Remote sensing, as a tool to aid in the control of water pollution, offers a means of making rapid, economical surveys of areas that are relatively inaccessible on the ground. At the same time, it offers the only practical means of mapping pollution patterns that cover large areas. Detection of oil slicks, thermal pollution, sewage, and algae are discussed

White, P. G.

English

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302
Peter G. White
Mr. White is manager of the Electro-Optical Sensors Department
of TRW Systems in Redondo Beach, California. He has directed
a number of programs in the remote sensing of water bodies, in
order to relate the remotely sensed data to the quality of the
water. Most of these programs have involved the use of spectro-
meters, cameras, and thermal infrared imagers flown in aircraft
in conjunction with suitable surface truth measurements obtained
from boats.
Mr. White was registered as a Professional Engineer in 1954
by the Association of Professional Engineers of Ontario, Canada.
He has published a number of papers on the subject of remote
measurement of water quality, and holds patents on two instruments
specifically designed to make such measurements.
https://ntrs.nasa.gov/search.jsp?R=19730007671 2020-03-17T07:43:37+00:00Z
303
REMOTE SENSING OF WATER POLLUTION
By
Peter G. White
TRW Systems Group
Remote sensing, as a tool to aid in the control of water pollution, may
prove to be exceptionally valuable in the next few years. It offers a means
of making rapid, economical surveys of areas that are relatively inaccessible
on the ground. At the same time, it offers the only practical means of
mapping pollution patterns that cover large areas.
New sensors and techniques, under development for the past several years,
will obtain information about the nature and extent of pollution that previously
could only be obtained by water sampling. Some of these sensors operate in
daylight only, while others will work either day or night.
Data are presently being obtained from aircraft in conjunction with
analyses of water samples taken on the ground to correlate the airborne data
with such parameters as sediment count, algae concentration and type, degree of
thermal pollution, and oil slick extent and thickness.
Figure 1* shows the regions of the electromagnetic spectrum of most inter-
est in this type of remote sensing. The right-hand bar indicates the relation-
ship of the ultraviolet, visible, and infrared portions of the spectrum. It
should be noted that the radiation which is detected by a passive remote
sensor has two different sources - reflected or scattered sunlight, and thermal
emission of the water itself. As with the atmosphere, at short wavelengths,
light is heavily scattered by the molecules of water, and above 0.7 microns
water abosrbs most of the energy which reaches it.
Finally, the two left-hand bars show the regions in which we can perform
our passive remote sensing after allowing for all the previous effects. The
only radiation from beneath the surface which reaches the sensor is constrained
to the visible and near ultraviolet. Material floating at the surface, such as
oil slicks, can be detected at longer wavelengths, including the infrared
region around 10 microns. As the source energy at 10 microns is emitted, rather
than reflected, its absolute value is indicative of the temperature and emissi-
vity of the surface, whether it contains floating matter or not.
Detection of oil slicks, thermal pollution, sewage and algae will be
discussed in the following paragraphs.
Unless otherwise credited, all illustrations and data used in this
report were obtained by TRW Systems Group, in large part under the
sponsorship of the Spacecraft Oceanography Project.
304
OIL SLICKS
As oil floats on the water surface, it offers the possibility of detec-
tion in any spectral region in which a suitable atmospheric windiw and adequate
detectors exist. Optical contrast between oil and water is high in the ultra-
violet and sometimes in the thermal infrared regions of the spectrum, as shown
in Figure 2. In Figure 2, the upper two images were obtained by a multispectral
linescan sensor which simultaneously images many different spectral regions. The
lower image is a photograph of the same scene, in this case the large oil spill
in the Santa Barbara Channel in early 1969. It can be seen from Figure 2 that
the Panchromatic film/K-2 filter combination (sensitive from 0.48 to 0.65 microns)
does a poor job of rendering contrast. On the other hand, the Kodachrome II film
used in Figure 3 shows good contrast between the oil and water. This is because
this film (used without a filter) has good sensitivity in the near ultraviolet
region of the spectrum. Some purple tones are evident in Figure 3, indicating
a contribution to the signal in the red as well as blue/ultraviolet. This is in
agreement with some spectral reflectance measurements made of the oil slick with
a spectrometer that indicate a relatively high oil-to-water contrast in the 0.60
to 0.67 micron region of the spectrum.
