Course Title

IEQ-303 Earthquake Geology and Geoinformatics

(Dept. of Earthquake Engineering, IIT Roorkee)

Remote Sensing Studies

 Remote sensing is the science of acquiring information about the Earth's
surface without actually being in contact with it.

 This is done by sensing and recording reflected or emitted energy and

processing, analyzing, and applying that information.

 Remote sensing process involves an interaction between incident

radiation and the targets of interest.

 This is exemplified by the use of imaging systems where the following

seven elements are involved.

With the advent and tremendous technological progress in the field of remote
sensing technique it has now become possible to study several things happening
on the earth surface. Earth observing satellites have the capability of producing
synoptic view of the earth and can generate wealth of information. Remote
sensing offers this perspective and allows a researcher to examine other reference
ancillary data simultaneously and synergistically. Nature and pattern of deformation
that the earth has undergone are beautifully displayed by the satellite images
enabling us to study these in details.

Deformation mapping is the identification and characterization of structural expression.

Structures include faults, folds, synclines and anticlines and lineaments. Understanding
structures is the key to interpreting crustal movements that have shaped the present
terrain. Structures can indicate potential locations of oil and gas reserves by
characterizing both the underlying subsurface geometry of rock units and the amount of
crustal deformation and stress experienced in a certain locale. Structures are also
examined for clues to crustal movement and potential hazards, such as earthquakes,
landslides, and volcanic activity. Identification of fault lines can facilitate land use
planning by limiting construction over potentially dangerous zones of seismic activity.

Certain remote sensing devices offer unique information regarding structures, such as in
the relief expression offered by radar sensors. A benefit of side looking radar is that the
illumination conditions can be controlled, and the most appropriate geometry used for
type of terrain being examined.

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1. Energy Source or Illumination (A) – the first requirement for remote sensing is to
have an energy source which illuminates or provides electromagnetic energy to the
target of interest.
2. Radiation and the Atmosphere (B) – as the energy travels from its source to the
target, it will come in contact with and interact with the atmosphere it passes through.
This interaction may take place a second time as the energy travels from the target to
the sensor.

3. Interaction with the Target (C) - once the energy makes its way to the target through
the atmosphere, it interacts with the target depending on the properties of both the target
and the radiation.

4. Recording of Energy by the Sensor (D) - after the energy has been scattered by, or
emitted from the target, we require a sensor (remote - not in contact with the target) to
collect and record the electromagnetic radiation.

5. Transmission, Reception, and Processing (E) - the energy recorded by the sensor
has to be transmitted, often in electronic form, to a receiving and processing station
where the data are processed into an image (hardcopy and/or digital).

6. Interpretation and Analysis (F) - the processed image is interpreted, visually and/or
digitally or electronically, to extract information about the target which was illuminated.

7. Application (G) - the final element of the remote sensing process is achieved when
we apply the information we have been able to extract from the imagery about the target
in order to better understand it, reveal some new information, or assist in solving a
particular problem.

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Electromagnetic Radiation : The first requirement for remote sensing is to
have an energy source to illuminate the target (unless the sensed energy is
being emitted by the target).

All electromagnetic radiation has fundamental properties and behaves in

predictable ways according to the basics of wave theory. Two characteristics of
electromagnetic radiation are particularly important for understanding remote
sensing. These are the wavelength and frequency.

Electromagnetic radiation consists of an electrical field (E) which varies in

magnitude in a direction perpendicular to the direction in which the radiation is
traveling, and a magnetic field (M) oriented at right angles to the electrical field.
Both these fields travel at the speed of light (c).

Wavelength is measured in meters (m) or some factor of metres such as

nanometres (nm, 10-9 metres), micrometres (µm, 10-6 metres) (µm, 10-6
metres) or centimetres (cm, 10-2 metres).

Electromagnetic Spectrum:

For most purposes, the ultraviolet or UV portion of the spectrum has the
shortest wavelengths. The light which our eyes - our "remote sensors" - can
detect is part of the visible spectrum. The visible wavelengths cover a range
from approximately 0.4 to 0.7 µm. The longest visible wavelength is red and the

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shortest is violet. Common wavelengths of what we perceive as particular colours
from the visible portion of the spectrum are listed below. It is important to note
that this is the only portion of the spectrum we can associate with the concept of
colours.

