A TWO-PROBE NUCLEAR DEVICE FOR DETERMINING THE DENSITY OF SEDIMENTS ( )

Transcription

1 A TWO-PROBE NUCLEAR DEVICE FOR DETERMINING THE DENSITY OF SEDIMENTS ( ) J. ROGER McHENRY (* ) RÉSUMÉ Des techniques nucléaires ont été utilisées pour mesurer sur place la densité des sédiments et la concentration des sédiments en suspension. Le détecteur double est'1'object de cette étude. Cet instrument a deux tubes d'accès qui entourent la source radioactive (césium-137) dans un des tubes et le système de détection dans l'autre. Le détecteur est composé d'un cristal de scintillation en iodure de sodium (thallium activé) et du système électronique qui contient un discriminateur intégral qui n'enregistre que les rayons gamma ayant plus de 0.65 Mev. La précision obtenue dans le laboratoire avec le détecteur double a été excellente. La largeur maximum des volumes de sédiments mesurés était plus petite qu'un pouce. Ce qui permet l'emploi de cette technique pour mesurer les densités dans des couches sédimentaires minces et dans des surfaces où il existe un changement de phase. La performance du détecteur double a été comparée en dehors du laboratoire à celle d'un détecteur simple, qui, lui, utilise le principe de l'atténuation des rayons gamma absorbés par réflexions. Le détecteur double a fonctionné d'une manière satisfaisante en dehors du laboratoire et de plus, celui-ci a l'avantage particulier de pouvoir faire une mesure dans une petite région. Par exemple, des mesures de densité ont été faites à des intervalles d'un pouce à travers une épaisseur de sédiments de trois pieds, le tout sous une profondeur d'eau de quinze pieds. Dans son stade de développement actuel, le détecteur double n'est pas aussi robuste que le détecteur simple et le détecteur double n'est pas recommande pour les mesures courantes en dehors du laboratoire. Le détecteur double permet, cependant, une mesure précise de variations de densité et augmente ainsi l'étendue et l'utilité de la méthode de la mesure des densités des sédiments au moyen de l'atténuation des rayons gamma. ABSTRACT Nuclear techniques were employed to measure the density of reservoir sediments in place, and the concentration of suspended sediments. A dual probe was used in these studies. This instrument uses two access tubes which house the radioactive, cesium-137, source and the detector system, respectively. The detector is a sodium iodide (thallium activated) scintillation crystal and the electronic read-out system employs a modified integral discriminator so that only the gamma rays exceeding 0.65 Mev are recorded by the scalcr-ratemeter. Excellent precision and accuracy were obtained with the dual probe in laboratory experiments. The maximum thickness of the measured volume of sediment was less than 1-inch, permitting the use of this technique for the measurement of densities of thin sediment layers and at phase interfaces. The performance of the dual probe was compared in a field survey with the single gamma probe which utilizes the reflection technique of measuring gamma attenuation. The dual probe performed satisfactorily in the field and in addition offered the unique advantage of determining densities of narrow sediment bands and at interfaces. Sediment densities were determined at 1-inch intervals through sediments up to 3 feet deep beneath as much as 15 feet of water. In its present state of development the dual probe is not as rugged as the gamma probe and is not recommended for use in routine field surveys. However, the dual probe provides the means of measuring accurately the variation of density within a sediment and near interfaces and as such extends the utility of the gamma attenuation method of measuring sediment density. (*) Research cooperative with the University of Mississippi and Mississippi State University. For presentation at meeting of International Union of Geodesy and Geophysics, Berkeley, California, August, (* ) Soil Scientist, USDA Sedimentation Laboratory, Southern Branch, Soil and Water Conservation Research Division, Agricultural Research Service, US Department of Agriculture, Oxford, Mississippi. 189

