Bernd Bobertz

Linkage between acoustic parameters and seabed sediment properties in the south-western Baltic Sea

Acoustic profiling methods are widely used to provide a rapid view into geological structures. For the interpretation of acoustic profiling results (single- and multi-beam), reliable geo-acoustic models are needed. Suitable geo-acoustic models covering a wide range of sediment types do not exist to date for the Baltic Sea. Based on surface sediment datasets, geo-acoustic models have been set up for the prediction of acoustical parameters derived from sedimentological data for south-western Baltic Sea surface sediments. Empirical relationships were created to predict key in situ parameters (p-wave velocity, wet bulk density) from sedimentological core data, notably grain density and water content. The Gassmann-Hamilton equations were used to set up a more generic physically based model. For the first time semi-empirical equations for the calculation of the elastic frame modulus and the solid sediment particle modulus were established by an iterative Gassmann-Hamilton fitting procedure. The resulting models have a remarkably good performance with, for example, a calculated sound velocity accuracy of about 17–32 m s–1 depending on model input data. The acoustic impedance of seafloor sediments can be estimated from single-beam echosounding if the contribution of seafloor reflectivity is extracted from the total acoustic signal. The data reveal a strong linkage between acoustic impedance and selected sediment properties (e.g. grain size, water content). This underlines the potential for effective mapping of seafloor sediment properties (e.g. habitat mapping). Furthermore, these geo-acoustic models can be used by marine geologists for a precise linkage between sediment facies identified in longer cores and corresponding acoustic facies recorded by high-resolution seismic profiling in future work.

Classification of the Pearl River Estuary via Principal Component Analysis and Regionalisation

Abstract This paper deals with the subdivision of the LingDingYang Estuary of the Pearl River Estuary (South China at the South China Sea) using geostatistical methods. The division obtained is characterised by the dominating components and interpreted in terms of ruling environmental conditions. The mathematical procedures used are regionalised classification via hierarchical cluster analysis and principal component analysis. While regionalisation determines areas characterised by different sedimentological and geochemical parameters, principal component analysis identifies the influencing factors in the different parts of the estuary. Three main subareas within the LingDingYang Estuary were discriminated. Here, marine-influenced regions can be clearly separated from those determined by the geological catchment area and the ones dominated by river discharge. The samples for this work were taken during cruises in 2004 and 2005. This work has been integrated into a German–Chinese cooperation project between the Leibniz Institute for Baltic Sea Research (Warnemünde, Germany) and the South China Sea Institute of Oceanology (Guangzhou, China).

Surface Sediments of the Pearl River Estuary (South China Sea) - Spatial Distribution of Sedimentological / Geochemical Properties and Environmental Interpretation

ABSTRACT Heise, B.; Bobertz, B.; Tang, C.; Harff, J., and Zhou, D., 2013. Surface sediments of the Pearl River Estuary (South China Sea) – spatial distribution of sedimentological / geochemical properties and environmental interpretation. In: Harff, J., Leipe, T., Waniek, J.J., and Zhou, D. (eds.), Depositional Environments and Multiple Forcing Factors at the South China Seas Northern Shelf, Journal of Coastal Research, Special Issue, No. 66, pp. 34–48. Coconut Creek (Florida), ISSN 0749-0208. The Pearl River Delta (South China) is one of the densest populated regions of the world. This study aims at the investigation and interpretation of the spatial distribution of grain size parameters and geochemical parameters obtained from surface sediment samples. These samples have been taken during cruises in 2003, 2004 and 2005. Investigations of the spatial correlations of the parameters obtained reveal an approximately north-south directed trend for the majority of the parameters. The trend was removed before applying Ordinary Kriging for interpolation. The maps obtained show non-uniform distribution patterns of the sedimentological and geochemical parameters. Here e.g. the concentrations of the As, Co, Cu, Hg and Ni decrease to the more marine influenced southeastern parts and show a higher concentration in the central part and at the western shoals of the estuary.

Sediment Properties in the Western Baltic Sea for Use in Sediment Transport Modelling

Abstract To simulate transport of clastic material in the Baltic a sediment transport module is linked to a Baltic Sea Model that is based on the Modular Ocean Model—MOM3. In order to describe the properties of the seabed sediment parameters as mean grain size, critical shear velocity and bed roughness length must be provided as input data to the numerical model system. To obtain maps of these quantities for the Baltic Sea area the proxy-target concept is applied. As proxy-variable the mean grain size of the sediment types is used. For different sediment samples the critical shear velocity was measured and serves as the target variable. Using the relation between the sediment classifications based on the mean grain size (proxy) and the measured critical shear velocity (target) a map of the critical shear velocity in the Baltic is derived. In January 1993 several extreme strong storm events occurred in the Baltic. Using this period for a model calculation maximum values of current and wave induced bottom shear velocities were obtained. Comparing these model results with the critical shear velocity distribution provided by the proxy-target concept we identify potential erosion areas. Further we show the transport path of material initially deposited in the Mecklenburgian Bight.

Enhancement of Seafloor Maps for Mecklenburg Bay, Baltic Sea, Using Proxy Variables

A common situation in the earth sciences is the unfortunate availability of data in inverse abundance to needs. The geostatistical method of cokriging is used here to address the situation of lack of measurement for geophysical and geotechnical characteristics of the seafloor of the Baltic, where, in contrast, there is abundant information about the bathymetry and the granulometry of the seafloor sediments. New maps for porosity, bulk density, grain density, p-wave velocity, acoustic impedance, and critical shear stress velocity show consistency among themselves and good agreement with old maps prepared with direct measurements, but covering about one third of the total area of interest. These maps show seafloor properties controlled by the effect of erosion and redeposition of glacial tills that have resulted in the accumulation of sediments of decreasing coarseness from coastal areas to a depocenter in the middle of an embayment.