Violet: 0.400 - 0.446 µm

Blue: 0.446 - 0.500 µm
Green: 0.500 - 0.578 µm
Yellow: 0.578 - 0.592 µm
Orange: 0.592 - 0.620 µm
Red: 0.620 - 0.700 µm

The next portion of the spectrum of interest is the infrared (IR) region which can
be divided into two categories based on their radiation properties - the reflected
IR, and the emitted or thermal IR. Radiation in the reflected IR region is used for
remote sensing purposes in ways very similar to radiation in the visible portion.

The portion of the spectrum of more recent interest to remote sensing is the
microwave region from about 1 mm to 1 m. This covers the longest
wavelengths used for remote sensing. The shorter wavelengths have properties
similar to the thermal infrared region while the longer wavelengths approach the
wavelengths used for radio broadcasts.

Interactions with the Atmosphere : Before radiation (used for remote sensing)
reaches the Earth's surface it has to travel through some distance of the Earth's
atmosphere. Particles and gases in the atmosphere can affect the incoming light
and radiation. These effects are caused by the mechanisms of scattering and
absorption.

Rayleigh scattering occurs when particles are very small compared to the
wavelength of the radiation. These could be particles such as small specks of
dust or nitrogen and oxygen molecules. Rayleigh scattering causes shorter
wavelengths of energy to be scattered much more than longer wavelengths.
Rayleigh scattering is the dominant scattering mechanism in the upper
atmosphere.

Mie scattering occurs when the particles are just about the same size as the
wavelength of the radiation. Dust, pollen, smoke and water vapour are common
causes of Mie scattering which tends to affect longer wavelengths than those
affected by Rayleigh scattering. Mie scattering occurs mostly in the lower
portions of the atmosphere where larger particles are more abundant, and
dominates when cloud conditions are overcast.

Nonselective scattering The final scattering mechanism of importance is called

nonselective scattering. This occurs when the particles are much larger than
the wavelength of the radiation. Water droplets and large dust particles can

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cause this type of scattering. Nonselective scattering gets its name from the fact
that all wavelengths are scattered about equally.

Absorption is the other main mechanism at work when electromagnetic

radiation interacts with the atmosphere. In contrast to scattering, this
phenomenon causes molecules in the atmosphere to absorb energy at various
wavelengths. Ozone, carbon dioxide, and water vapour are the three main
atmospheric constituents which absorb radiation. Carbon dioxide referred to as
a greenhouse gas. This is because it tends to absorb radiation strongly in the far
infrared portion of the spectrum - that area associated with thermal heating -
which serves to trap this heat inside the atmosphere. Water vapour in the
atmosphere absorbs much of the incoming longwave infrared and shortwave
microwave radiation (between 22µm and 1m).

Atmospheric Window

Those areas of the spectrum which are not severely influenced by atmospheric
absorption are useful to remote sensors, are called atmospheric windows.

The visible portion of the spectrum, to which our eyes are most sensitive,
corresponds to both an atmospheric window and the peak energy level of the
sun. Energy emitted by the Earth corresponds to a window around 10 µm in the
thermal IR portion of the spectrum. The large window at wavelengths beyond 1
mm is associated with the microwave region.

Radiation - Target Interactions

There are three (3) forms of interaction that can take place when energy strikes,
or is incident (I) upon the surface. These are:

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 Absorption (A) Transmission (T) Reflection (R)

The total incident energy will interact with the surface in one or more of these
three ways. The proportions of each will depend on the wavelength of the energy
and the material and condition of the feature.

In remote sensing, we are most interested in measuring the radiation reflected

from targets. We refer to two types of reflection, which represent the two extreme
ends of the way in which energy is reflected from a target:

Specular reflection and Diffuse reflection.

When a surface is smooth we get specular or mirror-like reflection where all (or
almost all) of the energy is directed away from the surface in a single direction.

Diffuse reflection occurs when the surface is rough and the energy is reflected
almost uniformly in all directions.

If the wavelengths are much smaller than the surface variations or the particle
sizes that make up the surface, diffuse reflection will dominate. Most earth
surface features lie somewhere between perfectly specular or perfectly diffuse
reflectors.

