Indian Journal of Radio & Space Physics Vol. 29, June 2000, pp. 140- 145

Study of emissivity of dry and wet loamy sand soil at microwave frequencies

0 P N Calla

Internati onal Centre for Radio Science, "OM NIWAS" A-23, Shastri Nagar, Jodhpur 342 003 and

B Deka

Department of Electronics and Telecommunication, Assam Engineering Institute, Guwahati 781 003

Received 8 November 1999; revised 7 February 2000; accepted /March 2000

Results of esti mation of emi ssivity from dielectric constant of dry and wet soil using emissivity model for microwave frequencies ranging from 2 GHz to 20 GHz are presented. The measurements of the dielectric constant of d1y and wet soil had been carried out using HP network analyzer and HP dielectric probe employing co-axial probe method. The estimations are made with incidence angles 0°, 30°, 45° and 60° for both hori zontal and vertical polarization. The emissivity, which is less than one, increases with frequency and decreases with moi sture content. In both the cases emissivity for vertical polarization is hi gher than that for horizontal polarization.

1 Introduction Electromagnetic emtsston characteristics from

either naturally ex isting or man-made ground features vary widely depending upon their physical properties as well as the environment in which they would .ex ist' . The moisture content of soil in the near surface regions is of major importance in a number of disciplines inc luding agriculture, hydrology and meteorology. Detect ion and analysis of radiated energy emitted by so il in the near surface are possible by passive microwave remote sensing techniques by measuring the emitted radiation of the object. One of the sa lient characteristics of microwave passive sensors, (radi ometers) is their capability to estimate the degree of wetness of the soil. The microwave remote sensors aboard the earth orbiting satellites have potential for providing soil-moisture information over a large area. In order to facilitate the interpretation of data obtained by airborne or satellite-borne sensors , the ground-based investigation of mi crowave radiation properties of so il with different moisture content is essential. A large number of ground-based experiments on the remote sensing of soil moi sture content have been conducted with the passive microwave sensors in the past few decades and there has been a significant interest in in vestigating the potential for using microwave remote sensing to estimate soil-moisture information. Extensive studies were also conducted by several researchers 2 in USA and Europe, using

truck-mounted passive microwave sensors, to estimate the emissivity of soil in the near surface region. In thi s paper, a comprehensive study of emission properties of typical Indian soil from its dielectric constant (in laboratory) has been undertaken . The Jielectric constant is already measured in the laboratory with the help of HP network analyzer and HP dielectric probe using co­axial probe method for frequencies ranging from 2 to 20 GHz in steps of 0.9 GHz for dry as we ll as with different moisture content. The estimation of emissivity has been carried out with the empirical formulae of the theoretical emissivity model. The emission properties are studied for four different look angles (0, 30, 45 · and 60 degrees) for both horizontal and vertical polarization .

2 Absorptivity, emissivity and brightness temperature of material

All natural objects with temperature above absolute zero are capable of emission , absorption and transmission of electromagnetic energy . Al so, natural objects do not absorb all the incident energy as a black body does. Some amount of energy is absorbed and some amount is reflected . The 'absorptance ' is defined as the ratio of the energy absorbed by an object in re lation to the energy absorbed by a perfect black body. The ' emissivity' is the ratio of the energy emitted by an object as compared to that emitted by a perfect black body maintained at same physical

CALLA & DEKA: EMISSIVITY OF DRY & LOAMY SAND SOIL AT MICROWAVE FREQUENCIES 141

temperature. A quality related to brightness is the 'brightness temperature', which is corresponding physical temperature of black body with the same brightness . Neglecting the effects of transmission and reflection observed in a practical situation, the brightness temperature is the product of the physical temperature and the emissivity of the object. Emissivity is a function of dielectric properties, surface roughness, chemical composition, physical temperature, frequency, angle of observation, etc. By appropriate choice of one or more frequencies, the properties pertaining to the parameters can be studied separately. Emissivity is a stronger function of surface roughness, moisture content and reflection property of the objects.

