PROPERTIES OF EARTH SURFACES AND THEIR

INTERACTIONS WITH ELECTROMAGNETIC RADIATION

Energy InteractionsElectromagnetic 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 transmittedthrough 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 EMREarth-Atmosphere System Energy Budget

ABSORBTIONThe areas of the EM spectrum 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 singlesurface 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 absorbmore 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 increaseswith wavelength in the visible and near infrared spectral regions. You can see from Figure 1 that maximum soil reflectance occurs at near infraredwavelengths.

Most important: moisture contentorganic matter contenttexturestructure

Least important: iron oxide content

Soil moisture contentthe 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 soilmoisture; an increase in soil moisture will result in a particularly rapiddecrease in reflectance due to water (H20) and hydroxyl (HO) absorptionfeatures 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 contentSoil organic matter is dark and its presence will decrease the reflectance fromthe soil up to an organic matter content of around 4 -5%. When the organicmatter content of the soil is greater than 5%, the soil is ‘black’ and any furtherincreases in organic matter will have little effect on reflectance (Curran,1985).

Texture and structureTexture (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 contentIron oxide gives many soils their ‘rusty’ red colouration by coating or stainingindividual 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 thatVincent (1973) used a ratio of red to green reflectance to locate iron oredeposits (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 EMRwith 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 reflectedand 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.

1. 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:

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 tanninproduced 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 EMRwith 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, physiologicalstructure 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 completelyindependent of organic matter content, texture or structure.

Rock spectral reflectance primarily depends on their mineral composition.

• Lower reflecting minerals, such asgeothite, have similar spectral properties to

soil surfaces – low to moderatereflectance at visible wavelengths

increasing into the near-infrared.• Whereas high reflecting minerals, such as

quartz and calcite, exhibit almost uniformlyhigh 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

Absorption Band

Atmospheric Transmission Band

Scattering

INTERACTION OF EMR WITH WATER

EMR

–molecular

0.5

transmission

تفاعل الماء الشعة المرئية

•

INTERACTION WITH ICE

near infrared

infrared

INTERACTION WITH PLANTS

Strong influenceEMR

SolarFuel Photosynthesis

0.450.68

Fluoresce 0.690.74

•Red edge

•

near-infrared

SCATTERING

.1

Rayleigh Scatter

.2Mie

Scatter

.3Non-selective

scattering

•

PROPERTIES OF EARTH SURFACES AND THEIRINTERACTIONS WITH ELECTROMAGNETIC RADIATION ATVISIBLE AND NEAR INFRARED WAVELENGTHS

Spectra

Electronic transition

Vibration

•

SpecularDiffusive

Specular

Diffusive