ON APPLICATIONS FOR
OF REMOTE
ENVIRONMENTAL
SENSING
MONITORING
D. SPITZER Netherlands Institute for Sea Research, Postbox 59, 1790 A B Den Burg, the Netherlands
Abstract. Modern airborne and satellite remote sensingtechniques offer attractive opportunities to coastal monitoring systems. Improvements of the evaluation of larger scales phenomena and processes due to the synopticity of the remote sensing data are of particular interest. However, some uncertainties and limitations about remote sensing must be considered. Microwave, infrared and visible radiation methods and their applications are briefly discussed and some applications are demonstrated. Special attention is paid to the remote sensing of various pollutants in the sea, in particular with respect to oil pollution. Promising developments of the remote sensing methods for coastal monitoring are to be expected from the European remote sensing satellite missions ERS 1 and ERS 2. Combination of these observations with simultaneous in situ measurements from ships (sea truth) appears to be most advantageous for the interpretation of the collected data.
1. Introduction D e t e c t i o n o f earth surface features a n d d y n a m i c a l p h e n o m e n a f r o m the aircrafts a n d satellites, so called remote sensing, has already s h o w n useful in m a n y fields of the earth sciences a n d survey a n d m o n i t o r i n g applications. H o w e v e r most m a r i n e a p p l i c a t i o n s are still in a n e x p e r i m e n t a l or p r e - o p e r a t i o n a l stage a n d need f u r t h e r d e v e l o p m e n t . Some features which r e n d e r remote sensing p r o m i s i n g for m a r i n e m o n i t o r i n g are: (1) p r o v i s i o n of synoptic data, (2) R e p e t i t i o n o f (satellite) o b s e r v a t i o n with intervals f r o m several h o u r s to several weeks, (3) S c a n n i n g o f large scale processes, (4) A c q u i s i t i o n o f d a t a over r e m o t e a n d / o r difficultly accessible regions. H o w e v e r due to the distance b e t w e e n the s c a n n e d area a n d the sensor, uncertainties a n d l i m i t a t i o n s o f the r e m o t e sensing m u s t also be considered: (1) T e c h n i c a l l i m i t a t i o n s o f the detection system as well as the a t m o s p h e r i c i n f l u e n c e s c a n cause i m p o r t a n t errors in the q u a n t i t a t i v e data derived f r o m remote sensing, (2) S u p p o r t i n g in situ (sea t r u t h ) m e a s u r e m e n t s are o f t e n needed for proper c a l i b r a t i o n a n d c o r r e c t i o n o f the remotely a c q u i r e d data.
2. Methods and applications R e m o t e sensing t e c h n i q u e s can differ according to p l a t f o r m type, altitude, r a d i a t i o n Environmental Monitoring and Assessment 7 (1986) 263-271. 9 1986 by D. Reidel Publishing Company.
264
D. SPITZER
wavelength region
platform (altitude) aircraft
visible
$ rare?vo
ZTsZ
300 k m - 40.000 km
radiation source sun
~laser
sun ~
r
a
d
a
r
Fig. 1. Aspects of remote sensing. wavelength region and source o f the incident radiation. This is summarized in Figure 1. The platform's flight and instruments determine the spatial and temporal resolution o f the observations. The ground resolution can vary from several tenths o f meter to several km, the repetition o f the (satellite) observations can vary from several hours to weeks. When the backscattered, reflected or re-emitted solar radiation is being detected, the remote sensing is called passive, contrary to the active techniques when the platform carries the radiation source (laser, radar). Each o f the present remote sensing methods offers specific opportunities for coastal monitoring. Expected a n d / o r proven abilities o f remote sensing o f natural waters are displayed in Table I. Radiation emitted by the (sea) surface at infrared and microwave wavelengths is characteristic for its temperature providing data about gradients and fronts. Incident microwave radiation reflecting at the seasurface can interfer with the waves, thus giving information on the wind and wave fields. TABLE I Application and merits of various remote sensing methods. The expected or proved suitability is indicated by the number of crosses (+). ~ feature
method ~
optical
passive optical
++
++
+
+
++
+++
active
passive
microwave
microwave
seasurface
+ +
infrared
active
+ + +
temperature
sea surface topography (wave, currents) bottom topography composition of the w a t e r column suspended and dissolved materials
+++
ON A P P L I C A T I O N S O F R E M O T E S E N S I N G F O R E N V I R O N M E N T A L M O N I T O R I N G
265
Using special imaging techniques (synthetic aperture radar, SAR), the sea surface t o p o g r a p h y and related p h e n o m e n a (wind, currents, b o t t o m topography, internal waves), can be reproduced f r o m space. Contrary to the infrared and microwave, visible radiation (light) can penetrate the sea surface. The variations of the backscattered solar radiation (ocean colour) depend on the composition of and the amounts of the various suspended and dissolved materials in the watercolumn. In some cases the influence of the b o t t o m can be recorded. Bottom reflectance detected from an aircraft equiped with a laser instrument can be used for bathymetry. Also the laser induced fluorescence of natural substances (chlorophyll) and pollutants (oil) can be remotely measured and thus serve for mapping their distribution. Examples of the collected, processed and interpreted imagery are shown in Figures 2, 3, 4. Figure 2 (Nimbus-7 CZCS Experiment Team, 1984) presents the phytoplankton and temperature patterns recorded simultaneously at the east coast of the l I.S.A.
