Indian Journal of Radi o & Space Phys ics Vo l. 3 1. October 2002. pp. 285-292

Emission characteristics of dry and wet loamy sand so il layered pack at microwave frequencies

0 P N Ca lla

Internati onal Ct: ntre for Radi o Scit: nee. "OM - II WAS". /\ -23 Shastri Nagar. Jod hpu r 342 003

and

R K Singh

Sanjay Gandhi Co lony. 1-lanuman agar. Kankar Bagh. Patna 800 020

l?eceil ·ed -1 .lunumT 200 I: rn·ised I 0 Dece111ber 200 I: accepletf 23 April 2002

The emissiv it y <.:s timat.:d at ditlcrent moisture cont ent and at different freq uencies is reported for a typica l loamy ~and ~o il. It is ob:-.e rv~:d that th o.: <.:m iss ivit y of so il pack dt: pt: nds on frt:qu ency as wo.: ll as on moistun; co nten t and the depth of laye r at whi ch moisture is pro.:sc nt. The estimated emi ssivit y al so shows dependence on look ang lo.: and polari zati on. The o.:~­timatcd value' of emi s~ ivit y ca n be: w,..:d for des ignin g pass ivt: remote sensing instrument s.

1 Introduction Emissiv it y is the very important parameter, which

prov ides information about so il. All the natu ral ob­

jects such as soil w ith temperature above 0° absolute arc capable of emission, absorpti on and tran smiss ion. The cmitlcd rad iation from so il clcpcncl s upon its eli­electric cons tant, surface roughness. chemical compo­siti on. phys ical temperature, frequency polarization and angle of observati on. The di electric cons tant or soil varies with the amount or moisture con tent pres­ent in the so il. The emi ss iv ity or the so il al so varies w ith different moisture content s. The knowledge of the emi ss ivity or the so il is use ful for th e efficient usc or so il. as thi s wou ld be very helpful for building mi ­crowave instrument for appli ca ti ons in agriculture.

The es timati on of emiss i vit y o f the soil pack has been carri ed out using coherent model 1 for multilaye r med ia which is considered to have three layers, whe re first layer is air, second one is moist so il and third layer is dry soil. The estimation is ca rri ed ou t by us­ing complex dielectri c con stant of all layers at micro­wave frequencies, ran ging from 2 to 20 G Hz. as well

as at five look an gles (0°, 15°, 30°, 45° and 60°) for both horizontal and verti cal polarization. for different soil samples.

In thi s paper, the data o f dielec tric constant ~ whi ch arc already measured using HP network analyzer with coaxial probe method ha ve been used for es timati on of emissivity of so il pack . The die lectric constants were measured for different gravemctri c moisture

content by weight over the microwave freq uencies 2-20 G Hz.

The so il sample was loamy sand soi l wi th ave rage texture of 83.3% fine sand, 3.4% coarse sand, 3.337c si l t and 9.85 % clay w ith w ilting coefficient of 0.06. The so il sampl es with 13.62%, 2 1.15%, 26.08% and 30.-+3% gravimetric moisture content were used for cst i mat ion of cmi ss i vity .

2 1icrowave emission model There are basically five differen t models for es ti ­

mation o f bri ghtness temperature or emissivity. They arc:

(i ) Zero order non-coherent radiati ve rran sl"cr model (ii ) First order non-coherent radiati ve transfer model (iii ) Coherent model (i v) Emi ssiv ity model , and (v) Coherent model for multilayer medium.

The model s used for surface layer of moisture conten t ha\'e been discu ssed' and estimates o f emi s­sivity using emi ss i vity model have been made. Table I gives comparison of diflcrent models. To find the cl"fccl of depth of mois ture content in the so il pack. the coherent model for mu ltilayer has been used.

3 Coherent approach for multilayer medium The coherent approach for multilayer media is

based on Kong8 model. It is very effic ient model for

286 INDI A J RA DIO & SPACE PH YS, OCTOBER 2002

T ab le ! - Compari son of eli fTerent models

Zero-order non­coherent rad iative tran sfer mode l4

f'i rst-orcler non coher­ent radiative tra nsfer model'

Coheren t mode l6

Emissivity mode l7

Coheren t model for mul ti -l ayer mecl iu m8

·9

ll'rnposed by Schmugg & Chouclhry ( 198 1) I

I Proposed by Burke e/ a/. ( l lJ79)1

I Proposed by Stogryn ( 1970) 1

I Propos..:d by Peak ( l lJ59) 1

[Proposed by Moor ( 1%0) & Kong ( 1975) 1

( i } Non-coherent Non-coherent propaga- Coheren t propag:llion on-coherent prop:.~ga - Coherent propagation propagat ion ti on tion