2 INTRODUCTION The convervcrsion of a measured volume of sediment to a weight basis is dependent on the density of the sediment. The bulk densities, or volume weights, of sediments are readily determined when such deposits occur above low-water elevations. Many types of samplers have been developed for taking undisturbed samples for use in determining the density of sediments. However, the conventional methods of determining the densities of underwater sediments have not been entirely satisfactory. The operation of core-type samplers in underwater sediments, particularly those sediments of low density,has not produced consistent results (Hvorslev, 1949). The use of various nuclear techniques for measuring the densities of sediments has been reported. This paper reports the development, calibration, testing, and application of a dual probe employing radioactivity for the measurement of sediment density and concentration in situ. 2. DEVELOPMENT AND THEORY OF OPERATION Nuclear techniques have been employed in industry for nondestructive measurements of density and thickness of various materials (Miller, 1958 ; Kcinath, 1958 ; Nickerson, 1958). Single-probe devices, utilizing the reflection principle, have been used for measuring sediment densities in situ (Trimblin & Florey, 1957; Caldwell, 1960; Cameron & Bourne, 1958). Excellent results have been obtained with the single gamma probe in measuring the bulk densities of soils in place (Timblin, 1955 ; Phillips, Jensen, & Kirkham, 1960 ; Homilius, 1958),"and recently the principle has been applied successfully in the study of underwater sediments (McHcnry, 1962 a; Hcinemann, 1962). One disadvantage of the single-probe device is that both the radioactive source and the detector are contained in the same tube (probe). Consequently, it is necessary to have shielding within the probe to prevent the direct transmission of the gamma rays from the source to the detector. This physical separation of source and detector in the vertical axis prevents the accurate determination of densities of narrow bands of sediment or near interfaces (McHenry, 1962a; Van Bavel, 1960). The transmission method of measuring density, or thickness, by gamma attenuation has been widely adapted commercially (Miller, 1958; Keinath, 1958; Kohl et al, 1961). This method requires two access tubes ; one tube contains the radioactive source, the other contains the detection system. The attenuation of the gamma rays emitted from the radioactive source is measured by the detection system. With the proper apparatus and instrumentation it is possible to determine the m situ density of thin or narrow layers of underwater deposits. Van Bavel and co-workers (Van Bavel, et al 1958; Van Bave!, 1959) have utilized the direct transmission for the measurement of soil density. The possibilities for using a dual-probe system for the measurement of sediment densities have been investigated by the USDA Sedimentation Laboratory (McHcnry, 1962b). The absorption, or attenuation, of gamma rays on passage through a medium is expressed by the equation : / = / oe -^ (1) where / is the intensity of the transmitted beam of photons of initial intensity, / 0, passing through a thickness x of a material with an absorption coefficient of //,. The absorption coefficient is a function of both the energy of the photons and of the atomic number of the absorber. In this study, the thickness factor, x, was maintained constant. The emission energy of the gamma rays used was also constant as only cesium-137 gamma rays were 190

3 produced by the radioactive source. Variations in the intensity of the transmitted gamma rays were therefore a function of changes in the absorption coefficient. The absorption coefficient of a material is the product, N a, of the number of atoms, N, per unit volume and the cross section, a. The mass absorption coefficient is represented by N ajq where g is the density of the absorber. The linear absorption coefficient is defined as N a total- In the reported study similar materials were used so that N a did not change significantly. Changes in the measured photon intensity of the transmitted beam were, therefore, an inverse function of g, the density of the absorber medium. 3. METHODS AND MATERIALS 3.1. Dual probe The dual-probe system used in this study consisted of a gamma source, a scintillation detector, a fixed dual access-tube assembly for spacing the probe and source, and a suitable power supply and electronic read-out device (Fig. 1). The gamma radiation was produced by a 7-millicurie cesium-137 source placed in the smaller access tube. A sodium iodide (thallium activated) scintillation crystal (1-inch diameter x 1/2-inch high) was employed to detect the gamma emissions. The distance between the source and detector crystal was maintained constant, 12-inch center to center, by the jig arrangement shown in figure 1. The rods that supported the source and the probe in the access tubes were marked at 1-inch intervals for ease in positioning Fig. 1 A dual-probe system for determining density by the attenuation of cesium- 137 gamma emissions. (1) Lead cask for storage of radioactive source when not in use; (2) aluminum rod used to position source; (3) aluminum tubing with cable connections and probe attached; (4) scaler-ratemeter; (5) dual access tube assembly with 12-inch spacers ; (6) charger for sealer battery ; (7) connecting cables ; and (8) magnesium bar for calibration. Personnel exposure badges are not shown. 191

