Download PROPERTIES OF EARTH SURFACES AND THEIR INTERACTIONS

PROPERTIES OF EARTH SURFACES
AND THEIR
INTERACTIONS WITH
ELECTROMAGNETIC RADIATION
Energy Interactions
Electromagnetic waves that originate on the sun are radiated through
space and eventually enter the Earth's atmosphere. In the atmosphere,
the radiation interacts with atmospheric particles, which can absorb,
scatter, or reflect it back into space.
Much of the sun's high-energy radiation is absorbed by the atmosphere,
preventing it from reaching the Earth's surface. This absorption of
energy in the upper atmosphere is an important factor in allowing life to
flourish on the Earth. Atmospheric particles such as dust, sea salt, ash,
and water droplets will reflect energy back into space.
Visible light can be scattered by particles in the atmosphere, allowing
only selected wavelengths to penetrate to the surface.
 A portion of the energy is able to penetrate the atmosphere, allowing it
to reach the Earth's surface.
 Radiation that is able to penetrate the material and pass through it is
said to be transmitted.
 Most wavelengths of visible light energy from the sun are transmitted
through the atmosphere, allowing it to come into contact with the
Earth's surface.
 Once this radiation reaches the surface, it interacts with the surface
materials where it can be reflected back into space or absorbed and
red-emitted as thermal infrared energy.
Effect of Atmosphere on EMR
Earth-Atmosphere System Energy Budget
The areas of the EM spectrumABSORBTION
that are absorbed by atmospheric gases such
as water vapor, carbon dioxide, and ozone are known as absorption
bands. In the figure, absorption bands are represented by a low transmission
value that is associated with a specific range of wavelengths.
In contrast to the absorption bands, there are areas of the electromagnetic
spectrum where the atmosphere is transparent (little or no absorption of
radiation) to specific wavelengths. These wavelength bands are known as
atmospheric "windows" since they allow the radiation to easily pass through
the atmosphere to Earth's surface.
Most remote sensing instruments on aircraft or space-based platforms operate in
one or more of these windows by making their measurements with detectors
tuned to specific frequencies (wavelengths) that pass through the atmosphere.
INTERACTIONS WITH ELECTROMAGNETIC
RADIATION AT
VISIBLE AND NEAR INFRARED WAVELENGTHS
 the particular properties of a surface will determine how
much EM energy is radiated in the direction of a sensor.
 The reflectance varies with wavelength.
 In the early 1970s it was thought that every surface would
have a unique spectral signature and that
 once this had been established from laboratory
experiments it would be possible to map any surface type
from multispectral imagery.
there are general differences between surfaces there
exists considerable variation in the reflectance of a single
surface type.
The interaction of visible and near infrared
EMR with soil
In general, soil surfaces are brown to the human eye. ‘Brown’ colouring is
a product of green and red EM radiation such that ‘brown’ surfaces absorb
more blue EMR than either green or red.
Furthermore, very little energy is transmitted through soils, the majority of
the incident flux is absorbed or reflected.
The technical term for these types of surface is single scatterer.
In the case of soil surfaces the level of reflectance gradually increases
with wavelength in the visible and near infrared spectral regions. You can
see from Figure 1 that maximum soil reflectance occurs at near infrared
wavelengths.
Most important:
moisture content
organic matter content
texture
structure
Least important:
iron oxide content
Soil moisture content
the presence of soil moisture reduces the surface reflectance of soil at all
visible wavelengths (Jensen, 1983). This occurs until the soil is saturated,
at which point further additions of moisture have no effect on reflectance.
Reflectance at near infrared wavelengths is also negatively related to soil
moisture; an increase in soil moisture will result in a particularly rapid
decrease in reflectance due to water (H20) and hydroxyl (HO) absorption
features at 0.9 μm, 1.4 μm, 1.9 μm, 2.2 μm and 2.7 μm.
The effect of water and hydroxyl absorption is more noticeable in clay
soils because they have much bound water and very strong hydroxyl
absorption properties.
Organic matter content
Soil organic matter is dark and its presence will decrease the reflectance from
the soil up to an organic matter content of around 4 -5%. When the organic
matter content of the soil is greater than 5%, the soil is ‘black’ and any further
increases in organic matter will have little effect on reflectance (Curran,
1985).
Texture and structure
Texture (the proportion of sand, silt and clay particles) is related to
structure (the arrangement of sand, silt and clay particles into
aggregates).
A clay soil tends to have a strong structure which leads to a rough
surface on ploughing, causing small shadows and lowering reflectance
values. In contrast a sandy soil exhibits weak structure which leads to a
