Scholarly article on topic 'Measurement of Soil Moisture Content at Microwave Frequencies'

Measurement of Soil Moisture Content at Microwave Frequencies Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Procedia Computer Science
OECD Field of science
Keywords
{"Complex Relative Permittivity" / "Microstrip Patch Antenna" / "Moisture Content" / "Free-space Transmission Technique" / "Topp's Equation"}

Abstract of research paper on Materials engineering, author of scientific article — R. Rajesh Mohan, Binu Paul, S. Mridula, P. Mohanan

Abstract This paper discusses the measurement of moisture content present in soil by computing the dielectric constant of soil samples at three frequencies, using microwave free-space transmission technique. This leads to the determination of inherent moisture content in the samples by applying Topp's Equation. The change in the value of dielectric constant and the subsequent increase in the moisture content for each of the soil samples with added moisture levels at 20%, 23.1% and 28.6% are noted. Dielectric constant of some common materials such as free-space, FR4, acrylic and water are measured to check the efficacy of the proposed method.

Academic research paper on topic "Measurement of Soil Moisture Content at Microwave Frequencies"

(8)

CrossMark

Available online at www.sciencedirect.com

ScienceDirect

Procedia Computer Science 46 (2015) 1238 - 1245

International Conference on Information and Communication Technologies (ICICT 2014)

Measurement of Soil Moisture Content at Microwave Frequencies

Rajesh Mohan Ra*, Binu Paula, S. Mridulaa, P Mohananb

aDivision of Electronics, School of Engineering. CUSAT, Kochi-682022, India. bDepartmentof Electronics, CUSAT, Kochi-682022, India.

Abstract

This paper discusses the measurement of moisture content present in soil by computing the dielectric constant of soil samples at three frequencies, using microwave free-space transmission technique. This leads to the determination of inherent moisture content in the samples by applying Topp's Equation. The change in the value of dielectric constant and the subsequent increase in the moisture content for each of the soil samples with added moisture levels at 20%, 23.1% and 28.6% are noted. Dielectric constant of some common materials such as free-space, FR4, acrylic and water are measured to check the efficacy of the p rop osed method.

© 2015 TheAuthors.PublishedbyElsevierB.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-reviewunder responsibility of organizing committee of the International Conference on Information and Communication Technologies (ICICT 2014)

Keywords: Complex Relative Permittivity; Microstrip Patch Antenna; Moisture Content; Free-space Transmission Technique; Topp's Equation

1. Introduction

When microwaves are directed towards a material, energy gets reflected or transmitted through the surface or absorbed by it. The proportions of energy, which fall into these three categories, have been defined in terms of material properties. Permittivity s and permeability ^ are the key parameters describing the interaction of materials with electromagnetic fields1. It has been found that permittivity is not only frequency dependent but also dependent on density, water content, profile, sampling depth, mineral composition, granular size distribution, porosity, boundary conditions, vegetation canopies and geographic conditions; some of these parameters, especially the last

* Corresponding author. Tel : +91 98461 38648. E-mail address: rm rcek@yahoo. com.

1877-0509 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of organizing committee of the International Conference on Information and Communication Technologies (ICICT 2014) doi:10.1016/j.procs.2015.01.040

few, are typical in the case of Soil. Dielectric profiles of materials are investigated in different parts of the frequency spectrum. Since recent research applications are concentrated at microwave frequencies, the work focuses in this range. At microwave frequencies, various non-resonant and resonant techniques are available for the measurement of dielectric constant of materials, which include transmission line, free-space, coaxial probe and cavity techniques.

The methods for measurement of soil moisture content are discussed in Section 2. The experimental setup and methodology are described in Section 3 and the results are discussed in Section 4. This is followed by the conclusion.

2. Methods of Measurement of Soil Moisture Content

The objective of this work is to discuss the measurement of moisture content present in soil by computing the dielectric constant of soil samples using microwave free-space transmission technique. Application of an electric field changes the electric charge distribution of a material. Dielectric permittivity is a measure of this change It is commonly expressed in relation to that of free space and is therefore termed as complex relative permittivity, s r. er, given in equation (1), describes the material behaviour in the electric field. The real part, s' is called the dielectric constant and the imaginary part, sr" is called the loss factor.

sr =sr'- jzr" (1)

Dielectric constant represents the ability of a material to store electric energy, while the loss factor represents the loss of electric-field energy in the material. Another parameter frequently employed is the loss tangent, defined as the ratio of the loss factor to the dielectric constant, as defined in equation (2).

tan 5 = Sr "/ Sr ' (2)

sr varies with frequency and characteristics of materials such as moisture content, temperature and density.

