Design and implementation of a programmable, wideband sonar for sediment mapping

Abstract: To be able to explore different signal types and processing techniques, apt for sediment mapping, programmable, fully digital profiling systems has been built and tested. The systema are designed to be deployed also at small water depths from small boats, without special handling equipment. The usable frequency band is 100-25000 Hz, limited to approximately 2-10 kHz depending on the projector used. Measurements have been made with the sonar programmed to cyclically repeat a sequence of signals, producing profiles that are (virtually) made in one and same track, but with different signals. Properties of the reflex from the seabed are then extracted from the different profiles, combined and compared. In particular a measure of the frequency content in the reflex, the centroid, is evaluated. The centroid is the mass center of a power line spectrum. Also the energy content in the bottom reflex is used, as well as other features. The first chapters are a definition of the historical and engineering framework in which the rest of the work is made. Some references to basic work in the field are given throughout the text. The physical background is briefly outlined, including a description of the more basic components and concepts in sonar technology. Digital signals and the shaping of a signal, in order to assign to it certain properties is briefly covered. One chapter is dedicated to the concepts of positioning and navigation. The final sections deal with data analysis, with emphasis on a description of the methods used and their implementation. The main goal is to produce a map of the bottom from the measured profiles. Mapping procedures and examples are shown, that finally can be used to produce a three dimensional computer model of the investigated area. It is followed by a description of some field measurements and a short discussion. The examples are one traditional, profiling, measurement from Kalmarsund, and two mapping examples from other sites. Here a qualitative measure, spatial coherence is used to describe the maps, meaning that areas that after the mapping process are interpreted as one area with same property, are considered to be spatially coherent. In one of the investigated sites, lake Möckeln, a glaciofluvial esker crosses a level area with mud and gyttja. Here, areas with spatial coherence show up in maps of the spectral center of gravity (centroid), and in maps of the energy content. The areas are different for different frequencies in the case of centroid maps. Another area, Skillinge, possibly with a more monotonous material distribution, shows small spatial coherence.