4 the probe and source at the desired depth. The two rods were held by a common clamp so that when one rod was raised or lowered the other moved accordingly. In operation the source was placed in the plane which passed through the center of the detector crystal. The projection of the area presented to the source by the scintillation crystal was a rectangle, 1-inch x 1/2-inch. The solid angle subtended from the source to the detector crystal approximated a pyramid whose base was the projection of the scintillation crystal and whose altitude was 12-inches. Any gamma ray emitted in a direction outside this solid angle, and which was deflected into this solid angle, could not arrive at the detector with an energy of the emitted ray. By electronically discriminating against all photons of energies less than about 0.66 Mev, the energy of the photons emitted by the cesium-137 source, the detection of photons by the scintillation system becomes a function of the mass of the medium through which the solid angle is subtended. Electronic discrimination of the type described is usually achieved by the use of a pulse height analyzer. At the time this study was performed, a pulse height ana- PREAMPLIFIER PHOTÖMULTIPLIER TUBE SCINTILLATION CRYSTAL - Fig. 2 Schematic representation of the two-probe system employed for measuring density of sediment utilizing attenuation of gamma rays. lyzer compatible with the electronic units used was not available. (*). It was possible, however, to achieve partial discrimination by adjustment of the voltage and amplifier gain controls. Since the manufacturer had determined the radiation intensity transmitted through the standard magnesium bar from the cesium source by the use of a pulse height analyzer, it was possible to attain the desired discrimination by reproducing ( ) Tests are currently being made using a pulse height analyzer designed for use with the described dual-probe system. 192

5 this radiation intensity. By adjusting the high voltage and amplifier gain controls, discrimination against pulses of energy less than about 0.65 Mev was obtained. In effect, integral discrimination occurred. Most of these photons transmitted directly from the source to the detector, within the solid angle subtended by the scintillation crystal and which suffered no loss in energy, would be counted. Those photons absorbed, scattered, or reflected in passage through the medium between source and detector would not be measured because of their lowered energy. The sealer used in this study was a Troxler Model 200 B. (*) This instrument combines an adjustable high voltage power supply, a sealer, a ratemeter, a voltmeter, and an amplifier in one unit. It is possible to operate the unit from a dry cell or from alternating current The sealer is transistorized and in the field operates from the rechargeable silver-cadmium dry cells The unit, complete with batteries, weighs 22 pounds and is easily portable. A sketch of the equipment in use is shown in Fig EXPKRIMENTAL RESULTS 4.1. Calibration The dual probe was calibrated in the laboratory using the density of a magnesium bar, specific gravity = 1.75, and of water, specific gravity = 1.00, as the two primary measurements. The known density was plotted as a function of the log of the ratio of the intensity of the gamma rays passing through the test medium to the intensity of the gamma rays passing through the magnesium bar. A calibration curve was constructed through these points (Fig. 3) whose equation was : X = (log Y) (2) where y was the ratio, measured counts/counts through magnesium, and A"thedensity Laboratory determination of sediment density One of the objectives of this study was to develop a technique for determining densities of thin layers of sediment. To this end a three-phase system of sediment, water, and air was prepared. Soil and water were mixed in a 55-gallon steel drum and allowed to stand. After 24 hours, the soil had settled to the bottom of the drum, as sediment, and was covered by relatively clear water. The dual probe was placed in the drum and measurements of radiation intensity were made at 1-inch increments from the bottom of the container to the top. The results arc presented in figure 4. The observed counts per minute plotted against depth indicate three distinct levels of radiation intensity representing sediment and water, water, and air The observed radiation intensity readings in the sediment-water mixture are constant from 0 to 5 inches. The recorded measurement at 6 inches indicates the transition from a wet sediment to water occurs from 5 1/2 to 6 1/2 inches. The measurements from 7 to 10'inches indicate another region of homogeneous medium; in this case, water. The radiation intensity at 11 inches is slightly higher. At 12 inches the radiation intensity is much greater it is that observed with the dual probe in air. The transition from water to air occurred between 11 and 111/2 inches. These data substantiate the calculation that the thickness of the solid angle subtended from the source to the detector crystal did not exceed 1 inch. ( ) Trade names and company names are included for the benefit of the reader and do not infer any endorsement or preferential treatment of the product by the United States Department of Agriculture. 193

