DIELECTRIC PROPERTIES OF NORTH INDIAN OCEAN SEAWATER AT 5 GHZ

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1 DIELECTRIC PROPERTIES OF NORTH INDIAN OCEAN SEAWATER AT 5 GHZ A.S. Joshi 1, S.S. Deshpande 2, M.L.Kurtadikar 3 1 Research Scholar, J.E.S. College, Jalna, Maharashtra, India. 2 Rashtramata Indria Gandhi College, Jalna, Maharashtra, India. 3 P.G. Department of Physics and Research centre, J.E.S. College, Jalna, Maharashtra, India. ABSTRACT This study presents dielectric properties of North Indian Ocean seawater. In all, fourteen seawater samples are collected from Arabian Sea, Lakshadweep Sea, Tip of Bay of Bengal Sea, deep Indian Ocean and Equatorial region. The Von Hipple method is used to measure dielectric properties, both real part ε' and imaginary ε'', at 5 GHz and 30 C using automated C-Band microwave bench set up. The dielectric constant ε' and dielectric loss ε'' are calculated using least square fitting technique. The salinity measurement of seawater samples are done on autosalinometer. Making use of salinity values of all samples and for 5 GHz and 30 C, static dielectric constant and dielectric loss are estimated by Klein-Swift model and Ellison et al. model. Experimental and theoretical results are compared. This study emphasizes latitude and longitudinal variations of salinity and dielectric properties. The laboratory data obtained are significant for microwave remote sensing applications in physical oceanography. KEYWORDS: Seawater Permittivity, Salinity, North Indian Ocean, 5 GHz microwave frequency. I. INTRODUCTION Indian Ocean is third largest ocean of the world and has unique geographic setting. The Tropical India Ocean (TIO), in particular is significant to oceanographers and meteorologists as it experiences the seasonally reversing monsoon winds and is land locked on northern side. Remote sensing [1-2] of ocean sea surface salinity, sea surface temperature is important in the areas like seawater circulations, climate dynamics, atmosphere modeling, environmental monitoring etc. For microwave remote sensing applications over ocean radar and radiometer, precise values of emissivity and reflectivity are required. The surface emissivity is a complex function of dielectric constant of surface seawater. This complex function is composed of two parts, the real part is known as the dielectric constant (ε ) and is a measure of the ability of a material to be polarized and store energy. The imaginary part (ε ) is a measure of the ability of the material to dissipate stored energy into heat. The two are related by the expression: = 1 The dielectric constant in turn is governed by electrical conductivity and microwave frequency under consideration. The conductivity is governed by salinity and temperature of seawater [3-4]. There are variations in salinity and temperature of ocean resulting variation in dielectric properties and hence in emissivity at that particular location. These variations follow certain pattern latitude and longitude of the location, due to dynamic features of the ocean. This work focuses on measurement of dielectric properties of seawater samples at 5 GHz at 30 C. The study emphasizes on latitude and longitudinal variations in salinity and dielectric properties. Knowing the dielectric constant and dielectric loss, the parameters like emissivity, brightness 220 Vol. 2, Issue 1, pp

2 temperature, scattering coefficient can be interpreted, as they are interdependent. Making use of the measured salinity values of all samples, static dielectric constant and dielectric loss are estimated by Klein-Swift model [5-6] and Ellison et al. model [7-8] for 5 GHz and 30 C. The laboratory data obtained are significant for interpretation of microwave remote sensing applications, and helps in designing active and passive microwave remote sensors. II. MATERIAL AND METHODOLOGY 2.1. Seawater Sampling By participating in ORV Sagar Kanya scientific cruise SK-259, organized by NCAOR in May-June 2009 that is summer monsoon period, seawater samples were collected from Arabian Sea, Lakshadweep Sea, Tip of Bay of Bengal Sea, deep Indian Ocean and from equatorial regions of Tropical Indian Ocean. Surface seawater at different locations were drawn through bucket thermometer and two bottles of the samples were preserved around 4 C by standard procedure. Out of two bottles, one of the samples was used to determine the salinity parameter at that location using an Autosalinometer 8400B in the laboratory onboard Sagar Kanya vessel and the other sample of the same location was brought to the Microwave Research Lab, J.E.S. College, Jalna, Maharashtra for dielectric measurement Temperature and Salinity Measurement The bucket thermometer is used to measure the temperature of surface seawater. Salinity measurements of seawater samples were done using 8400B AUTOSAL onboard ORV Sagar Kanya laboratory. This instrument is semi-portable, semi-automatic and is used in the land based or seaborne laboratory to determine salinity levels of saline seawater samples and standard seawater sample by measuring their equivalent conductivity. The instrument reading is displayed in terms of conductivity ratio. Inputting the conductivity ratio to the software available in the computer lab, salinity value of the sample is calculated. The software calculates salinity using the following formula. The equation is based on the definitions and the algorithm of practical salinity formulated and adopted by UNESCO/ICES/SCOR/IAPSO Joint Panel on oceanographic tables and standards, Sidney, B.C., Canada, 1980 [9-10]. S= a +a R +a R +a R +a R +a R + S 2 T 15 S = T 15 b +b R +b R 3 +b R +b R +b R Where a = , b =0.0000, For, 2 S 42, and for 2 C T 35 C. Table 1. Values of the coefficients a and b 2.3. Measurement of Dielectric Properties i a b There are several methods of dielectric measurement of liquid [11]. In present work, the dielectric properties of seawater samples are measured using Von Hipple Method [12] for which automated C- 221 Vol. 2, Issue 1, pp

