A MANUAL

Second, revised edition, 1994

by

Valter Axelsson

Introduction

X-ray radiography of sediment cores is a fast, non-destructive scanning and recording technique, which simplifies the determination of sedimentary properties, and facilitates the calculation of sediment accumulation. 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. The absorption in different types of water-saturated sediment cores (I/I_o) may be calculated by the formula:

I/I_o = e^{-(u/d . d . x)}w ^{- (u/d . d . x)}s₁....^{-(u/d . d . x)}s_n

This manual is a revised edition of a manual, published both in the English and in the Spanish version of the "Cachí Report" (Axelsson, 1992a). That manual with examples from the Nam Ngum reservoir was also published as an appendix in the "Nam Ngum Report" (Axelsson, 1992b), and was partly based on a manual in Swedish, originally prepared at the Geomorphology Laboratory, Uppsala University by Axelsson and Bodbacka (1982). A comprehensive study of the deposits in the Lakes Lilla Ullfjärden and Stora Ullfjärden, based on the use of X-ray radiographic techniques, described in this manual, was published by Fältman, L. Bodbacka (1993).

If possible digital X-ray equipment should be used for core analysis. The procedure during coring and X-raying depends on the equipment used. This manual is based on the use of rectangular, transparent coring tubes and on conventional X-radiography. Therefore, the procedures described below may only be regarded as guidelines when using other types of coring tubes and X-ray equipment. Calculations by different versions of the computer program "XRAY.FOR" (Axelsson and Hårdén, 1987) are based on the use of Agfa-Gevaert X-ray film Structurix D7Pb, and on X-raying at 50 kV. The principal components of the radiographic techniques are illustrated in the flow diagram, Fig. 1 below.

Fig. 1. Flow diagram of radiographically based sediment analysis (modified after Axelsson 1983).

Coring

The gravity corer and the rectangular, transparent coring tubes described in this manual are mainly designed for the sampling of unconsolidated modern deposits, and for the purpose of X-raying in stereo before extrusion of the cores from the coring tubes. When compacted sediment is sampled with gravity corers, the internal friction along the coring tube may become large enough to terminate further penetration. In this case, the core forms a plug inside of the tube and the sediment below is pushed aside and is thereby not recovered. Such incomplete recovery is not the result of sediment compaction, as is sometimes assumed, but is rather a manifestation of plug formation (Wright, 1991).

Normally, the gravity corer used, Fig. 2 below, is equipped with rectangular coring tubes made of transparent acrylic glass, with an inside section of 30 mm x 60 mm, and a wall thickness of 4 mm. The brass cutter at the lower end of the tubes is sharpened on the outside. The lower end of the tubes may be closed by inserting a rectangular plate through a slit in the brass cutter. The tubes, 0.7 or 1.0 m long, are surmounted by a hollow, circular disk of acrylic glass, which fits into the coring head.

Fig. 2. A gravity corer with a rectangular coring tube, lead weights, core catcher, and a corer head fitted with a lid-shaped valve

High compacting pressures may arise within the tubes of gravity corers if the flow-through of water is hindered during the coring. Therefore, the corer head is fitted with a valve system, see Fig. 3 below , that allows an unrestricted flow of water through the coring tube both during descent and during penetration into the sediment. On lowering, when the lowering wire is taut, the valve is held in open position one by means of two inner hooks attached to the flange of the valve haft, and by the weight of the sampler. Upon contact with the bottom, when the wire slackens, the position of the valve changes to open position two. It remains in this position as long as the wire is slack and the corer penetrates the sediment, by means of two outer hooks attached to the top disc on the valve haft, and by the weight of the valve. The moment the wire tightens and the withdrawal of the corer starts, the hooks are automatically loosened, whereby the valve opening is closed.

Fig. 3. The closing mechanism in position 1, 2, and 3 (from Axelsson and Håkanson, 1978).