Leaves strongly absorb radiation in the red and blue wavelengths but reflects
green wavelengths producing green appearance. Water absorbs more longer
wavelength radiation than shorter visible wavelengths thus water typically looks
blue or blue-green due to stronger reflectance at these shorter wavelengths.

Passive vs. Active Sensing

Passive Sensors : Remote sensing systems which measure energy that is

naturally available are called passive sensors. For all reflected energy, this can
only take place during the time when the sun is illuminating the Earth. Energy
that is naturally emitted (such as thermal infrared) can be detected day or night,
as long as the amount of energy is large enough to be recorded.

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 Active sensors, on the other hand, provide their own energy source for
illumination. Some examples of active sensors are a laser and a synthetic
aperture radar (SAR).

Satellites Orbits and Swaths

Geostationary Orbits : The orbit of a satellite is elliptical in shape, but remote

sensing satellites are usually put in orbits that are very close to approximations to
a circle. Satellites at very high altitudes, which view the same portion of the
Earth's surface at all times have geostationary orbits. These geostationary
satellites, at altitudes of approximately 36,000 kilometres, revolve at speeds
which match the rotation of the Earth so they seem stationary, relative to the
Earth's surface. This allows the satellites to observe and collect information
continuously over specific areas. Weather and communications satellites
commonly have these types of orbits.

Geostationary Satellite

Nearpolar Orbits : Many remote sensing platforms are designed to follow an

orbit (basically north-south) which, in conjunction with the Earth's rotation (west-
east), allows them to cover most of the Earth's surface over a certain period of
time. These are nearpolar orbits, so named for the inclination of the orbit relative
to a line running between the North and South poles.

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Sun-synchronous orbits: Many of these satellite orbits are also sun-
synchronous such that they cover each area of the world at a constant local time
of day called local sun time. At any given latitude, the position of the sun in the
sky as the satellite passes overhead will be the same within the same season.

Swath : As a satellite revolves around the Earth, the

sensor "sees" a certain portion of the Earth's surface.
The area imaged on the surface, is referred to as the
swath. Imaging swaths for spaceborne sensors
generally vary between tens and hundreds of
kilometers wide.

Sensor Technology

Most remote sensing instruments (sensors) are designed to measure photons.

The fundamental principle underlying sensor operation centers on the concept of
the photoelectric effect. The magnitude of the electric current produced (number
of photoelectrons per unit time) is directly proportional to the light intensity. Thus,
changes in the electric current can be used to measure changes in the photons
(numbers; intensity) that strike the detector during a given time interval.

CCD Detector

An individual CCD is an extremely small silicon (micro) detector, which is light-

sensitive. Many individual detectors are placed on a chip side by side either in a
single row as a linear array or in stacked rows of linear arrays in X-Y (two
dimensional) space. When photons strike a CCD detector, electronic charges
develop whose magnitudes are proportional to the intensity of the impinging
radiation during a short time interval (exposure time). The number of elements
per unit length, along with the optics, determine the spatial resolution of the
instrument. Using integrated circuits each linear array is sampled very rapidly in
sequence, producing an electrical signal that varies with the radiation striking the
array. Each individual CCD corresponds to the "pixel". The size of the CCD is
one factor in setting spatial resolution (smaller sizes represent smaller areas on
the target surface); another factor is the height of the observing platform (satellite
or aircraft); a third factor can be the use of a telescopic lens.

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Resolution
Spatial Resolution
Spectral Resolution
Radiometric Resolution
Temporal Resolution

Spatial Resolution :Spatial resolution, the capability of distinguishing closely

spaced objects on an image or a photograph. Image resolution is determined by
the size and number of picture elements or pixels used to form an image. The
smaller the pixel size, the greater the resolution.

Spatial resolution of passive sensors depends

primarily on their Instantaneous Field of View
(IFOV). The IFOV is the angular cone of visibility of
the sensor (A) and determines the area on the
Earth's surface which is "seen" from a given altitude
at one particular moment in time (B).
The size of the area viewed is determined by
multiplying the IFOV by the distance from the ground
to the sensor (C). This area on the ground is called
the resolution cell and determines a sensor's
maximum spatial resolution.