6 I g is cc X= (logy) DENSITY Fig. 3 Calibration curve obtained for the two-probe system using water and a magnesium metal bar as control points. A number of soil-water suspensions of various densities were prepared by thoroughly stirring weighed amounts of soil with a known weight of water. The total volume of the suspension was measured. The densities of the various suspensions were then measured with the dual probe and these values were compared with the computed density values. The results are shown in figure 5. The agreement between calculated and measured density values of the suspensions is good, although some variation is noted at the lower density values. In these tests, a 5-minute count was used for the measurement of radiation intensity. The soil used in the suspension contained sand and coarse silt so that some settling of these particles occurred during the period of measurement ; hence the variation in observed radiation intensity. The maximum observed variation between the calculated and observed densities was 0.87 absolute density units. This corresponded to 4.4 pounds per cubic 194

7 OBSERVED COUNTS PER MINUTE, IN THOUSANDS Fig. 4 The variation of observed radioactivity with changes in density of the surrounding medium UJ tu v> 5 UJ S g V) 1.3 < 12 m il (O 1.0 / Q 1.0 o h/ c c I.I CALCULATED DENSITY Fig. 5 Comparison of measured to calculated densities of prepared sediments using the two-probe system. t 9 /

8 foot. It was possible to reduce the maximum observed difference to less than 0.03 density units either by eliminating sand from the sediment material or by increasing the viscosity of the suspension by the addition of bentonite as sediment. Total erosion in terms of soil weight is predicted from sediment density data. A test was made to determine the efficiency of the dual probe for this purpose. Weighed amounts of sand and/or clay were added to a known volume of water, the contents thoroughly mixed, and allowed to settle. Measurements of the density of the sediments were made with the dual probe at 1-inch vertical increments, from the bottom to the top of the system. The total weight of the sediment was computed on the basis of the observed density readings with the dual probe and the known volume of the system at each depth. The results are summarized in table 1. Good agreement between the added and calculated soil weights may be noted. Not all of the observed difference in soil weight, of course, can be ascribed to the dual-probe measurements. Errors are also involved in the volume and weight measurements. TABLE 1 The determination of total sediment weight from density measurements made with the dual probe (Static laboratory tests using summation of 1-inch increments) Total Sediment Weight (Dry) System Amount Added lb Amount Calculated lb Grenada silt loam soil Sand Sand, duplicate Sand, 100%, clay, 0 Sand, 75%, clay, 25% Sand, 50%, clay, 50% Sand, 25%, clay, 75% Sand, 0, clay, 100% Laboratory determination of sediment concentration The use of the gamma transmission method for measuring the concentration of suspended sediments in water was also investigated. Because of the low concentration of the absorber material of interest, that is sediment, and the high penetrating power of the relatively energetic cesium-137 photons, Mcv, the transmission method was of questionable sensitivity. Previous workers (Morgen, et al, 1955) as well as current investigators (*) have considered the use of beta bremsstrahlung, which is more highly absorbed on passage through sediments and sediment-bearing water. Preliminary work performed in the USDA Sedimentation Laboratory indicated that gamma attenuation techniques could be employed for the measurement of sediment concentration provided integral or differential scintillation spectrometry was used (McHenry, 1962b). (*) Parametrics, Inc., Waltham, Mass., is currently working on the development of equipment for this purpose. 196

9 A series of samples was prepared in which the percentage by weight of suspended material ranged from 5 to In order to maintain the larger particles in suspension, montmorillonite was added with the soil material used as the sediment source. After thoroughly mixing the suspensions, measurements of the concentration of suspended sediment were made with the dual probe. The results are plotted in figure 6. In this particular test the plot of the measured activity density as a function of the percentage of sediment in suspension was nearly linear from 5 to 0.25% sediment >- o "o~ - ^ C) < STATIC PERCENTAGE OF SEDIMENT IN SUSPENSION Fig. 6 The determination of the percentage of sediment in suspension by the dual probe in a static system. Studies were made in a 50-foot recirculating flume of the effectiveness of the dual probe in measuring sediment concentration of a flowing stream. The range of sediment concentrations used in the flume was the same as that used in the previously described static test. The flume was operated at a discharge of 0.65 cfs. After each addition of sediment material to the flume the flume was operated at the given discharge rate for a period of 30 minutes before measurements were made. The results are shown in figure 7. Each recorded point is the average of five I-minute counts of radiation density. The degree of linearity of the observed datais less than that observed for a static system. These preliminary studies were considered a measure of the feasibility of the gamma transmission method of determining sediment concentrations. As such the results are preliminary and conclusions tentative. Additional studies are underway and a complete report will be made upon their completion Determination of moisture The measurements of sediment density and concentration were reported on a total, wet, basis. All conversions to dry weight were based on an assumed specific gravity of For a given sediment whose density is constant, a variation from time to time in the observed gamma-ray attenuation of the total mass would be a function of the moisture content. A measure of gamma attenuation as a function of moisture content is shown in figure 8. The results indicate excellent linearity for a plot of radia- 197

