Photonlrvachak Journal of the Indian Society of RemoteSensing, Vol. 20, No. 4, 1992

Remote Sensing For Ocean Resources* s z QASIM Member, PlanningCommission,New Delhi

I feel greatly honoured to be invited to deliver the second Vikram Sarabhai Memorial lecture. First of all, I would like to pay the homage to Dr. Vikram Sarabhai whom I consider the founder of remote sensing and space research in India. Whatever infrastructure, facility and manpower we have in the country today is all because of his vision and foresight. Moreover, our wonderful achievement in the field of space research is a glowing tribute to his imagination, dedication and hard work. He was also one of our freedom fighters and a Gandhian. He devoted his entire life to the service of our people. His simplicity, modesty and humanity always marked his life. It is for such a man that this lecture is dedicated and we have assembled here to commemorate his memory. Activities related to ocean such as shipping, fishing, oil drilling, mining and pollution control require a detailed knowledge of the various processes taking place in the ocean. Nearly three decades ago, the only mechanism at our disposal to study the oceans and their variability was the ship. Taking observation from the ship is expensive, time-consuming and requires a great deal of effort and manpower. The new tech-

noiogy of remote sensing, which makes use of aircrafts, radars and satellites, has proved to be most valuable due to its speed, large coverage and frequency of data collection and involves lesser cost and time. Remote sensing can be defined as "deriving information on a distant object without actually coming into direct contact with that object". Human eye, for example, is the best remote sensing instrument we have, as it is constantly feeding us with information without directly having contact with objects. As time passes, the awareness about the ocean and its resources is increasing. This is because the land resources are diminishing very rapidly and our increasing demand for more and more resources make us increasingly dependent on the sea. Oceanography is a multi-disciplinary science and it encompasses practically all sciences including engineering and medicine (special medical facilities needed for underwater work). Broadly the ocean-related studies can be classified as physical oceanography, chemical oceanography, biological oceanography, geological and geophysical oceanography, marine instrumentation, ocean engineering etc. Physical oceanography includes the state of the sea which is governed by surface

*Second Vikram Sarabhai Memorial Lecture delivered on the occasion of Annual Symposium of ISRS, at UP-RSAC, Lucknow on November 18, 1992.

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wind, waves, tides, surface temperature etc. Ocean dynamics means the study of currents, circulation pattern, eddies, internal waves, air-sea interaction and land-sea interaction. Chemical oceanography covers salinity, oxygen, other gases, fertility, quality of water, contamination of the sea etc. Biological oceanography refers to the study of phytoplankton, zooplankton, fish and other living resources. Geological and geophysical oceanography embraces nonliving resources, the sea bottom topography and ,the sea bed structure. Marine instrumentation deals with various kinds of equipment used in the ship as probes and ocean engineering includes structures built at sea and the submarine technology for the transportation of materials (pipeline) and communication (cables). Other oceanrelated studies which are of much importance for remote sensing are erosion and accretion, wetland vegetation, coastal morphology and physiography. HISTORY Remote sensing of the ocean began to be talked about in the 1960s and it became operational in the late seventies. Much advancement in this sector was made in the eighties. The first operational oceanographic satellite SEASAT was launched in 1978. The SEASAT carried with it several microwave sensors namely altimeter, scatterometer, visible and infrared scanner, synthetic aperture radar, scanning multichannel microwave radiometer etc. This satellite functioned only for three months but during this period it gave us a wealth of information. It demonstrated the feasibility of measuring global surface winds, wave height, sea surface temperature distribution, ocean currents, coastal topography etc. In addition to the SEASAT, several meteorological satellites launched during the seventies gave useful information on oceanrelated data and information. Particular mention may be made about the Nimbus-7

