Accurate monitoring of seabed structures
The harmful effects of toxic waste in the marine environment have direct effects on the quality of life for instance in fisheries. In line with European conventions, including London Dumping Convention (1975) and MARPOL Convention (1978), dumping at sea has been substantially prevented, yet no measures were taken against past dumping practice. The Baltic Sea where toxic dumping occurred in the post World War II era has been considered as one of the key sites in Europe. For this reason, the Helsinki Commission recommended further in situ investigations on the toxicity and bioaccumulation effects in water, sediment and biota. Under this perspective, the SITAR project focused on developing, testing and applying instrumentation to meet these needs. More specifically, the project developed a series of acoustic models and instruments for imaging waste barrels/containers of small dimensions buried in unconsolidated sea sediments. Additionally, biological testing methods were generated for specifying the related in-situ bioaccumulated toxicity at a contaminated site. Most importantly, both acoustical imaging data and biotoxicological information were integrated in a user-friendly way to benefit end-users and decision makers. One of the key project results involved the geoacoustic inversion of monostatic data from the main sea trial of the project work. The so-called FARIM method offers estimation of the roughness and impedance of the sea bottom on the basis of calibrated sonar data. It relies on describing roughness as a stochastic variable with a Gaussian probability density as there is incoherent scattering of energy in the presence of roughness at the sea-bottom. Comparison of the observed shift in the received signal spectrum, represented by the centre of gravity frequency, towards lower frequencies and the theoretical shift provides an estimate for roughness. Unlike currently used methods where calculation of the bottom impedance from the ratio of received to transmitted energy, FARIM allows a better estimation of impedance. Thereby, improved calculation of the bottom impedance heavily relies on the estimated roughness. The FARIM method offers accurate and reliable data for improved detection of waste structures buried in unconsolidated sea sediments.