Soil is defined as the unconsolidated mineral material on the immediate surface of the earth that (i) serves as a natural medium for the growth of land plants and (ii) has been subjected to and influenced by genetic and environmental factors of parent material, climate (including moisture and temperature effects), macro and micro organisms and topography, all acting over a period of time and producing a product that differs from the material from which it has derived all the properties and characteristics. Microwaves are capable of penetrating more deeply into vegetation and soil as compared to optical waves. The depth of penetration is more for dry vegetation and dry soil. The penetration decreases with increase in the moisture content in vegetation and soil. This is due to the variability of dielectric constant of the dry material in the presence of water. The dielectric constant of dry soil varies between 2-3 depending upon the texture whereas the dielectric constant of pure water is around 80 at room temperature and at around 1 GHz. Thus the dielectric constant will vary between dielectric constant of dry soil and of the saturated soil which is around 30. Hence one can measure the moisture content in soil by computing the dielectric constant of the soil. The electromagnetic (EM) properties of soil are not only frequency dependent but also dependent on density and water content. Surface soil moisture is the water in the upper 10 cm of soil. In the two-way interaction between land and atmosphere, soil moisture is the second most important forcing function - the first being sea-surface temperature - and it becomes a significant factor in the summer months. Therefore the systematic study about the microwave sensing of soil properties has evolved as a major need of the hour 2-7.

Various mixture equations have been published for soil with different ranges of application. There are two categories of such equations. The first category is empirical equations for determining water content and/or soil density8. The second category is classified as volumetric mixing models, which are derived from discrete capacitor network theories or continuum mean field theories9.

Soil water content is expressed on gravimetric basis or volumetric basis.

• Gravimetric water content (0g) is the mass of water per mass of dry soil

Qg _ mwater _ mwet ~ mdry (3)

mSo,l mdry

• Vo lumetric water content (0v) is the ratio of volume of water to the total volume.

volume of water

0v =----(4)

total volume

The Topp's Equation connecting dielectric constant of soil with volumetric water content is derived from the second theory and is given as :

9v = 4.3 x 10~6xer '3- 5.5 x 10"4xer '2 + 2.92 x 102xer'- 5.3 x 102 (5)

3. The Experimental Setup and Methodology

As mentioned, the experimental setup is based on the free-space transmission technique, developed and tested successfully for the measurement of the dielectric properties of granular materials10. The measurement of attenuation and phase shift of microwaves traversing a layer of the soil sample gives the sr' and sr". In this work, a pair of identical coaxial probe-fed microstrip patch antennas (MPA) fabricated on a substrate (with sr' = 4.4, tan 8 = 0.02 and thickness = 1.6 mm) are made to transmit/receive. A slab-holder containing the soil samples, whose dielectric properties are to be determined, is placed in between. The study is conducted at three frequencies, viz. 1.85 GHz, 2.45 GHz and 5.35 GHz, in the L, S and C bands. Rohde & Schwarz ZVB8 Vector Network Analyzer (VNA) is used for measurement.

The antennas are connected to the VNA. The VNA is calibrated in transmission mode (response-type calibration) with a bandwidth of 300MHz, centred around the respective resonant frequency, with the empty sample holder between the two antennas. The sample holder is a box of rectangular cross-section made of acrylic, a material with a dielectric constant close to 3. Its dimensions are 10cm x 10cm x 0.3cm. After calibrating the VNA, each soil sample is inserted into the sample holder. Measurements of magnitude and phase of transmission coefficient (|S21| and O), give the attenuation A and phase shift O according to equations (6) and (7), where n is an integer to be determined.

A = 20log|S21|dB (6)

§ = §0-2nn deg (7)

The dielectric constant of each sample is computed using equation (8).

where c is the velocity of light (m/s), f is the frequency (Hz) and d is the thickness of the layer of the soil (m). Typical antenna geometries are shown in Fig. 1; dimensions for the three frequencies are given in Table 1.