3 Band microwave bench, as shown in figure 1, is used. The MW bench consists of a low power tunable narrow band VTO-8490 solid-state microwave source; having frequency range of GHz. Tuning voltage is kept at 7 volts, throughout the experiment, which corresponds 5 GHz frequency. The other components of the bench setup are: an isolator, coaxial to waveguide adapter, attenuator, SS tuner, slotted line and the liquid dielectric cell. Figure 1. Block diagram of a C-band microwave bench. Microwave generated by the VTO propagate through the rectangular waveguide to the liquid cell. A desired power level in the line is adjusted with the attenuator. A slotted section with a tunable probe is used to measure the power along the slot line. The crystal detector (1N23) in the probe is connected to a microammeter and to the PC to read, acquire and store the data. The empty liquid dielectric cell is connected at the output end of the bench. The bench is tuned to get symmetrical standing wave pattern in the slot line. The positions of minima are noted from the pattern from which wavelength λ g of the wave-guide can be calculated. The probe position on the slot line is kept constant at the first minima of the standing wave pattern in the slot line. The liquid dielectric cell is then filled with the sample under consideration. The plunger of the liquid cell is initially set in a position such that the thickness of the liquid column below the plunger is zero. By moving the plunger away from this position, data of microwave power is recorded for different plunger positions. The data of plunger positions and the corresponding power are acquired and stored in a file which is further used to calculate dielectric constant ε and dielectric loss ε using the least square fit program. The parameters α, β, P 0, δ are used as the fitting parameters, where α= attenuation factor, β=propagation constant, P 0 =maximum power, and δ= phase factor. The computer program also takes care of calculating error in dielectric constant, ε, and error in dielectric loss, ε. The dielectric properties of seawater samples can be calculated using the relations ε =λ 1 λ + α β 4π 4 and ε = λ αβ 5 2π where λ is the free space wavelength which can be calculated using the formula 1 λ = 1 λ + 1 λ 6 Whereλ =2a= cm=9.46 cm, a being the broader side of the C-band rectangular waveguide. 222 Vol. 2, Issue 1, pp

4 III. RESULTS AND DISCUSSIONS The Sea Surface Temperature of collected samples is found to be between 27 C to 30 C (Table 1.). Winds over the North Indian Ocean reverse twice during a year. They blow from the southwest during May September and from the northeast during November January with the transition-taking place during the months in between. Forced by these winds, circulation in the Indian Ocean has a general eastward direction during summer (May September) and westward during winter (November January). During summer, period when seawater samples were collected the monsoon current flows eastward as a continuous current from the western Arabian Sea to the Bay of Bengal [13-14]. These circulations are shown in Figure 2. Figure 2. Schematic diagram of major surface currents in the TIO during the southwest (summer) monsoon. The thickness represents the relative magnitude of the current (adapted from Shenoi et al., 1999a) [15]. The Arabian Sea has high salinity (usually in the range 35 to 37) due to excess of evaporation over rainfall. In Table 2, the samples S-01 and S-03 are from Arabian Sea and have higher salinity values compared to other samples. Table 2. The temperature and salinity values of seawater samples. Sample Latitude Longitude Temperature Salinity C S-01 N ' E ' S-02 N ' E ' S-03 N ' E ' S-04 N ' E ' S-05 N ' E ' S-06 N ' E ' S-07 N ' E ' S-08 N ' E ' S-09 N ' E ' S-10 N ' E ' S-11 N ' E ' S12 N ' E ' S13 N ' E ' S14 N ' E ' Vol. 2, Issue 1, pp