Spectral Resolution : Spectral resolution describes the ability of a sensor to

define fine wavelength intervals. The finer the spectral resolution, the narrower
the wavelength range for a particular channel or band. Many remote sensing
systems record energy over several separate wavelength ranges at various
spectral resolutions.

Radiometric Resolution : Radiometric characteristics describe the actual

information content in an image. Every time an image is acquired by a sensor, its
sensitivity to the magnitude of the electromagnetic energy determines the
radiometric resolution. The radiometric resolution of an imaging system
describes its ability to discriminate very slight differences in energy. The finer the
radiometric resolution of a sensor, the more sensitive it is to detecting small
differences in reflected or emitted energy. Imagery data are represented by
positive digital numbers which vary from 0 to (one less than) a selected power of
2. This range corresponds to the number of bits used for coding numbers in
binary format. Each bit records an exponent of power 2 (e.g. 1 bit=2 1=2). The
maximum number of brightness levels available depends on the number of bits
used in representing the energy recorded. Thus, if a sensor used 8 bits to record
the data, there would be 28=256 digital values available, ranging from 0 to 255.

Temporal Resolution: Temporal resolution is also important to consider in a

remote sensing system which refers to the length of time it takes for a satellite to

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complete one entire orbit cycle. The revisit period of a satellite sensor is usually
several days. Therefore the absolute temporal resolution of a remote sensing
system to image the exact same area at the same viewing angle a second time is
equal to this period.

Thermal Imaging
All matter of the earth radiates energy at Thermal Infrared wavelength (3 µm to
15 µm) both day and night. Thermal sensors use photo detectors sensitive to
the direct contact of photons on their surface, to detect emitted thermal radiation.
The detectors are cooled to temperatures close to absolute zero (0K) in order to
limit their own thermal emissions.

Thermal sensors essentially measure the surface temperature and thermal

properties of targets. Thermal IR images generally record broad spectral bands,
typically 8.0 µm to 14.0 µm for images from aircraft and 10.5 µm to 12.5 µm for
images from satellites. To interpret thermal IR images, one must understand the
basic physical processes that control the interactions between thermal energy
and matter, as well as the thermal properties of matter that determine the rate
and intensity of the interactions.

 On most thermal IR images, the brightest tones represent the warmest

radiant temperatures, and the darkest tones represent the coolest ones.

 Clouds typically show the patchy warm-and-cool pattern.

 The thermal inertia of water is similar to that of soils and rocks, but in
daytime, water bodies have a cooler surface temperature than soils and
rocks. At night the relative surface temperatures are reversed, so that
water is warmer than soils and rocks.

 If water bodies have warm signatures relative to the adjacent terrain, the
image was acquired at night. Whereas, relatively cool water bodies
indicate daytime imagery. Damp soil is cooler than dry soil, both day and
night.

 Green deciduous vegetation has a cool signature on daytime images and

a warm signature on nighttime images. During the day, transpiration of
water vapor lowers leaf temperature, causing vegetation to have a cool
signature relative to the surrounding soil. At night the insulating effect of
leafy foliage and the high water content retain heat, which results in warm
nighttime temperatures.

Applications of Thermal IR Sensing

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Land cover classification and mapping
 Estimate sea surface temperatures
 Estimate soil moisture
 Monitor plant stress
 Detect ground water and geological
Structures and materials
 Detect and map thermal discharges
 Measure heat loss of buildings
 Assess urban heat island effects
 Map forest fires
 Monitor volcanic activity

Microwave Remote Sensing

Microwave sensing encompasses both active and passive forms of remote
sensing. The microwave portion of the spectrum covers the range from
approximately 1cm to 1m in wavelength. Because of their long wavelengths,
compared to the visible and infrared, microwaves have special properties that are
important for remote sensing. Longer wavelength microwave radiation can
penetrate through cloud cover, haze, dust, and all but the heaviest rainfall
as the longer wavelengths are not susceptible to atmospheric scattering which
affects shorter optical wavelengths. This property allows detection of microwave
energy under almost all weather and environmental conditions so that data can
be collected at any time.

A passive microwave sensor detects the naturally emitted microwave energy

related to the temperature and moisture properties of the emitting object or
surface within its field of view.

Active microwave sensors provide their own source of microwave radiation to

illuminate the target.

The most common form of imaging active microwave sensors is RADAR.