fairly smooth surface on ploughing with few shadows.
The effects of soil structure complement other properties such as low
moisture and organic matter content to increase the level of sandy soil
reflectance.
Iron oxide content
Iron oxide gives many soils their ‘rusty’ red colouration by coating or staining
individual soil particles. Iron oxide selectively reflects red light (0.6 - 0.7 μm)
and absorbs green light (0.5 - 0.6 μm). This effect is so pronounced that
Vincent (1973) used a ratio of red to green reflectance to locate iron ore
deposits (Curran, 1985).
Look again at Figure 1 - can you identify the increase in contrast between
red and green reflectance of the iron dominated soil?
The interaction of visible and near
infrared wavelengths of EMR
with water
 The majority of the radiant flux incident upon water is not
reflected but is either absorbed or transmitted.
 At visible wavelengths of EM radiation little energy is
absorbed, a small amount, usually under 5%, is reflected
and the majority is transmitted.
 Water absorbs strongly at near infrared wavelengths,
leaving little radiation to be either reflected or transmitted
(Figure 2). You would not ‘see’ through water as clearly at
these wavelengths.
 Although water surfaces may be more homogeneous than
soil surfaces we can still expect some variability in the
reflectance of a body of water.
 The two most important factors are the depth of the
water and the materials within the water.
In shallow water some of the radiation is reflected
not by the water itself but from the bottom of the
water body.
 the three most common materials suspended in water
are non-organic sediments, tannin and chlorophyll:
1.
 Non-organic silts and clays increase the reflectance at visible
wavelengths due to interaction with and scattering by soil-like
particles. This is caused by the high concentration of fine ‘rock
flour’ sediment suspended in the water.
 In agricultural scenes the main colouring agent is tannin
produced by decomposing humus. This is yellowish to brown in
colour and results in decreased blue and increased red
reflectance. A good example of the effects of tannin can be found
in streams that drain peat moorlands.
 Chlorophyll content must be very high before changes in
reflectance can be detected (Piech et al., 1978). Water bodies that
contain excessive levels of chlorophyll have reflectance properties
that resemble, at least in part, those of vegetation with increased
green and decreased blue and red reflectanc
The interaction of visible and near
infrared wavelengths of EMR
with vegetation
Reflectance of a leaf
 A leaf is built of layers of structural fibrous organic
matter, within which are pigmented, water-filled cells
and air spaces (Figure 3).
•
Subsequently, three features - pigmentation, physiological
structure and water content - have an effect on the reflectance,
absorption and transmittance properties of a green leaf (Curran, 1985).
• Each feature dominates the reflectance in particular
spectral regions .
• Pigment absorption at visible wavelengths,
physiological structure at near infrared
wavelengths and absorption by water molecules at
specific wavelengths in the near infrared region.
The interaction of visible and
near infrared wavelengths of
EMR with rocks
 Rocks, like soils, are single scatterers and exhibit relatively
simple spectral properties. Unlike soils rock reflectance is
less dependent on water content and completely
independent of organic matter content, texture or
structure.
 Rock spectral reflectance primarily depends on their
mineral composition.
• Lower reflecting minerals, such as
geothite, have similar spectral properties to
soil surfaces – low to moderate
reflectance at visible wavelengths
increasing into the near-infrared.
• Whereas high reflecting minerals, such as
quartz and calcite, exhibit almost uniformly
high reflectance throughout the visible,
near- and shortwave infrared spectrum.
• Differences between high reflectance minerals
occur at specific wavelengths
End
INTERACTION BETWEEN
INFRARED WITH WATER
Infrared
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Absorption Band

Atmospheric Transmission Band
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
Scattering
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INTERACTION OF EMR WITH WATER

EMR
molecular
–
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‫تفاعل الماء الشعة المرئية‬
‫‪‬‬
‫‪0.5‬‬
‫‪‬‬
‫‪transmission‬‬
‫‪‬‬
‫•‬
INTERACTION WITH ICE
near infrared
infrared
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INTERACTION WITH
PLANTS

Strong influence
EMR
Fuel
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Solar
Photosynthesis
0.68
0.45

Fluoresce
0.74
0.69
•
near-infrared
•
Red edge
SCATTERING

.1
Rayleigh Scatter
.2
Mie
Scatter
.3
Non-selective
scattering
•
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PROPERTIES OF EARTH SURFACES AND THEIR
INTERACTIONS WITH ELECTROMAGNETIC
RADIATION AT
VISIBLE AND NEAR INFRARED WAVELENGTHS
Spectra

Electronic transition
Vibration
•
Specular
Diffusive
Specular
Diffusive
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