusters. A parabolic equation acoustic transmission model is then applied on the WOD2005 statistical profiles and on the fuzzy cluster populations. Conclusions will be drawn about the suitability of this clustering to capture the variability of acoustic response at a given time and place.
A large shoal of fish occurring in the vicinity of the 220-m isobath was observed using a standard 38-kHz fisheries echosounder and a 1.5- to 10-kHz low-frequency fish sonar (LFFS) for several days in July 2000. The fish behaved like herring, exhibiting a rapid rise to the sea surface at dawn, formation into schools, and a rapid descent to the sea floor. Schools remained at depth through the day and gradually rose to the sea surface at dusk and then rapidly descended and spread out into a diffuse scattering layer at 75- to 125-m depth. Shifts in resonance frequencies during migration, release of gas bubbles during migration, and strong avoidance of the vessel when maneuvering, all suggest the fish were most likely round herring, Etrumeus teres, which are common at these depths in the NMFS historical trawl survey data. An examination is made of some of the scattering characteristics of the schools and layers of these fish and comparisons of the 38-kHz data to scattering at 500-Hz bands from 1.5 to 5 kHz and 1-kHz bands from 5 to 10 kHz. [Work supported by ONR.]
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