Fig. 2. Phytoplankton (top) and temperature (bottom) patterns remotely measured above the North Atlantic off the U.S. coast (Nimbus-7 CZCS Experiment Team, 1984).
266
Fig. 3.
D. SPITZER
Synthetic Aperture Radar (SAR) image of proliferation of internal wave trains west of Portugal (Allan, 1983)
using the Coastal Zone Color Scanner (CZCS) aboard N A S A ' S Nimbus 7 satellite. The CZCS measures light backscattered f r o m the upper layer o f the watercolumn in four visible wavelenght bands, as well as the sea surface temperature in an infrared band. Detected pigment concentrations (Figure 2 top) vary f r o m above 1 mg m - 3 (dark) to less than 0.01 mg m - 3 light. Sea temperature (Figure 2 bottom) varies from about 25 ~ (dark) to about 6 ~ (light). Conspicious in both images is a circular warm core ring of the Gulf stream south of Cape Cod. Trains o f internal waves influence also the sea surface structure (Allan, 1983) as imagined by the SAR o f the SEASAT satellite above the west coast of Portugal presented in Figure 3. Bottom features of shallow coastal waters can als influence the distribution of small surface waves and thus can be recorded by SAR as shown in Figure 4. Potentially, remote sensing offers perspectives for monitoring discharges, distribution and impact of various pollutants; expected and demonstrated applications are outlined in Table II. The most promissing application seems to be airborne remote sensing of oil spills at the seasurface, as currently operational in various coastal regions. The presence of oil film at the seasurface changes its roughness, temperature, reflection0 coefficient and colour, so that all the mentioned remote sensing techniques might be employed. However, best achievements have been reached up to now mainly by using the radar imaging instruments (Side Looking Airborne
ON APPLICATIONS OF REMOTE SENSING FOR ENVIRONMENTAL MONITORING
267
Fig. 4. Synthetic A p e r t u r e R a d a r ( S A R ) i m a g e d i s p l a y i n g gullies in the e a s t e r n W a d d e n Sea ( u p p e r p a r t o f the i m a g e ) a n d the I J s e l m e e r (lower part o f the image), (by c o u r t e s y o f N L R , A m s t e r d a m ) .
T A B L E II D e m o n s t r a t e d a n d p o t e n t i a l r e m o t e sensing a p p l i c a t i o n s for d e t e c t i o n o f sea p o l l u t i o n . Utility is i n d i c a t e d b y the n u m b e r o f crosses ( + ) . ~ ~ pollution
method ~
oil surface
active microwave
passive microwave
infrared
active optical
passive optical
+++
++
++
++
+
++
+
oil subsurface eutrophication
+
++
+++
chemical c o m p o u n d s
+
+
++
268
D. SPITZER
Fig. 5. Infrared image of wind dispersed (within 4 km) oil spill patches on the seasurface as recorded from an aircraft at 1000 ft altitude 3 hours after discharge (Parker and Cormac, 1984).