Although dramatic in appearance, the reverse Infrared Ektachrome print of
Figure 4, shows less contrast between oil and water.
In summary, oil slick photography is best accomplished with ultraviolet
sensitive film such as unfiltered Kodachrome II or 103-0. If the altitude is
very high or haze conditions are such that atmospheric scattering is a problem,
thermal infrared imagery obtained with an infrared linescanner should probably
be substituted for photography.
THERMAL POLLUTION
Thermal anomalies are characteristic of many types of pollution. Biochem-
ical action at sewage outfalls is capable of raising the temperature of the
surrounding water many degrees. Discharge of cooling water from electrical
generating plants has a similar effect. Oil slicks are frequently at a different
temperature than the surrounding water, resulting in the contrast shown in the
infrared image of Figure 2.
As water, or any other substance with high emissivity and temperature
within the range of 0 to 25 degrees Centigrade emits maximum radiation in the
spectral region of 10 microns, a detector which is sensitive in the atmospheric
window of 8 to 13.5 microns is well suited to this type of measurement.
A simple radiometer containing a temperature controlled reference black
body is capable of remotely measuring temperature within an absolute accuracy of
better than +_ 0.5°C. Infrared linescanners can give good imagery of thermal
gradients. Figure 5 is a picture showing the thermal diffusion from the cooling
water discharge of a large electric generating station. This picture was obtain
obtained with an infrared linescanner operating between 8 and 13.5 microns in an
aircraft at 1000 ft. altitude. In the picture, higher temperatures are indicated
by lighter tones. Temperature differences of about 0.2°C are probably evident.
305
One of the primary advantages of thermal imagery of the type shown, is that
it may be obtained either day or night. As the long wavelength infrared radia-
tion being sensed is emitted by the water itself, sunlight is not required. In
fact, sunlight contains virtually no energy in this region of the spectrum.
Figure 5 was obtained in the daytime, and as a result, the land areas appear
lighter (warmer) than the water as they have been heated by the sun. A similar
picture taken on a cool night would show the land areas as darker (cooler) than
the water. The thermal patterns in the water itself, however, would remain
unchanged.
SEWAGE AND ALGAE
Particulate and dissolved matter released from sewage outfalls is charac-
terized by various changes in the visible region of the spectrum. Some of the
more dramatic visual effects are shown in Figure 6, which was taken with
Kodachrome II film. Usually, however, the visual appearance is much less
pronounced. Work is currently unqerway to attempt to quantitatively relate
these color changes to the condition of the water, with the intention that type,
concentration, and distribution of the pollutants may be determined remotely.
As water transmits light in the visible spectrum, the signal received by the
sensor gives information about the condition of subsurface water instead of only
the surface condition.
Probably the most useful instrument for this type of measurement is the
rapid scanning airborne spectrometer. When pointed at an area on the surface of
the water, it gives a continuous scan through the visible spectrum in about one
second, measuring spectral radiance with good spectral resolution. When the
resulting signal is divided by a similar scan of a known surface of constant
reflectance, the spectral reflectance of the water body is obtained.
Current tests are underway to relate these remotely obtained spectral sig-
natures to measurements made at the water surface. Figure 7 is a map of
Los Angeles Harbor showing areas in which spectrometer scans were made from 1000
feet altitude (shown as rectangles on the map) and points at which turbidity
measurements were made from a boat (shown as dots on the map). Figure 8 is a
photographic montage of the area taken from 8000 ft. altitude within 30 minutes
of the spectrometer measurements. The positions of the spectrometer scans are
marked on the picture. Two distinct types of matter can be seen in the water.
Particulate matter from three sewage outfalls is evident near the top of the
picture. Algae of a type known as gonyaulax or "red tide" may be seen in the
lower half of the harbor and also outside of the breakwall.
Spectral signatures from scans 2, 5, and 11 (positions indicated on Figure 8)
are shown in Figure 9. Scans 2 and 5 are from regions of high particulate matter
from the sewage outfalls, while scan 11 is from an area with a high algae concen-
tration. Absorptions in the algae signature indicate the presence of pycobilin
pigment and chlorophyll-a, both present in the algae gonyaulax. Characteristic
features of the other signatures are still under investigation.