10 o o o o 33 0 < a. 3.2 FLUME -o PERCENTAGE OF SEDIMENT IN SUSPENSION Fig. 7 The determination of the percentage of sediment in suspension by-the dual probe in a recirculating flume Xo I-O.O73Ï>X d fi1* r inches of water per foot % Moisture by Volume N X 50 Fig. 8 The determination of the percentage of moisture by volume with the dual ' probe.

11 tion intensity as a function of moisture content. For the experimental system used a I % change in moisture was equivalent to a differential of 88 counts per minute, assuming the total system activity approximated 10,000 c.p.m. The standard deviation of a count this size is 1%. Thus the dual-probe system could be used for determining moisture of sediments (if the density were known) but the sensitivity of the method would be in the order of 2% Field determination of sediment density Field measurements of sediment density were undertaken with the dual probe as a result of the very encouraging laboratory tests. In 1959 and 1960 sedimentation surveys were made on a small reservoir near Oxford, Mississippi. A single gamma probe was used in these surveys to measure the sediment density (McHenry, 1962a). This reservoir, known as the "Power Line Dam", offered a suitable and convenient test area for a study of the applicability of the dual probe for field use. Accordingly, a survey of the Power Line Dam was made in Density measurements were made with both the single and dual probes on the established ranges. The agreement between densities obtained by the two probes was generally good. Typical of the data obtained is that shown in figure 9. The measured total wet density was plotted as a function of depth. The measurements with the single probe 8 ui ui UI o < 10 SINGLE PROBE x ui UJ m i UJ û Fig. 9 dual POWER LINE '62 RANGE: N-M LOCATION^ 50 FEET I.I MEASURED WET DENSITY A comparison of the total wet densities as determined with the single and probes. The densities arc plotted as a function of profile depth. were taken at 6-inch intervals. Measurements with the dual probe were made at 3-inch intervals except that at the water-sediment interface and near the bottom of the access tube, the measurements were made at 1-inch increments. As reported, the measurements with the dual probe represent the density of a 1-inch layer, whereas 199

12 the single probe integrates the density of an 18-inch section of the profile (McHenry, 1962a; Heinemann, 1962). This latter range, or sensitive volume, is indicated in figure 9 for two single-probe measurements by the vertical lines drawn through the center point. Actual variations of sediment density within a vertical profile are readily illustrated by the dual-probe results shown in figure 10. Variation of density with depth was obtained with the single probe, but the results were inaccurate because of the long sensitive length of the probe. A true representation of the variation of density within the vertical profile cannot be obtained with the single probe. it 5 o if rr =5 o UJ m tui o POWER LINE '62 RANGE: C-D LOCATION: 100 FEET DUAL PROBE MEASURED WET DENSITY Fig. 10 A comparison of changes in density in a sediment profile as determined by the single and dual probes. Information concerning the existence of a viscous, or semi-solid, layer above consolidated sediments could not be ascertained with the single probe because of this lack of sensitivity. However, the dual probe has shown positively such existence in this particular reservoir. In both figures 9 and 10 the dual-probe measurements indicate the existence of 3 to 4 inches of materials markedly less dense than the bulk of the underlying sediment. The data presented in figures 9 and 10 are typical of that obtained in the 1962 survey of Power Line Dam. The agreement in the data obtained with the two probes was in the order illustrated. However, the field operation of the dual probe was less dependable than that of the single probe. It was necessary to repeat measurements frequently with the dual probe in order to obtain the required precision. A scintillation detector is highly dependent on the high voltage applied and so frequent checks were necessary to ascertain the operating characteristics of the scintillation detector in the dual probe. Frequent adjustments of the high voltage gain control were necessary to maintain the proper.sensitivity and precision. 200

