In situ measurement of marine sediment density by gamma radiation

Deep-Sea Research, 1968, Vol. 15, pp. 637 to 641. Pergamon Press.

Printed in Great Britain.

In s l t u measurement of marine sediment density by gamma radiation

K . PREISS*

(Received 8 May 1968)

INTRODUCTION

BULK density is an important mass physical property of the water-saturated sediments of the sea floor, Research on the bulk density distribution within 1 or 2 meters of the water-sediment interface has been severely hampered by the difficulty of measuring the weight and volume of small quantities of soft sediment. To overcome this difficulty, gamma-ray transmission equipment was designed to measure, non-destructively, the bulk density of sediment cores in the laboratory (PREISS, 1968). However, because the water-sediment interface may be disturbed in even the best of cores, it was considered necessary to obtain in situ measurements. This paper describes the equipment used to make such in-place density measurements, together with the results of observations made during July 1967 from the U.S.C.G.S. Davidson, in the Wilkinson Basin, 100 k m east of Boston. The sediment in the Basin is a silty clay at a water depth of 280 m. Density may be measured either by gamma-ray scattering or transmission. With the former method the thickness of sediment layer " s e e n " by the probe is greater than 50 c m (KELLER, 1965). With the transmission technique the vertical thickness of the layer of sediment " s e e n " is equal to the thickness of the scintillation (detection) crystal, which is usually between 2 and 5 cm (PREISS, 1967). Since measurement of vertical variation of density was an important requirement for this equipment, the transmission method was selected. The principle of this method is well-established and has been described in previous papers (PREISS, 1968; MCHENRY and DENDY, 1964).

THE

APPARATUS

The apparatus consisted of a two-legged probe, one leg of which contained a gamma-ray source and the other a scintillation detector, attached to a 4-meter high tower. The tower was lowered from the ship at the end of a six-conductor armored cable until it stood on the sea floor; the probe was then pushed into the sediment by a n electric motor and drive system. The apparatus was controUed from the ship. The detected count rate, which was a measure of density, was observed on the ship, and a profile of the density obtained as the probe was pushed into the sediment. The probe is attached below the tower (Fig. 1). It consisted of two aluminum legs, 51 m m (2 in.) o.d. and 44 m m (1.75 in.) i.d., held 267 m m (10.5 in.) apart by a stainless steel bridge piece. One leg contained a tungsten alloy shield, within which was a source of 50 mc cesium-137. The cesium source was sealed in a stainless steel welded capsule, which had been pressure tested up to 70 kg/cmL A hole in one side of the shield permitted the radiation to penetrate through the sediment to the detector, while being shielded in other directions. The second leg contained a scintillation detector, within a stainless steel pressure-resisting container. The detector included a sodium iodide crystal, 25 m m (1 in.) thick by 19 m m (0.75 in.) diameter, and a photomultiplier tube. The detector was connected by a single underwater coaxial cable to an electronic package, which was in a pressure-resisting aluminum container on the tower. The source and detector were arranged so that the center of the crystal was opposite the hole in the tungsten shield. *Negev Institute for Arid Zone Research, Beer-Sheva, Israel. Illinois, Urbana, Illinois 61801, U.S.A. 637

Previously at the University of

638

Instruments and Methods

The electronic detector system consisted of the usual circuits for a scintillation counter (high voltage supply, linear amplifier, single channel pulse height analyser), but included also a special stabilizing circuit to minimize instability. This instability would be primarily due to temperature changes as the apparatus was lowered through the sea. After the pulse height analyser the pulses were passed to a count-down circuit which was arranged so that at every 32nd detected pulse, a single~ square, 12V pulse was sent up the armored cable to the ship. This pulse was then passed into am oscilloscope, which triggered and passed a pulse to a standard scaler-timer. The probe was attached to a 5 cm (2 in.) dia. stainless steel shaft, which was held in the tower as shown in Fig. 1. When the tower was lowered onto the sea floor an electric motor was activated by a relay system operated from the ship, and the shaft was driven into the sediment. Magnetic proximity switches were provided at approximately 30 cm intervals on the tower; each of these automatically stopped the motor when it was reached. By monitoring the drive motor with a voltmeter aboard the ship, the depth of the penetration of the probe could be recorded. The motor could be reversed to retract the shaft. Power for the motor was supplied by a modified lead-acid battery on the tower. l-he detector was powered by low temperature mercury cells within the detector electronic package. CALIBRATION

