of Remote Sensing Techniques in
Remote Sensing is the science and art of acquiring information
(spectral, spatial, temporal) about material objects, area, or phenomenon,
without coming into physical contact with the objects, or area, or phenomenon
under investigation. Without direct contact, some means of transferring
information through space must be utilized. In remote sensing, information
transfer is accomplished by use of electromagnetic radiation (EMR). EMR is a
form of energy that reveals its presence by the observable effects it produces
when it strikes the matter. The electromagnetic spectrum is the
continuum of energy that ranges from meters (radio waves) to fractions of
nanometers (gamma rays) in wavelength, travels at the speed of light, and
propagates through a vacuum such as outer space. 1 nanometer (nm) = 10-9
When electro-magnetic energy is incident on any given earth surface
feature, three fundamental energy interactions with the feature are possible
it is either reflected, absorbed or transmitted. The proportions of energy reflected, absorbed, and
transmitted will vary for different earth features, depending upon their
material type and conditions. These differences permit us to distinguish
different features on an image. Even
within a given feature type, the proportion of reflected, absorbed, and
transmitted energy will vary at different wavelengths. Thus, two features
may be distinguishable in one spectral range and be very different on another
wavelength brand. Within the visible portion of the spectrum, these spectral
variations result in the visual effect called COLOUR. For example we call blue
objects 'blue' when they reflect highly in the 'green' spectral region, and so
on. Thus the eye uses spectral variations in the magnitude of reflected energy
to discriminate between various objects. A
graph of the spectral reflectance of an object as a function of wavelength is
called a spectral reflectance curve. The configuration of spectral
reflectance curves provides information about the identity of an object, and if
it has a high resolution, it is unique to an object. Resolution of remotely
sensed imageries is of different types:
resolution refers to the width or range of each spectral band being
recorded. As an example, panchromatic imagery (sensing a broad range of all
visible wavelengths) will not be as sensitive to vegetation stress as a narrow
band in the red wavelengths, where chlorophyll strongly absorbs electromagnetic
resolution refers to the discernible detail in the image. Detailed
mapping of wetlands requires far finer spatial resolution than does the regional
mapping of physiographic areas.
resolution refers to the time interval between images. There are
applications requiring data repeatedly and often, such as oil spill, forest
fire, and sea ice motion monitoring. Some applications like geological mapping
need imaging only once.
In respect to the type of Energy Sources:
Passive Remote Sensing:
Makes use of sensors that detect the reflected or emitted electro-magnetic
radiation from natural sources.
Active remote Sensing:
Makes use of sensors that detect reflected responses from objects that are
irradiated from artificially generated energy sources, such as radar.
In respect to Wavelength Regions:
Remote Sensing is
classified into three types in respect to the wavelength regions
Visible and Reflective
Infrared Remote Sensing. (400-1400 nm)
Thermal Infrared Remote
Sensing. (3-5 and 8-14 µm)
Sensing. (0.1-30 cm)
band of information collected from a remote sensing sensor contains important
and unique data. We know that different wavelengths of incident energy are
affected differently by each target - they are absorbed, reflected or
transmitted in different proportions. In many applications, using information
from different bands ensures that target identification or information
extraction becomes fairly accurate.
sensing gives the overview required to 1) construct regional unit maps, useful
for small scale analyses, and planning field traverses to sample and verify
various units for detailed mapping; and 2) understand the spatial distribution
and surface relationships between the units. VIR remote sensing provides the
multispectral information relating to the composition of the unit, while radar
can contribute textural information. Multiple data sources can also be
integrated to provide a comprehensive view of the lithostratigraphy.
imagery can also facilitate delineation and identification of units by providing
a three dimensional view of the local relief. Some rocks are resistant to
erosion, whereas others erode easily. Identification elements such as weathering
manifestations may be apparent on high or medium resolution imagery and
airphotos. Images or airphotos can be taken into the field and used as
basemaps for field analysis. Two
different scales of mapping require slightly different imaging sources and
site specific analysis, airphotos provide a high resolution product that can
provide information on differential weathering, tone, and microdrainage. Photos
may be easily viewed in stereo to assess relief characteristics.
overviews require large coverage area and moderate resolution. An excellent data
source for regional applications is a synergistic combination of radar and
optical images to highlight terrain and textural information.
either case, frequency of imaging is not an issue since in many cases the
geological features of interest remain relatively static. Immediate turnaround
is also not critical.
for remote sensing application do not differ significantly around the world. One
of the biggest problems faced by both temperate and tropical countries is that
dense forest covers much of the landscape. In these areas, geologists can use
remote sensing to infer underlying lithology by the condition of vegetation
growing above it. This concept is called "geobotany". The underlying
principle is that the mineral and sedimentary constituents of the bedrock may
control or influence the condition of vegetation growing above.
reality, the topography, structure, surficial materials, and vegetation combine
to facilitate geologic unit interpretation and mapping. Optimal use of remote
sensing data therefore, is one that integrates different sources of image data,
such as optical and radar, at a scale appropriate to the study.
in Geological Mapping
have seen that geological mapping provides the ground for any mineral
exploration programme. It involves
the study of landforms, structures, and the subsurface, to understand physical
processes creating and modifying the earth's crust. Geological mapping is
absolutely essential for the exploration and exploitation of mineral,
hydrocarbon and other energy resources. Remote sensing is used as a tool to
extract information about the land surface structure, composition or subsurface.
