Improving Accuracy in Studying the Interactions of Seismic Waves with Bottom Sediments

The emerging tasks of determining the features of bottom sediments, including the evolution of the seabed, require a significant improvement in the quality of data and methods for their processing. Marine seismic data has traditionally been perceived to be of high quality compared to land data. However, high quality is always a relative characteristic and is determined by the problem being solved. In a detailed study of complex processes, the interaction of waves with bottom sediments, as well as the processes of seabed evolution over short time intervals (not millions of years), we need very high accuracy of observations. If we also need significant volumes of research covering large areas, then a significant revision of questions about the quality of observations and methods of processing is required to improve the quality of data. The article provides an example of data obtained during high-precision marine surveys and containing a wide frequency range from hundreds of hertz to kilohertz. It is shown that these data, visually having a very high quality, have variations in wavelets at all analyzed frequencies. The corresponding variations reach tens of percent. The use of the method of factor decomposition in the spectral domain made it possible to significantly improve the quality of the data, reducing the variability of wavelets by several times.

Acoustic remote sensing monitoring of morphological and sedimentological seabed evolution of small and medium-scale French estuaries

<p>Estuarine benthic habitat quality health is integrated within the framework of the EU Water Directive and Marine Strategy Framework Directive. The long-term monitoring of small and medium-scale estuarine benthic habitat is based on recurrent observation of several factors, mainly bathymetry and seabed nature. Numerous studies have already addressed the performance and limitations of acoustic remote sensing and mapping techniques. However, most of these studies are limited to the marine and coastal domains and do not include the estuarine domain. Estuaries are considered as transitional domains, with various seabed morphologies (from rocky reefs to hydraulic dunes with anthropic modification overlap) and subtle granulometric variations of the seabed nature.</p><p>The objectives of our study are to explore the mapping performance of several acoustic remote sensing techniques and to determine which physical factors are the most representative of morphological and sedimentological characteristics of subtidal estuarine environment and of its evolution. The exploration of these cartographic variables has been performed for three small and medium-scale French estuaries: the Orne estuary, the Baie de Somme and the Belon estuary. These estuaries have been chosen to cover different morphological and sedimentological estuarine contexts.</p><p>Firstly, we evaluate the capacity of the main variables derived from bathymetry (slope, curvature, ruggedness) to map seabed morphology. We extend the variable exploration to the “Terrain Variable” GIS category and BTM (Benthic Terrain Modeler Toolbox) as well. Secondly, we explore the capacity of several cartographic variables, extracted from bathymetric, seabed acoustic backscatter and acoustic ground discrimination system (i.e. RoxAnn©), to map seabed sediment characteristics and variations. The seabed nature mapping is validated with ground truthing data, namely grab samples and seabed video profiles. Moreover, quantitative (D90, roughness, sorting) and qualitive information (apparent roughness of the seabed, benthic habitat) are extracted from the grab samples and seabed video profiles, respectively. The capacity of these variables to produce seabed nature maps is also explored.</p><p>Mapping results on the three areas are compared, in terms of mapping precision and reproducibility, and transposed into recommendations for small and medium-scale estuaries monitoring. The next step of the AUPASED project is he exploration of image analysis and machine learning classifications and their comparison to manual morphological and sedimentological maps produced.</p><p>The AUPASED project is funded by the AFB (French Agency for Biodiversity) as part of a convention between the AFB and the CNRS (UMR 6143, M2C).</p>

WAVE-CURRENT INTERACTION, SEDIMENT TRANSPORT AND SEABED EVOLUTION IN THE PIRAQUÊ-AÇU/PIRAQUÊ MIRIM ESTUARY (BRAZIL)

The medium-term seabed evolution of Piraquê-Açú/Piraquê-Mirim estuary (ES/Brazil) is studied numerically in this work. The hydrodynamics is induced by the tide, the river discharges, and the incident water wave. The wave-tide-current interactions are obtained by coupling the shallow water equations with the radiation stress tensor introduced by Longuet-Higgins & Stewart (1960). In this way, the influence of both the tidal current and the current induced by gravity waves on the sediment transport are taken into account. We utilized the Exner (1925) equation, based on the conservation of seabed sediment mass, to calculate the morphological evolution. Seabed morphological changes are accelerated by introducing a time scale factor. Four bedload sediment transport formulations were tested and compared. We found an excellent agreement when numerical results are compared with currents measured in the upper estuary and with sediment transport rates measured at the river’s mouth when using the Engelund and Hansen (1967) sediment transport formulation. We also found that the main morphological changes occurring at the estuary mouth are due to the action of gravity waves. Between the head and mouth of the estuary, the sediment transport rate and morphological seabed changes are controlled exclusively by the tidal currents and the river discharge. In this latter case, we found that the large sandbank located at the estuary mouth is responsible for the absence of wave.

EXPERIMENTAL AND NUMERICAL MODELING OF FLUID-SEABED INTERACTION IN SHALLOW WATER

For the design and the construction of the defence structures against beach erosion, there is a need to predict and un- derstand the evolution in time of the shape of the sea sandy bed, using different numerical models and control theories. In the present paper a numerical model based on shallow water equations, which is an application of control theory to the evolution of sandy bed, is used in order to propose a formulation for the wave motion based on fluid and structure coupling, using minimization principles. Furthermore, measurements from a physical experiment are used in order to verify the accuracy of the model. The experiment was carried out in a multi directional wave basin at the SOGREAH (LHF facility, G-INP, France) and provided extensive measurements and detailed analysis of combined hydrodynamics and morphodynamics, suggesting a subtle interplay between several feedback mechanisms associated to wavedriven rip current circulations, wave nonlinearities, sediment transport, and seabed evolution. Using the numerical model, wave characteristics and depth profiles have been calculated and compared to experimental results.

STABILITY AND ENGINEERING EFFECT OF SHOALS AND CHANNELS IN CAOFEIDIAN DEEP-WATER HARBOR AREA, CHINA

Caofeidian sea area is the only natural site suitable for building a large-sized berth of 300 000 tons without excavation of waterways and harbor basins in Bohai Bay. In Caofeidian harbor area, the stability of the shoals-inlets coastal system under natural conditions is a key point. The harbor could be developed only if the coastal system is stable. In this paper the characteristics of waves, tidal currents, sedimentation and seabed evolution in the Caofeidian shoal area is briefly described, and the trend of stability of shoals and channels is predicted based on research on the coastal system formation. Engineering effect of some projects, such as the dam linking the Caofeidian Island and the coastline, the shoal reclamation and ore terminal construction, is also studied. The analysis show that deposition and erosion of the seabed in front of Caofeidian foreland will reach its equilibrium in 2～3 years after the implementation of the proposed scheme.