Two or four smaller or larger lead weights, weighing 2.5 or 5 kg, are added to a movable brass frame mounted externally on the coring tube before sampling. Thus the total weight of the sampler can be increased in 5 kg steps. A core catcher of the type described by Niemistö, L., 1974 in Merentutkimuslait. Julk/Havsforskningsinst. Skr. 238: 33 - 38, is also added to the brass frame before sampling.

Procedure

Screw the coring tube into the corer head. Mount the brass frame on the coring tube and add lead weights and the core catcher to the brass frame. Move the brass frame and fastened it so that the core catcher can close the lower end of the coring tube. Check that the valve is held in open position one on lowering, and also that the core catcher is held in open position by squeezening the core catcher cord above the valve.

The energy available for forcing the coring tube into the sediment may be increased by increasing the weight and the downward velocity of the gravity corer. The optimum free-fall distance is 2-3 m. However, it is important not to disturb the sediment surface during sampling, and the sediment cores should be taken with attention being paid to minimizing any compaction. If the surface deposits are loose the sampler should be lowered slowly. Two smaller lead weights (2 x 2.5 kg) are sometimes enough when sampling 0.5 m of very soft sediment, but often it is useful to use two larger lead weights (2 x 5 kg). Check immediately the result of coring, which is possible since the coring tubes are transparent, see Fig. 4 below.

Fig. 4. Part of a transparent coring tube with an inside section of 30 mm x 60 mm and showing the result of sampling.

If the result is acceptable, close the lower end of the tube by inserting the rectangular plate when moving the core catcher. Remove the corer head and the brass frame with lead weights and core catcher from the coring tube. Replace the plate in the cutting head of the coring tube with a "bottom" cork in a plastic bag. Use a syringe to decrease the amount of water on top of the sediment core. Close the coring tube with a "surface" cork.

If possible, use a winch, an echo sounder for depth sounding and for observing the corer during sampling, and DGPS for location of the sampling stations. The gravity corer as well as the equipment used for extrusion and dissection of the sediment cores are constructed at the Geomorphology Laboratory, Uppsala University.

Core handling and preliminary density determination

After sampling the coring tubes with bottom water on top of the sediment cores are transported, stored, and X-rayed in an upright position. If possible they should be stored in a cold-storage.

As previously pointed out, the absorption of X-rays varies with the density of the absorbing material. Therefore, in order to select the appropriate exposure according to the exposure chart, Fig. 8, it is necessary first to calculate the wet bulk density of the sediment cores.

Procedure

Use Form 1, Fig. 5 above, for a preliminary core description, and for calculating mean wet bulk density of the sampled sediment cores.

After sampling the sediment core may increase in length due to the formation and expansion of gas bubbles. Therefore, measure the distance between the sediment surface and the tube collar (the circular disk, surmounting the tube) as soon as possible after sampling, and later also in connection with the X-raying and dissecting of the sediment core. Note the values (air + water) at the times of coring, X-raying, and core dissection on lines 14-16 in Form 1. Note also the number of the coring tube (line 2), date and place of sampling, the water depth and the initials of the operator (lines 4 - 8) as well as the number of the surface cork (line 18) and the bottom cork (line 20). Note also visible sedimentary structures, etc. below line 26.

Use a spring balance for determining the total weight of the coring tube including sediment, water and corks (line 9). Measure also the distance between the water surface and the tube collar (line 17), and the projecting part of the bottom cork (line 23). The weight and height of the empty coring tubes and of the corks (lines 3, 13, 19, 21, and 22) should be listed in a file, that should be stored together with the instructions for sediment sampling and sediment analyses.