5 In contrast, the Bay of Bengal has much lower salinity due to the large influx of fresh water from river discharge and high amount of rainfall. The samples S-02 and S-04 although located on similar latitude as S-01 and S-03 respectively, differ longitudinal wise and for these Lakshadweep Sea samples, drawn at mouth of Bay of Bengal Sea, decrease in salinity is seen. The samples S-05, S-07, S-08 are from deep IO and S-06, S-08, S-10, although located on similar latitude, differing in longitude, are towards east, and are from border of Bay of Bengal Sea and Arabian Sea. The salinity values of these samples are found less than the former ones. As we move towards Equator there is slight decrease in salinity in case of the samples S-11, S-12, but near the equatorial regions, S-13, S-14, a sudden slight increase in salinity value is found. This is due to high evaporation in low-pressure equatorial regions [16]. The dielectric constant ε', dielectric loss ε'', error in dielectric constant ε' and error in dielectric loss ε'', at 5 GHz, at 30 C and with varying salinity, latitude and longitude wise in North Indian Ocean are given in Table 3. The magnitude of dielectric constant is found to be 66. It is found that dielectric constant is decreased with increase in salinity. The dielectric loss values are in range of 53 to 58. Table 3. The experimentally measured values of dielectric constant, dielectric loss, error in dielectric constant, error in dielectric loss of all seawater samples at 5 GHz. Sample Latitude Longitude Salinity ε' ε'' ε' ε'' S-01 N ' E ' S-02 N ' E ' S-03 N ' E ' S-04 N ' E ' S-05 N ' E ' S-06 N ' E ' S-07 N ' E ' S-08 N ' E ' S-09 N ' E ' S-10 N ' E ' S-11 N ' E ' S12 N ' E ' S13 N ' E ' S14 N ' E ' The values calculated in Tables 4 and 5 are by using Klein and Swift and Ellison et al. models respectively. Comparison of measurement results with these respective models shows that real part, dielectric constant ε' values are well in agreement. However, our experimental loss factor is higher by a magnitude of about 20 as compared with the theoretical models. The percentage error in measurement in dielectric constant and loss is of the order of 7 and 2 respectively. Table 4. The calculated relaxation time, τ ps, static dielectric constant ε, dielectric constant ε and dielectric loss ε using Klein-Swift Model at 5 GHz. Sample Latitude Longitude Salinity ε' ε'' S-01 N ' E ' S-02 N ' E ' S-03 N ' E ' Vol. 2, Issue 1, pp

6 S-04 N ' E ' S-05 N ' E ' S-06 N ' E ' S-07 N ' E ' S-08 N ' E ' S-09 N ' E ' S-10 N ' E ' S-11 N ' E ' S12 N ' E ' S13 N ' E ' S14 N ' E ' Table 5. The calculated relaxation time τ ps, static dielectric constant ε, dielectric constant ε and dielectric loss ε using Ellison et. al. model at 5 GHz. Sample Latitude Longitude Salinity ε' ε'' S-01 N ' E ' S-02 N ' E ' S-03 N ' E ' S-04 N ' E ' S-05 N ' E ' S-06 N ' E ' S-07 N ' E ' S-08 N ' E ' S-09 N ' E ' S-10 N ' E ' S-11 N ' E ' S12 N ' E ' S13 N ' E ' S14 N ' E ' ACKNOWLEDGEMENTS We are thankful to ISRO for providing the C-Band Microwave Bench Setup under RESPOND project of Dr. M.L. Kurtadikar. Special thanks to NCAOR, Goa, for allowing participation in SK-259 cruise of ORV Sagar Kanya, for seawater sample collection. REFERENCES [1] Fawwaz T Ulaby, Richard K Moore and Adrian K Fung (1986). Vol. 3 Artech House Inc. [2] Eugene A. Sharkov (2003). Passive Microwave Remote Sensing of the Earth, Springer, Praxis Publishing, UK. [3] Smyth, C.P., (1955). Dielectric Behaviour and structure, McGRAW-HILL Book company Inc, New York. 225 Vol. 2, Issue 1, pp

