Indian Journal of Radio & Space Physics
Vol 40, October 2011, pp 267-274
Estimation of emissivity and scattering coefficient of low saline water
contaminated by diesel in Cj band (5.3 GHz) and Ku band (13.4 GHz)
O P N Calla1,$,*, Nima Ahmadian2,# & Sayeh Hasan2,†
1
International Centre for Radio Science, Jodhpur 342 003, Rajasthan, India
2
Department of Geoinformatics, University of Pune, Pune 411 007, India
E-mail: $opnc06@gmail.com, #ahmadian.n@gmail.com, †sayeh.h@gmail.com
Received 15 July 2010; revised 5 August 2011; accepted 11 August 2011
In case of low salinity or variable salinity, there is a very shallow radius of investigation. The dielectric constant of low
saline water can be significantly modified by the presence of diesel. Techniques based on the propagation of electromagnetic
waves may be used to detect contaminant and evaluate decontamination processes. Microwave remote sensing of diesel oil
contaminated low saline water requires the study of electrical parameters of low saline water as well as diesel such
as dielectric constant, emissivity and scattering coefficient along with their physical parameters like surface roughness, etc.
The measurement of dielectric constant is very essential for estimating the emissivity and scattering coefficient.
The measurement of dielectric constant of low saline water in combination with diesel has been carried out using waveguide
cell with shift in minima method in 5.3 and 13.4 GHz. Tests are conducted with samples of different salinity of water with
various amount of diesel oil. The amount of salinity is 5, 10 and 15 kppm and the amount of diesel contamination is from
40 to 280 percent with the interval of 80 percent for Cj band (5.3 GHz) and 40 to 160 percent with the interval of 40 percent
for Ku band (13.4 GHz). The estimation of emissivity and scattering coefficient have been done for incident angles varying
from 10 to 80 degree with the interval of 5 degree for both horizontal and vertical polarization. The value of Brewster angle
has been calculated and the values of emissivity and scattering coefficient for three look angles (45, 50 and 55) degree is
presented which are useful for designing space borne active and passive sensors. Furthermore, this database is useful for
detecting oil spills in low saline water.
Keywords: Microwave remote sensing, Emissivity, Scattering coefficient, Diesel contaminated water, Low saline water
PACS Nos: 84.37.+q; 84.40.xb; 92.05.Hj
1 Introduction
The essence of remote sensing is the measuring and
recording of the electromagnetic radiation emitted or
reflected by the earth's surface and its features.
Diagnosis of water contamination by diesel in any
region is an essential pre-requisite to plan a water
reclamation project. Diagnosis with conventional
techniques has been a time consuming and laborious
exercise. With the advent of remote sensing,
diagnostic procedures have been made easier and
cheaper. In spite of this progress, it was found that
an appropriate investigation for the identification
and diagnosis of diesel oil contamination in low saline
water has been lacking.
Annually, fuels account for 48% of the total oil
spilled into the sea worldwide, while crude oil spills
account for 29% of the total1. Tanker accidents
contribute with 5% of all pollution entering into the
sea2. Moreover, numerical models are being used to
predict the drift and dispersion of oil following an
accident3. Between 1988 and 2000, there were 2,475
spills which released over 800,000 liters of oil in
Toronto and surrounding regions4. The impact of not
monitoring oil spills is presently unknown, but the
main environmental impact is assumed to be seabirds
mistakenly landing on them and the damage to the
coastal ecology as spills hit the beach5.
Recent research shows that even if the operators go
through extensive training to learn manual oil spill
detection they can detect different slicks and give
them different confidence levels6, so a laboratory
research has been made to measure the dielectric
constant of diesel and then estimating the scattering
coefficient and emissivity of low saline water
contaminated by diesel and calculation of Brewster
angle as well for better detection of diesel oil
contaminated low saline water.
