IL NUOVO CIMENTO VoL. 8 C, N. 6 Novembre-Dicembre 1985

Remote Sensing of Oils on the Sea Surface with an

Airborne Infra-Red Scanner (*).

F. CASTAGiNOLI, ]~. PAINTA~'I, I. PIPPI and B. RADIGATI

Istituto di Ricerca sulle Onde Elettromagnetiche del C.N.R. - tZirenze, Italia

(rieevuto 1'11 Febbraio 1985)

Summary. - - The ability of an infra-red scanner to detect oil spills over the sea surface is discussed on the basis of the first results of the Archi- medes experiment 1983. During this experiment an infra-red scanner was flown over two oil spills and a real-time detection was achieved. The IR images were also recorded and processed in order to improve the film-thickness evaluation. An infra-red visible package was then developed which allows a real-time remote sensing of oil spills, eventually from small aircrafts.

PACS. 92.60. - Meteorology.

1 . - I n t r o d u c t i o n .

A n e x p e r i m e n t on r e m o t e sens ing of oil spills was ca r r i ed ou t on Oc tober

21-22, 1983 in t he ~North Sea a b o u t 100 k m :Nor th-West f rom R o t t e r d a m .

The e x p e r i m e n t was p a r t of t h e A R C H I M E D E S p r o g r a m m e of t he JRC-

E u r a t o m a n d h a d t he pu rpose of t e s t i n g the genera l a b i l i t y of r e m o t e - s e n s i n g

devices in de tec t ion , c lass i f ica t ion a n d q u a n t i f i c a t i o n of oil spills on t he sea

surface (1).

Two oil spills were m a d e a b o u t 9 k m a p a r t f rom each o ther , one wi th pu re

fuel oil a n d t he o the r w i th <~ chocolate mousse >>, a s t ab i l i zed m i x t u r e of fue l oil

(*) Paper presented at the lo Congresso del Gruppo Nazionale per la Fisica dell'At- mosfera e dell'Oceano, Rome, June 19-22, 1984. (2) R.C. SCHnI~L : The logistic o/the Archimedes exercise, in The Archimedes 1 Experiment, EUR 10216 EN (Office for Official Publications of the European Communities, Luxem- burg, 1985), p. 19.

762

REMOTE SENSING OF OILS ON THE SEA S'[IRFACE W I T H AN AIRBORNE ETC. ~ 6 ~

a n d w a t e r 4 0 + 6 0 , a n d d i f f e r en t sensors (~-7) were flow]l ove r t h e spi l ls . The

on ly I t a l i a n o r g a n i z a t i o n wh ich p a r t i c i p a t e d to t h e e x p e r i m e n t was t h e / R O E -

CI~R w i t h a n i n f r a - r e d s c a n n e r (7) i n s t a l l e d on a D o r n i e r D-28 of DI~VLI~.

The s c a n n e r was u s e d b o t h for r e a l - t i m e m o n i t o r i n g of t h e sp i l l a n d for r e c o r d i n g

t h e i n f r a - r e d i m a g e s for p o s t - f l i g h t p roces s ing .

The e x p e r i m e n t has shown t h e a b i l i t y of a n i n f r a - r e d s c a n n e r in r e a l - t i m e

m o n i t o r i n g of oi l sp i l l s ove r t h e sea su r face d a y a n d n i g h t w i t h t h e on ly r e q u i r e -

m e n t of a t r a n s p a r e n t a t m o s p h e r e .

2 . - O i l s p i l l d e t e c t i o n .

W h e n a n i n f r a - r e d sensor is f lown a t a n a l t i t u d e Z o v e r t h e sea su r face

(fig. 1) t h e t o t a l s p e c t r a l r a d i a n c e wh ich r e a c h e s t h e i n p u t op t i c s a t a f r e q u e n c y

v a n d a z e n i t h ang le 0 c a n be e x p r e s s e d as t h e s u m of fou r t e r m s , t h e su r face

emis s ion , t h e a t m o s p h e r i c emi s s ion , t h e d o w n w a r d r a d i a n c e r e f l ec t ed b y t h e

su r face a n d t h e r e f l ec t ed so la r f lux. T h e su r face c h a r a c t e r i s t i c s wh ich in f luence

t h e t o t a l s p e c t r a l r a d i a n c e a re , t h e r e f o r e , t h e su r face e m i t t a n c c s(v) a n d t h e

su r face r e f l ec tance . The l a s t one can be c o n s i d e r e d (s) as t h e s u m of t w o t e r m s :

a di f fuse L a m b e r t i a n r e f l ec t ance o(v) ~ ] - - ~(v) a n d a s p e c u l a r r e f l e c t a n c e ~'(v)

wh ich fol lows F r e s n e l laws.