Radar, SLAR) preferably in combination with the termal infrared. The advantage of using the microwave techniques is that they are relatively little disturbed by atmospheric conditions (clouds). When the oil layer sinks under the seasurface, the surface scanning techniques (microwaves, infrared) are less useful. Methods for remote measurements of laser induced fluorescense o f surface and subsurface oil layers are in an advanced stage of development. Abundancy of phosphate and nitrogen compounds can result in increased biological activity (eutrophication) in water and thus in a change of its colour, which can be remotely observed. A m o u n t s of organochlorine and heavy metal compounds do not change physical (optical) properties of the seawater, and thus cannot be detected directly by remote
ON APPLICATIONS
OF REMOTE
SENSING
FOR ENVIRONMENTAL
MONITORING
269
Fig. 6. Radar (SLAR) image of an oil slick in the North Atlantic 24 hours after dumping. The length of the plume is about 10 km, the white spots are the ships, (Backlund, 1984).
sensing. However, as these compounds often occur in various binding forms to optically traceable materials like suspended sediments and dissolved organic matter, that fact offers a possibility for an indirect remote sensing. On the other hand, the dumping and incineration of chemical wastes f r o m ships do influence the optical properties and temperature of the surrounding sea and the air layer just above its surface, (sometimes even over hundreds of kilometers) and thus can be observed directly. More than 100 000 tonnes of industrial wastes are incinerated annually from special vessels in the open seas. The developing patch or plume cannot be sufficiently measured f r o m the vessel itself and hence remote sensing m a y b e more efficient. Several examples of the remote detection of pollution are presented in Figures 5, 6, 7. Figure 5 is an image of an oil discharge recorded by an infrared scanner f r o m aircraft (Parker and Cormac, 1984). An airborne radar image of an oilslick is presented in Figure 6 (Backlund, 1984). Satellite detection supplying large scale imagery is demonstrated in Fig. 7. This CZCS image in a visible wavelenght band (green) shows a wind driven plume containing HCI gas developed by the incineration of chemical wastes at the North Sea. The eddy patterns in the sea north of Holland visualize the variations of the content of suspended matter in the watercolumn.
3. Developments The demonstrated opportunities of remote sensing can be fully exploited when quantitative relationships between the remotely detected signals and the sea truth,
270
D. SPITZER
Fig. 7. Satellite image (CZCS) showing turbidity patterns in the North Sea and gas plume (arrow) developed by incineration of chemical wastes,
as well as the atmospheric correction procedures are further developed and refined. This requires thorough study o f the relevant processes concerning theoretical and experimental investigations in situ and from aircrafts. A rapid and regular data distribution system for the users is imperative for an effective application of any of the remote sensing methods. Many European initiatives are combined in the preparation for launching and performance, of the first European remote sensing satellite devoted t o the sea observations, ERS 1. This satellite is planned to be launched in 1989 and will carry on board active microwave and thermal infrared instruments. The following ERS 2
ON APPLICATIONS OF REMOTE SENSING FOR ENVIRONMENTAL MONITORING
271
satellite mission, on schedule for the end of the eighties, would include a multispectral (visible light) scanner, the Ocean Colour Monitor (OCM). Despite of the advantages and demonstrated and expected achievements of remote sensing, one should realize that this tool cannot completely replace in situ measurements from ships. Both types of observations, preferably combined and synchronously performed, would support and supplement each other, resulting in a vice versa increased accuracy of both systems of observations. References Allan. T. D.: 1983, ' A Review of S E A S A T ' , in T. D. Allan (ed.), Satellite Microwave Remote Sensing, Ellis H o r w o o d Ltd., pp. 16-44. Backlund, L.: 1984, 'Airborne Oil Spill Surveyance Systems in Sweden', in J. M. Massin (ed.), Remote Sensing for the Control of Marine Pollution, P l e n u m Press, pp. 135-152. Nimbus-7 CZCS Experiment Team: 1984, Oceanography from Space, folder, NASA. Parker, H. D. and Cormack, D.: 1984, 'Evaluation o f Infrared Line Scan (IRLS) and Side Looking Airborne Radar (SLAR) over Controlled Oil Spills in the North Sea', in J. M. Massin (ed.), Remote Sensing for the Control of Marine Pollution, P l e n u m Press, pp. 237-256.