Although the vast majority of seawater has salinity
in the range 3.1-3.8%, seawater is not uniformly
saline throughout the world. Where mixing occurs
INDIAN J RADIO & SPACE PHYS, OCTOBER 2011
268
with fresh water runoff from river mouths or near
melting glaciers, seawater can be substantially
less saline. Unfortunately, in case of low salinity
(brackish water), there is very small amount of
investigations. Thus, this paper presents the result of
emissivity and scattering coefficient estimation as
well as Brewster angle in low water salinity
contaminated by diesel. The measurement works
well because diesel and water have dissimilar
dielectric properties; the value of dielectric constant
for diesel is 1.7 for Ku band and 2.3 for Cj band.
The water has a typical value of 80. So, in other
words, water is capable of storing much more energy
per volume unit than the average oil. There is a
special classification for the amount of salinity,
which is presented in Table 1.
2 Methodology
Tests have been conducted with samples having
5, 10 and 15 kppm salinity and the diesel
contamination from 40 to 280 percent with the
interval of 80 percent for Cj band and 40 to 160
percent with the interval of 40 percent for Ku band
(13.4 GHz). The estimation of emissivity and
scattering coefficient have been done for incident
angles varying from 10 to 80 degree with the interval
of 5 degree for both horizontal and vertical
polarization. The value of Brewster angle has been
calculated and the values of emissivity and scattering
coefficient for three look angles (45, 50 and 55)
degree has been presented, which are useful for
space borne active and passive sensors.
Table 1—Classification of various amount of salinity in water
Fresh water
< 0.05 %
< 0.5 ppt
Brackish water
0.05 – 3 %
0.5 – 30 ppt
Saline water
3–5%
30 – 50 ppt
Brine
>5%
> 50 ppt
2.1 Measurement of dielectric constant
There are four methods of measurement of
dielectric constant, viz. transmission line method,
resonant cavity method, wave guide cell method and
free space method. This study covers the estimation of
emissivity and scattering coefficient in Cj band as
well as Ku band. The data of dielectric constant,
which is already measured using waveguide cell
with shift in minima method, has been implemented
for this purpose. The dielectric constant was measured
for different water salinity and different weight
percentage of diesel in Cj band as well as Ku band
in the laboratory.
2.2 Microwave emission model
Various theoretical models have been developed
to estimate microwave emission of materials. These
models include zero order non-coherent radiative
transfer mode, first order non-coherent radiative
transfer model, coherent model and emissivity
model. Table 2 gives comparative depiction of the
four models. The estimation of emissivity using the
available data of dielectric constant and emissivity
model is presented in this paper.
2.2.1 Estimation of emissivity using emissivity model
The emission of microwave energy is proportional
to the surface temperature and the surface emissivity,
which is referred to as the microwave brightness
temperature (TB). Thus, the brightness temperature
of an emitter of microwave radiation is related to
the physical temperature of the source through the
emissivity:
TB= (1-R) ∗ Teff= e ∗ Teff
…(1)
where, R, is the hemispherical-directional reflectivity
from the surface; Teff, the effective physical temperature
Table 2—Comparative study of four models
Zero order non-coherent radiative First order non-coherent radiative
transfer model [Proposed by transfer model [Proposed by Burke
Schmugge11]
et al12]
Coherent model
[Proposed by Stogryn13]
Emissivity model
[Proposed by Peake14]
Non-coherent propagation and
reflection in material medium
Non-coherent propagation
Non-coherent propagation
Non-coherent propagation
Considers material as a
homogenous medium
Considers material as a
homogenous medium
Considers material as a
homogenous medium
Considers material as a
homogenous medium
Assumes material medium as a
single layer
Material medium is considered to
be horizontally stratified into
N-layers, each layer having uniform
electrical & thermal properties.
Assumes radiation arises from the Assumes radiation from N-layers.
material surface interface.
Considers material medium
as a single layer
Assumes radiation arises from a
surface close to the surface.
CALLA et al.: EMISSIVITY & SCATTERING COEFF. OF LOW SALINE WATER CONTAMINATED BY DIESEL
of the surface; and e = (1-R), the emissivity, which
depends on the dielectric constant of the medium.
This model is the simplest to use with reasonable
accuracy for the radiation within a range close to
the surface.