Before proceeding to calibrate the equipment in materials of different density, it was necessary to check the stability of the equipment and to determine exactly what volume o f material was seen by the probe. The latter was checked by placing the probe in water and moving a bar of aluminum in front of the detector. The count rate decreased only when the bar was in front of the detector crystal, as was expected. Stability was checked by taking a number of counts with the probe in a constant situation, for instance, in a tub of water, and checking the standard deviation of these counts. A greater scatter than expected would indicate instability; a smaller scatter might indicate that an unwanted regular noise signal was being detected. Checking of stability was an intrinsic factor in planning an experiment. Calibration counts, for instance, were repeated twenty times and a statistical test made on the readings to check that the scatter observed could reasonably have been that expected. The probe was calibrated by observing the count rate with the probe in liquids of different densities. The liquids used were sea-water, and solutions of CaCl2 and ZnClz in water. The density of each calibration liquid was corrected for the difference in composition between the liquid and the sediment, by using the " equivalent density" (PREISS, 1968) of each liquid, rather than the actual density. The equivalent density is the density of liquid normalized to the composition of the sediment. It depends both upon the elemental composition of the sediment and its water content; however (HARMSand CHOQUETrE, 1965; PREISS, 1967), calculations show that for most minerals the chemical composition effect is negligible. In this particular instance a chemical analysis of a sediment which had been obtained previously from the Wilkinson Basin (PREISS, 1968) was used. It would probably be sufficiently accurate to use this analysis for any sediment which does not b.ave a high. concentration of organic hydrogen, or of heavy elements such as lead; if the sediment does include these elements, a calculation of equivalent density based on the actual composition should be made, as shown in a previous paper (PP,Eiss, 1968). In order to obtain an absolute accuracy of calibration of the order of 1 ~ , the water content of the sediment must be known to an accuracy of about 3 0 ~ (Premiss, 1968). However, only the absolute value of density, and not the shape of the density profile, is affected by errors in the water content or chemical composition used to calculate the equivalent density. The equipment was calibrated on three different occasions and gave identical calibration curves. The sea water was held in a drum 56 cm dia. and 36 cm deep. Figure 2 shows the calibration curve obtained. Even with the stabilization system referred to previously there was still a small change of count rate with temperature. This effect of temperature is equivalent to multiplying the count rate by a factor almost equal to one. Since count rate in Fig. 2 is on a logarithmic scale, multiplication by a factor is equivalent to displacement by a constant quantity. In order to convert the reading at the sea temperature to density, the reading obtained in the sea just above the floor was used as a bench mark. From this point a line parallel to the previously obtained calibration curve was drawn and this new line used to convert count rate to density.

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Fig. 1. The apparatus. The probe has been extended and is seen below the base of the tower. The motor driving system and the detector electronics are on the platform o f the tower. [facing p. 638]

Instruments and Methods

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CALIBRATION CURVE COUNT RATE vs ~'e • Seo Woter in Container [] Ca C12 Solution zx Zn Cl 2 Solution 5(;

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Count Per Sec.

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Fig. 2. The calibration curve : B - - i n the sea just above the sea floor. USE

AT

SEA

After arriving on station the tower was lowered from the ship, and held several meters off the bottom until twenty 10-see counts were obtained in the cold water. These were checked for stability. The tower was then lowered until it rested on the sea floor. When the tower was upright two marks were observed on the precision fathometer recorder on the ship. One was the sonic echo from the sea floor and the other the echo from the top of the tower 4 meters above. The echo sounder chart was therefore checked to see if it appeared that the tower was vertical. Readings obtained from a tilt gauge mounted on the same tower, when it was used with vane shear apparatus, had shown that the tower usually did stand vertical. The drive motor and scaler-timer were then simultaneously started. Counts were accumulated for an interval of ten seconds, with five seconds allowed between counts for recording. The probe penetrated 2'7 cm during the 10 sec for which counts were taken, and 1"3 cm during the 5 sec between counts. Counting continued until the drive stopped after 30 cm, when a magnetic proximity switch was reached. This driving and counting process was repeated for each 30 cm of depth, until the full penetration was reached. The probe was then retracted, the tower lifted off the sea floor and twenty 10-see counts takert to see that the count rate in the sea water had not changed. By using this procedure the location of the radiation beam, and thus the sediment " s e e n , " was known relative to the tower for each reading. The experiments did not always go as smoothly as indicated above. Net penetration time was 10 min in all experiments, but the time required to retract the probe reached as much as 15 rain. On one occasion the tower was pulled over while the probe was fully extended; the shaft was bent and could not retract. The first count o f each 30 cm was unreliable; on some occasions it was noticed that pulses in the conductor to the control relays induced spurious pulses in the conductor carrying the nuclear information. The first count of each 30 cm drive was therefore rejected, even wb.en it did not appear anomalously large.