(ii) Consider soi I as Consider so i I as homo- Consider so i l Consider soi I as homo- Consider soi I as homo-homogenou ~ gcneous medium as homogeneous me- gcneous medi um gencous med ium medium diu m

(i ii ) Single layer Multilayer medium Single layer med ium M ulti layer medium medium

(iv) Radi ati on comes Radiation comes fro m Radiat ion from N- layer Rad iati on from single Radiati on from from the soi I N-layers lay..: r N-layer surface i tllerface

(v) Brightness temp.

is: Tn (011 P )

= I I - f"'" (011, P)l

Forjth lay..:r:

Tu COu. P) = 11 - r ,(Ou. P) l X 7;.11 (00 )

Tu (Ou. P)

= 11 - r dOo. P) I

xTs

Tn= 11- rp( El) ] T+ r r (0) x7:,Y

x7 ~rr (Ou)

estimating the emiss i vity of the layered media. For specul:.lr surface, the specul ar emi ss i vity e can be ex­pressed in terms of the coherent refl ecti vity as

. .. ( I )

where,

8 1 = A ngle of incident

p = Polari zati on [for verti cal (v) and hori zontal (H)]

Now the relati onship between fi eld reflecti on coef­f icient Rc(8 1, p ) at the terrain boundary and the coher­

ent refl ecti v ity rcce,, p ), when volume scattering is absent, is given by

(2)

The above approach is ca lled the coherent ap­

proach, because Rc (8 1, p ) accounts for both the am­plitude and phases o f the retlecti on in the medium.

A s the terrain medium is, indeed, composed of uni ­form b yers, Rc(8 1, p) can be calculated using the equi valent transmiss ion line formul ati on whi ch was

. b M 9 gtven y oor .

Let us consider the three- layer case as shown in Fig. I. Layer I is air, layer 2 is the sample layer which

ex tends from ~ = 0 to ~ = d, whereas layer 3 is con­sidered to ex tend from ~ = d to ~ = oo. Here, ~ is the coordinate in the verti ca l direc ti on representing the

, thi ckness of the layers. The equi valent transmiss ion line is al so shown in Fig. I.

Since layer 3 extends up to oo, i t presents an im­

pedance Z3 at ~ = d. Layer 2 is characteri zed by the impedance Z2 and layer I by Z 1.The impedance of the transmiss ion line into the medium ~= 0 is obtained by transforming z3 through layer 2 to the input at ~ = 0. Therefore, we have

where,

I + R2 exp (- j y; d ) l 1- R2 exp (- j y; d )

R, = (-1)"(

.. . (3)

. .. (4)

CALLA & SINCi !-1 : EMISS ION CHARACTERISTICS OF DRY & WET LOAMY SAND SOIL 287

Layer } Air

Layer•2: Wet soil

f.ayer 3; Dry soil

z I = Z ""

Z in ----1 .... ;

~ = 0 Fig. !- Coherent app lication l'or multi layer medium

which is the refl ec ti on coe ffi cient fo r a wave in me­dium 2 incident upon boundary 2.

Here. 11 = 0 for hori zontal po lari zati on and n = I for verti ca l po lari zati on.

j y~ = j (2rc //..0) JE': where.

/..0 = Free space wave length c~ = Dielec tric constan t of the medium 2 y2 = Propagation fac tor y·~ =Component of y~

(5)

(6)

Va lues of Z 1, Z2 an d Z.1 were found by using the fol­low ing eq uati on:

lll i cos 0 i ; for verti ca l polari zation z = ' ll i sec 0 i ; for horizon tal polari za tion

... (7)

w ith i = I or 2. The refl ecti on angle Eh is related to the angle of incidence 81 as

. .. (H)

and

... (9)

where, 11 and E are the relati ve permeability and rela­

ti ve dielec tri c constant. respect ively. A ssume ~~ as unity for all media.

In the case of lossy medium, c~ is complex. Hence

Y2 is al so complex and becomes

where,

~2 =(2 rr/A.u) R,.[ c:~ ] l a2 = (2 rr/A. 0 ) l im[~ C: 2 J I

.. . ( I 0)

. .. ( ll )

Here, c = c', since rati o of£" and E' is less than unity . So, c" is ignored.