OF REMOTE
ENVIRONMENTAL
SENSING
MONITORING
D. SPITZER Netherlands Institute for Sea Research, Postbox 59, 1790 A B Den Burg, the Netherlands
Abstract. Modern airborne and satellite remote sensingtechniques offer attractive opportunities to coastal monitoring systems. Improvements of the evaluation of larger scales phenomena and processes due to the synopticity of the remote sensing data are of particular interest. However, some uncertainties and limitations about remote sensing must be considered. Microwave, infrared and visible radiation methods and their applications are briefly discussed and some applications are demonstrated. Special attention is paid to the remote sensing of various pollutants in the sea, in particular with respect to oil pollution. Promising developments of the remote sensing methods for coastal monitoring are to be expected from the European remote sensing satellite missions ERS 1 and ERS 2. Combination of these observations with simultaneous in situ measurements from ships (sea truth) appears to be most advantageous for the interpretation of the collected data.
1. Introduction D e t e c t i o n o f earth surface features a n d d y n a m i c a l p h e n o m e n a f r o m the aircrafts a n d satellites, so called remote sensing, has already s h o w n useful in m a n y fields of the earth sciences a n d survey a n d m o n i t o r i n g applications. H o w e v e r most m a r i n e a p p l i c a t i o n s are still in a n e x p e r i m e n t a l or p r e - o p e r a t i o n a l stage a n d need f u r t h e r d e v e l o p m e n t . Some features which r e n d e r remote sensing p r o m i s i n g for m a r i n e m o n i t o r i n g are: (1) p r o v i s i o n of synoptic data, (2) R e p e t i t i o n o f (satellite) o b s e r v a t i o n with intervals f r o m several h o u r s to several weeks, (3) S c a n n i n g o f large scale processes, (4) A c q u i s i t i o n o f d a t a over r e m o t e a n d / o r difficultly accessible regions. H o w e v e r due to the distance b e t w e e n the s c a n n e d area a n d the sensor, uncertainties a n d l i m i t a t i o n s o f the r e m o t e sensing m u s t also be considered: (1) T e c h n i c a l l i m i t a t i o n s o f the detection system as well as the a t m o s p h e r i c i n f l u e n c e s c a n cause i m p o r t a n t errors in the q u a n t i t a t i v e data derived f r o m remote sensing, (2) S u p p o r t i n g in situ (sea t r u t h ) m e a s u r e m e n t s are o f t e n needed for proper c a l i b r a t i o n a n d c o r r e c t i o n o f the remotely a c q u i r e d data.
2. Methods and applications R e m o t e sensing t e c h n i q u e s can differ according to p l a t f o r m type, altitude, r a d i a t i o n Environmental Monitoring and Assessment 7 (1986) 263-271. 9 1986 by D. Reidel Publishing Company.
264
D. SPITZER
wavelength region
platform (altitude) aircraft
visible
$ rare?vo
ZTsZ
300 k m - 40.000 km
radiation source sun
~laser
sun ~
r
a
d
a
r
Fig. 1. Aspects of remote sensing. wavelength region and source o f the incident radiation. This is summarized in Figure 1. The platform's flight and instruments determine the spatial and temporal resolution o f the observations. The ground resolution can vary from several tenths o f meter to several km, the repetition o f the (satellite) observations can vary from several hours to weeks. When the backscattered, reflected or re-emitted solar radiation is being detected, the remote sensing is called passive, contrary to the active techniques when the platform carries the radiation source (laser, radar). Each o f the present remote sensing methods offers specific opportunities for coastal monitoring. Expected a n d / o r proven abilities o f remote sensing o f natural waters are displayed in Table I. Radiation emitted by the (sea) surface at infrared and microwave wavelengths is characteristic for its temperature providing data about gradients and fronts. Incident microwave radiation reflecting at the seasurface can interfer with the waves, thus giving information on the wind and wave fields. TABLE I Application and merits of various remote sensing methods. The expected or proved suitability is indicated by the number of crosses (+). ~ feature
method ~
optical
passive optical
++
++
+
+
++
+++
active
passive
microwave
microwave
seasurface
+ +
infrared
active
+ + +
temperature
sea surface topography (wave, currents) bottom topography composition of the w a t e r column suspended and dissolved materials
+++
ON A P P L I C A T I O N S O F R E M O T E S E N S I N G F O R E N V I R O N M E N T A L M O N I T O R I N G
265
Using special imaging techniques (synthetic aperture radar, SAR), the sea surface t o p o g r a p h y and related p h e n o m e n a (wind, currents, b o t t o m topography, internal waves), can be reproduced f r o m space. Contrary to the infrared and microwave, visible radiation (light) can penetrate the sea surface. The variations of the backscattered solar radiation (ocean colour) depend on the composition of and the amounts of the various suspended and dissolved materials in the watercolumn. In some cases the influence of the b o t t o m can be recorded. Bottom reflectance detected from an aircraft equiped with a laser instrument can be used for bathymetry. Also the laser induced fluorescence of natural substances (chlorophyll) and pollutants (oil) can be remotely measured and thus serve for mapping their distribution. Examples of the collected, processed and interpreted imagery are shown in Figures 2, 3, 4. Figure 2 (Nimbus-7 CZCS Experiment Team, 1984) presents the phytoplankton and temperature patterns recorded simultaneously at the east coast of the l I.S.A.