Reflectivity is described by the Fresnel equation
that defines the behaviour of electromagnetic waves at
a smooth dielectric boundary. For horizontal (H) and
vertical (V) at non-nadir incidence (θ), the Fresnel
reflectivity may be derived from electromagnetic
theory as7:
R ( H ,θ ) =
2
cos θ − ε H − sin 2 θ
…(2)
cos θ + ε H − sin 2 θ
and
R(V , θ ) =
εV cosθ − εV − sin 2 θ
2
εV cosθ + εV − sin 2 θ
…(3)
where, εp, is the polarization-dependent dielectric
constant of the emitter; and θ, the angle of
observation. The emissivity can be calculated using
Eqs (2) and (3) and then the brightness temperature
can be obtained using Eq. (1).
2.3 Backscattering behaviour of the surfaces
When a target is moist or wet, scattering from the
topmost portion (surface scattering) is the dominant
process taking place. The type of reflection (ranging
from specular to diffuse) and the magnitude will
depend on how rough the material appears to
the radar. If the target is very dry and the surface
appears smooth to the radar, the radar energy may
be able to penetrate below the surface, whether
that surface is discontinuous (e.g. forest canopy
with leaves and branches), or a homogeneous surface
(e.g. soil, sand or ice). For a given surface, longer
wavelengths are able to penetrate further than shorter
wavelengths.
If the microwave does manage to penetrate
through the topmost surface, then volume scattering
269
may occur. Volume scattering is the scattering
of radar energy within a volume or medium, and
usually consists of multiple bounces and reflections
from different components within the volume.
2.3.1 Scattering models for different surfaces
Depending upon the surface pattern, three different
models are used for the estimation of scattering
coefficient. These models are:
1. Perturbation model
2. Physical optics model
3. Geometric optics model
The perturbation model is appropriate for slightly
rough surface where both the surface standard
deviation and correlation length are smaller than
the wavelength, thereby, this model has been
implemented for estimating the scattering coefficient
of low saline water in combination with diesel.
Selecting a model for computation for a particular
surface will depend upon the validity of a model
for the surface concerned. The validity condition for
each model is given in Table 3.
2.3.2 Perturbation model
In the perturbation model, the standard deviation
should be at least 5% less than that of the
electromagnetic wavelength. In addition to this, the
slope of the surface should be of the same order
of magnitude as the wave number times the surface
standard deviation.
In the present paper, the wavelength is 5.66 cm for
Cj band and 2.66 cm for Ku band, so the standard
deviation should be less than 0.3 mm (ref. 8).
Mathematically,
kσ < 0.3 and (2)1 / 2 σ / l < 0.3
where, K = 2π/λ; σ = r.m.s. surface height;
l= correlation length; and M = r.m.s. surface slope.
The backscattering coefficient is given by:
σ0
ppn
( θ ) =8k 4 σ 2 cos 4 θ
α pp θ
2
W (2k sin θ )
…(4)
Table 3—Validity conditions for different models
Model
Validity condition
Physical optics model (Kirchoffs’ model with scalar approximation)
M < 0.25 and Kl > 6
Geometric optics model (Kirchoffs’ model with stationary phase approximation)
(2Kσ cosθ)2 > 10, and l 2 = 2.76σλ
Perturbation model
Kσ < 0.3, M < 0.3
Note: K = 2π/λ; σ = r.m.s. surface height; l = Correlation length; M = r.m.s. surface slope
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INDIAN J RADIO & SPACE PHYS, OCTOBER 2011
where, p = polarization; v = vertical polarization; and
2
h = horizontal polarization. Also α nn (θ ) = Γn (θ )
is the Fresnel reflection coefficient. The value
for Fresnel reflection coefficient for horizontal
polarization is given by:
cos θ − (ε s − sin 2 θ )1 / 2
α hh ( θ ) =
cos θ + (ε s − sin 2 θ )1 / 2
…(5)
For vertical polarization, the Fresnel coefficient σ vv
is given by:
α vv ( θ ) = ( ε s - 1)
sin 2 θ − ε s (1 + 2 sin 2 θ )
[ε s cos θ + (ε s − sin 2 θ )1 / 2 ] 2
…(6)
where, ε s , is the dielectric constant of emitter; and θ,
the angle of incident. W (2ksinθ is the normalized
roughness spectrum which is the Bessel transform of
the correlation function ρ (ξ ) , evaluated at the surface
wave number of 2ksinθ. For the Gaussian correlation
function ρ (ξ ) = expo (- ξ 2 / l 2 ) , the normalized
roughness is given by:
W (2ksin θ ) = 1/2 l 2 exp [-(k l sin θ ) 2 ] ...(7)
2.3.3 Computation of scattering coefficient
For estimation of scattering coefficient, the
validation condition can be considered as kσ = 0.25
and M = 0.25. Estimation of scattering coefficient
for low saline water in combination with diesel with
slightly rough surface has been done in Cj band
(5.3 GHz) and Ku band(13.4 GHz) for different
look angles ranging from 10 to 80 degree with the
interval of 5 degree and for both horizontal and
vertical polarizations. Fortuny-Guasch9 discusses the
potential of polarimetric active microwave remote
sensing data for improved oil spill detection and
classification.