In order to calculate the average wet bulk density of the sampled sediment core (line 26), you first have to calculate the tare weight (line 10), the water weight (line 11), and the sediment weight (line 12), and also the height (line 24) and volume (line 25) of the sediment core. The tare weight is equal to the weight of the coring tube plus the weight of the surface and bottom corks (sum the values on lines 3, 19, and 21) + about 10 g due to the difference between the dry and the wet weight of the bottom cork with its plastic cover. The weight (in g) of the water is equal to the height of the water column (the difference between the values on lines 15 and 17) times 18 (the inside cross-sectional area in cm² of the coring tube). The weight of the sediment core is equal to the total weight minus the tare weight and the water weight (the value on line 9 minus the values on lines 10and 11). The core length is equal to the sum of the values on lines 13 and 23 minus the values on lines 15 and 22. Multiply the core length by the cross-sectional area (18 cm²) to get the sediment volume. The mean wet bulk density of the sediment core (line 26) is equal to the sediment weight divided by the sediment volume (the value on line 12 by the value on line 25). If you have access to a computer these calculations may preferably be done by the use of a simple computer program.

X-raying

X-ray units are almost everywhere available in clinics and hospitals, physics laboratories, and in many industrial laboratories. It is desirable and sometimes obligatory to have the radiation installation and the safety scheme examined by a qualified radiation expert. If you yourself wish to carry out radiography, you should carefully study the directions for radiographical work.

This manual is adapted to the use of a mobile Philips K 140 X-ray apparatus. The tubehead, weighing 29 kg, is equipped with a Beryllium window, and used in combination with a standard control box, weighing approximately 20 kg. A diaphragm and a protective cover is used to limit the useful beam of the tube head, and also in order to reduce the scattered radiation, and to improve the image quality of the radiographs. Use an output from the tube head of 50 kV and 2 mA when X-raying rather loose, modern deposits.

Fig. 6. X-ray radiographs of the upper part of sediment core G7 from the Nam Ngum reservoi in Laos. The lead letters O and B are used for marking corresponding levels on the partly overlapping radiographs. The linear brass scale, divided in cm, and the optical density steps, caused by the Al-wedge, are partly visible along the right edge of the left radiograph. The black circle on the brass scale marks the centre level of X-ray projection. Mean annual sedimentation rate 1971-1991 at this sampling station: 37 mm and 16.3 kg of solids/m².

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 the core length for every cm as well as the centre of X-ray projection on the radiographs, see Fig. 6 above. The protective cover, the frame, the tube holder, and the 7-stepped aluminium wedge are constructed at the Geomorphology Laboratory, Uppsala University. The setup for radiography is shown by Fig. 7 below.

Fig. 7. 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 coring tubes. From Axelsson 1979 (Ymer 99).

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:

y_z = 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 on top of the sediment core, and c a factor which varies with the composition of the solids and with the energy of radiation (Axelsson, 1979). For 30 mm thick, silty and clayey sediment cores with fresh bottom water, X-rayed through a Beryllium window at 50 kV, the value of b is about 3.3 and the value of c in the order of 20. The assumption that the density of the pore water is equal to the density of the bottom water often gives a satisfactory accuracy when calculating the wet bulk density.

Fig. 8. Exposure chart for X-raying sediments in rectangular coring tubes made of acrylic glass (modified after Axelsson, 1979

By radiographic exposure is meant the intensity of radiation, multiplied by the exposure time. The required time of exposure increases with the density of the sediments, and in proportion to the square of the film-to-focus distance. The exposure chart, Fig. 8 above is, for example, based on a value of c in equation (2) = 18, and on a film-to-focus distance = 100 cm.

Fig. 9. The relationship between the film density and the relative exposure for X-ray film Structurix D7Pb. Assume D = 1.35 = 100% = desired film density.

If necessary use the diagram for correcting the time of exposure, Fig. 9 above, to get the desired average film density.

Fig. 10. Schematic diagram of experimental setup for radiography of a sediment sample. From Axelsson and Händel 1972 (Geogr. Ann. 54 A).

As shown by Fig. 10 above the X-rays are diverging from the focus of the X-ray tube. The X-ray radiograph is thus a central projection of the X-rayed sediment core. Thin, horizontal layers in thick sediment cores may therefore be projected as thick and diffuse on the X-ray radiographs if their distance to the centre of projection is great. Therefore, it is important to use a large film-to-focus distance during X-raying, to use rather thin coring tubes, and to restrict the vertical movement of the coring tube between the exposures, so that the image interpretation may be restricted to areas close to the centre of X-ray projection. The sharpness of the radiographs also increases with decreasing size of the focal spot.