satellite which was launched in 1978 and carried with it an infrared and microwave radiometer in addition to ocean colour scanner CZCS (coastal zone colour scanner). INDIAN P R O G R A M M E Satellite-based remote sensing programme in India began when the first satellite Aryabhata was launched in 1975. Following this, Bhaskara-1 was launched successfully in June 1979 and an improved version of Bhaskara-2 was launched in November 1981. These two experimental satellites gave a lot of experience to Indian scientists in building and launching the satellites and generated a large volume of data and information to be handled and used. Soon afterwards, rapid advancements were made in the satellite launch programme with APPLE, Rohini satellite series, INSAT and IRS series. Appreciating the importance and usefulness of remote sensing, the Government of India has planned to launch a series of remote sensing satellites. The first in the series, IRS-IA was launched in March 1988. The success of IRS-IA represents a key step in planning and implementation for the survey and management of various natural resources. About an year ago the IRS-IB has been successfully launched which is sending excellent data and information. O C E A N - R E L A T E D DATA AND INFORMATION Oceans cover roughly 70% of the Earth. It is rather surprising that this vast area still remains largely unexplored. With the massive population growth, the demand for food has grown enormously. As the per capita land availability decreases, the world's population has to look towards the sea for food, fresh water, minerals and energy. The associated economic benefits

Remote Sensing for Ocean Resources that could accrue from these resources are not easy to estimate. A judicious assessment of the ocean resources and its management is called for in order to ensure sustainable development of a country's ocean resources. Mismanagement could lead to rapid depletion of marine resources causing upheaval in the marine ecosystem and ultimately in the entire global environment. For this purpose, monitoring the oceans becomes very important. There are two approaches for monitoring, (1) in situ observations, and (2) remote sensing observations. Some of the parameters/processes to be monitored are: (1) Population distribution of marine organisms, the type and species and an assessment of their economic importance. (2) Availability of the economically important materials. (3) Location and availability marine materials.

of

(4) Location and distribution of fish schools.

(5)

Meteorology related parameters such as sea surface temperature, fluctuations in sea level heights, ocean currents etc.

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of the man-induced chemicals with the marine organisms. How does the introduction of these harmful substances into the ocean waters affect the composition of the water? In turn how will it affect the biological resources and the productivity of the ocean organisms? What are the long term impacts of this interference of man with the ocean waters? Some of the subjects which are of direct interest to man and for which remote sensing gives a lot of information, are as follows:

Climate It is well known that the oceans and the atmosphere by mutual exchange of heat and water essentially determine the earth's weather and climate. In fact this exchange has an impact on storms and their movement. The surface circulations in the world oceans is closely related to the wind patterns. Further the distribution of marine life is closely linked with the ocean circulation. Recent years have seen some important developments in the global climate programme and these are likely to play an important role in the future. These include extensive use of weather satellites and the large computer capabilities for simulating ocean-atmosphere exchanges. India is using both these very effectively.

Micro-organisms (6) Population levels of organisms in the ocean and their impact. An understanding of the phenomenon/ process in the ocean waters is important for an effective management of the resources. The aqueous medium provides tremendous scope for chemical reactions and serves as a solvent for a number of naturally occurring materials (such as sediment particles, biological wastes, etc.) as well as man-induced materials such as industrial wastes. Thus it becomes difficult to predict the interaction

The distribution and number of different micro-organisms and the inter-relationships between these organisms are important parameters to be monitored to ensure that no biological imbalances take place. It has been found that in some cases interaction between the organisms such as competition for food and space are more important in determining the levels of a particular species rather than the physical aspects of the oceanic environment. The utility and economic importance of a particular species would primarily

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determine the frequency and the type of monitoring required. Conventionally, monitoring of these parameters is done by carrying out sample analysis of the ocean waters, based on the samples collected from different locations. Certain marine plants have the ability to trap large quantities of solar energy, convert it into living matter and store it in accessible forms. The type of marine plants that could be harvested for energy production are theoretically numerous. However, the desirable characteristics of the plant material for an efficient fuel processing is not clearly known.