2.4 Brewster angle
An angle at which total transmission occurs for the
vertically polarized wave is called the Brewster angle.
Note that the total transmission does not occur for
horizontal polarization10. The value of Brewster angle
can be obtained from the following equation:
Tan δ = ε
…(8)
Calculations of Brewster angle of low saline water
contaminated by diesel in Cj and Ku band has been
done. Emissivity and backscattering coefficient is
calculated for three different look angles (45°, 50°
and 55°), which are desirable for satellite sensors such
as NIMBUS, AMSRE, SSMI and MSMR as well as
QuikSCAT and OSCAT (India’s first scatterometer).
3 Results and Discussions
The amount of change in dielectric constant of
combination of low saline water and diesel has been
around 25.74, 24.67 and 23.46 for 1% change in
weight percentage of diesel in low saline water for 5,
15 and 25 kppm, respectively in Cj band. On the other
hand the amount of change in dielectric constant of
combination of low saline water and diesel has been
around 29.9, 29.85 and 29.69 for 1% change in weight
percentage of diesel in low saline water for 5, 15 and
25 kppm, respectively in Ku band.
The variations of emissivity of low saline water in
combination with different weight percentage of
diesel are presented in Figs 1 and 2 in different look
angles from 10 to 80 degrees with the interval of
5 degree for both horizontal and vertical polarization
in Cj band and Ku band, respectively. These figures
indicate that the variation follows a polynomial curve
of third order for horizontal polarization and fourth
order for vertical polarization. The observable fact
in Figs 1 and 2 is that the value of emissivity for
horizontal polarization is decreasing as the look
angles increase and the minimum values for all the
curves take place at 80 degree for both horizontal
and vertical polarization. On the other hand, the
values of vertical polarization are always greater than
the values of horizontal polarization and are highest
at Brewster angle.
Figure 3 represents the emissivity at three different
special angles (45°, 50° and 55°) for 15 kppm
which are desirable for space borne remote
sensing sensors, such as NIMBUS, AMSRE, SSMI
and MSMR as well as QuikSCAT and OSCAT
(India’s first scatterometer), for Cj and Ku band,
respectively. This figure indicates that the emissivity
increases as the combination of low saline water and
diesel increases in both horizontal and vertical
polarization. It is also observed that the emissivity
values for vertical polarization are more than those
for horizontal polarization and at 55 degree, there is
largest difference in emissivity values for horizontal
and vertical polarization.