Stereoradiographs facilitate the image interpretation and may be used to calculate the real thickness of sediment layers also in rather thick sediment cores, projected at a relatively great distance from the centre of projection on the X-ray radiographs. The stereoradiographs should be presented also in a reduced size. This makes it possible to study the sedimentary structures in three dimensions without the help of a stereoscope. See, for example, the stereoradiographs of core 502, Fig. 11 below.

Fig. 11. Stereoradiographs of Sandy and gravelly storm layers in the predominantly clayey core 502 from a depth of 42 m, north of Örskär in the south-western part of the Bothnian Sea. Core depth: 19-41 cm.

Try to study the stereoradiographs without the help of a stereoscope. Start with your nose close to the monitor, pull your face slowly away from it, and look through the image.

In order to take these stereoradiographs, the coring tube and its holder were moved 62 mm horizontally at a film-to-focus distance of 1.0 m.

Click here to get a better resolution of the stereoradiographs.

Procedure

Use Form 2 (Fig. 12 above) to record the exposure data. Use Agfa-Gevaert industrial X-ray film Structurix D7 Pb, and film size 6 x 24 cm. Determine the number of films needed for X-raying the sediment core. At least 6 films are needed for X-raying a one meter long sediment core in normal (n) position, and at least 12 films if also stereo (s) position is selected. The film used for X-raying the uppermost core segment (u) should reach about 5 cm above the sediment-water interface. There should be an overlap of at least 5 cm between the positions in relation to the sediment core of the films used for X-raying the uppermost, the middle (m1, m2, etc), and the bottom (b) core segments.

Put the X-ray film under the right edge and at the upper or lower part of the tube holder depending on whether the upper or lower part of the sediment core is to be X-rayed. Place the coring tube in the tube holder and attach the 7-stepped aluminium wedge in front of the right edge of the X-ray film. Attach a 5 mm thick aluminium bar behind the wedge if the chosen exposure time, using a tube current of 2 mA, is longer than 4 minutes. The use of this aluminium bar is marked as "ds" or "double Al wedge" in Forms 2 and 4 (Fig. 12 above and Fig. 14 below). Use lead letters and numbers and fasten them with tape on the front wall of the coring tube for marking film no. and the same levels on the overlapping parts of the X-ray films.

Place the tube holder on the upper or lower part of the frame attached to the X-ray tube depending on the position of the X-ray film in the tube holder. Place it at the position marked "normal" or "stereo" at a film-to-focus distance of 100 cm. The distance between the positions "normal" and "stereo" is 62 mm.

Use the exposure chart (Fig. 8) to select the approximate time of exposure at a tube current of 2 mA to get an average film density of 1.2 -1.5. Use a somewhat shorter time of exposure for the upper, normally softer part of the sediment core than for the normally harder bottom part. X-ray at a voltage of 50 kV. To get good results when X-raying very hard and thick sediment layers it may be necessary to increase the tube voltage and the tube current and to decrease the film-to-focus distance.

Check the result of X-raying, if possible with the help of a transmission densitometer. If the X-ray film was over- or underexposed, you may use the diagram, Fig 9, for correcting the time of exposure to get the desired film density.

It is good practice to make a film negative of the radiograph and to make prints from the negative.

Manual processing of X-ray film

Film processing should preferably be done with an automatic film processor. However, if you yourself have to develop the X-ray film and you have access to a darkroom, you may use the instructions given below as guidelines.

Procedure

Core dissection and some simple laboratory analyses

The absorption of X-rays varies not only with the density of the absorbing material but also with its atomic number. Therefore the value of c in the equations used by different versions of the computer program "X-ray" to calculate sediment density and values of density related parameters, varies with sediment composition. The radiographically calculated bulk density values should be compared with those which were gravimetrically determined, and the difference used to correct the assumed value of c. The simplest way is to compare the radiographically calculated mean dry bulk density of the whole sediment core with the dry bulk density value corresponding to the wet bulk density (the value on line 26 in Form 1, Fig. 5), obtained by measuring the volume and weight of the core when it still is in the coring tube.