(2) S. MADSEN : Detecting oil at sea by means o] HH pola~'ized side looking airborne radar, in The Archimedes 1 Experiment, EUR 10216 EN (Office for Official Publicat ions of the European Communities, Luxemburg, 1985), p. 51; N. SKy: Measuring oil at sea by means o] airborne microwace radiometry in the range 5-34 GHz, in The Archimedes 1 Experiment, EUR 10216 EN (Office for Official Publicat ions of the European Communi- ties, Luxemburg, 1985), p. 83. (3) D. DIEBEL-LA~-GOH~, T. HEX(;STE~m.~, R. REL'TL~, G. CECCm and L. PANTA~I: Measuring oil at sea by means o] an airborne Laser Fluorosensor, in The Archimedes 1 Experiment, EUR 10216 EN (Office for Official Publications of the European Communi- ties, Luxemburg, 1985), p. 163. (4) A. LOFFET: Correlation and synthesis o] the ~'esults obtained by di]ferent sensor types, in The Archimedes 1 Experiment, EUR 10216 EN (Office for Official Publicat ions of the European Communities, Luxemburg, 1985), p. 163. (~) K. GRU.~ER: Measuring oil at sea by means o] an airborne 90 GHz microwave radiometer, in The Archimedes 1 Experiment, EUR 10216 EN (Office for Official Publica- tions of the European Comnmnities, Luxemburg, 1985), p. 105. (6) F. WITTE : Detection oil at sea by means o] VV polarized side looking airborne radar, in The Archimedes 1 Experiment, EUR 10216 EN (Office for Official Publicat ions of the European Communities, Luxemburg, 1985), p. 65. (7) Y. CASTAGNOLI, L. PANTANI, I. PIPPI and B. RADICATI: I~emote sensing oil at sea by means o] an airborne in]rated scanner, in The Archimedes 1 Experiment, EUR 10126 EiY (Office for Official Publicat ions of the European Communities, Luxemburg, 1985), p. 143. (s) P . Y . DESCHAMPS, H. HEn.~tA_,,-, J. LENOBLE, D. TAURE and ~M. VIOLLIER: Atmos- pheric e]]ects in remote sensing o] ground and ocean re]lectances, irr Remote Sensing of Atmospheres and Oceans (Academic Press, New York, N .Y . , 1980), p. 411.

764 F. CASTAGNOLI, L. PANTANI, I. PIPPI and B. RADICATI

l i l

7(v ~ O) I I I

/ I

/ I

/ I

i I

I /

i I I

I /

I I I

i I /

I r

I s

i t I

I I

/ I

i i

i I I

I i

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i t sea

Fig. 1. - The geometry of sea surface observation with an airborne infra-red scanner.

Under the assumpt ion of a ver t ica l downward observa t ion with small scan- ning angles, the dependence f rom 0 can be neglected and the t o t a l spectra l radiance can be wr i t t en {9)

(1) I(v) ~- z(v, Z){e(v)B(v, T) + [1-- e(v)]I~,i(v) +

+ CB(v, T~) cos O~q'(v)~(v~, 0~)} + Io~(v),

where B(v, T) is the well-known Planck funct ion,

Z

( 2 ) ~a~(~)) : f B [ ~ ) , T ( z ) ] ~T(~) ' 0 , z ) ~z dz, o

z

fB ~v(~, O, z) (3) I~4(u ) ---- 2 [v, T z ) ] ~ z - - - dz,

o

(4) C = 2 .16 ~ 1 0 - 5 c .

(s) M . T . CI~AI~]~: In]rared remote sensing o/ sea sur]ace temperature, in Remote Sensing o/ Atmospheres and Oceans (Academic Press, N e w York, N . Y . , 1980), p. 411.