Fig. 1. Probe-fed MPA

Two different cases are taken up for study :

• 62 different samples of 20 ml each of soil are tested; of these, 31 samples are acidic in nature (pH below 7.0), while the rest are alkaline (pH above 7.0). All the samples are dry and have varying Total Soluble Salt (TSS) and organic contents; the samples are the 'Udayamperoor' series from Ernakulam district which are moderately acidic in nature (pH between 4.7 and 6.4) and 'Anuppur'/'Agali' series from Palakkad district, which are moderately alkaline (pH between 7.0 and 7.5)11.

• All these samples are individually mixed with 5ml, 6ml and 8ml of water; the effect of extra-added moisture in soil from the variation in the dielectric constant is thus analysed. 5ml water in 20 ml soil corresponds to a 0v of 5/25 = 20%; similarly 6ml and 8ml water in 20 ml soil correspond to a 0v of 6/26 = 23.1% and 8/28 = 28.6% respectively.

Sr' of each dry soil sample is then calculated using equation (8). 0v in each case is computed using equation (5). This gives the amount of inherent moisture content present in the soil sample. Then s r' of each wet soil sample corresponding to the amount of water extra-added is calculated. 0v is computed again for the three water levels mentioned above. Dielectric constant of some common materials such as free-space, FR4, acrylic and water is also measured to verify the efficacy of the proposed method.

4. Experimental Results and Discussion

Experiment is done on 62 samples of soil. Significant results of 9 samples are presented. Table 2 shows the dielectric constant of the soil samples at the three frequencies. A plot of this is shown in Fig. 2.

Table 2. Dielectric Constant of soil samples at three frequencies Variation of Dielectric Constant of various soil samples at different frequencies

I > t 4 I I Fatwr.M

Fig. 2. Plot of dielectric constant of soil samples at 1.88, 2.45 and 5.35 GHz

On analysis of these values, it is seen that there is a variation in the sr' with frequency for all samples; the

variation is between 3.14 and 3.98. This more or less follows the inverse relationship between sr' and frequency12.

However no fixed pattern for the variation can be noted, which is suggestive of the fact the specific value of sr' depends on the electrical, agronomic and organic properties of the soil sample. The data sheet says that there is a variation in the TSS content and also in the contents of the organic compounds in each of the samples. Also, even though a degree of dryness is claimed in the data sheet, there is always a possibility of some 'moisture' in any sample of soil, which certainly affects sr'.

Figs. 3 (a to i) show the dielectric constant of all 9 samples of soil computed with inherent moisture content levels and extra-added moisture levels at the three frequencies.

pH = 4.7

pH = 4.9

c pH = 5.0

pH = 5.2

pH= 5.8

inherent Moisture Content

pH = 6.1

S 3.0 » 3.6 1 3.4

S 15.5 I 15 « 14.5

2 3 4 5 6 Frequency, GHz 23.1% Moisture Content

1 2 3 4 5 6 Frequency, GHz

20% Moisture Content

2 3 4 5 6 Frequency, GHz 26.6% Moisture Content

19.5 19 10.5 18

1 2 3 4 5 6 Frequency, GHz

pH = 6.3

Inherent Moisture Content

20% Moisture Content

-2 3.8 2 3.6 I 3.4

1 23456 123456

Frequency, GHz 23.1 % Moisture Content

Frequency, GHz 26.6% Moisture Content

123456 123456

Frequency, GHz

Frequency, GHz

i pH = 7.4

nherent Moisture Content 20% Moisture Content

13.5 2 13

12 3 4 5 6 Frequency, GHz 23.1% Moisture Content

2 3 4 5 6 Frequency, GHz 23.6% Moisture Content

12 3 4 5 6 Frequency, GHz

2 3 4 5 3 Frequency, GHz

pH = 7.0

Inherent Moisture Content

23% Moisture Content

1 3.4 ö

I 15.5 5 15

S 14.5

2 3 4 5 6 Frequency, GHz 23.1% Moisture Content

2 3 4 5 3 Frequency, GHz 28.6% Moisture Content

2 3 4 5 6 Frequency, GHz

19.5 19 18.5 18

2 3 4 5 3 Frequency, GHz

Figs. 3 (a) to (i) Dielectric constant of all soil samples with inherent moisture content levels and extra-added moisture levels of 20%% 23.1% and 28.6 % at 1.88, 2.45 and 5.35 GHz

The above figures show that sr' decreases with increase in frequency for most of the soil samples, for both the dry and wet cases. However, some samples - especially the alkaline ones - show a different behaviour at 2.45 GHz. As the pH of these samples is close to 7.0 (the pH of pure water), it is inferred that water behaves differently at this frequency; this behaviour needs further investigation.