The relationship between the wet and the dry bulk density is given by some corresponding values in Table 1 above. The given values of organic content as a density-related parameter in the table are uncertain, and should only be regarded as median values for the correlation of water content to organic content in the 785 analysed sub-samples.

A more accurate way to determine the value of c is to compare the radiographically calculated amounts of solids between some selected sediment levels with the amount found by drying and weighing the deposits and to use the difference as a correction factor. Some instructions for core dissection and for some laboratory analyses, needed for the determination of sediment density and some density related parameters, are given below.

Procedure

Study the radiographs carefully before core dissection to select suitable sediment layers for the subsampling and the subsequent, desirable laboratory analyses. Place the radiographs on a light box so that you can study them also during the dissection. Form 3, see Fig. 13 above, is used as a laboratory worksheet to record some basic data. Note the selected sediment depths for sub-sampling and the numbers and dry weights of the required crucibles. Label the bottle for the bottom water sample with the film number.

Attach the coring tube to the holder for extruding and dissection of the sediment core in an upright position. Measure the distance between the sediment surface and the tube collar and note the value (air + water) on line 16 in Form 1 (Fig. 5). Loosen the bottom cork carefully and insert at the same time the rectangular plate shutter through the slit in the cutting head. Take away the bottom cork and fasten the frame used for locking the extruding rod at the bottom of the coring tube. Insert the extruding rod and remove the plate shutter.

Gently push up the sediment core with adjoining bottom water and remove some water until about 10 cm of water remains. Push up the sediment core until the water surface is level with the tube collar. Use a syringe to take a sample of the bottom water for the pH and conductivity measurements. Carefully remove the remaining water, the sediment surface must not be disturbed. Push up the core until the sediment surface is level with the upper surface of the tube collar. Measure the length of the sediment core from the underside of the tube collar to the underside of the extrusion plate of the extruding rod, and note the value on line 24 in Form 1 (Fig. 5). Attach a millimetre graded paper scale with tape on the front wall of the coring tube. The zero level should be at the level of the underside of the extrusion plate. (The thickness of the extrusion plate is the same as the thickness of the tube collar.)

Push up the sediment core and take sub-samples at selected sediment levels. During the extrusion and dissection make notes in the remarks column (Form 3, Fig. 13) of visible sedimentary structures, type and activity of bottom fauna, gas smell, etc.

Put the samples for the determination of water content in crucibles, weigh them on a balance, accurate to 0.1 g, and record the wet weight (sample + crucible) on Form 3 (Fig. 13). Dry the samples in an oven at 75^oC for 48 hours. Weigh them after cooling for 1 hour in a desiccator (with blue silica-gel granules) and note the dry weights. For determining residue on ignition, place the dried samples in a furnace at 550^oC for 2 hours, cool them in an oven at 105^oC for 30 minutes, and in a desiccator for 1 hour before reweighing them.

The particle density may be determined gravimetrically before and after ignition of the solids, using an air-comparison pycnometer. If you do not determine the particle density, you may assume the density of inorganic and organic particles to be 2.8 and 1.6 respectively. Then you may use the wet and dry sample weights in Form 3 (Fig. 13), and the equations given below to calculate (for recording on Form 3), the water content, the loss on ignition, the wet and dry bulk density, the porosity, the void ratio, the increase of solid with increasing sediment depth, and the effective overburden pressure.

Calculations

Express the water content (w) as a percentage of the wet weight of the sample (s_w). Then

0.01 w = (s_w - s_d)/s_w

where s_d = the dry sample weight.

Express the loss on ignition (i_l) as a percentage of the dry weight of the sample. Then

0.01 i_l = (s_d - i_r)/s_d

where i_r is the residue on ignition.

Wet bulk density (y) and dry bulk density (y_d) are the ratios of the wet and dry weight of the sample respectively to its volume expressed in grammes per cubic centimetre. The wet bulk density of freshwater-saturated sediments may be calculated by the equation:

y = p_d/[1 + w(p_d - 1)]

where p_d is the mean particle density of the solid particles.

