X-ray radiography of sediment cores is a fast, non-destructive scanning and recording technique which facilitates the calculation of sediment accumulation and simplifies the determination of sedimentary properties. The equipment for radiographing sediment cores, shown below, is portable, and has been used to X-ray about 1000 cores in connection with my field works both in Sweden and in other countries (Costa Rica and Laos) and also on board the Finnish research vessel Aranda. The radiographs of these cores may be used in core-to-core correlation and in monitoring sedimentation.
X-ray radiographic setup for radiographing sediment cores with adjoining bottom water in mono or stereo and before the sediment is extruded from the rectangular and transparent coring tubes. From Axelsson 1979 (Ymer 99).
A diaphragm and a protective cover are used to limit the useful beam of the tubehead, and also in order to reduce the scattered radiation, and to improve the image quality of the radiographs. With the help of a frame, attached to the tubehead of the X-ray unit, the position of the coring tube and its holder can be changed in relation to the tubehead. During the X-raying the coring tube, the X-ray film, and a 7-stepped aluminium wedge are placed in a holder equipped with a brass scale with slits and holes for marking a linear scale as well as the centre of X-ray projection on the radiographs.
The radiographs of selected sediment cores, from environments given in the frame to the left, show (with a few exceptions) the sedimentary sequence in rectangular, three cm thick and six cm wide cores, with bottom water on top, and X-rayed in mono or stereo before extrusion of the sediment from the transparent coring tubes. Most of the X-rayed cores were sampled in about 0.7 or 1.0 m long coring tubes and by a gravity corer with a special type of valve system, that allows an unrestricted flow of water through the coring tube both during descent and during penetration into the sediment.
In the seasonally laminated parts of the cores the border between the sedimentological years may sometimes be hard to identify. Therefore, the given chronology is somewhat uncertain.
The radiographs are negative copies of the developed X-ray films. This means that darker parts on the radiographs represent sediment of lower density than the lighter parts. The lead letters O, X, etc. are used for marking corresponding levels on the partly overlapping radiographs. A brass scale having slits and holes for marking the core length at every cm or at every half cm, as well as for marking the center level of the X-ray projection on the radiographs, is partly visible along the right edge of most of the radiographs. A seven-stepped aluminium wedge, used for density calibration, is also partly visible along the right edge of some of the radiographs.
The best way of monitoring sedimentation, both sediment accumulation and the formation of growing sedimentary sequences, may be to compare the sedimentary sequence in cores sampled at the same station area during different seasons and years.
Radiographic comparison between the uppermost part of sediment cores 740, 981, and 982 from Bråviken, a bay of the Swedish Baltic coast.
Coring sites 740 and 981 were situated rather close to each other at water depths of 29 and 26 m respectively. The distance between coring sites 981 and 982 amounted to about 200 m, but the difference in water depth was only 0.2 m. As shown, core-to-core correlations, based on X-ray radiographs, improve the possibilities of calculating spatial and temporal variations in sedimentation rate.
The stereoradiographs are shown also in a reduced scale. This makes it possible to study the sedimentary structures in three dimensions without the help of a stereoscope.
Stereoradiographs of sandy and gravelly storm layers in the predominantly clayey core 502 from the south-western part of the Bothnian Sea. Core depth: 19-41 cm.
Click here to get a better resolution of the stereoradiographs.
The principal components of the X-ray radiographic and X-ray densitometric
techniques I am using are illustrated in the flow diagram below.
Flow diagram of radiographically based sediment analysis. Modified after Axelsson 1983 (Hydrobiologia 103).
The use of X-ray radiographic techniques simplifies the determination of sedimentary properties and rates of sediment accumulation. The core-to-core correlation is facilitated as well as the identification of primary and secondary sedimentary structures, the dating of growing sedimentary sequences, and the monitoring of environmental changes.
Absorption of X-rays is dependent upon the wavelength of the radiation, the density and thickness of the sample, and the atomic number of the absorbing material. See further the manual: "X-ray radiographic techniques in studying sedimentary porperties, sedimentary sequences, and rates of sedimentation".
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