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York: W. H. Freeman, 2008. Encyclopedia of Astronomy and Astrophysics. CRC Press, Taylor and Francis Group Publishers. Available online. uRL: http://eaa.crcpress.com/. Accessed october 24, 2008.

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remote sensing Remote sensing is the acquisition of information about an object by recording devices that are not in physical contact with the object. Different types include 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. Common use of the term remote sensing refers 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 pho-togeologic 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. many geological mapping programs include 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. 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 the establishment of many satellite-based systems has provided 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 a linescanning technique, 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 picked up by the sensors. The digital data encodes this information, and digital image processing converts the strength of the signal from different bands 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 such as red, green, and blue to produce a colored image. Different electromagnetic bands may even be numerically or digitally combined or ratioed to highlight different geologic features.

optical and infrared imagery are now widely used for regional geologic studies, with common satellite platforms including the United States-based Landsat systems, the French SPOT satellite, and more recently some multispectral sensors including Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Advanced Very High Resolution Radiometer (AVHRR) data. Optical and infrared imagery can detect differences in rock and mineral types because the reflection is sensitive to molecular interactions with solar radiation, highlighting differences between Al-OH bonds, C-O bonds, and Mg-OH bonds, effectively discriminating between different minerals such as micas, Mg-sili-cates, 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 (of wavelengths less than 0.04 inches, 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 reflects it 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 proven to be immensely useful for both military and scientific purposes, producing images of topography, surface roughness, and structural features such as faults, foliations, and other features that are highlighted by radar reflecting off sharp edges. under certain circumstances, radar penetrates the surface of some geological materials (such as dry sand) and can produce images of what lies beneath the surface, including buried geologic structures, pipelines, and areas of soil moisture.

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