Simulation of tidal current components and oil spills spreading from Radarsat

Simulation of tidal current components and oil spills spreading from Radarsat

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Maged Marghany
International Institute For Aerospace Survey and Earth Science (ITC)
Division of Applied Geomorphological Surveys
PO Box 6, 7500 AA Enschede, The Netherlands
[email protected], [email protected]

Abstract
This paper presents work done utilizing Radarsat fine mode data to model the tidal current component effects on oil spills spreading. In this study, the utility of oil spills detection is examined using fractal dimension methods. The finite difference model is used to simulate the tidal current velocity from Radarsat data. The tidal current velocity modeled by using a finite difference model of Doppler shift frequency. The current velocity components are modeled by applying Lagarangian interpolation. The statistical model is used to find out the significant correlation between tidal current components and oil spills spreading.

The results show the tidal current components have effects on the oil spills spreading. The northward tidal current components are effected the length of the oil spills while the eastward components are effected the width of oil spills.

In conclusion, Radarsat data has good potential for oil spills detection and mapping. Radarsat data could be used to model the tidal speeds and their effects on oil spill patterns. The tidal current components are in good correlation with oil spill lengths and widths.

Introduction
Modeling of oil spills movements by remote sensing is in early stage of investigation. Up to now the scientists could not be used the full capability of remote sensing techniques for modelling and predicting oil spill movements. Using remote sensing techniques for oil spill detection was just focused on the improvement of detection algorithms. These studies cannot provide any sufficient information for identification the effects of physical parameters (wind, current, waves…etc.,) on the oil spill patterns.


fig1

The modelling of water movement on radar data effects have been studied by Maged and Ibrahim [3]. They found that the strong tidal current induced the large oil spill width. Maged [4] modelled the ship speed effects on oil spill spreading. Maged [4] found that the increases of oil spill lengths are due to the effect of ship wakes. As the ships created a turbulent area along the oil spill tracks.

This study aims to model the effect of tidal current components on oil spill patterns (width and length) on Malacca Straits. This is because of the fact that tidal current movements considered significant in narrow waterway such as the Malacca Straits, which is a near 12 hourly sinusoidal phenomenon arising from the semi-diurnal motion of the M2 and S2 tidal components.

Methodology

Study Area
The study area is located in the Malacca Straits between 102° 16′ E to 103° 48′ E and 1° 16’N to 2° 13′ N. According to Wyrtki [5] the water movements are in general directed towards the northwest direction and are strongly related to the surface gradient of the sea level. Furthermore, Wyrtki [5] stated that the period of strongest flow is from January to April, during the northeast monsoon with current velocity of 0.95 m/s.


fig2

Oil spills Detection
According to Benelli and Garzelli [1] the fractal dimension could be estimated from power spectra. This means that fractal dimension can be characterized by a random-phase Fourier description in the form of power density. The power density can given by

P (W1,W2)�����=�����1/[�W12+W22]b���������� (1)

where W is the frequency domain of the Fourier Transform and b= 2H +2 as H is Hurst or persistence parameter, controlling the roughness of the surface. H is 1 corresponding to smooth surface and H is zero which corresponding to a very rough texture. The fractal dimension D and persistence parameter are related by

D�����=�����3-H����������(2)

Thus D can be computed by applying a linear regression on log [P(W1,W2)] vs. Log [�W12, �W22]. Furthermore, H can be computed from the linear regression of ratios of powers.
Current Speed Model
The current velocity modeled by applying the azimuth velocity equation. The azimuth velocity component of current movements was obtained by

Vx�����=����� – Dx dx v2 / Df l R����������(3)

where dx is the pixel spacing in azimuth direction and Df is the difference between the look center frequencies of the two successive images and l is the wavelength and R is the distance between antenna and the target [3].

The current speeds modeled from RADARSAT image were related to the tidal current modeled by using M2 components from tidal table (1997). The Lagrangian method was applied with finite difference grids to simulate tidal current components from Radarsat image. These tidal current components are divided into their x and y components as:

u�����=����� Ui + Vj����������(4)

U and V are determined from surrounding grid nodes by means of Lagrangian interpolation [2]. The statistical regression model was applied in order to investigate the tidal currents components effects on oil spill pattern.


Fig – 3

Table 1: Fractal Dimension Estimation
Area Fractal Dimension Oil Spills
A 2.02
B� 2.11
C �2.32
D �2.43
Surface Water� 2.63

Results and Discussion
The composite image of texture analysis, LEE filter and Gamma filter shows that a heavy ship traffics near to Johor Barua which it may be caused oil spills (Fig. 1). The result of LEE filter shows that the oil spills curved along the coastal water of Malacca Straits (Fig. 1). It is interesting to find that the LEE filter is suitable for oil spills detection. This is because of the fact that LEE filter can reduce the noise variance beside multiplicative noise and additive noise. Table 1 shows the summaries of fractal dimension results. The sea surface is dominated by steady value of fractal dimension of 2.63 while the oil spills have a different value of fractal dimension ranged between 2.03 to 2.43. This finding is similar to study of Benelli and Garzelli, [1]. Fig. 2 shows the simulated current velocity from Radarsat image. The simulation results are in conformity that the largest current speeds are found in the largest area of oil spill width. The tidal current components U and V have a higher positive frequency (Fig. 3). This means that the strong tidal current movements was towards the northwest direction. The V components have a larger velocity than U components. The maximum V velocity is 1.34 m/s while the maximum U velocity is 0.15 m/s. It was noticed that the U and V components move in two opposite directions. The effects of tidal current components on oil spill patterns can observed from Figs. 4a and 4b. It is obvious that the V components have good effects on oil spill lengths than U components. However, U components have good effects on oil spill widths than V components. It can explain that due to the effects of V components, the oil spills moved towards the north direction. This means that the spreading of oil spills towards the west coast of Malaysia will be due to the effects of U components.


Fig – 4

Conclusion
It can be concluded that the fractal dimension method is good method for oil spills detections from radar data. RADARSAT data shows a good potential for detection oil spills and modelling of tidal current effects on oil spill patterns. Tidal currents have two directions on the oil spill patterns. The V components have a most effects on the oil spill lengths toward the north and the U components could be effected the spreading of the oil spills toward the west.

References

  1. Benelli, G. and A.Garzelli, ” Oil-spills detections In SAR Images By Fractal Dimension Estimation”. Proceeding of IGARSS’99, 28 June to 2 July Homburg, Germany,1999
  2. Hadi, S. , k. Dadang, Michardja and Totoks, “Oil spill Model of Makassa Straits”. Proceeding of the Regional Workshop on Oil Spill Modeling, 31 may to 3 July, Puasan, Republic of korea, 1996.
  3. M. Maged and M.H. Ibrahim, “Tidal Current Effects on the oil Spills movement By Radarsat. Paper Presented at International Conference on the Straits of Malacca, 19-22 April 1999, Equatorial Hotel, Malacca, Malaysia.,1999.
  4. M. Maged, “Simulation of Ship Effects on Oil Spills Spread 2nd International Symposium on Operationalization of Remote Sensing. 16-20 August, 1999,ITC,Enschde,Netherlands, 1999
  5. W. Wyrtki, “Physical Oceanography of the Southeast Asian Waters”. In NAGA Report Vol.2, Univ.Calif-Scripps.Ins.Ocean,1961