Tables 3 (a to c) show the error, expressed as a percentage, between actual and experimental values of moisture content levels (20%, 23.1% and 28.6 %) at the three frequencies respectively, for the experimentally measured dielectric constant. The error percentage is calculated (for example, for the 20% moisture case for pH=4.7, Table (a) as (20+5.48) - 25.33 / (20+5.48) = 0.59%.

Table 3 (a) to (c) % error between actual and experimental values of moisture content levels

(a) fr = 1.88 GHz

'Wet' (Moisture Added) 'Wet' (Moisture Added) 'Wet' (Moisture Added)

Soil Sample (pH) Sr' 'Dry' (Inherent) Moisture Content (%) Sr' (obtained) = 20 % Moisture Content (%) (obtained) Error % Sr' (obtained) = 23.1 % Moisture Content (%) (obtained) Error % Sr' (obtained) = 28.6 % Moisture Content (%) (obtained) Error %

4.7 3.98 5.48 13.61 25.33 0.59 15.63 28.54 0.14 19.61 34.05 0.09

4.9 3.62 4.57 13.06 24.42 0.61 15.04 27.64 0.11 18.90 33.15 0.06

5.0 3.79 5.00 13.32 24.86 0.56 15.31 28.06 0.14 19.23 33.57 0.09

5.2 3.83 5.10 13.38 24.95 0.6 15.38 28.16 0.14 19.31 33.67 0.09

5.8 3.45 4.14 12.81 23.99 0.62 14.75 27.19 0.18 18.57 32.71 0.09

6.1 3.64 4.62 13.09 24.47 0.61 15.07 27.68 0.14 18.94 33.19 0.09

6.3 3.48 4.21 12.86 24.07 0.58 14.81 27.28 0.11 18.62 32.78 0.09

7.0 3.32 3.80 12.62 23.66 0.59 14.55 26.87 0.11 18.31 32.37 0.09

7.4 3.78 4.98 13.31 24.83 0.6 15.30 28.04 0.14 19.21 33.55 0.09

_(b) fr = 2.45 GHz_

'Wet' (Moisture Added) 'Wet' (Moisture Added) 'Wet' (Moisture Added)

'Dry' = 20 % = 23.1 % = 28.6 %

(Inherent) Moisture Moisture Moisture

Moisture Sr' Content Error Sr' Content Error Sr' Content Error

(pH) Content (%) (obtained) (%) (obtained) % (obtained) (%) (obtained) % (obtained) (%) (obtained) %

4.7 3.90 5.28 13.49 25.13 0.59 15.50 28.34 0.14 19.45 33.85 0.09

4.9 3.43 4.09 12.78 23.94 0.62 14.73 27.15 0.15 18.53 32.66 0.09

5.0 3.53 4.34 12.93 24.19 0.62 14.89 27.40 0.15 18.73 32.92 0.06

5.2 3.75 4.90 13.26 24.75 0.6 15.25 27.96 0.14 19.15 33.47 0.09

5.8 3.76 4.92 13.27 24.77 0.6 15.27 27.99 0.11 19.17 33.49 0.09

6.1 3.47 4.19 12.84 24.04 0.62 14.79 27.25 0.15 18.61 32.76 0.09

6.3 3.55 4.39 12.97 24.25 0.57 14.92 27.45 0.15 18.76 32.96 0.09

7.0 3.72 4.82 13.22 24.68 0.56 15.20 27.88 0.14 19.10 33.40 0.06

7.4 3.80 5.03 13.34 24.88 0.6 15.33 28.08 0.18 19.25 33.60 0.09

(c) fr = 5.35 GHz

Soil (Inherent)

Sample Sr' Moisture

(pH) Content (%)