T he power loss factor or simply loss fac tor L fo r layer 2 is given by

... ( 12)

T herefore, the impedance o f the equi va lent transmi s­sion line is gi ven as

where,

~· 2 = ~~ sec8 2

/?c(Ot,p)=(- 1)"( zi,-zt l Zi, +2 1

... ( 13)

.. . ( 14)

... ( I 5)

288 INDIAN J RA DI O & SPACE PHY S. OCTOBER 2002

4 Calculation of emissivity For es timating the emissi vity by using Eqs ( 1)­

( 15) . the dielec tri c cons tant of dry and wet so il used in the es timati on or emiss i vity was obtained by using network analyzer. The dielec tric constants were measured for di l'fcrent moisture contents by wei ght or gravimetri c mois ture content over a w ide range of rrcqucncies from 2 G Hz to 20 G Hz. The so i I sample was co ll cc tecl from wes tern part o f Rajasthan. The sample was loamy sand so il w ith average tex ture o f 83.30% fine sand. 3.40r;(J coarse sand. 3.33 % silt and 9.85 o/c clay w ith w ilting coefficient of 0.06. The so il samples \Nith 13.62%. 2 1. 157c, 26.08% and 30.43% moisture content , were used for measurement of di­elec tri c constant.

The es timati on of emi ss ivity has been carri ed out by using ·'coherent model for multi layer med ia" for

eli ITercnt look angles (0° , 15° , 30°, 45° and 60°) as we ll as for clif'ferent sample thi ckness. From polari za­ti on point of view both hori zontal and vert ica l polari­za ti ons have been cons iclerccl .

5 Results and discussion The graphi cal presentat ions o f variati on of emi s­

si vity o f the soil pack w ith dillercnt frequencies, clif-

ferent moist•Jre contents as we ll a. dil'fcrent layer thi ckn esses (thi ckness or miclcll e layer) arc shown in Figs 2-9 . The variab ility or emiss ivi ty w ith different look ang les as well as w ith both hori zontal and ve rti­ca l po lari zations is also shown w i th soil hav ing sam­ple thi ckness of 0.5 em.

The results show that the value of emi ss ivity of soil pack is always less th an unity for al l conditi ons and it increases w ith increasing frequencies and decreases with increas ing moisture content in '>o il sample. The value of emi ss iv ity increases slow ly w ith increasi ng so il sample thi ck ness at lower frequencies bu t is con­stant at higher frequenc ies as shown in Figs 7-9.

In Fig. 2, it is observed that the value of emi ssiv ity increases w ith increasing frequency at all look angles at 30.43% of moisture content. It is also observed that the va lue o f emiss ivity increases wi th increasing look ang le at all frequencies at 30.43% moisture content for vertical polari zation, whereas i t decreases w ith increa. ing look angle for horizontal po lari za ti on.

In Figs 3-6 the graphs are plotted between emi ss iv­ity and moisture content of the soil sample of 0.5 em thi ckness at 2 G Hz and 20 GHz for verti cal and hori ­zontal polari zations. It is observed that values arc

r--------------------------------------------------------------------0.9 f-

X X ~ * ~~ * ~ ~ ~ .X ::.:: 0.8 f-

* * )I( ~

::f(-)1( ~ )I(

0.7 1- : : ~ : : :~ )( )( ~ )(

i'= )( X )( ,_>r a

.....,. ,...... - O.tl

r:::lo ~ -~ :::. - ~ ....... (f) 1 ~ u

! (f) : : "! ~ : - - : =~ ~ 0.5 w@ w 0 0 0 &

¢ ¢ ~ 0 0.4 ~¢- & <>

ld g 8 -13 8 0 -e-El g [;J g

Q. 8 f) D El 0 0 0.3 El

0 0° - V 0 0° - H 0 15°- v - 15°- H

0.2 ~ ll30~ - v - 30°- H X 45°- V 045° -H

0.1 f- % 60° - v 0 60°- H

0 I I I l I I --L-- I I I I I J I J. I I

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 1'7 18 19 20

FREQUENCY,GHz Fig. 2-Variali on of emiss ivity w it h frequency at 30.43% moisture content and 0.5 em soil sample thickness

CALLA & SINGH: EMI SS ION CHA RACTERI STICS OF DRY & WET LOAMY S/\ND SOIL 289

lower for horizontal polari zation as compared to ver­tical polarizations. I t also shows that the va lue of em­iss ivity decreases as the moisture content increases for all frequencies. However, it is lower at 2 GHz as compared to the va lues at 20 GHz. It is clue to the

i': 2: 0.6 (/) (/)

5! w

13.6 2% 21.15%

presence of higher percentage of free water molecules in the soil wi th higher moisture content. Since free water molecu les have high value of dielec tri c con­stant, the lower value of emiss i vity is obtained with higher moisture content.