Chlorophyll Chlorophyll is the primary photosynthetic pigment contained in almost all plants. The distributional patterns of chlorophyll as well as its spatial and temporal variability are useful indicators of the physical, chemical and biological processes occurring in the oceans. In the ocean, microscopic organisms called phytoplankton contain most of the ocean chlorophyll. Particularly, location of fish schools and their abundance is linked with the distribution of phytoplankton. Locating the positions of currents and oceanic fronts is also very relevant in sit!ng fish schools. Where water masses of different temperatures, meet, some fishes, including high-seas tuna as well as smaller fish, aggregate. For most marine fishes, the period of egg and larval drift is beliexted to be the most critical survival period in the life cycle. When surface currents do not provide favourable transport, fisheries may be severely affected.

Food The zone of maximum importance to mankind today for the exploitation of food resources is the uppermost layer of 100 m. This is the zone where most of the photosynthetic production of organic matter occurs. From this zone, more than 50 percent of the world's fish catch is obtained

at present. The regions occupying this zone are either fairly close to the coast or are in very fertile areas of the coastal or offshore upwelling regions. Since such areas constitute only 25 percent of the total oceanic area, it can be assumed that 75 percent of the ocean area may be termed as oligotrophic with moderate to low production rates. In India, sea food consists largely of capture fisheries and for these the intensively exploited areas are found in the narrow coastal belt. In 1947, fish production in India was about 0.4 million tonnes. In 1979-80, it had increased to 1.4 million tonnes. Today our marine fish production is of the order of 2.16 million tonnes. At present India ranks sixth in the list of fish producing countries. Within the last 30 years, India has also emerged as one of the foremost exporters of seafood in the world. The export of seafood from India, which started in 1962 with a modest value of Rs.4 crores, has shown a very high rate of increase and in 1991-92, it has reached Rs.1375 crores in value.

Seaweeds Seaweeds are one of the important living resources exploited by man for food, animal feed, fertilizers and for chemicals and pharmaceutical products. The total marine algal yield of the world has been estimated as 172,000 tonnes per year. Of this, India contributes only about two percent of the total. The demand for agarproducing seaweeds (agarophytes) and alginin-producing seaweeds (alginophytes) by the industry in India and abroad is increasing very rapidly. Unfortunately, India has not yet fully utilized its seaweed resources.

Mangroves In India, mangrove ecosystems, as in several other countries, have been severely

Remote Sens|ng for Ocean Resources depleted during the last two decades. In the past, they have been treated as unwanted plants and were largely used as a source of timber and charcoal. It is only in recent years that they have been recognized as ecologically vital areas. Mangroves play a very important role in protecting the shoreline from major erosion damage. The ecosystem forms an ideal nursery for juveniles of many economically important specics such as mullets, sea trout and shrimps. We have two very famous mangrove forests in our country - the Sunderbans forest in West Bengal which is the home of the Bengal tiger and the lush mangrove tbrests of the Andaman and Nicobar Islands. Both these require conservation.

Coral Reef Coral reefs are among the most biologically productive, taxonomically diverse and aesthetically important living communities. While their massive occurrence provides the much needed protection for the coastline from waves, their biological productivity yields a multitude of fauna and flora dependent on the coral-reef ecosystem. The communities also form the main attraction for diving, underwater photography, sport fishing and shell collecting. They thus provide a vital stimulus to the tourist industry. Due to population pressure, most of the coral reefs have become extremely vulnerable to pollution and industrial activities along the coastline. Hence, unless protection is offered to coral reefs in the future, most of them will shrink in size and will ultimately die. The reefs that would probably flourish would be on the atolls of Lakshadweep and on some of the islands of Andaman and Nicobar. Some states such as Tamil Nadu and Gujarat have declared certain areas of coral reefs as protected.

