DEFINITION OF REMOTE SENSING a

The acquisition of information about an

object by recording devices that are not in physical contact

with the object. There are many types of remote sensing,

including airborne or spaceborne techniques and sensors that

measure different properties of Earth materials, ground-based

sensors that measure properties of distant objects, and techniques

that penetrate the ground to map subsurface properties.

The term remote sensing is commonly used to refer only

to the airborne and space-based observation systems, with

ground-based systems more commonly referred to as geophysical

techniques.

Remote sensing grew out of airplane-based photogeologic

reconnaissance studies, designed to give geologists a vertically

downward-looking regional view of an area of interest,

providing information and a perspective not readily appreciated

from the ground. Most geological mapping now includes

the use of stereo aerial photographs, produced by taking

downward-looking photographs at regular intervals along a

flight path from an aircraft, with every area on the ground

covered by at least two frames. The resolution of typical aerial

photographs is such that objects less than 3.2 feet (1 m)

across can be easily identified. The camera and lens geometry

is set so that the photographs can be viewed with a stereoscope,

where each eye looks at one of the overlapping images,

producing a visual display of greatly exaggerated topography.

This view can be used to pick out details and variations in

topography, geology, and surface characteristics that greatly

aid geologic mapping. Typically, geologic structures, rock

dips, general rock types, and the distribution of these features

can be mapped from aerial photographs.

Modern techniques of remote sensing employ a greater

range of the electromagnetic spectrum than aerial photographs.

Photographs are limited to a narrow range of the

electromagnetic spectrum between the visible and infrared

wavelengths that are reflected off the land’s surface from the

Sun’s rays. Since the 1960s a wide range of sensors that can

detect and measure different parts of the electromagnetic

spectrum have been developed, along with a range of different

optical-mechanical and digital measuring and recording

devices used for measuring the reflected spectrum. In addition

many satellite-based systems have been established, providing

stable observation platforms and continuous or repeated coverage

of most parts of the globe. One technique uses a mirror

that rapidly sweeps back and forth across an area measuring

the radiation reflected in different wavelengths. Another technique

uses line-scanning, where thousands of detectors are

arranged to electronically measure the reflected strength of

radiation from different wavelengths in equally divided time

intervals as the scanner sweeps across the surface, producing

a digital image consisting of thousands of lines of small picture

elements (pixels) representing each of the measured

intervals. The strength of the signal for each pixel is converted

to a digital number (dn) for ease of data storage and

manipulation to produce a variety of different digital image

products. Information from the reflected spectrum is divided

into different wavelength bands that correspond to the narrow

wavebands measured by the sensors. The digital data

encodes this information, and during digital image processing,

the strength of the signal from different bands is converted

into the strength of the mixture of red, green, and blue,

with the mixture producing a colored image of the region.

Different bands may be assigned different colors, and bands

may even be numerically or digitally combined or ratioed to

highlight different geological features.

Optical and infrared imagery are now widely used for

regional geological studies, with common satellite platforms

including the United States–based Landsat systems, the

French SPOT (Système Pour l’Observation de la Terre) satellite,

and more recently some multispectral sensors including

ASTER (Advanced Spaceborne Thermal Emission and Reflec-

tion Radiometer) and AVHRR (Advanced Very High Resolution

Radiometer) data. Much optical and infrared imagery is

able to detect differences in rock and mineral types because

the reflection is sensitive to molecular interactions with solar

radiation that highlights differences between Al-OH bonds,

C-O bonds, and Mg-OH bonds, effectively discriminating

between different minerals such as micas, Mg-silicates,

quartz, and carbonates. Bands greater than 2.4 microns are

sensitive to the temperature of the surface instead of the

reflected light, and studies of surface temperature have

proven useful for identifying rock types, moisture content,

water and hydrocarbon seeps, and caves.

Microwave remote sensing (wavelengths of less than

0.04 inch, or 1 mm) uses artificial illumination of the surface

since natural emissions are too low to be useful. Satellite and

aircraft-based radar systems are used to shoot energy of specific

wavelength and orientation to the surface, which is then

reflected back to the detector. Radar remote sensing is very

complex, depending on the geometry and wavelength of the

system, and on the nature of the surface. The strength of the

received signal is dependent on features such as surface inclination,

steepness, orientation, roughness, composition, and

water content. Nonetheless, radar remote sensing has proved

to be immensely useful for both military and scientific purposes,

producing images of topography and surface roughness,

and highlighting structural features such as faults,

foliations, and other forms that are highlighted by radar

reflecting off from sharp edges. Under some special circumstances,

radar is able to penetrate the surface of some geological

materials (such as dry sand) and produce images of what

lies beneath the surface, including buried geologic structures,

pipelines, and areas of soil moisture.

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