26 .08%

MOISTURE CONTENT

• 0° - v 0 15° - v b. 30°- v X 45°- V :1: 60° - v

30.43%

Fig. 3- Var iation of emi ssi vity w ith moislllre content at 2 G l-l z frequency and 0.5 cm so il sample thickness ( for vertical polarizat ion)

0.9 t 0.8

0.7

0.6

~ 05

5 ~ 04

~ UJ

0.3

0.2

0.1

• 0° - H iJ 15' - H 6 30o- H X 45°-

0H

.i: 60°- H

0 ~----~------------~------------~------------~----~ 13.62% 21 .15% 26.08% 30.43%

PERCENTAGE MOISTURE CONTENT

Fig. 4- Var iat ion of emissi vity with moistu re content at 2 GHz frequency and 0.5 cm so il sample thi ckness (for hori zontal polarization)

290 I DIAN J RA DIO & SPACE PI-IYS. OCTOBER 2002

The emi ssiv ity va riati ons for different moisture content for different look angles and for both polari ­za tions are shown in Figs 3-6. It is observed that the emiss iv ity increases with the increase in look angle fo r ve rti ca l po lari zati on, whi le it decreases w ith the increase o f look angle for hori zontal polari zation.

The vari ati on of emiss ivity w ith moisture content has different gradient for different moisture contents. T hi s variability of gradient is very interest ing obser­va ti on. as this gives indication of the sensitiv i ty of em iss ivi ty with moisture content. The reason for var i­abi l i ty is cl ue to the fac t that the amount o f free water

i: 0.8

> U5 (/)

~ w 0.6

• 0° - v X 15° - V /', 30° - v IJ 45° - v • 60° - v

molecules increases w ith higher mois ture content and that changes the emissivi ty. The combination of bond water molecules and free water molecules play a ma­j or role in the grad ient vari abi li ty as observed in Figs 3-6 .

The linear relat ionsh ip from 2 to 20 GHz is clue to the fact that the dielec tri c constant of free water has l inear relationship from 2 to 20 GH z and the dielectri c constant of dry sand is constant and docs not vary with freq uency . The combination of these two for wet soi l g ives linear vari ati on or emiss ivity with fre­quency. The effec t of absorpti on band of water on

0.4 L_ __ _..J... _______ ..._ ______ --'--------.1..-------'

13.62% 21 .15% 26.08% 30.43%

PERCENTAGE MOISTURE CO TE NT

Fig. ) - Variation o f emiss ivity w ith moistu re content at 20 GHz frequency and 0.5 em so il sample thi ck ness ( for vertica l polari za ti on)

0.9 r-------------------------------------------------------. 0.8

0.7

~ 0.6

~ ~ 0 .5 :E w

0.4

0.3

0.2

0.1

+ cf- H o 15° - H A YJ0

- H X45° - H t&f-H

0 ~----J-------------~------------~-------------~~----~ 13.62% 21 .15% 30.43%

PERCENTAGE MOISTURE CONTENT

f' ig. 6- Same as Fig. 5. but for hor izontal polari zation

CALLA & Sl Gil : EM ISS ION CHARACTERISTICS OF DRY & WET LOAMY SA D SOIL 291

emi ss ivity is observed to be higher at higher fre­quency encl.

Figures 7-9 are the plots o f emiss ivity as a function of frequency for different thi cknesses of soil samples

at 0°, 30° and 60° look angles. It is observed that the va lue of emissi vity increases slow ly with increasing soil sample thickness at lower frequencies, but the emi ss ivity is almost same at higher frequencies. It happens clue to the fact that at higher frequencies the rati o of soil thickness to wavelength increases and the smaller thickness of sample acts as infinite or semi ­infinite thickness o f the sample. It is also observed that , as the look angle increases, difference in the

emissiv ity va lues for hori zontal and vertica l polari za­ti ons also increases .

The trend of the increase in emiss ivity for all the angles and polari zation is linear. The dielectri c con­stant o f water decreases linearl y with freq uency and as the water content in the so il is 30.46% by weight the soil is saturated and the free water present in the soil plays dominant role in the trend of emiss ion be­haviour.