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REMOTE SENSING Technique As noted earlier, the observation of a target by a device placed some distance from it is called remote sensing as against in situ sensing where the sensor is kept in contact with the target. Physical emanations such as electromagnetic radiation from the target is observed by the sensing device mounted on an aircraft or on satellite platforms. The source of energy for the sensing device is the sun and this energy reaches the target after passing through the intervening medium. In the intervening medium, some of the energy gets reflected, some gets absorbed and the remaining energy reaches the target. Depending on the type of the target, its surface characteristics, some part of the incident radiation falling on it is absorbed, some transmitted and the remaining energy is reflected back into the medium. This radiation together with the radiation emitted by the target and that emitted by the neighbourhood of the target, reaches the satellite sensor after passing through the medium. This process of absorption, transmission and reflection at the target is dependent on the wavelength of the incident radiation. Although sensing devices could be designed to observe the entire range of electromagnetic wavelengths, attenuation by atmospheric constituents preclude the utility of certain wavelengths. The actual wavelength band to which a sensor is made sensitive depends upon the application to which the data will be put to use. There are specific spectral regions called atmospheric windows in which most sensors of the earth resource satellites operate. The window regions are those with a high transmittance. When it is necessary to study the atmosphere itself using satellite data, sensors can be designed to operate in the region of the atmospheric windows.

Image Processing It is not possible to directly detect the

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surface features from the satellite images. These have to be inferred from the radiance measurements. The recognition of the features from the remote sensing image is based on the principle that every feature has a unique spectral sensitivity. Inferences on remote sensing data are aided by the spatial re.solution, radiometric resolution, spectral bands and their resolution and the temporal resolution of the satellite.

paved the way for a number of other applications carried out subsequently with the satellite remote sensing data. All ocean properties can be inferred from the satellite data. Satellite sensors operating in the 0.4 1.2 m and the 0.3 - 30 cm range of the electromagnetic spectrum measure four basic sea surface properties colour, temperature, wave height (gradient and slope) and wind speed.

Depending on the type of applications envisaged, the sensors have been designed and flown on board the satellites since 1960. These satellites mainly cater to the needs of resources monitoring of the land mass and to serve the meteorological and communication needs of the country. Table-1 gives the names of the satellites used for mapping the ocean and its resources.

Remote sensing sensors suitable for marine studies include infrared radiometers for mapping the surface temperatures; SAP, lor ocean waves, microwave radiometers for wind speeds, surface temperature, salinity measurements; and portions of thematic mapper and coastal zone colour scanner (CZCS) for detecting the factors relating to plankton densities.

Interestingly in India the importance of remote sensing as a monitoring tool was first recognized for the oceans. Prof. P.R. Pisharoty, the well-known meteorologist, was the first person in India to point out that the sea surface temperatures (SSTs) can be 'measured' using the IR sensors on board an aircraft. Measuring the SSTs prior to the monsoon helps in predicting the date of the onset of monsoon.

The principal applications of coastal and marine resource remote sensing are:

Application One of the first detailed investigations on resources monitoring using remote sensing was the Agricultural Resources Inventory and Survey Experiment (ARISE) carried out in the early seventies. The experiment was aimed at estimating the acreage of crops like groundnut and paddy in Annantpur district of Andhra Pradesh and wheat in Patiala district of Punjab. A variety of sensors, such as the IR scanners, multiband cameras and radiometers were flown on an aircraft. Extensive ground surveys were carried out to ascertain the feasibility of the technology to successfully accomplish the goals of the experiment. This experiment

.

Monitoring the changes in the coastal zone - impact of land use on the sediment contents of coastal waters and erosion. This will also indicate the state of health of the coral reefs and atolls.

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Determining the identity, concentration and dispersal patterns of the pollutants such as oil slicks, sewage and industrial effluents.

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Mapping the extent and quality of coastal vegetation and wetlands (seaweeds and mangroves), including the diversity of plant species and the effects of pollution.

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Mapping chlorophyll-rich upwelling regions, which are relevant to the studies of fish stock productivity and other biological processes.

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Mapping the ocean current patterns including the transport or aggregation of plankton, fish eggs and larvae and adult fish by mapping the surface water mass distributions.

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Mapping the sea floor topography as reflected by the sea surface elevations.