Fig. 2. Phytoplankton (top) and temperature (bottom) patterns remotely measured above the North Atlantic off the U.S. coast (Nimbus-7 CZCS Experiment Team, 1984).
266
Fig. 3.
D. SPITZER
Synthetic Aperture Radar (SAR) image of proliferation of internal wave trains west of Portugal (Allan, 1983)
using the Coastal Zone Color Scanner (CZCS) aboard N A S A ' S Nimbus 7 satellite. The CZCS measures light backscattered f r o m the upper layer o f the watercolumn in four visible wavelenght bands, as well as the sea surface temperature in an infrared band. Detected pigment concentrations (Figure 2 top) vary f r o m above 1 mg m - 3 (dark) to less than 0.01 mg m - 3 light. Sea temperature (Figure 2 bottom) varies from about 25 ~ (dark) to about 6 ~ (light). Conspicious in both images is a circular warm core ring of the Gulf stream south of Cape Cod. Trains o f internal waves influence also the sea surface structure (Allan, 1983) as imagined by the SAR o f the SEASAT satellite above the west coast of Portugal presented in Figure 3. Bottom features of shallow coastal waters can als influence the distribution of small surface waves and thus can be recorded by SAR as shown in Figure 4. Potentially, remote sensing offers perspectives for monitoring discharges, distribution and impact of various pollutants; expected and demonstrated applications are outlined in Table II. The most promissing application seems to be airborne remote sensing of oil spills at the seasurface, as currently operational in various coastal regions. The presence of oil film at the seasurface changes its roughness, temperature, reflection0 coefficient and colour, so that all the mentioned remote sensing techniques might be employed. However, best achievements have been reached up to now mainly by using the radar imaging instruments (Side Looking Airborne
ON APPLICATIONS OF REMOTE SENSING FOR ENVIRONMENTAL MONITORING
267
Fig. 4. Synthetic A p e r t u r e R a d a r ( S A R ) i m a g e d i s p l a y i n g gullies in the e a s t e r n W a d d e n Sea ( u p p e r p a r t o f the i m a g e ) a n d the I J s e l m e e r (lower part o f the image), (by c o u r t e s y o f N L R , A m s t e r d a m ) .
T A B L E II D e m o n s t r a t e d a n d p o t e n t i a l r e m o t e sensing a p p l i c a t i o n s for d e t e c t i o n o f sea p o l l u t i o n . Utility is i n d i c a t e d b y the n u m b e r o f crosses ( + ) . ~ ~ pollution
method ~
oil surface
active microwave
passive microwave
infrared
active optical
passive optical
+++
++
++
++
+
++
+
oil subsurface eutrophication
+
++
+++
chemical c o m p o u n d s
+
+
++
268
D. SPITZER
Fig. 5. Infrared image of wind dispersed (within 4 km) oil spill patches on the seasurface as recorded from an aircraft at 1000 ft altitude 3 hours after discharge (Parker and Cormac, 1984).