Combined with data from other sources, it provides complementary
measurements. Multispectral reflectance data can provide information on
lithology, rock composition or rock alteration, which is so often associated
with, and indicative of the presence of mineral deposits, particularly
epigenetic deposits. Radar
provides an expression of surface topography and roughness, and thus is
extremely valuable, especially when integrated with other data sources in
providing details of relief or physiography. Physiographic features, as we all know are excellent guides
to the presence of ore.
sensing is not limited to direct geology applications - it is also used to
support logistics, such as route planning for access into a mining area,
reclamation monitoring, and generating basemaps upon which geological data can
be referenced or superimposed.
applications of remote sensing include the following:
surficial deposit / bedrock mapping
sand and gravel (aggregate) exploration/ exploitation
sedimentation mapping and monitoring
event mapping and monitoring
in Structural Mapping
geology plays an important role in mineral and hydrocarbon exploration, and
potential hazard identification and monitoring.
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. Detailed examination of structure can be
obtained by geophysical techniques such as seismic surveying.
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.
main advantage of remotely sensed data in structural mapping are that they
provide some information on the spatial distribution and surficial relief of
the structural elements. Radar is well suited to these requirements with its
side-looking configuration. Imaging with shallow incidence angles enhances
surficial relief and structure. Shadows can be used to help define the
structure height and shape, and thus increasing the shadow effect, while
shallow incidence angles may benefit structural analysis.
remote sensing devices offer unique information regarding structures, such as
in the relief expression offered by radar sensors. Comparing surface
expression to other geological information may also allow patterns of
association to be recognized. For instance, a rock unit may be characterized
by a particular radar texture which may also correlate with a high magnetic
intensity or geochemical anomaly. Remote sensing is most useful in
combination, or in synergy, with complementary datasets.
areas where vegetation cover is dense, it is very difficult to detect
structural features. A heavy canopy will visually blanket the underlying
formation, limiting the use of optical sensors for this application. Radar
however, is sensitive enough to topographic variation that it is able to
discern the structural expression reflected or mirrored in the tree top
canopy, and therefore the structure may be clearly defined on the radar
analyses are conducted on regional scales, to provide a comprehensive look at
the extent of faults, lineaments and other structural features with which ore
deposits are frequently associated. Aerial photos can be used in temperate
areas where large-scale imagery is required, particularly to map relief which
is quite often determined by structure.
structural information provided by radar complements other spatial datasets.
When integrated together, SAR and spatial geological datasets provide a
valuable source of geological information
in Lithological Mapping
geologic units consists primarily of identifying physiographic units and
determining the rock lithology or coarse stratigraphy of exposed units. These
units or formations are generally described by their age, lithology and
thickness. Remote sensing can be used to describe lithology by the colour,
weathering and erosion characteristics (whether the rock is resistant or
recessive), drainage patterns, and thickness of bedding.
mapping is useful in oil and mineral exploration, since these resources are
often associated with specific lithologies. Structures below the ground, which
may be conducive to trapping oil or hosting specific minerals, often manifest
themselves on the Earth's surface. By delineating the structures and
identifying the associated lithologies, geologists can identify locations that
would most likely contain these resources, and target them for exploration.
terms of remote sensing, these "lithostratigraphic" units can be
delineated by their spectral reflectance signatures, by the structure of the
bedding planes, and by surface morphology.
is the study of water on the Earth's surface, whether flowing above ground,
frozen in ice or snow, or retained by soil. Hydrology is inherently related to
many other applications of remote sensing, particularly forestry, agriculture
and land cover, since water is a vital component in each of these disciplines.
Most hydrological processes are dynamic, not only between years, but also
within and between seasons, and therefore require frequent observations.
Remote sensing offers a synoptic view of the spatial distribution and dynamics
of hydrological phenomena, often unattainable by traditional ground surveys.
Radar has brought a new dimension to hydrological studies with its active
sensing capabilities, allowing the time window of image acquisition to include
inclement weather conditions or seasonal or diurnal darkness.
Examples of hydrological applications include:
wetlands mapping and monitoring,
soil moisture estimation,
snow pack monitoring / delineation of extent,
measuring snow thickness,
determining snow-water equivalent,
river and lake ice monitoring,
flood mapping and monitoring,
glacier dynamics monitoring (surges, ablation)
river /delta change detection
drainage basin mapping and watershed modelling
irrigation canal leakage detection
compiled from information provided through the website of Canada Centre for
Remote Sensing at http://www.ccrs.nrcan.gc.ca/ccrs/homepg.pl?e