REMOTE SENSING OF OILS ON THE SEA SURFACE W I T H AN AIRBOR2N-E ETC. 7 6 5

~(~, z) und "~(r, 0~,) are the a tmospher ic t r ansmi t t ances be tween the sea surface and the a l t i tude z and the Sun, respect ively , T is the t e m p e r a t u r e of the sea surface, T(z) is the t e m p e r a t u r e of the a tmosphere a t an a l t i tude z, Th ---- 5600 K, and c the f rac t ion free of clouds of the solid angle under which the Sun is seen f rom the sea surface.

The spectral-radiance difference be tween the spill and the free wa te r can be ob ta ined f rom cq. (1):

(5) Do,,(v ) ---- x(v, Z){[so(v)B(v , T,o ) - - e , , (r)B(v, T ) ] -

- - [eo(v ) - - s .(r)]I4(v) q- [e~(v) - e'w(v)] CB(r , T~,)cos 0nT(v , 0h)},

Fig. 2. - Example of an infra-red image recorded at night. The oil spill is shown as a darker (colder) region. Flight No. 1, image 11.

where o and w indicate oil and water , respect ively. The solar t e r m is negligible for observat ions done in the wa te r vapour windows at 3.7 [zm and 11 [~m, and obviously for obserw~tions done a t night or iu ve ry c loudy days. _it n ight the spill can general ly be assumed at the same t e m p e r a t u r e of the surrounding wate r and eq. (5) is simplified in

(6) Do.(~) = v(~, Z)Eeo(V)- s.(v)]EB(v, T ) - I~(v)];

in these condit ions the spill shows a radiance lower t h a n the wa te r because of its lower emi t t ance (fig. 2). Dur ing the day the s i tua t ion is more complicate .

49 - I I N u o v o C i m e n t o C,

7 6 6 F. CASTAGI~OLI, L. PANTANI, I. PIPPI a n d B. RADICATI

The oil absorbs t he solar rad ia t ion more t h a n the water , and its t e m p e r a t u r e m a y be significantly higher in the th icker p a r t of the spill (10) (fig. 3). The radiance m a y , therefore , pass f rom a va lue lower t h a n the surrounding wa te r in the th inner p a r t of t he spill, to a value higher t h a n the wa te r in the th icker pa r t s (fig. 4). On this basis an indicat ion of the thickness changements can be obtained.

Fig. 3. - Example of an infra-red image recorded in daytime. The oil spill is shown with different gray levels corresponding to different thicknesses of oil film. Flight No. 2, image 10.

I f the sea would be a fiat surface t he solar reflection t e rm , if p resent 9 would influence Do,(U) only when Oh is lower t h a n the i n s t r u m e n t field of view. The presence of sea waves gives rise to a f luc tuat ing g u t t e r t e r m which m a y p lay an interes t ing funct ion in oil spill identif icat ion because of the change in waves behav iour caused b y the oil film (11).

(lo) H.D. P ~ K ~ and D. CORMACK: Requirements ]or remote sensing o/ oil on the sea, in Airborne Remote Sensing o] Oil in Coastal Waters (U.S. Coast Guard, Washington, D.C., 1979), p. 65. (11) R. CIN~, P .P . LOMBA~DINZ and H. HVH~ERFVSS: Int . J . Remote Sensing, 4, 101 (1983).

R E M O T E S E N S I N G OF OILS ON T H E S E A S U R F A C E W I T H A N A I R B O R N E E T C . 767

o w

J

day f i lm thickness

night

Fig. 4. - Qualitative behaviour of the spectral radiance difference between the oil spill and the water as a function of the oil film thickness.

3 . - T h e e x p e r i m e n t .

The s canne r used in t h e Arch imedes e x p e r i m e n t was a n A G A T h e r m o v i s i o n

782-SW h a v i n g t h e charae te r i s t i e s s u m m a r i z e d in t a b l e I . The s c a n n e r was

i n s t a l l ed over t he floor t r a p of t he D-28 wi th in f r on t a 45 ~ go ld -p la t ed

mi r ro r in o rder to ach ieve a v e r t i c a l d o w n w a r d view. The in f ra - r ed images

were recorded wi th a pho tog raph i c c a m e r a m o u n t e d in f r on t of t he m o n i t or

screen. Since the f luorescence l ida r of t he O l d e n b u r g U n i v e r s i t y (3) was car r ied

on t he same p lane , t h e c a m e r a was e lec t r i ca l ly c o n n e c t e d to t he l id~r c o m p u t e r

for t i m i n g purposes .

