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The composition of the underwater sediment bed is often defined in terms of the mud (63 and 63 mum) and sand (>63 mum) as: 46.2 +/- 1.9 Bq/kg U-238 and 9.3 +/- 0.9 Bq/kg U-238 and 45.6 +/- 1.9 Bq/kg Th-232 and 9.7 +/- 0.9 Bq/kg Th-231, respectively. The assessed radiometric fingerprint allows a quantitative interpretation of the mud and sand content through the total U-238 + Th-232 activity. This interpretation is confirmed by the high correlation (R-2 = 0.96) for the mud percentages obtained from radiometric analysis and those by laser diffraction in the laboratory. It can be concluded that radiometric sedimentology provides a tool for reliable sediment characterisation. New perspectives are opened when radiometric fingerprints are combined with in situ radiometric characterisation of sediments, which will be presented in part B of this paper (this issue). (C) 2002 Elsevier Science B.V. All rights reserved.

Traditionally, obtaining accurate spatial information about the textural composition of heterogeneous aquatic sediments requires extensive sediment sampling. To avoid a costly and time-consuming operation, a new in situ technique has been investigated. This technique characterises sediment components by the activity concentrations of natural radionuclides. In situ activity concentrations are continuously measured with the Multi-Detector system for Underwater Sediment Activity (MEDUSA), a detector trailed over the bottom by a towing vessel, containing a highly sensitive gamma-ray BGO-detector, a water-depth sensor and a microphone. From the collected gamma-ray spectra, the activity concentrations of K-40, U-238 and Th-232 are derived.During two separate field surveys of 5 days each, MEDUSA was applied to the Hollandsch Diep and Haringvliet, two fresh water basins, each approximately 20 km long and 2-3 km wide in the Rhine-Meuse Estuary, the Netherlands. In part A of this paper (this issue), it has been illustrated how the radiometric fingerprint of sand and mud in the area was determined. This fingerprint was then used to calculate the sand-mud ratio from the U-238 and Th-232 activity concentrations of about 25,000 MEDUSA data points.The interpolated sand-mud distribution of the top layer of the aquatic sediment shows a distinct correlation with bathymetry: the deeper channels are mud-rich, whereas the shallow zones are predominantly sandy. In general, the mud content decreases in The seaward direction. This corroborates well with the morphological development of the area in which the deposition of mud is highest near the rivers' outflow in the cast. The absolute total random error in the sand map varies between 6% and 18%, where the largest errors are caused principally by spatial variability of the sediment composition. At present, the radiometric results underestimate the mud content by 10-30% due to a higher water content of sediment with high mud contents (>60%). The simultaneously recorded friction sound levels provide qualitative information with respect to the sediment composition. Sandy areas show a higher sound level than muddy areas; with the sound and sand map showing a striking similarity. (C) 2002 Elsevier Science B.V. All rights reserved.