Vrije Universiteit Brussel; Faculty of Engineering; Fundamental Electricity and Instrumentation (ELEC), co-ordinator
Van Biesen, Leo, contact
Abstract
During the last few years, an important number of research activities related to bottom sediments in a marine environment were developed. The fundamental scientific research, however, has been stimulated by new questions, which arise from the pollution of the environment and from large infrastructure construction operations, which were erected on the continental shelf and in estuaries of industrialized countries or which are still under development. The concern of the pollution of the European Seas and estuaries, together with the large civil constructions for the many European harbours are the principal agents for a more profound knowledge of the marine environment. The methodology of the investigation of the sea floor has changed drastically in the distinct disciplines involved in a marine study. This holds for sedimentology and stratigraphy in the discipline of the marine geology, but also for the biology, geochemistry, hydrography etc. The availability of data has also increased enormously. An important aspect of these investigations is that they require more or less an a priori knowledge of the sediments of the subbottom. In some cases, the sea bed could be investigated by taking samples or cores on-site, but these methods provide significantly less information than a continuous measurement. An increasingly environmentally conscious community would prefer a non-invasive method to reduce the impact on the sea bed environment and, hence, its fauna and flora. An acoustically based remote sensing method for determining the sediment properties, using equipment either on a research vessel or a near-surface tow fish, would provide such detailed information, and is likely to be less time consuming than a method requiring equipment to be deployed on the bottom. Acoustical time domain reflectometry is one of the most important investigation techniques to perform a remote sensing of the sea bed. It can be envisaged to measure the depth (echo sounding to obtain the bathymetry) of a marine system, to map the bottom morphology (side scan sonar) and to study the subbottom (subbottom profiling) or the deeper layers of the sea bottom (seismic investigations). The distinction between a seismic approach, which yields information about the geological structures of the ocean bottom, although with a limited vertical resolution, and the sub bottom profiling technique is important, because the latter becomes extremely meaningful in the study of the water sediment interface and surficial layers, which occur only up to a few meters depth and which contain, of course, also the youngest deposits. These youngest deposits are important for the environmental problems and the sustainable management. Therefore, an appropriate systematism needs to be developed for inventory purposes (mapping) and for the accurate location of the various sediment types, but also for the gathering of information of the layers thickness and stratigraphic features. This all contributes to enhance the knowledge about the sediments origin and the dislocations and movements induced by tidal currents and waves, the discharge of a river (case of estuaries, deltas), storm conditions, etc. Commercially available acoustical measurement instruments can only provide some information about the thickness of the sediment layers. The intrinsic sediment properties can not up to now be defined, unless sampling or coring is foreseen. During the last few years, an important number of research activities related to bottom sediments in a marine environment were developed. The fundamental scientific research, however, has been stimulated by new questions, which arise from the pollution of the environment and from large infrastructure construction operations, which were erected on the continental shelf and in estuaries of industrialized countries or which are still under development. The concern of the pollution of the European Seas and estuaries, together with the large civil constructions for the many European harbours are the principal agents for a more profound knowledge of the marine environment. The methodology of the investigation of the sea floor has changed drastically in the distinct disciplines involved in a marine study. This holds for sedimentology and stratigraphy in the discipline of the marine geology, but also for the biology, geochemistry, hydrography etc. The availability of data has also increased enormously. An important aspect of these investigations is that they require more or less an a priori knowledge of the sediments of the subbottom. In some cases, the sea bed could be investigated by taking samples or cores on-site, but these methods provide significantly less information than a continuous measurement. An increasingly environmentally conscious community would prefer a non-invasive method to reduce the impact on the sea bed environment and, hence, its fauna and flora. An acoustically based remote sensing method for determining the sediment properties, using equipment either on a research vessel or a near-surface tow fish, would provide such detailed information, and is likely to be less time consuming than a method requiring equipment to be deployed on the bottom. Acoustical time domain reflectometry is one of the most important investigation techniques to perform a remote sensing of the sea bed. It can be envisaged to measure the depth (echo sounding to obtain the bathymetry) of a marine system, to map the bottom morphology (side scan sonar) and to study the subbottom (subbottom profiling) or the deeper layers of the sea bottom (seismic investigations). The distinction between a seismic approach, which yields information about the geological structures of the ocean bottom, although with a limited vertical resolution, and the sub bottom profiling technique is important, because the latter becomes extremely meaningful in the study of the water sediment interface and surficial layers, which occur only up to a few meters depth and which contain, of course, also the youngest deposits. These youngest deposits are important for the environmental problems and the sustainable management. Therefore, an appropriate systematism needs to be developed for inventory purposes (mapping) and for the accurate location of the various sediment types, but also for the gathering of information of the layers thickness and stratigraphic features. This all contributes to enhance the knowledge about the sediments origin and the dislocations and movements induced by tidal currents and waves, the discharge of a river (case of estuaries, deltas), storm conditions, etc. Commercially available acoustical measurement instruments can only provide some information about the thickness of the sediment layers. The intrinsic sediment properties can not up to now be defined, unless sampling or coring is foreseen. However, parameters such as the sand-silt-clay ratios, density and porosity are of the greatest importance in correlating results with other research disciplines and for practical marine engineering. Some attempts (e.g. by former MAST-projects) were made to propose techniques to classify the sea bed by acoustic means. These attempts, however, are based on empirical c
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