Sedimentkarakterisering genom dubbelfrekvensekolodning.
Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Sonar is a technique used in hydrography to map and visualize the depth and bottom topography in marine areas. The technique is based on the differences in the different material's ability to propagate pressure waves, which means that reflections occur in the transition between different media, such as between water and sediment. The sonar converts electrical energy into acoustic energy and a pressure wave spreads in water until the wavefront hits a medium whose acoustic impedance differs from that of water. At the interface, a part of the wave is reflected back to the water surface, some to continue in the new medium and some will spread uncontrollably due to imperfections along the bottom. The amount of energy reflected back to the water surface is dependent on differences in acoustic impedance between water and sediment, the bottom roughness and the sonar frequency. The purpose of this study is to immerse in sonar technology as well as study how the sonar signal behaves at the interface between water and different bottom types and sediments. Measurements were made with dual frequency sonar at five different objects in northern Sweden with varying bottom types and sediment sampling was done in parallel with it. The sediments were analyzed on factors that were considered to affect the sonar response, such as grain size distribution, density, pressure wave velocity and water content. The goal was to determine whether these geotechnical and geophysical parameters could be related to a particular type of response in the echogram for sonar measurements of the same point. Next to this, studies were made to establish whether dual frequency sonar is a appropriate technique to determine the thickness of different types of sediments by relating the sonar responses from deeper down in the sediment profile to the measurements of sediment thickness made with steel rods. The study showed characteristic features in the echogram, which could be related to the acoustic impedance and thus the strength of the reflected sonar signal for the current bottom and sediment type. In addition, the study indicates that the acoustic impedance correlates well with the dry content, water content and density of the current sediment. Comparisons between the echogram and measurements made with steel rods in the same point also shows that the dual frequency sonar can be used to, with reasonable precision, identify the thickness of mainly organogenic sediments which occur in many lakes in the Swedish pine forest landscape.
Place, publisher, year, edition, pages
2014. , 41 p.
Teknik, ekolod, sediment, akustisk impedans, hydrografi
IdentifiersURN: urn:nbn:se:ltu:diva-52837Local ID: 9ed59c0d-81ba-46fb-93fa-b34ca04785ffOAI: oai:DiVA.org:ltu-52837DiVA: diva2:1026209
Subject / course
Student thesis, at least 30 credits
Natural Resources Engineering, master's
Validerat; 20140901 (global_studentproject_submitter)2016-10-042016-10-04Bibliographically approved