Radar, SLAR) preferably in combination with the termal infrared. The advantage of using the microwave techniques is that they are relatively little disturbed by atmospheric conditions (clouds). When the oil layer sinks under the seasurface, the surface scanning techniques (microwaves, infrared) are less useful. Methods for remote measurements of laser induced fluorescense o f surface and subsurface oil layers are in an advanced stage of development. Abundancy of phosphate and nitrogen compounds can result in increased biological activity (eutrophication) in water and thus in a change of its colour, which can be remotely observed. A m o u n t s of organochlorine and heavy metal compounds do not change physical (optical) properties of the seawater, and thus cannot be detected directly by remote
ON APPLICATIONS
OF REMOTE
SENSING
FOR ENVIRONMENTAL
MONITORING
269
Fig. 6. Radar (SLAR) image of an oil slick in the North Atlantic 24 hours after dumping. The length of the plume is about 10 km, the white spots are the ships, (Backlund, 1984).
sensing. However, as these compounds often occur in various binding forms to optically traceable materials like suspended sediments and dissolved organic matter, that fact offers a possibility for an indirect remote sensing. On the other hand, the dumping and incineration of chemical wastes f r o m ships do influence the optical properties and temperature of the surrounding sea and the air layer just above its surface, (sometimes even over hundreds of kilometers) and thus can be observed directly. More than 100 000 tonnes of industrial wastes are incinerated annually from special vessels in the open seas. The developing patch or plume cannot be sufficiently measured f r o m the vessel itself and hence remote sensing m a y b e more efficient. Several examples of the remote detection of pollution are presented in Figures 5, 6, 7. Figure 5 is an image of an oil discharge recorded by an infrared scanner f r o m aircraft (Parker and Cormac, 1984). An airborne radar image of an oilslick is presented in Figure 6 (Backlund, 1984). Satellite detection supplying large scale imagery is demonstrated in Fig. 7. This CZCS image in a visible wavelenght band (green) shows a wind driven plume containing HCI gas developed by the incineration of chemical wastes at the North Sea. The eddy patterns in the sea north of Holland visualize the variations of the content of suspended matter in the watercolumn.
3. Developments The demonstrated opportunities of remote sensing can be fully exploited when quantitative relationships between the remotely detected signals and the sea truth,
270
D. SPITZER
Fig. 7. Satellite image (CZCS) showing turbidity patterns in the North Sea and gas plume (arrow) developed by incineration of chemical wastes,
as well as the atmospheric correction procedures are further developed and refined. This requires thorough study o f the relevant processes concerning theoretical and experimental investigations in situ and from aircrafts. A rapid and regular data distribution system for the users is imperative for an effective application of any of the remote sensing methods. Many European initiatives are combined in the preparation for launching and performance, of the first European remote sensing satellite devoted t o the sea observations, ERS 1. This satellite is planned to be launched in 1989 and will carry on board active microwave and thermal infrared instruments. The following ERS 2
ON APPLICATIONS OF REMOTE SENSING FOR ENVIRONMENTAL MONITORING
271
satellite mission, on schedule for the end of the eighties, would include a multispectral (visible light) scanner, the Ocean Colour Monitor (OCM). Despite of the advantages and demonstrated and expected achievements of remote sensing, one should realize that this tool cannot completely replace in situ measurements from ships. Both types of observations, preferably combined and synchronously performed, would support and supplement each other, resulting in a vice versa increased accuracy of both systems of observations. References Allan. T. D.: 1983, ' A Review of S E A S A T ' , in T. D. Allan (ed.), Satellite Microwave Remote Sensing, Ellis H o r w o o d Ltd., pp. 16-44. Backlund, L.: 1984, 'Airborne Oil Spill Surveyance Systems in Sweden', in J. M. Massin (ed.), Remote Sensing for the Control of Marine Pollution, P l e n u m Press, pp. 135-152. Nimbus-7 CZCS Experiment Team: 1984, Oceanography from Space, folder, NASA. Parker, H. D. and Cormack, D.: 1984, 'Evaluation o f Infrared Line Scan (IRLS) and Side Looking Airborne Radar (SLAR) over Controlled Oil Spills in the North Sea', in J. M. Massin (ed.), Remote Sensing for the Control of Marine Pollution, P l e n u m Press, pp. 237-256.
