Sediment mapping with an underwater gamma-ray spectrometer

New approaches in monitoring and mapping the dispersal of fine sediments

In recent years, laws and regulations in the field of ecology became more strict due to public pressure. Therefore, the discharge of fines during dredging and land reclamation works and its impact on seafloor habitats is an important aspect in environmental impact assessments.

Traditionally, the content of fines in seafloor sediments is determined by taking sediment samples by for example (box)coring. These measurements give accurate information of one spot, but spatial variation in the concentration of fines, e.g. due to the presence of small-scale morphological features such as ripple structures, can result in data that is not representative for large areas. Spatial variation can be mapped by taking large amounts of sediment samples, which is often too expensive.

Radionuclides as a proxy for sediment composition

We learned that the concentration of natural occurring radionuclides correlates strongly to the concentration of fine material (clay) in sediment. These radionuclides (⁴⁰K, ²³²Th and ²³⁸U) are present in rocks and sediments since the origin of the earth. Most sediments are composed of sand, clay and organic matter. Due to the differences in chemical composition of these minerals, the concentration of radionuclides varies for each fraction. This relation between the concentration of radionuclides and sediments is established by calibration in the laboratory. See our application models.

Sandcontent, measured with the MS-700 sub, draped over bathymetry

The map shows the sand content of the sediments in the Lauwersmeer in the Netherlands, a former tidal channel that is closed by a dam. The map of the sand content is draped over the morphology that was simultaneously measured with a multibeam system. The combined map clearly shows how fine material is settled in the deeper parts of the former tidal channel.  

Map the concentration of silts in sediments

The concentration of radionuclides can be measured in the field by a seaborne gamma spectrometer (MS-700 SUB). This system was successfully used to measure variation of grain size, to measure the sediment transport of tracer material, to map the concentration of silts in sediments and to estimate the pollution of fine sediments for dredging.

This approach opens new approaches in monitoring and mapping the dispersal of fine sediments.