The emitted radiation from bare-soil is a strong function of surface roughness, dielectric constant, angle of observation and physical temperature. The effect of soil moisture is directly reflected on the dielectric constant and hence, emissivity.

3 Measurement of dielectric constant There are basically three methods for the

measurement of dielectric constant in laboratory'. Co­axial probe method is the fastest and accurate method for the measurement of dielectric constant of soil for a very large range of frequency. In this paper the data of dielectric constants, which are already measured using HP network and HP dielectric probe using the coaxial probe method, have been used for the estimation of emissivity . The dielectric constants were measured for different moisture content by weight (or gravimetrically) over the frequency range 2-20 GHz in steps of 0.9 GHz. The soil sample was loamy sand soi l with average texture of 83.30% fine sand, 3.40% coarse sand, 3.33% silt and 9.85% clay with wilting coefficient of 0.06. The moisture content of the soil sample was found to be 31%, 27%, 23%, 17%, II %, 7 .5 %, 4.5 % and 1.5%. In this paper, the emission behaviour of soi l samples with moisture contents of 0% (dry soil) and 30.33% for all freqtJencies ranging between 2 GHz and 20 GHz has been reported for all look angels for both horizontal and vertical polarizations.

4 Microwave emission models Various theoretical models have been developed to

estimate microwave emission of soil from its surface. These models include zero-order non-coherent radiative transfer model4

, first-order non-coherent

radiative transfer model5, coherent model6 and

emissivity model7. Table I gives comparative

description of the four models . From Table I it can be seen that the first-order radiative transfer model and emissivity model are quite similar.

In this paper, the emissivity model is used for estimation of emissivity, as this model is simple to use with reasonable accuracy.

5 Calculation of emissivity from emissivity model This model is the simplest to use with reasonable

accuracy for the radiation from within a range close to the surface. In this model the brightness temperature, T8 , of soil is given by

. . . (I)

where, ep(8) is the emissivity of the surface layer, 'p' refers to the polarization either vertical or horizontal , rp (8) the reflectivity at air-soil interface, T the surface temperature and T sky is the brightness temperature equivalent of the sky and atmospheric radiation incident on the soil.

The emissivity ep(8) can be written as

. .. (2)

In the case of smooth surface, over a homogeneou medium, rp(8) can be obtained from the Freshnel reflection coefficient Rp (8) as

rp(8) = IRP(8)12

... (3)

where,

cos8-~t:-sin 2 (}

Rr (8) = ---'-;==== cos 8+~t:-sin 2

(}

for horizontal polarization, and

R a t:cos8-~t:-sin 2 8

(u) = -------,:.=== r I . '

E cos(} + v E - sm- (}

for vertical polarization.

... (4)

.. . (5)

where, 8 is the angle of observation from the nadir and £ is the dielectric constant of the soi l. Equations (3)-(5) can be used for calculation of emissivity, provided the dielectric constant of the soil with moisture content is known. The brightness temperature T8 can be computed using Eq . (I) after knowing the values of rp (8), T and T sky·

In the present work, the emissivity of dry and wet loamy sand soil at microwave frequencies is estimated by using Eqs. (2)-(5). The values of

142 INDIAN J RADIO & SPACE PHYS, JUNE 2000

Table 1-A comparative study of the four models

Zero-order non-coherent radiative transfer model

[ Proposed by Schmugge & Choudhur/ ( 1981 )]

(i) Non-coherent

(i i)

(ii i)

(iv)

propagation and reflection in soi l medium

Considers soi l as homogeneous medium

Assumes soil medium as single layer

Assumes radiation arises from the soil­surface interface

First-order non-coherent radiative transfer model

[ Proposed by Burke eta/. 5

( 1979)]

Non-coherent propagation

Considers soil as homogeneous medium ,

Soil medium is considered horizontally stratified into N-layers. Each layer having uniform electrical and thermal properties

Assumes radiation from N-layers

dielectric constant used in this computation, for both dry and wet so il , have been measured using co-axial probe method in the laboratory.

