Appl. Radiar. Isot. Vol. 31, No. 7, pp. 563-565, 1986 ht. J. Radiat. Appl. Instrum. Part A Printed in Great Britain

0883-2889/86 $3.00 + 0.00 Pergamon Journals Ltd

A New Method for Simultaneous Measurement of Soil Bulk Density and Water Content

G. S. MUDAHAR and H. S. SAHOTA

Soil Science Laboratories, Department of Physics, Punjabi University, Patiala-147002, India

(Received 12 August 1985; in revised form 4 February 1986)

A new idea is given for the dual-energy method of measuring the soil-water characteristics in which two different energies are taken from a single source. Compared with the previous dual-energy as well as the single-energy methods, this proposed single-source dual-energy method is more accurate and convenient, for the simultaneous measurement of bulk density and water content of soil.

Introduction

Gamma-ray-transmission methods have been very accurate and useful for the study of soil-water properties in the laboratory as well as in the field. Experiments have been conducted using a single monoenergetic source as well as two sources simul- taneously. Soane”) proposed a y-ray dual-energy method for the simultaneous measurements of water content and soil density using a broad beam. He studied the transmission of y-rays from the decay of 241Am and 13’Cs in turn through the same soil sample. Corey et ~1.‘~) attempted to improve upon this method by combining the two sources in a single collimator of l-cm diameter. The Compton contribution of higher energy photons to the low energy peak was also determined and corrected.

The use of two energies from two different sources can create problems such as positioning of the sources and proper collimation etc.

In the present work the two energies have been taken from a single source. For this purpose, a *03Hg source, emitting 74-keV x-rays and 279-keV y-rays was used. This single-source dual-energy method has been tested for the study of soil-water characteristics.

Theoretical Equations

The attenuation equation for a soil-water system using 74-keV photons is given as

(Z/Z,) = exp -(w, + @)x, (1)

where I, is the initial photon intensity, whereas Z is the intensity after transmission through moist soil of thickness x. pS and pW are the mass attenuation

coefficients of soil and water respectively. p, is the soil density (g/cm3) and 6 is the water content (g/cm3).

Similarly the attenuation equation for 279-keV photons through soil-water system is written as:

(Z’/Z;) = exp -@:p, + ~&,6)x (2)

From equations (1) and (2),

Pa = hln(ZlZ;) - &ln(Z/ZO)

(P&S + Z&Z&)x (3)

and

6 = p,ln(Z’/Z;) - Z&ln(Z/l,)

tiWP(: -Z&)x (4)

Thus if (Z/Z;) and (Z/Z,) are measured by experiment, pI and f3 can be computed.

Experimental Arrangement and Studies

The ‘OsHg source emitting 74-keV x-rays and 279-keV y-rays was housed in a lead container and properly shielded. A narrow beam of 0.5 cm diameter was used with the help of lead collimators (Fig. 1).

For the spectroscopy of photons a 3.81 cm height and 3.8 1 cm diameter NaI(Tl)-crystal scintillation spectrometer was used in conjunction with a lOOO- channel pulse-height analyzer (MCA), ECIL 38B. The resolution of the arrangement for 662-keV photons from the decay of 13’Cs was 12.5%. The MCA was calibrated using y-rays from the decay of *03Hg, ‘“Ba, **Na and 13’Cs sources.

A loamy sand soil sample was used in a 5 x 5 cm cross-sectional and 25cm-length perspex column that could be moved down and upward by rack and pinion arrangement. The bottom of the column was

563

564 G. S. MUUAHAK and H. S. SAHOTA

Pb shield rPb shwld

I!- No1 ITI) detector

Fig. 1. Lay out of the experimental set-up (not to scale).

pyramidal in shape with the provision of an outlet hole that could be closed (Fig. 1).

Dry soil (assumed to have no moisture) was packed in the perspex column at an average known density of 1.41 g/cm3. Water was applied to the surface up to the saturation point. When water was ponding on the surface, transmission measurements were made at different depths down the column.

Then the bottom of the column was opened, and the system was drained of water until there was a depletion of water. The trensmission measurements were repeated.

Results and Discussion

From the measured values of Saksena et u/.,(~’ Fishman et al.,@) and Mudahar and Sahota”’ it has been found that the mass attenuation coefficient pLs is approximately independent of moisture content and type of soil. According to Saksena et u/.,‘~) the variation in p(s for fine sand, medium sand and clay samples at a particular energy is less than 2%. Fishman et ~1.‘~) reported the spread in the p5 values for different soils to be less than 5% both for photons from the decay of 13’Cs and 6oCo. Our results”) on loamy sand and sandy soil confirm the above findings for three photon energies covering the region. In a separate set of experiments the values of p, and pw in

cm’/g were found to be equal to:

11~ = 0.1955 f 0.0030 IL,, = 0.1656 &- 0.0026

p: = 0.1030 + 0.0009 p; = 0.1107 f 0.0011

To test the applicability of this single-source dual- energy method separate experiments were carried out with known values of soil density and water content taking soil in 5-cm-cube perspex boxes, Applying a correction for the Compton contribution of higher energy under the low-energy peak, values of p, and 0 calculated from equations (3) and (4) are listed as follows with one estimated standard deviation:

Soil densit)~

I&cm’)

Known Measured 1.39 f 0.02 I .39 + 0.02 1.42 f 0.02 1.41 * 0.03 1.49 * 0.02 1.48 _t 0.05

Wuter content

(s/cm”)

Known Measured 0.002 * 0.0001 0.000 + 0.003 0.080 _+ 0.003 0.078 * 0.005 0.223 + 0.007 0.226 _t 0.010

Having confirmed the satisfactory performance of the single-source dual-energy method in the case of known water content and soil density, the method was used to measure the water content and soil density following infiltration in the column.

Measuring the values of (I/I,) and ([‘jr;) (as given in the previous section) and applying a correction for the Compton contribution of the higher-energy to the lower-energy peak, the values of water content and soil density were calculated from equations (3) and (4). The method for making the Compton-scattering correction is illustrated as follows.

The spectra were recorded first without any ab- sorber, except the soil sample, between the source and detector, and then with a graded absorber of lead, tin and copper to absorb the x-rays completely, and finally with a lead shadow shield of suitable thickness to stop the 279-keV y-rays also from reaching the detector. The lead stopper was placed in the path of the narrow beam from the source to the detector behind the soil column. It was larger in diameter than the beam and its purpose was to determine the scattered counts if any. The last pulse-height distribu- tion when subtracted from the previous two gives background subtracted spectra for (a) the sample in position with x-rays unattenuated and (b) the x-rays absorbed in Fig. 2. When x-rays are eliminated using graded absorbers, the y-ray peak is also attenuated. The ratio of the areas of the unattenuated and attenuated peaks gives a normalizing factor to correct spectrum (b), over the whole range of energy, to give spectrum (c), as shown in Fig. 2. The net x-ray-peak

Simultaneous measurement of soil bulk density and water content 565

4200 r

Channel number

Fig. 2. Different spectra from a l”jHg source used to correct for Compton-scattering, (a) spectrum of radiation trans- mitted through the soil sample, (b) x-ray suppressed spec- trum using graded absorbers of lead, tin and copper. and (c) corrected spectrum after raising each point of spectrum (b) with the normalizing factor found from the ratio or areas

of full energy peaks in (a) and (b).

0 (a) Water - saturated soil

2

4

6

i

. 0

. 0

. 0

8 t

. 0

10 1 . 0

12 . 0

14 - . 0

2 water SolI 16- content . 0 density

0 1 I I I

c 0

z (b) Soil at field capacity El 2 F l 0

0

16 . 0 I I 1 0.0 0.5 1.0 1.5 2.0

Water content or soil density (g/cm31

Fig. 3. Water content and soil densities determined by single-source dual-energy method for different moisture

stages.

area is now easily determined. The photopeak area of the y-ray is easier to compute.

The results of our experiments are shown in Figs 3a and b, where both the saturated and after-drainage condition results of column studies are shown. From these results, it is clear that the soil density at both stages (saturated and after-drainage) remains the same: this is because the soil particles do not rearrange during a wetting and drying process. Although this is the only major assumption of the y-ray-transmission method it offers no limitation for many agricultural soils. This point is also clear from the derivation of equation (3) because the moisture factor is eliminated.

This single-source dual-energy method is very use- ful and accurate and is suggested for in situ studies.

Kumar and Singh@) described a quick and accurate method of calibration for the measurement of soil moisture using a single-energy transmission method. Unlike that, our method does not require any calibration since well-collimated photon beams have been used. In comparison with previous dual-energy attempts (Soane”’ and Corey et al.‘*)) this is an im- proved method as the two energies are taken from the same source; because only one source is used, there is no problem of proper location and collimation, as may pertain for the measurements with two sources.

Applications

Besides the measurements of water content and soil density the single-source dual-energy method is

applicable to such studies as the swelling of soils, as well as the freezing and thawing processes.

1. 2.

3.

4.

5.

6.

References

Soane B. D. Nature 214, 1273 (1967). Corey J. C., Peterson S. F. and Wakat M. A. Proc. Soil Sci. Sot. U.S.A. 35, 215 (1971). Saksena R. S., Chandra S. and Singh B. P. J. Hydrol. 23, 341 (1974). Fishman A., Notea A. and Segal Y. Nucl. Instrum. Methods 184, 571 (1981). Mudahar G. S. and Sahota H. S. J. Hydrol. SO, 265 (1985). Kumar B. and Singh B. P. Int. J. Appl. Rudiat. Isot. 30, 493 (1979).