TABLE I. -- Characteristics o] the in/ra-red scannter used during the Archimedes experi~nent.

Scanner head

detector: photovoltaie, indium antimouide (liquid N 2 cooled) lenses: 12 ~ scanned field at 10m: 2.1 m geometrical resolution : 1.9 mrad spectral sensitivity: from 3 vm to 5.6 ~m aperture: ]/1.8 field frequency: 25 Hz line frequency: 2500 Hz line • frame : 280 interlaced 4:1 resolving power: 100 elements/line

Black and white monitor

image : 50 mm • 50 mm measurement scale: on the screen thermal range: 9 calibrated ranges thermal level: 10 tu rn potentiometer picture mode: five-step gray scale

768 F. CASTAGNOLI, L. PANTANI , I . P I P P I a l l d B. RAI)ICATI

The a i rcraf t was flown at an a l t i tude of 200 m wi th a speed of 50 m/s. As it can be deduced f rom the da ta of t ab le I , a foo tpr in t of 42 m • 42 m was ob ta ined with a theore t ica l spa t ia l resolut ion of 0.38 m. Since the scanner field f requency is 25 I t z and the resolving power is 100 elements/ l ine, the ac tua l resolut ion was 2 m along the course direct ion and 0.42 m in the cross-course direction.

Two flights were done dur ing the expe r imen t : the first one on October 21st which crossed the fuel-oil spill be tween 6.15' and 6.42', the second one on October 22 which crossed the chocolate-mousse spill be tween 12.15' and 12.36'. A to t a l n u m b e r of 96 p ic tures were t a k e n dur ing the first flight, one each 20 l idar shots. Since the l idar p.r .f , was 5 t tz , this corresponds to a 200 m dis tance be tween the centres of two subsequent pictures. Dur ing the second flight 36 pic tures were t a k e n with m a n u a l opera t ion of the camera .

I n bo th flights the oil spill was visual ly detected on the moni to r screen and good images were recorded. Some interferences were presen t on the screen, and were recorded on the film. These i~lterferences did not d is turb the spill detect ion.

Unfor tuna te ly the a i rcraf t did not offer the possibi l i ty of posi t ion da ta recording, and a careful compar ison with the da ta of o ther sensors is, therefore , difficult.

4. - Analysis o f the images.

I n bo th flights the oil spill c learly appea red on the moni to r screen and was identified b y the change ia t he g ray levels, and also b y the modificat ion of the wave s t ructure . More detai led images were t a k e n on flight ~o . 2. This fac t can be a t t r i bu ted to two dif ferent causes: t he first one is the difference be tween the images t a k e n a t n ight (flight 1) and dur ing the day (flight 2) which was po in ted out in the previous section, the second one is the t i m e in te rva l of abou t 30 hours be tween the two flights dur ing which the spills divided in compl ica te s t ruc ture of th ick pa tches separa ted b y th in oil films or clear wa te r as it was observed also b y o ther image sensors (4).

The images of ten show a pa r t i t ion in two or three bands of different br ight - ness, with s t raight- l ine boundar ies a t abou t 15 ~ wdth the hor izonta l axis. This pa r t i t ion was caused b y a lack in the syncronizat ion be tween the photographic camera and the screen scanning, and was corrected in image processing as i t will be shown.