6 Results and discussion The variation of emissivity of dry and wet soil with

respect to the frequency moisture content as well as

emissivity with four different look angles 0°, 30°, 40°

and 60° for both horizontal and vertical polarization are presented in Figs 1-4.

Figures I and 2 show the variation of emissivity with respect to different frequencies. The general trend of it is linear.

From Fig. I, it is observed that the emissivity of dry soil is very close to one at all frequencies . The variation of the emissivity with frequency is very small for all the look angles. The emissivity values for the vertical polarization are more than those for horizontal polarization . The dry soil which is a mixture of air and sand particles behaves like a black body and so, in the vertical polarization, the emissivity for all angles is close to one as compared to the emissivity value for horizontal polarization .

From Fig. 2 it is observed that the emissivity increases with frequency. It is also observed that the

Coherent model

[Proposed by Stogryn~ ( 1970)]

Non-coherent propagation

Considers soil as homogeneous medium

Emissivity model

[Proposed by Peak7 ( 1959)]

Non-coherent propagation

Considers soil as homogeneous medium

Considers soil medium as single layer

Assumes radiation from a surface close to the surface

emtsstvtty values of soil with moisture content 30.76% are less than those for dry soil. The increase of emissivity with frequency for moist soil is due to water as the emissivity of water increases with frequency. Again the lower value of emissivity of so il with higher percentage of moisture content is due to the lower value of emissivity of water than that of dry soil.

From the observation of the other percentages of moisture content it was observed that for moisture content up to 16.92%, there is a very little increase in emissivity with frequency. This s low increase in emi­ssiv ity is due to the presence of more bound water molecules compared to free water molecules. The bound water molecules have very little contribution to the dielec tric constant as compared to free water molecules2

. For moisture content 16.92% and above, the increase of emissivity with frequency is rapid for all frequencies. This happens because, as the moi sture content increases, the number of free water molecules ava ilable in the soil-water mixture increases. Therefore, the value of dielectric constant for soi l­water mixture increases. The variation of dielectric constant with frequency above 16.92% of moi sture content is also more than the lower percentages of

~ > iii Ul

~ w

0.9

0.8

0.7

0.6

0.5

04

0.3

0.2

0.1

0

2

CALLA & DEKA: EMISSIVITY OF DRY & LOAMY SAND SOIL AT MICROWAVE FREQUENCIES

0 9

X X X

X X

• O_deg

• 30_deg (H)

A 45_deg(H)

X 60_deg(H)

o 30_degM

• 45_degM

+60_degM

2.9 3.8 4 .7 5.6 6.5 7.4 8 .3 9.2 10.1 11 11 .9 12 .8 13.7 14.6 15 .5 16.4 17 .3 18.2 19.1 20

FREQUENCY, GHz

Fig. !-Variation of emissivity with frequency for dry soil (solid lines are for horizontal polarization and dotted lines are for vertical polarization. Dash line is for 0° angle)

0

0.78j ... •. .. , ... . • .. . .• •. • . . . • ... • ....• ....• ... • .... • ....•. ...• . . · • .. + . . ~ ... . . .......... .

• ... • - ... e .. --• ·· . .a .. - • · .. -• · .. -e . . . • .. - e · ·· ·e · · I

l ... .... . ---···· ······ •··· •···•-- --•··· · --- ...: · ·· x···· x ··-1

>- 0.6 i ___ x __ · x· _. ·x · __ ~ ___ • · . .. x -.- .x . · · ~ · · · : · · -: -~ ~:- ~-~~ -~ :- ~-: -~-: _ -.-- ~- - .-- i

i ,, r: · , · : : = ~ : · · · : : : : : : : i ~ 0 4 r : : : : . · · - -- :

~ -~~~~~~~~~~~~~x~~x~~x~~x~~x~~x~~~x~~x~~x~~w~~w-:=:w~=:x~~x 03 ~ W X X X X M

1 i • 0 deg I -

I 1• 30 deg (H) 0.2 J • 45_deg(H)

1 I X 60_deg(H)

x 30_deg(V) 0.1

29 3 8 47 56 6.5 7 4 8.3 9.2 10 1 11 11 .9 12.8 13.7 14.6 15.5 16.4 17 .3 18.2 19.1 20