CALLA et al.: EMISSIVITY & SCATTERING COEFF. OF LOW SALINE WATER CONTAMINATED BY DIESEL
Fig. 1—Variation of emissivity of low saline water: (a) 5 kppm;
(b) 15 kppm; and (c) 25 kppm with respect to different look
angles in Cj band
271
Fig. 2—Variation of emissivity of low saline water: (a) 5 kppm;
(b) 15 kppm; and (c) 25 kppm with respect to different look
angles in Ku band
Fig. 3—Variation of emissivity of low saline water with respect to weight percentage of diesel in water in 15 kppm for three look angles
(45, 50 and 55 degree) in: (a) Cj band; and (b) Ku band
Figures 4 and 5 show the variation in scattering
coefficient of low saline water with different weight
percentage of diesel with respect to different look
angles ranging from 10 to 80 degrees for slightly
rough surface and for both horizontal and vertical
polarization at 5.3 and 13.4 GHz, respectively. From
the figures, it is observed that the scattering
coefficient for slightly rough surface decreases as
the look angle increases for horizontal polarization.
From the figures, it is observed that the values of
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INDIAN J RADIO & SPACE PHYS, OCTOBER 2011
Fig. 4—Variation of scattering coefficient of low saline water: (a)
5 kppm; (b) 15 kppm; and (c) 25 kppm with respect to different
look angles in Cj band
Fig. 5—Variation of scattering coefficient of low saline water: (a)
5 kppm; (b) 15 kppm; and (c) 25 kppm with respect to different
look angles in Ku band
scattering coefficient for VV polarization are higher
than the values for HH polarization. It can be seen
that as the weight percentage of diesel in low saline
water increases, the value of scattering coefficient
for both horizontal and vertical polarization
decrease.
Figure 6 shows the variation of backscattering
coefficient with respect to different weight percentage
of diesel in low saline water for three different look
angles (45°, 50° and 55°) for 15 kppm, which are
desirable for space borne sensors.
Figure 7 represents the Brewster angle of low
saline water contaminated by diesel at different
percentages ranging 0-280% in Cj band and 0-160%
in Ku band, respectively. It can be seen that as
the percentage of diesel contamination in low saline
water increases, the Brewster angle decreases.
It may be concluded from this study that the
scattering coefficient decreases with increase in the
weight percentage of diesel in low saline water
whereas the emissivity increases with increase in the
weight percentage of diesel in low saline water for
both horizontal and vertical polarization at a specific
look angle. Furthermore, emissivity and scattering
coefficient for vertical polarization is higher than
that for horizontal polarization. On the other hand,
emissivity and scattering coefficient decreases with
increase in the value of look angles for horizontal
polarization. There is a peak value for vertical
polarization for emissivity and scattering coefficient
for both Cj and Ku band. With increase in the
weight percentage of diesel in low saline water, the
maximum values of vertical polarization shifts
towards lower look angles. Emissivity and scattering
CALLA et al.: EMISSIVITY & SCATTERING COEFF. OF LOW SALINE WATER CONTAMINATED BY DIESEL
273
Fig. 6—Variation of scattering coefficient of low saline water with respect to weight percentage of diesel in water in 15 kppm for three
look angles (45, 50 and 55 degree) in: (a) Cj band; and (b) Ku band
Fig. 7—Variation of Brewster angle of low saline water with respect to weight percentage of diesel in: (a) Cj band; and (b) Ku band
coefficient for vertical polarization is higher than that
for horizontal polarization.
4 Summary and Conclusions
This paper presents the estimated emissivity
and scattering coefficient at Ku band (13.4 GHz) and
Cj band (5.3 GHz) frequency and for different
samples of low saline water contaminated by diesel.
The data obtained suggest the following:
1. Scattering coefficient decreases with increase
in weight percentage of diesel in low saline
water at a specific look angle.
2. Emissivity increases with increase in weight
percentage of diesel in low saline water for a
particular look angle.
3. Scattering coefficient and emissivity decrease
with increase in look angle for horizontal
polarizations and the value of scattering
coefficient for horizontal polarization is less
than that for vertical polarization.
Clearly, the problems associated with water
pollution have the capabilities to disrupt life on our
planet to a great extent. The database of this study
will help in determining the behaviour of low saline
water contaminated by diesel all over the world.
The study will provide good results in designing
microwave sensors for remote sensing of low
saline water and the water bodies with low salinity.
An extensive study on this subject is required for
monitoring the spatial and temporal variations
of contamination of low saline water.
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