Ocean colour measurements have a lot of relevance in monitoring the distribution patterns of chlorophyll in general and phytoplankton in particular. Chlorophyll absorbs strongly the blue region of the electromagnetic spectrum and this has resulted in several algorithms using the sensors operating in this region to monitor the phytoplankton in the oceans. Currently the satellites offer the only means to globally monitor the growth, fate and transport of phytoplankton in the oceans. Satellite derived chlorophyll estimates account for 65% of the variability of the ship data measurements (ground truth). Its accuracy, however, decreases with low angles of the sun. Surface temperature measurements (from IR images) can also be used in conjunction with the ocean colour observations. The Coastal Zone Colour Scanner (CZCS) onboard the NIMBUS satellite provided images of ocean colour for the period 1978 to 1986. Presently there is no satellite that carries a sensor meant exclusively for monitoring the ocean colour. However, a few studies with Landsat TM, Indian Remote Sensing Satellites IRS-IA and IRS-IB and SPOT have proved to be a valuable source of information on this parameter. A sensor similar to CZCS called Sea Viewing Wide Field of View Space Sensors (SEAWIFS) will probably be put into orbit in 1993 by the NASA. The oceans transport a large fraction of solar energy from the equator to the poles.

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Climate models for the 'Earth' have to be based on meteorological observations spaced regularly with respect to time and space. Although it is possible to have dense ground observation system over the land and over the oceans, such observations on a regular periodicity are difficult to make. Considering the important role the oceans play in determining the Earth's climate, the climate modellers rely increasingly on the remote sensing observations. As of now, satellites are able to provide Sea Surface Temperatures (SST), surface wind parameters, and rainfall estimates over the oceans. The AVHRR onboard NOAA is the one sensor that operationally provides global sea surface temperatures. The National Remote Sensing Agency (NRSA) in Hyderabad (India), regularly generates the SST maps over the Indian Ocean and the adjoining seas. These maps have been extremely useful in predicting the onset of monsoon. The Along Track Scanning Radiometer (ATSR) deployed on the ERS-1 is expected to provide much improved SST maps. CONCLUSION Monitoring of the oceans is important for the following reasons: i)

to assess and manage the ocean resources; ii) to keep track of the changes that occur m the ocean processes from time to time; iii) to understand the role of oceans in determining the Earth's climate. Location and distribution of fish schools is becoming an increasingly important activity for man. Fish egg and larval drift patterns may be modelled from the information obtained from remote sensing. Surface winds and currents can be monitored by microwave sensors. Although the fish schools cannot be seen directly from the satellite altitudes, satellites have been

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widely used to locate the areas of high probability for fish availability. This is done by mapping the areas which are characterised by strong thermal gradients caused by colder upwelling water, and spectrally different nutrient and chlorophyll contents. land,

Remote sensing in India for monitoring ocean or ocean resources has

developed so quickly that we have not be able to keep pace with the full utilisation ot information it has generated during the last ten or twelve years. We are hoping that during the eighth five year plan (1992-97), reliable information from remote sensing will be utilised fully for the management of our land and ocean resources.

Table-1. Marine Satellit~ and their Sensors used tbr mapping - Past, Present (Nop - not operational; O p - operational)

SATELLITE AND STATUS

SENSORS

APPLICAIlONS

SEASAT (Nop)

ALTI METER

Wind speed, Sea surface

SCAITEROMETER, SMMR

Temperature, Atmospheric water vapour, Wave height

SMMR CZCS

Sea surface temperature, Wind speed, Fisheries,

NIMBUS (Nop)

Pollution

GEOSAT (Nop)

ALTIMETER

Wind speed, Wave height

NOAA (Op)

AVHRR, TOVS, MSI.J

Sea surface temperature, Vertical temperature, Atmospheric water vapour, Cloud and rainfall estimates

MOS (Op)

MESSR, VTIR, MSR

Fisheries, Marine pollution, Sea surface temperature, Water vapour, Suspended sediment

LANDSAT (Op)

TM

Coastal landuse, Sedimentation, Chlorophyll mapping

SPOT (Op)

HRV

Coastal landuse, Sedimentation, Chlorophyll mapping

|RS (Op)

LISS I & 11

Coastal landuse, Sedimentation, Chlorophyll mapping

ERS-1 (Op)

SAR, SCATrEROMETER, RADAR ALTIMETER, SCANNING RADIOMETER, MICROWAVE SOUNDER

Pollution monitoring, Sea surface topography, Sea surface temperature, Weather modelling