Calculate the dry bulk density (the amount of solids in grammes per cubic centimetre) with the equation:

y_d = (1-w)y

If you have not determined the particle density you may calculate its probable value by assuming the inorganic particles (the residue on ignition) to have a particle density of 2.8, and that the organic particles (the loss on ignition) have a particle density of 1.6. The correspondence between the density-correlated values in Table 1 is based on this assumption.

In water-saturated sediments the void ratio (e) is the ratio of the volume of the water (w_v = the volume of the pores) to the volume of sediment particles forming the skeleton of the given mass, 1 - w_v. Thus

e = w_v/(1-w_v)

Then, if the volume of the water = the volume of the pores, the porosity, p, expressed in percentage is given by the equation:

p = 100 wv = w ^. y

The effective overburden pressure (= total overburden pressure minus acting pore pressure) is computed according to the relationship:

p_z = (y-w_d)z

where p_z = effective overburden pressure at sediment depth z, and w_d is the density of water.

If necessary (due to core expansion), recalculated the core data to represent in situ values.

Film density

Variations in textural as well as chemical composition throughout the sediment core result in differential attenuation of the incident X-ray radiation before it reaches the X-ray film. Therefore, the spatially variable radiation arriving at the film plane creates differences in photographic density. By photographic density or film density is meant the logarithm to base 10 of the ratio: intensity of incident light / intensity of transmitted light. It is of special interest to record variations in the film density along the centreline of the radiation images of the sediment cores, where also the variations of radiation intensity, due to radiation scatter, and in terms of angle of emergence, are at a minimum. The recorded density values may be used to illustrate and to calculate the vertical variation in density and in density-related sedimentological parameters.

Variations in the film density should preferably be recorded by a densitometer, which scans and plots density versus position in the form of a graphical trace on an integral chart recorder, and also records the data in a digital form suitable for data processing. Instructions for manual film-density measurements with an ordinary black and white transmission densitometer are given below.

Procedure

Form 4, see Fig. 14 above, is used to report the results with a resolution of 2 mm in the vertical direction. Enter film no., type of densitometer, and size of measuring slit at the top of the form. Check the 0-point of the densitometer. Start by measuring the film density caused by the seven steps of the aluminium wedge (single or double), and by the bottom water. Attach a mm-graded paper scale to the X-ray film with the zero at the sediment surface. This mm-graded paper should first be enlarged by 4 % due to the central projection, that causes an enlarged image during the X-raying. Measure and record the film density at every second (enlarged) mm along the centreline of the radiograph.

Fig. 15. Film-density curves for the aluminium wedge on the radiographs G2u, m, and b. The bottom water gives the same film density as 3.3 mm aluminium

Draw a film density curve for the aluminium wedge, as exemplified by Fig. 15 above. Use the diagram (Fig. 15) to determine the value of b, i.e. the thickness in mm of the aluminium wedge which gives the same film density as the bottom water. Calculate the mean film density (from Form 4, Fig. 14) for sediment layers with laboratory determined bulk density (y). Use Fig. 15 also to determine the value of x, i.e. the thickness in mm of the aluminium wedge, which gives the same film density as the sediment core at level z. Then you may calculate the value of c. Illustrate the relationship between the bulk density of the sediment layers and the thickness of the aluminium wedge for the same film density values, as exemplified by Fig. 16 below, and calculate the mean c-value.

Fig. 16. 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 b. From Axelsson, 1979.

Density related parameters

The vertical variation in density-related sedimentological parameters is calculated from the variations in the film density along the radiation images of the sediment cores and the aluminium wedge. The relationship of film density (x) to bulk density (y) is illustrated by Fig. 17 below. The calculations are also based on the median values for the correlation of water content to organic content and to particle density given in Table 1. The accuracy of the calculations increases with decreasing density of the deposits, since the results are a function of the difference in density between bottom water and sediment.