In order to utilize the airborne spectrometer measurements to obtain quanti-
tative data about the type and concentration of the water pollution, correlation
must be obtained between certain characteristics of the spectral signatures and
the data obtained at the water surface itself. In the tests currently underway,
306
water samples are taken from one foot beneath the surface and at the surface.
Portions of the samples are fixed with formaldehyde and counts made under a
microscope of different organisms and particles. As another measure of water
turbidity, a Secchi disk is lowered into the water until the depth is reached
at which it is no longer visible to observers in the boat.
Certain parameters have been selected to characterize the spectrometer
curves. Figures 10, 11, and 12 show these parameters plotted against the Secchi
depth, which is a measure of water turbidity. The parameters have been defined
as follows:
K =
 R4000A - R5250A
1 R5700fl
v . ,
 R5250A
? ' n oK5700A
K =
 R7000A - R6700A
3 R5700fi
In all cases R^ is the reflectance of the water at wavelength A.
K, (Figure 10) was selected to indicate the amount and direction of the
slope of the reflectance curves in the blue-green region of the spectrum. High
concentrations of sewage result in a rising reflectance with increasing wave-
length in this spectral region whereas high concentrations of algae produce a
decreasing reflectance. The parameter can be seen to relate to concentrations
of sewage, but not to concentrations of algae, at least in high concentrations.
(Figure 11) is a measure of the drop in reflectance due to absorptions
around 5250A. It is in this region that pycobilin pigment of the type from the
algae gonyaulax is found. In this case a correlation is apparent with high
concentrations of algae as well as a different correlation with sewage.
K, measures the chlorophyll -a absorption at 6700A and again shows differ-
ent correlations with sewage and algae.
It is expected that as better, faster, methods of analyzing many water
samples become available, better correlations with spectrometer data will be
obtained. However, the present data is most promising in that it indicates a
capability to differentiate between types of pollution and at the same time
give a measure of the concentration of each type.
A factor which can contribute to the inaccuracy of this type of measure-
ment is the speed of the spectrometer in taking each spectral scan. As the
aircraft is moving at a velocity of some 150 ft/sec., within a one-second scan
time, the area on the surface being viewed by the spectrometer has moved 150 ft.
To illustrate this effect, Figure 13 is a photographic montage of the northern
portion of the test area in Los Angeles Harbor described earlier. The distance
307
that the aircraft has moved during each scan is marked on the picture. In some
areas, these lines cross boundaries in the water. This will result in a spec-
trum composed at short wavelengths from one type of water and longer wavelengths
from another. To overcome this difficulty, faster scanning spectrometers are
under development.
NEU DEVELOPMENTS
Certain other instruments and techniques are in various stages of develop-
ment. One of the most promising new instruments is the imaging spectrophotometer.
This device is capable of sensing radiation from many small areas on the surface
simultaneously and obtaining a spectral signature from each. After properly
processing the data, maps of pollution concentration may be prepared. The
imaging spectrophotometer and associated data processing techniques are currently
being developed.
Certain other techniques are in early stages of investigation. Image
intensifies and low light level television cameras which are sensitive to
extremely low levels of illumination may be used to detect phosphorescence of
decaying material at night. Some investigations currently underway are attempt-
ing to utilize the very narrow regions of the solar spectrum in which no light
reaches us from the sun (Fraunhofer lines) to measure fluorescence during day-
light. These techniques offer interesting possibilities for the future.
In summary, remote sensing of water pollution is rapidly progressing to
the point where quantitative measurements may be obtained from aircraft. The
benefits which will be derived from the ability to rapidly monitor and evaluate
the condition of our environment will be of major value.
308
GLOSSARY
Algae - Microscopic living organisms found in oceans and lakes
Atmospheric Window - Portion of the electromagnetic spectrum ii
which the atmosphere does not attenuate radiation.
Emissivity - The fraction of radiation emitted by a body,
compared to a perfect radiator of the same temperature.
Oil Slick - The film formed on a water surface by oil.
Reference Black Body - A perfect thermal radiator.
Sediment Count - The number of particles per unit volume of
water sample.
Thermal Anomaly - A localized region of different temperature
from the surrounding water.
Thermal Emission - Radiation given off by a body due to its
temperature.
Thermal Pollution - Water of a higher temperature which is adde
to a body of natural water, usually as a by-product of an
industrial process.
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Remote Sensing of Water Pollution

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