640

Instruments and Methods

The batteries powering the radiation detector were unreliable and did not always survive an experiment. Of seven profiles measured, the final count in water was observed to remain stable on only two occasions. On three occasions the count rate changed due to battery failure after the profile had been observed, while retraction was in progress. For one experiment the count was stable, but at a value I 0 ~ lower than usual; and on one occasion battery failure resulted in rejection of the observations. R E S U L T S

Figure 3 shows a typical profile observed. The count rates were converted to density by using the calibration curve B of Fig. 2. A core had been obtained from the same location oll a previous cruise. Densities measured on the core by laboratory gamma-ray apparatus referred to previously are shown in Fig. 3 as squares. No allowance was made for core shortening in these plots. Positions indicated on the figure were from Loran observations, with accuracies of about 1 mile (1.60934 km). Density, g/era ~

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Fig. 3. Density vs. depth into the sediment. Solid circles represent in situ readings; hollow circles represent repeated in situ readings (see text); solid squares represent measurements made on cores by laboratory gamma-ray apparatus. No allowance has been made for tower settlement or core shortening. Table 1. Differences between density measured in situ and on cores. Difference in situ - core (4 g/cm 8)

Number o f comparisons (n = 57)

0.03 0.02 0.01 0.00 --

0.01

0.02 -- 0-03 --

R.m.s. difference J ( ~ - ~ ) =

4 10 8 13 14 6 2 0-016g/cm 3.

The first reading obtained, when driving, was in water. As the probe entered the sediment the reading changed. The zero position of the depth readings, as plotted, was inferred from the density readings, and may be in error by as much as 3 cm. Irt Fig. 3 the probe was driven 59 cm, retracted fully, then driven the full 149 cm. The solid circles are the readings in undisturbed sediment, i.e., the first 59 cm and the last 90 cm of the second

Instruments and Methods

641

penetration. Observations from the first 59 cm of the second penetration are shown as hollow circles. When the probe was driven the first 59 cm the bridge piece penetrated so that its lower edge was 16 cm lower than the base plate of the tower. On the second penetration, lower readings were obtained for the first 13 cm, and then the readings matched the previous run. This leads to the conclusion that repeated observations gave the same density in undisturbed sediment. It may also be inferred that at this location the tower settled approximately 3 cm during the first retraction. No allowance was made for tower settlement in plotting Fig. 3. Altogether, six profiles were measured in the Wilkinson Basin, all at locations from which cores had. been previously taken, and the densities measured by the gamma-ray apparatus in the laboratory. To check the in situ values of density obtained, the differences between 57 measurements on cores and the in situ observations were examined. Density readings were taken at 10 cm intervals on the cores, starting from the sea-sediment interface. These were compared with in situ readings from nominally the same locations, at the same depths. Readings for the upper 30 cm were neglected since the results showed a possibility that the cores had. been disturbed over this region. The results are shown in Table 1 from which it may be seen that all the comparisons were within 0.03 g/cm 3. The r.m.s, difference between the two methods was 0.016 g/cm 3, which is about 1 ' 2 ~ . It is difficult to infer the absolute accuracy of the in situ probe from this information, since the cores may have been disturbed, and were not from precisely the in situ locations. However the accuracy of the apparatus may be taken as of the order of 1.2 ~ . CONCLUDING

REMARKS

Although some difficulties were encountered in the first sea trials, the in situ density probe operated successfully. The probe penetrated the full design distance, and the densities obtained agreed well with densities obtained from cores. The r.m.s, difference between 57 comparisons of core and in situ density was 1.2%.

Acknowledgements--This work was carried out as a cooperative effort between the Departments of Civil Engineering and Geology, and the Nuclear Engineering Program, at the University of Illinois, Urbana. It was sponsored by the U.S. Office of Naval Research as part of a project for which Professor A. F. R~CHARDS is principal investigator. The help of Professor V. J. McDONALD, Mr. J. STERNER and Mr. F. CLARtI~E in designing and operating the tower with its electrical and mechanical systems is acknowledged. The cooperation of Dr. G. H. KELLER of the Institute for Oceanography, ESSA, Miami, and L C D R K. W. JEFFERS and the company of the U.S.C.G.S. Davidson were instrumental in getting the in situ results. The measurements on the cores were made by Mr. R. PISKIN and Dr. A. F. RICHARDS. The nuclear detection system was supplied by the Texas Nuclear Corporation. REFERENCES

HARMS J. C. and P. W. CnOQUEa~rE (1965) Geologic evaluation of a gamma-ray porosity device. Society of Professional Well Log Analysts 6th Annual Symposium, 2, 37 pp. KELLER G. H. (1965) Deep-sea nuclear sediment density probe. Deep-Sea Res., 12, 373-376. MCHENRV J. R. and F. E. DENDY (1964) Measurement of sediment density by attenuation of transmitted gamma rays. Proc. Soil Sci. Soc. Am. 28, 817-822. PREISS K. (1967) Non-destructive measurement of the water content and density of sediment using radioisotopes. In: Marine Geotechnique, ADRIANF. RICHARDS, editor, Univ. Illinois Press, 307-318. PREISS K. (1968) Non-destructive laboratory measurement of marine sediment density in a core barrel using gamma radiation. Deep-Sea Res., IS (3), 401-407.