FREQUENCY, GHz

Fig. 2-Yariation of emi ssivi ty with frequency for 30.76% moisture content (solid lines are for hori zontal polarization and dotted lines arc for vertical polarization. Dash line is for oo angle)

143

144

0.9

0.8

0.7

0.6

~ 5 iii 0.5 VI ~ w

0.4

0.3 ~deg O_deg(H)

0.2 "45_deg(H)

x60_deg(H)

:x 30_deg(V) 0.1 • 45_deg(V)

+ 60 deg(V)

0

INDIAN 1 RADIO & SPACE PHYS, JUNE 2000

+

• ··-. ....

+

·· ---• ·---..

------- • ·····.. ... :X

---

X

--­• --

+ . ...

• ....

0% 4.61% 7.69% 10.76% 12.30% 14.62% 16.92% 23 .70'/: 26.92%

>-1-

2: VI VI

:E w

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

MOISTURE CONTENT

Fig. 3-Variation of emissivity of soil with moisture content at 2 GHz (solid lines are for horizontal polarization and dotted lines are for vert ical polarization. Dash line is for oo angle)

• O __ deg

• 30_deg(H)

"45_deg(H)

x 60_deg(H)

:x 30_deg(V}

• 45_deg (V}

+ 60 deg(V}

X

·---··· X"- ·· .. ... ··· >~:..... --· ----- ----.. -----.. ____ _

.... _+ - ·····

• :X

..........

---

+ ·--. •

---:X . ---+

• ------ X

X

----

0% 4.51% 7.69% 10.76% 12.30% 14.62% 16.92% 23.70% 26.92%

MOISTURE CONTENT

Fig. 4-Yariation of emissivity with moisture content at 20 GHz (solid lines are for horizontal pol arization and dotted lines are for vertical polarization. Dash line is for oo angle~ ·

30.76%

30.76%

CALLA & DEKA: EMISSIVITY OF DRY & LOAMY SAND SOIL AT MICROW AYE FREQUENCIES 145

moisture content. Hence, the slope of the emissivity with frequency is also more at higher percentages of moisture content.

From Figs 1 and 2 it is also observed that for vertical polarization, as the look angles increases from 0°, the emissivity values with frequency also increase for a particular moisture content. In case of horizontal polarization, it is observed that as the look angle increases from 0°, the values of emissivity decreases for that particular moisture content.

Figures 3 and 4 indicate that the variation follows a polynomial curve of second order. From these curves it is observed that the emissivity decreases with moisture content at all frequencies.

Figure 3 shows that emissivity decreases lfS the moisture content increases. The same trend is also seen in case of 20 GHz (Fig. 4). This is also due to water, as higher percentage of moisture content in soil means higher percentage of free water in it. Emissi­vity of dry soil is close to one for all frequencies . Higher the percentage of moisture content, less is the value of emissivity, because emissivity of water is less than emissivity of dry soil. ..

From Figs 3 and 4 it can be observed that the values of the emissivity at 20 GHz are more than the values at 2 GHz. Thus, the ~missivity that decreases with higher percentage of moisture content is of higher values at higher frequencies. It is also observed that the emissivity for vertical polarization is greater than the horizontal polarization.

7 Conclusions This paper presents the estimated emissivity for

2-20 GHz frequency range and for · different percentage of moisture content assuming the soil to be bare and homogeneous.

From the above results and discussion, it can be concluded that the value of emissivity of soil with respect to either frequency or moisture content depends on the dielectric constant of soil which is related to its bound water as well as the quantity of free water molecules in it.

It can also be concluded that the study of emissi­vity of soil for either in situ or microwave remote sensing is suitable at higher look angles in vertical polarization for higher microwave frequencies .

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1 Remote Sens (UK), 6 (1985) 1153.

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6 Stogryn A, Radio Sci (USA), 12 (1970) 1397. 7 Peak W, IEEE Trans Antennas & Propag (USA), A P 7

(1959) 324.