RADAR is an acronym for RAdio Detection And Ranging, which essentially
characterizes the function and operation of a radar sensor.

RADAR Basics
A radar is essentially a ranging or distance measuring device. It consists
fundamentally of a transmitter, a receiver, an antenna, and an electronics system
to process and record the data. The transmitter generates successive short
bursts (or pulses of microwave (A) at regular intervals which are focused by the
antenna into a beam (B). The radar beam illuminates the surface obliquely at a
right angle to the motion of the platform.

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The antenna receives a portion of the transmitted energy reflected (or
backscattered) from various objects within the illuminated beam (C). By
measuring the time delay between the transmission of a pulse and the reception
of the backscattered "echo" from different targets, their distance from the radar
and thus their location can be determined. As the sensor platform moves
forward, recording and processing of the backscattered signals builds up a two-
dimensional image of the surface.

The microwave region of the spectrum is quite large, relative to the visible and
infrared, and there are several wavelength ranges or bands commonly used.

Irrespective of wavelength, radar signals can transmit horizontal (H) or vertical

(V) electric- field vectors, and receive either horizontal (H) or vertical (V) return
signals, or both. There can be four combinations of both transmit and receive
polarizations as follows:

HH - for horizontal transmit and horizontal

receive,
VV - for vertical transmit and vertical receive,
HV - for horizontal transmit and vertical
receive, and
VH - for vertical transmit and horizontal
receive.

Viewing Geometry and Spatial Resolution

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While ground range is the horizontal distance between the emitter and its target
and its calculation requires knowledge of the target's elevation. Since the waves
travel to a target and back, the round trip time is divide by two in order to obtain
the time the wave took to reach the target.

Spatial resolution in both the range (look) direction and azimuth (flight) direction
is determined by the engineering characteristics of the radar system. Depression
angle defined as the angle between a horizontal plane and a beam from the
antenna to a target on the ground. The depression angle is steeper at the near
range side of an image strip and shallower at the far-range side. The average
depression angle is measured for a beam to the midline of an image strip.
Incidence angle is defined as the angle between a radar beam and a line
perpendicular to the surface.

Spatial Resolution
The spatial resolution of a radar image is determined by the dimension of ground
resolution cell, which are controlled by the combination of range resolution and
azimuth resolution.

Range Resolution
Range Resolution (Rr) or resolution in the
radar look direction is determined by the
depression angle and by the pulse length.
Pulse length () is the duration of the
transmitted pulse and is measured in
microseconds.
It is converted from time into distance by
multiplying by the speed of electromagnetic
radiation.

Range Resolution is determined by the relationship

 Rr =  . c / 2 cos  (c = speed of electromagnetic radiation)

Target A and B, spaced 20 m apart, are imaged with a depression angle of 50 o

and a pulse length of 0.1 sec. For these targets range resolution is 23.4m.

Targets A and B are not resolved because they are closer together (20m) than
the range resolution distance. They are within a single ground resolution cell and
cannot be separated of the image.

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Targets C and D are also spaced 20m apart but are imaged with a depression
angle of 35o. For this depression angle, range resolution is calculated as 18m.
Targets C and D are resolved because they are more widely spaced than the
ground resolution cell.
One method of improving range resolution is to shorten the pulse length,
but this reduces the total amount of energy in each transmitted pulse.

Azimuth Resolution
Azimuth resolution (Ra), or resolution in the
azimuth direction, is determined by the width
of the terrain strip illuminated by the radar
beam.
To be resolved, targets must be separated
in the azimuth direction by a distance
greater than the beam width as measured
on the ground.
The fan-shaped beam is narrower in the
near range than in the far range, causing
azimuth resolution to be smaller in the near-
range portion of the image.

The equation of azimuth resolution (Ra) is

Ra = 0.7 . S .  / D
S = Slant-range distance
D = Antenna length.
For a typical X-band system ( = 3.0 cm; D = 500 cm) with a slant-range distance
of 8 km, Ra is calculated to be 33.6 m. Estimated resolution is close to target
separation i.e. 35m so it is resolved.
At the far range position the slant range distance is 20 km, and Ra is
calculated as 84 m, thus targets C and D, also separated by 35m, are not
resolved.