From Figs 7-9 it is seen that the emiss ivity is dif­ferent for different depths up to 4 GHz. After 4 GHz the emiss ivity is same for all the layers cons idered in

0 .7~----------------------------------------------------------------,

0.6

r: 05

~ r/) r/)

~ 0.4 UJ

0.3

2 6 8 10 12 14

FREQUENCY,GHz

0 0.5cm- v 0 1.5cm- V t:,. 3.0cm- V X 0.5cm- H

* 1.5cm- H 0 3.0cm - H

16 18 20

f-i g. 7- Variati on of emissivit y with frequency at oo look angle and 30.46'!1• moisture content so il sample (for both hori zontal and ve rti ­ca l polari za ti ons)

0.7 r----------------------------------------------------------------~

0.6 f-

~ ~ 0.5 r/) r/)

~ UJ 0 .4 1-

0.3 f-

: : :: 0 o.scm- v 0 1.5cm- V I~ 3.0cm-V ~ 0.5cm -H * 1.5cm - H 0 3.0cm- H

0.2 ~~------~----J-----~----~------~----~----~L-----J_-----L~ 2 4 6 8 10 12 14 16 18 20

FREQUENCY,GHz

Fig. 8- Variation of emissivit y with frequency at 30° look angle and 30.46% moisture content so il sample ( for both hori zontal and verti ­ca l polari zati ons)

292 INDIAN J RADIO & SPACE PI-IYS. OCTOBER 2002

0.9r--------------------------------------------------------------.

0.8

0.7

0.6

~ ~ 0.5

(f) (f)

~ 04 w

0.3 rs 8 a D 13 E! -£1

0 O.Scm. V 0 1.5cm · V

0.2 6 3.0cm. V X O.Scm · H ;( 1.5CIT' . H

0.1 c 3.0cm · H

0 l_ 1 1 I _j_

i 6 10 12 14 16 18 20

FREQUENCY,GHz

Fig. 9- Var iationuf em iss ivity w ith frequency at 60° look angle and :10.46% moistu re content so i l sample ( for bmh horizontal and l't:rt i­cal polariza ti ons)

the presen t study, though the increase in emiss i vity is observed w ith increase in frequency . That is clue to the fact that the dielectri c constant decreases with fre­quency for wet soil. In Figs 7-9, the observation with regard to the look angle and two polari zations is im­porl<mt. For higher look angle the separation between two po lari zati ons indicates that , for the study of so il , higher look angle with verti ca l polari zati on gives higher em iss ivity as compared to hori zontal polari za­tion. Thi s happens due to the fact that, for wet so il w ith larger moisture content, the ori en tation of dipole in the water molecules in free water has better corre­lation !'or ve rt ica l po lari zation.

6 Summary (i) The dielectric constant or the so il is one of the

most important parameters for es timating emis­sivity.

(ii ) The emi ss ivity of so il pack is always less than unit y !'or all cond itions.

(iii ) The emissivity of soil pack is never less th an 0.3.

(i v) The va lue of emiss ivity or the soil pack in­creases with increasing frequency and decreases w ith increas ing moisture content.

(v) Thickness of the so il sample al so plays an im­portant ro le in estimating the emissivity. The va lues of Clll iSs ivily increaseS \Vith increas ing

soi l sample thick ness at lower frequencies and becomes con<;~ant at hi gher frcqu nci es.

(v i) For vertica l polari za ti on, as the look angle in­

creases from 0°_ the va lues or emissivity also in­creases with frequency for all percentage of moisture content. However, in case of hori zontal

polari za ti on, as the look angle increases from 0° , the va lue of em iss iv ity decreases ;vith fre­quency .

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3 Ca lla 0 P & Dcka B. Indian J Radio & Space Pln·s. 29 (2000) 1-10.

4 Schmuggc T J & Chaudhuri 13. J Radio s , ·i (USA). 16 ( 19X I ) 927.

5 f3Lu-kc W. Schmuggc T J & Pari s J F. J Geophr.1 Res (USA) . !)4 (1979) 287.

6 Strogryn A. Radio Sci (USA). 12 ( 1970) 1397.

7 Peak W. IEEE Tmns An1enno.1· & Pmpag (USA) . A P 7 ( 19:59) 32-l .

8 Kong J A, Th e il1eon• o( elecllwllllgneli<· H·m ·es (John Wih.:) & Sons Inc. New York ). 1975.

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