Fig. 17. The film-density curve for the aluminium wedge and for corresponding bulk densities of the sediment layers on the reference film (delta y = 0). After calculating the delta y-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-related parameter. From Axelsson, 1983.

The computer program XRAY is written in Fortran 77 and in different versions, but ought to be replaced by a more modern computer program.. Before film-density values can be entered in a suitable application of the old XRAY program (via the keyboard or the paste command), a dataset must be created to hold it. The instructions given below are for XWG7, a version of the XRAY program without plotting functions, for the use of a Macintosh computer and a MacFortran compiler and debugger (version 2.4), and with the film-density expressed in real values for every second mm in a vertical direction.

Procedure

Create a new file by using the EDIT command to invoke an editor. Type the number of film-density values in the first row, FORMAT (I3), and then 10 film-density values/row, FORMAT (10F3.2). Save the document and use Film no. and core segment (u, b, m1, m2, etc.) as file name.

Duplicate and rename the file to XRAY.DAT. Open the relevant application of the computer program (in this case XWG7 apl). Answer the instructions and questions such as "Give the film number", "Give the zero-point correction" (= 0 if you do not need to correct the densitometer data), "Do you want to have the film-density values on the lineprinter? Yes or No [N] ", "Give the film-density difference" (= the film-density value for the bottom water, or the film-density value for the thickness of the aluminium wedge (often 3.3 mm), which gives the same film density as the bottom water), "Do you want to change the c-value?", "Give a new c-value" (if c in equation (2) is higher or lower than 18), "Do you want to use the film-density data for the calculation of dry bulk density?", "Do you want to have the cumulated amount of solids on the lineprinter?", "Give the wet bulk density correction value" (= the density of the bottom water), "Give the weight reduction for calculating the effective overburden pressure in g/ square meter" (also = the density of the bottom water), etc.

Open the file "Calculations", where the calculated density-related values are stored, study and correct if necessary the tabulated values before printing. An example of values calculated and printed with the computer program XWG7, is given by Table 2 below.

Rate of sediment accumulation

Rapid and detailed core-to-core correlations are made possible by the use of radiographs of unextruded sediment cores. This simplifies the determination of contemporary rates and spatial variations in sediment accumulation, as exemplified by Fig.18 below.

Fig. 18. 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 rate of sedimentation is often expressed in terms of thickness per unit of time. However, the porosity, the water content, and thus the original thickness of a sediment layer will decrease with increasing depth of burial because of compaction. It is therefore often necessary also to express the rate of sedimentation of solids in weight per unit of time, i.a. when comparing sediment accumulation in a water reservoir with sediment yield from its drainage area.

Procedure

Collect sediment cores from selected locations during different seasons and years. Accept only sediment cores with an undisturbed sediment-water interface. X-ray the sediment cores, the uppermost part preferably in stereo. Use the X-ray densitometric technique to determine the variations and the increase of solids with increasing sediment depth. Compare radiographs of the uppermost part of the sediment cores in order to identify cyclic and young, event types of stratification, which may be used as time markers for the determination of contemporary rates and spatial variations in sediment accumulation. Sometimes peaks of the film-density curves, corresponding to minimum values in bulk density, may be used to determine the border between identified sedimentological years. Use the tabulated values giving the increase of solids with increasing sediment depth (calculated and printed by the selected version of the computer program XRAY), to determine the rate of sedimentation between datable sediment levels in weight of solids per unit of time.

Final remarks

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.

As previously pointed out, the procedures described in this manual may only be regarded as guidelines when using other types of coring tubes and X-ray equipment. Hopefully, however, this manual will stimulate future use of X-ray radiographic techniques in studying sedimentary properties, sedimentary sequences, and rates of sedimentation.

References

Axelsson, V., 1979: Sjöars sediment studeras med röntgenteknik. (Lake deposits are studied by X-ray technique). Ymer 99.

Axelsson,V., 1983: The use of X-ray radiographic methods in studying sedimentary properties and rates of sediment accumulation. Hydrobiologia 103.

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