 Angular beam width is directly proportional to the wavelength of the

transmitted energy. Therefore, azimuth resolution is higher for shorter
wavelengths, but the short wavelengths lack the desirable penetration
capability.
 Angular beam width is inversely proportional to antenna length, therefore
resolution improves with longer antennas.

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Slant Range Distortion

The slant-range distortion occurs because the

radar is measuring the distance to features in
slant-range rather than the true horizontal
distance along the ground. This results in a
varying image scale, moving from near to far
range.

Foreshortening occurs when the radar beam

reaches the base of a tall feature tilted towards
the radar (e.g. a mountain) before it reaches the
top.
Because the radar measures distance in slant-
range, the slope (a to b) will appear
compressed and the length of the slope will be
represented incorrectly (a' to b') at the image
plane.
Foreshortening

Layover occurs when the radar beam reaches

the top of a tall feature (b) before it reaches the
base (a).
The return signal from the top of the feature will
be received before the signal from the bottom.
As a result, the top of the feature is displaced
towards the radar from its true position on the
ground, and “lays over” the base of the feature
(b' to a').
Layover

The shadowing effect increases with greater

incident angle θ, just as our shadows lengthen
as the sun sets.

Shadowing

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SAR Image
A digital SAR image can be seen as a mosaic (i.e. a two-dimensional array
formed by columns and rows) of small picture elements (pixels). Each pixel is
associated with a small area of the Earth’s surface (called a resolution cell).

Each pixel gives a complex number that carries amplitude and phase
information about the microwave field backscattered by all the scatterers (rocks,
vegetation, buildings etc.) within the corresponding resolution cell projected on
the ground.

Different rows of the image are associated with different azimuth locations,
whereas different columns indicate different slant range locations.

The radiation transmitted from the radar has to reach the scatterers on the
ground and then come back to the radar in order to form the SAR image (two-
way travel). Scatterers at different distances from the radar (different slant
ranges) introduce different delays between transmission and reception of the
radiation.

Due to the almost purely sinusoidal nature of the transmitted signal, this delay 
is equivalent to a phase change  between transmitted and received signals.
The phase change is thus proportional to the two-way travel distance 2R of the
radiation divided by the transmitted wavelength .
In other words the phase of the SAR signal is a measure of just the last fraction
of the two-way travel distance that is smaller than the transmitted wavelength.

Real-Aperture and Synthetic-Aperture

Real-Aperture Radar uses an antenna of the maximum practical length to
produce a narrow angular beam width in the azimuth direction.

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A Sythetic Aperture Radar (SAR) is a coherent sidelooking airborne system
which utilizes the flight path of the aircraft to simulate an extremely large antenna
or aperture electronically and that generates high-resolution remote sensing
imagery.

The signal processing uses magnitude and phase of the received signals over
successive pulses from elements of a synthetic aperture. After a given number of
cycles, the stored data is recombined (taking into account the Doppler effects
inherent in the different transmitter to target geometry in each succeeding cycle)
to create a high resolution image of the terrain being over flown.

Interferometry
Interferometric SAR or InSAR, allows accurate measurements of the radiation
travel path because it is coherent.
Measurements of travel path variations as a function of the satellite position and
time of acquisition allow generation of Digital Elevation Models (DEM) and
measurement of centimetric surface deformations of the terrain.

A satellite SAR can observe the same area from slightly different look angles.
This can be done either simultaneously (with two radars mounted on the same
platform) or at different times by exploiting repeated orbits of the same satellite.

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  The distance between the two satellites (or orbits) in the plane
perpendicular to the orbit is called the interferometer baseline and its
projection perpendicular to the slant range is the perpendicular baseline.

 The SAR interferogram is generated by cross-multiplying, pixel by pixel,

the first SAR image with the complex conjugate of the second.

 Thus, the interferogram amplitude is the amplitude of the first image

multiplied by that of the second one, whereas its phase (the
interferometric phase) is the phase difference between the images.

Applications of remote sensing include the following:

Surficial deposit / bedrock mapping, Lithological mapping Structural mapping,
Sand and gravel (aggregate) exploration/ exploitation, Mineral exploration
Hydrocarbon exploration, Environmental geology, Geobotany, Baseline infrastructure
Sedimentation mapping and monitoring
Event mapping and monitoring
Geo-hazard mapping
Planetary mapping

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