13 The sediment depths shown in figures 9 and 10 are average for the present conditions in Power Line Dam which was constructed in It was possible to insert the two access tubes, necessary for dual-probe operation, to depths comparable to that attained with the single probe. To date the dual probe has not been operated at total depths below water surface greater than 20 feet nor in sediments greater than 4 feet in depth. Changes in techniques of operation will be necessary when such studies are initiated. 5. CONCLUSIONS With the above-mentioned exceptions the performance of the dual probe under field conditions has been equal or superior to that of the single probe. Both nuclear probes offer a unique means of measuring reservoir sediment densities in situ with an accuracy and precision not previously possible. In addition, the dual probe offers a means of determining densities of very thin layers and of determining accurately the boundaries between solid, semi-solid, and liquid phases. Other nuclear methods have been advanced for density measurements (Keinath, 1958 ; Morgen, et al, 1955; Kohl, et al 1961), but these have not shown the utility and ruggedness of gamma attenuation measurements as exemplified by the single and dual probes. In its present state the dual probe represents an early development stage. Modifications of, and improvements and additions to, the instrument will extend its utility and improve its performance and durability for use in reservoir sedimentation studies. REFERENCES CALDWKLL, J. M., Development and tests of a radioactive sediment density probe, Dept. of the Army, Corps of Engineers, Beach Erosion Board, Tech. Memo. 121, 29 pp., illus., appendix, CAMERON, J. F., and M.S. BOURNE, A gamma-scattering soil density gauge for subsurface measurements, Internatl. J. of Applied Radiation and Isotopes, 3, 15-19, HEINEMANN, H.G., Volume-weight of reservoir sediment, J. Hydraulics Div., Amer. Soc. Civil Engineers, Proc, (Sept.) , HOMILIUS, J., S. LORCH, and K. SEITZ, Radioactive density determination with the gamma-gamma-probe, AEC-tr-4099 (Trans, from Geologisches Jahrbuch, 75, ), HVORSLEV, M.J., Subsurface exploration and sampling of soils for civil engineering purposes, Dept. of the Army. Corps of Engineers. Waterways Expt. Sta. (Vicksburg), 521 pp., illus., KEINATH, G., The measurement of thickness, Nat. Bur. Stan. Cire., 585, 79 pp., illus., KOHL.J., R.D. ZENTNKR, and H. R. LUKENS, Radioisotope Applications Engineering, D. Van Nostrand Co., Inc., New York, 562 pp., illus., MCHENRY, J.R., Determination of densities of reservoir sediments with a gamma probe, U.S. Dept. Agri., ARS 41-53, 10 pp., 1962a. MCHENRY, J.R., Determination of sediment density by gamma attenuation, U.S. Dept. Agri., ARS 41-61, 16 pp., 1962b. MILLER, G.J., Radiation gage techniques for measurement of density variation, Sandia Corporation Tech. Memo , MORGEN, G.W., et al, Radiation analysis, U.S. Patent No. 2,722,609, Nov. 1, NICKERSON, R.A., The fundamentals of differential radiation measurements, J. Soc. Non-destructive Testing, 16, 24-7, 41, PHILLIPS, R.E., C. R. JENSEN, and D. KIRKHAM, Use of radiation equipment for plowlayer density and moisture, Soil Sei., 89, 2-7, TIMBLIN, L.O., Jr., Measurement of subsurface soil density by gamma ray scattering, U.S. Dept. Interior. Bur. of Reclamation, Chem. Eng. Lab. Rpt. SI-6, 12 pp., illus.,

14 TiMBLiN, L.O., Jr., and Q.L. FLORE Y, Density measurement of saturated submersed sediment by gamma ray scattering, U.S. Dept. Interior. Bur. of Reclamation, Chem. Eng. Lab. Rpt , 34 pp., illus., VAN BAVEL, C. H. M., Soil densitometry by gamma transmission, So/7 Sei., 87, 50-8, VAN BAVEL, C. H.M., Soil density measurement with gamma radiation, 1th Internatl. Cong. Soil Sei., Proc, Vol. 1, , VAN BAVEL, C. H.M., N. UNDERWOOD, and S. R. RAGAR, Transmission of gamma radiation by soils and soil densitometry, Soil Sei. Soc. Amer. Proc, 21, ,