Project record of Sediment Identification for Geotechnics by Marine Acoustics

Information

Type

Project

Name

Sediment Identification for Geotechnics by Marine Acoustics

Acronym

SIGMA

Description @en

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

Description @nl

De laatste jaren hebben zich een groot aantal onderzoeksactiviteiten ontwikkeld die betrekking hebben tot sedimentlagen in mariene milieus. Fundamenteel onderzoek werd gestimuleerd door nieuwe problemen die voorkomen uit de vervuiling van het leefmilieu evenals van grote infrastuctuur werkzaamheden die zich voltrekken op het continentaal plat of in de rivierbekken van geindustrializeerde landen. De bezorgdheid voor de vervuiling van de Europese wateren samen met de groter bouwkundige kunstwerken in vele Europese havens zijn de voornaamste drijfveren voor een dieper inzicht en kennis van het marien milieu. De onderzoeksmethodologie voor het bestuderen van de zeebodem is in de verschillende displines die zich hiermee bezig houden sterk veranderd. Dit geldt zowel voor de sedimentologie en de stratigrafie in de mariene geologie, maar evenzeer voor mariene biologie, geochemie, hydrografie, enz. De beschikbaarheid van gegevens is ook enorm toegenomen. Een belangrijk onderdeel van het zeebodem onderzoek is dat de methodes meer a priori kennis veronderstellen van de sedimenten aanwezig op de zeebodem. In bepaalde gevallen kan de zeebodem bestudeerd worden door gebruik te maken van staalnames of door bemonstering aan de hand van boorkernen. Deze methodes leveren evenals beduidend minder informatie op dan een continue metingen. De gemeenschap, die zich meer en meer bekommert om het welzijn van het milieu, verkiest niet invasieve methodes, die de impact op de zeebodem verminderen en aldus de fauna en flora niet verstoren. Een remote sensing methode gebazeerd op akoestische reflectometrie geplaatst op een onderzoeksschip of gesleept in een meetvis in de waterkolom boven de zeebodem zou zulke continue informatie kunnen opleveren. Verwacht wordt tevens dat deze meetprocedure minder tijd in beslag zal nemen dan het geval zal zijn voor uitrustingen die op de zeebodem neergeplaatst worden. Akoestische reflectometrie vormt een van de meest belangrijke beproevingstechnieken om een remote sensing van de zeebodem uit te voeren. Het kan aangewend worden om de diepte te meten van de zeevloer (bathymetrie via echolood), voor het in kaart brengen van de morfologie van de zeebodem (side scan sonar) en om de subbodem te bestuderen (seismisch of subbodem profielonderzoek). Het verschil tussen een seismische aanpak, die informatie oplevert aangaande de geologische struktuur van een oceaanbodem evenwel met een beperkte vertikale resolutie, en de subbodem profielmethode is belangrijk omdat de laatste heel erg geschikt is om de water-sediment interface en de eerste hieronder gelegen meters afzettingen, en dus de jongste sedimenten, te karakterizeren. De jongste afzettingen zijn van promordiaal belang voor milieuproblematiek en een duurzame ontwikkeling van de zeebodem. Het doel van het SIGMA project bestaat erin om de relaties tussen akoestische parameters van sedimenten (reflectiefactor, geluidssnelheid, demping, dispersie enz.) en de geotechnische parameters (sediment types, korrelgroottes, verdelingsfuncties, cohesie, gashoeveelheden, enz.) in functionele vorm af te leiden. De akoestische parameters van de sedimenten zullen geschat worden steunend op breedband metingen afkomstig van een stuurbare parametrische array die gecombineerd wordt met een gesleepte array ontvangers. Zulk een bron laat toe om zeer fijne spotmetingen van de zeebodem te verrichten, die verwerkt worden om de akoestische eigenschappen met hoge nauwkeurigheid hieruit af te leiden. Dit doel zal bereikt worden steunend op een multi-disciplinaire aanpak: akoestische modelering, ontwikkeling van inverse procedures, calibraties, opwekking van optimale testsignalen, systeem identificatie, validaties aan de hand van tankexperimenten, ontwikkeling van specifieke instrumentatie, campagnes op zee en

Start date

1997-9

End date

2000-8