7 [4] Hasted, J.B, (1973). Aqueous Dielectrics, Chapman and Hall Ltd, London. [5] Stogryn, A., (1971) Equation for calculating the dielectric constant of saline water, IEEE transactions on microwave theory and Techniques, vol.19, pp [6] Klein, L.A., and. Swift,C.T,(1977). An improved model for the dielectric constant of seawater at microwave frequencies, IEEE J. Oceanic Eng., OE-2: pp [7] Ellison, W., Balana, A., Delbos, G., Lamkaouchi, K., Eymard, L., Guillou, C., and Prigent, C., (1996). Study and measurements of the dielectric properties of sea water, Tech. Rep /94/NL/CN, European Space Agency. [8] Ellison, W., Balana, A., Delbos, G., Lamkaouchi, K., Eymard, L., Guillou, C., and Prigent, C. (1998). New permittivity measurement of seawater, Radio Science, Vol. 33: pp [9] Lewis, E.L. (1978). Salinity: its Definition and calculation. J. Geophys. Res. 83:466. [10] Lewis, E.L. (1980). The practical salinity scale 1978 and its antecedents. IEEEJ. Oceanic Eng. OE-5:3.pp 14. [11] Udo Kaatze (2010). Techniques for measuring the microwave dielectric properties of materials, IOP publishing, Metrologia, Vol. 47, pp [12] Von Hipple A (1954). Dielectrics & Waves, Wiley, New York. [13] Prasanna Kumar S, Jayu Narvekar, Ajoy Kumar, C Shaji, P Anand, P Sabu, G Rijomon, J Josia, K.A. Jayaraj, A Radhika and K.K. Nair (2004). Intrusion of Bay of Bengal water into Arabian Sea during winter monsoon and associated chemical and biological response, American Geophysical Research, vol. 31, L15304, doi: /2004 GL [14] Gangadhara Rao, L.V., Shree Ram, P., (April 2005). Upper Ocean Physical Processes in the Tropical Indian Ocean, monograph prepared under CSIR scientist scheme, National Institute of Oceanography regional centre, Visakhapatnam, pp [15] Shenoi, S.S.C, Saji, P.K and Almeida, A.M (1999a). Near-surface Circulation and kinetic energy in the tropical Indian Ocean derived from Lagrangian drifters, J Mar. Res. Vol. 57, pp [16] Shankar.D, Vinayachandra P.N, Unnikrishnan (2002). The Monsoon Currents in the North Indian Ocean, Progress in Oceanography, 52(1) pp Authors Anand Joshi was born in Aurangabad, India in He received B.Sc. degree in Physics, Mathematics, Computer Science and M.Sc. degree in Physics from Dr. Babasaheb Ambedkar Marathwada University, Aurangabad, Maharashtra, India in 2002 and 2004 respectively. He is currently pursuing a Ph.D.(Physics) degree under the guidance of Dr. M.L.Kurtadikar, Postgraduate Department of Physics and Research Centre, J.E.S. College, Jalna, Maharashtra, India. His research interests include Dielectric measurements, Microwave Remote sensing Applications and Astrophysics. Santosh Deshpande was born in Parbhani, India in He received M.Sc. degree in Physics from Swami Ramanand Teerth Marathwada University, Nanded, Maharashtra and M.Phil degree in Physics from Algappa University, Tamil Nadu, India in 2000 and 2008 respectively. He is currently working as Assistant Professor of Physics in the RMIG College, Jalna, Maharashtra, India. He is also pursuing a Ph.D. degree under the guidance of Dr. M.L.Kurtadikar, Postgraduate Department of Physics and Research Centre, J.E.S. College, Jalna, Maharashtra, India. His research interests include Dielectric measurements, Microwave Remote sensing Applications and Astrophysics. Mukund L. Kurtadikar was born in Nanded, India in He received the Master of Science (Physics) and Ph.D. (Physics) degrees from Marathwada University of Aurangabad, India in 1973 and 1983 respectively. He is currently working as Associate Professor of Physics in the Postgraduate Department of Physics of J. E. S. College, Jalna, Maharashtra, India. His research interests include Microwave Remote Sensing Applications, dielectric measurements of soils, seawater, rocks, snow, vegetation etc. He also works on Photometry of Variable Stars using Small Optical Telescope and Scientific Exploration of Historic Monuments. He is a Science Communicator. 226 Vol. 2, Issue 1, pp