The 20 mos t significant images were digi t ized on a magne t i c t ape a t the IEI -CNI~ in Pisa. The digi t ized images were then processed a t II%OE and displayed on a VDS-701 video-colour system. The processing included the cor- rect ion of the images for the pa r t i t i on caused b y the lack of syncronizat ion and the use of con t ras t e n h a n c e m e n t techniques in order to ev ident ia te the oil

:REMOTE SENSING OF OILS ON THE SEA SURFACE W I T H AN AIRBO:R.-N~ ETC, 7 6 9

spills and the i r th ickness changes. As was po in ted out before, the images of flight 2 showed a more detai led s t ructure , therefore a pa r t i cu la r a t t en t ion was

devoted to these images. A sequence of images wi th different processing is shown in fig. 5 for two

scenes of flight 2. F r o m the top to the b o t t o m : a) the image as is s tored on the magne t ic tape , b) the same image wi th the correct ion of the lack of syncroniz- a t ion and an enhancemen t of contras t , c) the same image as in b) bu t wi th a colour presenta t ion. I t m u s t be no ted t h a t each image contains 355 )<355 pixels each one wi th a level resolut ion of 8 bits. As can be seen the radiance difference be tween the pixels is more ev ident in the images of fig. 5b) ~nd par- t icu lar ly of fig. 5c) and allows a t least an ~pprox ima te e s t ima t ion of the spill

th ickness on the basis of the considerat ions of sect. 2.

5 . - C o n c l u s i o n s .

In f ra - red scanners were found sui table for rea l - t ime detect ion of oil spills

over the sea surface. The detect ion was possible 24 hours a day for bo th oil and chocolate mousse spill, while o ther sensors had problems with t he detect ion of the chocolate mousse dur ing the second day of the exper iment . The image analysis showed the possibi l i ty of a spill-thickness moni tor ing, a t least dur ing the day, wi th an appropr ia t e selection of the image presen ta t ion and processing.

Because the infra-red scanner detects the spill on the basis of the changes of surface emi t t ance and t e m p e r a t u r e , the detect ion is l i t t le affected b y the sea

state. W i t h respect to o ther sys tems the infra-red scanner has the hand icap of a lower m a x i m u m range and of the r equ i remen t of a t r a n s p a r e n t a tmosphere because of the t ransmiss ion t e r m which appears in eq. (1).

A n y w a y the inf ra- red scanner is a l ight weight sys tem which can be carr ied also b y smal l a i rcraf ts or used as an addi t ional sensor on large planes. On the basis of the results of the Archimedes expe r imen t a new a r r angemen t of the scanner was done. The AGA 782 SW was coupled with a s t andard colour TV- camera hav ing the same foo tp r in t as the AGA. The images in the visible and in t he infra-red are shown in real t ime on two moni to rs and recorded on s t andard video- tape. The sys t em is ba t t e ry -powered and has a weight lower t h a n 40 kg with the ba t t e ry . F igure 6 shows the sensor head and the moni tors . The sys t em was successfully t es ted dur ing Augus t 198~ on a Cessna 185 Skywagon.

The authors wish to t h a n k the collegues of D F V L R , of Oldenburg Uni- vers i ty , and of SMA S.p.A., for the help in pe r fo rming the exper iment , and Dr. L. XZZ~ELLI of the I E I - C ~ R for the digi t izat ion of the images.

7 7 0 F. CASTAGNOLI, L. PANTANI, I, PIPPI and B. RADICATI

�9 R I A S S U N T O

In questo lavoro b indagata la possibilit~t di individuare chiazze di idroearbm'i sulla superfieie marina per mezzo eli radiometri in infrarosso. L'indagine ~ effettuata sulla base dei primi risultati clell'esperimento Archimedes 1983. Durante tale esperimento

stato fatto volare un radiometro a scansione in infrarosso, ottenendo l ' individuazione in tempo reale di due ehiazze di idroearburi. Le immagini all'infrarosso sono state regi- strate e suecessivamente elaborate al caleolatore per migliorare la valutazione dello spessore del film. Sulla base dell'esperienza fat ta b stato sviluppato un sistema eompatto infrarosso/visibfle per l ' individuazione in tempo rale di ehiazze di idrocarburi, anehe da piecoli aerei.

PC31OMe HO IIOJI~OHO.

Fig. 5. - Digi t iza t ion and processing of two images. F r o m top to bo t tom : a) the digi t ized image in black and whi te presenta t ion , b) the same image af ter the correct ion of the lack of syncronizat ion, c) the same image as in b) in a five colour presenta t ion. The regions wi th different radiance are ev iden t in c).

Fig. 6. - The f inal s t r u c t u r e of t he I R / v i s i b l e i m a g e r : on t h e lef t t h e sensor h e a d , on t he r i g h t t h e moni to r .