4.7 3.84 5.13

4.9 3.32 3.80

5.0 3.27 3.68

5.2 3.52 4.32

5.8 3.68 4.72

6.1 3.14 3.34

6.3 3.21 3.52

7.0 3.56 4.42

7.4 3.30 3.75

'Wet' (Moisture Added) = 20 %

Moisture

Sr' Content Error

(obtained) (%) (obtained) %

13.39 24.97 0.64

12.62 23.65 0.63

12.54 23.52 0.68

12.91 24.16 0.66

13.15 24.57 0.61

12.35 23.19 0.64

12.46 23.37 0.64

12.98 24.27 0.61

12.59 23.61 0.59

'Wet' (Moisture Added) = 23.1 %

Moisture

Sr' Content Error

(obtained) (%) (obtained) %

15.40 28.19 0.14

14.55 26.87 0.11

14.46 26.73 0.19

14.87 27.38 0.15

15.13 27.78 0.14

14.26 26.40 0.15

14.37 26.58 0.15

14.94 27.48 0.15

14.52 26.82 0.11

'Wet' (Moisture Added) = 28.6 %

Moisture

Sr' Content Error

(obtained) (%) (obtained) %

19.33 33.70 0.09

18.32 32.38 0.06

18.22 32.25 0.09

18.71 32.89 0.09

19.01 33.29 0.09

17.97 31.91 0.09

18.11 32.09 0.09

18.78 32.99 0.09

18.28 32.33 0.06

From the above tables, it is evident that the method is able to correctly measure the amount of moisture added to the soil samples; the maximum error not going beyond 0.7% for any sample and for any frequency. The dielectric constant of some common materials such as free-space, FR4, acrylic and water is also measured to validate the effectiveness of the method, as illustrated in Table 4.

Table 4. Dielectric Constant of some common materials

Serial Material Sr' Sr' Sr'

No. at fr = 1.88 GHz at fr = 2.45 GHz at fr = 5.35 GHz

1 Free-space 1 1 1

2 FR4 3.29 2.75 2.67

3 Acrylic 4.38 4.17 3.99

4 Water 79.82 80.67 80.2

The values obtained are in good agreement with those available in literature. Also, the change in relationship of sr' with frequency in the case of water is evident and worth looking into for future study.

5. Conclusion

The method explores the relevance of usage of microwave frequencies for soil moisture estimation. The quantity of water in a soil profile can be used to determine the extent of irrigation required. Also, it is concluded that the freespace transmission technique, considered difficult at low microwave frequencies because of the need for a large structure and large amounts of soil samples, can be conveniently used along with a simple microstrip patch antenna and a suitable sample-holder, which is immune to microwave radiation.

Acknowledgments

The authors owe a great deal to the Soil Testing Labs at Nettoor, Kochi and Pattambi, Palakkad for providing the soil samples. The financial support of CERD, Govt. of Kerala, vide the project No. CERD/SP119/2012 dated 21/01/2013, is gratefully acknowledged.

References

1. Freedman G. The future ofmicrowave heating equipment inthe food industries. Microwave Power. 1973; 7: p. 161-166.

2. Bengtsson NE, Risman PO. Dielectric properties of foods at 3 GHz as determined by a cavity perturbation technique, Measurements on food materials. Journal ofMicrowave Power; 1971; 6: p.107-124.

3. Sucher M, Fox J. Handbook of Microwave Measurements. NewYork: Polytechnic Press Institute; 1963.

4. de Loor GP, Meijboom FW. The dielectric constant of foods and other materials with high water contents at microwave frequencies. Journal of Food Technology; 1966. 1: p. 313-322.

5. Thompson DR, Zachariah GL. Dielectric theory and bioelectrical measurements [Part II. Experimental (Apples)], Transactions of ASAE; 1971.14: p. 214-215.

6. Metaxas AC, Meredith R. Industrial Microwave Heating, IEEE Power Engineering Series, Piscataway, NJ: Peter Peregrinus; 1983.

7. Wastelands Atlas of India, Land Use Division, Land Resources Group, RS & GIS Applications Area, National Remote Sensing Centre, Indian Space Research Organisation. Hyderabad; 2010.

8. Hyoung-Sunyoun, Loon Yip Lee, Magdy Iskander. In-situ broadband soil measurements: Dielectric and Magnetic Properties, IEEE Geoscience and Remote Sensing Society; 2010.

9. Topp GC, Davis JL, Annan AP. Electromagnetic Determination of Soil Water Content and Electrical Conductivity Measurement Using Time Domain Reflectometry, Water Resources Research; 1980; 16: p. 574-582.

10. Trabelsi S, Nelson SO. Free-space measurement of dielectric properties of cereal grain and oilseed at microwave frequencies, Institute of Physics Publishing; 2003; p. 589-600.

11. Analysis report, Form ST-4, District Soil Testing Laboratory, Ernakulam and Palakkad Kerala; 2014.

12. Schmugge T, Jkckson TJ. Passive Microwave Remote Sensing of Soil Moisture, Advances in Remote Sensing; 1993.