X-ray densitometric methods

 

X-ray radiograph of the upper part of sediment core 510 from the coastal bay Hallsfjärden. Lead letters (O, etc.) are used for marking corresponding levels on the partly overlapping radiographs. A linear brass scale having slits for marking the core length at 0.5 cm intervals, and optical density steps caused by an aluminium wedge are visible along the right edge of the radiograph. The black circle on the brass scale marks the centre level of X-ray projection. The film density is recorded along the centerline of the radiograph.

The optical density steps on the radiographs, caused by the aluminium wedge, may be used as a scale for sediment density. The wet bulk density (y) at level z is given by: yz = a + (x-b)/c where a is the density of the pore water, x the thickness, in mm, of the aluminium wedge which gives the same film density as the sediment core at level z, b the thickness, in mm, of the aluminium wedge which gives the same film density as the bottom water in the coring tubes, and c a factor which varies with the composition of the dry substance and with the energy of radiation.

The relationship between the bulk density of the sediment layers and the thickness of the aluminium wedge for the same film-density values on the radiographs 234 u, m, and l of the upper, middle, and lower parts of sediment core 234. Modified after Axelsson 1979 (Ymer 99).

Recorded variations in the film density along the radiation images of the sediment cores and the aluminium wedge are used, together with the relationship, illustrated below, to calculate the vertical variation in density-related sedimentological parameters.

The film-density curve for the aluminium wedge and for corresponding bulk densities of the sediment layers on the reference film (fy=0). After calculating the fy value (which varies with the exposure) the given relationship may be used to calculate the probable bulk densities (y) from recorded film densities (x). The constants a, b, c, and d are dependent on the characteristic curve of the X-ray film and on the selected density-correlated parameter. Modified after Axelsson 1983 (Hydrobiologia 103).

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