Airborne Geophysics and Precise Positioning Scientific Issues and Future Directions (1995) / Chapter Skim
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1 Airborne Geophysics: A Powerful Tool for Studying the Earth
1 Airborne Geophysics: A Powerful Tool for Studying the Earth
Pages 5-28
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From page 5... ...
From the first photographs of Earth taken from aircraft with hand-held cameras to the highly refined swath terrain profiling systems now being developed, aircraft have provided a unique approach to studying earn science. Compared with ground- or space-based methods, airborne techniques offer the advantages of improved access, rapid sampling at scales that are optimal for many geophysical problems, and a tremendous potential for interdisciplinary studies at intermediate scales.
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From page 6... ...
6 ttM f -- ~? m:: -offs ~- a -- .- ~ =, ~ .
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From page 7... ...
A recent example Is an airborne survey over a strip mining region in Czechoslovakia that mapped the extent of uranium contamination in the local river system (Figure I.2~. Sampling Proper sample spacing is crucial for obtaining meaningful scientific results.
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In order to determine groundwater flow patterns, aircraft could carry instrumentation for both airborne magnetic capabilities and electromagnetic mapping. The airborne magnetic data would define the geologic structure, and the electromagnetic data would delineate the extent of saline groundwater.
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From page 10... ...
and marine gravity surveys (lines) of coastal Gabon.
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flown over Gabon in a 6 x 6 km grid. The sampling strategy is not limited by access to roads.
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The major drawback of using multi-instrumented aircraft, however, is the need to compromise individual measurement types. As noted above, the optimal altitude for collecting airborne gravity and magnetic measurements may differ.
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From page 13... ...
The accuracy required for navigation, positioning, and attitude varies among operations, but, in general, the exploitation of data obtained from airborne sensors increases with improved positioning and navigation capabilities. GPS is a powerful too} for determining aircraft positioning and navigation and is potentially useful for recovering orientation.
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From page 14... ...
. Carrier phase measurements are made by reconstructing the carrier signal, the fundamental frequencies of Ll (~.575 GHz)
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From page 15... ...
Differential carrier phase measurements are as accurate as 2 to 20 centimeters (cm) and generally require extensive postmission analysis, although real-time systems are under development (e.g., Frodge et al., 1994~.
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From page 16... ...
Other airborne tests have been conducted using inverse photogrammetry in which the camera positions are independently determined from aerotriangulation with ground control and then are compared to the estimated GPS positions (e.g., Cannon, 1991~. MEASURING THE EARTH'S GRAVITY HELD A striking example of the benefits of precise positioning of a research aircraft is the determination of the Earth's gravity field.
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From page 17... ...
Today, with the accumulation of tracking data from numerous satellites and with constantly improving tracking techniques, the Eagles gravity field can be derived from these data alone to wavelengths greater than about 500 to 600 km (Nerem et al., 1994~. Over the oceans, satellite altimetry yields measurements of the geoid with horizontal resolutions of tens of kilometers (Rapp and PavIis, 1990)
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From page 18... ...
Therefore, satellite altimetry is not the final answer to better marine gravity models. Gravity measurements made from a moving vehicle, such as an aircraft or ship, are contaminated by motion-induced and inertial accelerations due to the rotating coordinate frame, even when mounted on a stabilized platform.
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From page 19... ...
MEASURING THE EARTH'S SURFACE TOPOGRAPHY Measurements of surface elevations are fundamental to many applications of geology, geophysics, hydrology, terrestrial ecology, geomorphology, glaciology, and atmospheric physics Table I.3~. For example, the topography of the polar ice caps and mountain glaciers is important because it is a direct measure of ice flow dynamics and is closely linked to global climate and sea level change.
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x x FIGURE 1.4(a) Terrestrial gravity data distribution over Greenland as of 1992, reflecting more than 30 years of ground survey efforts.
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From page 21... ...
X; 21 FIGARO 1.4(b) Airborne gravity data distribution from the Greenland Aerogeophysics Project, flown during the summers of 1991 and 1992 by the Naval Research Laboratory and the Naval Oceanographic Once in collaboration with NOAA and the Danish National Survey and Cadastre.
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From page 22... ...
(jr) Geology/Geophys~cs Plate Boundaries and Intraplate Deformation Large-scale structures Rifts Diffuse extension Mountains Land Geology/Fault Zone Tectonics Mapping Surface structure Neotectonics 10 20 10 10 1 ,000 2,000 1,000 500 30 1 10 100 5-20 Volcanology Flow and ash volumes 0.5-3 30-100 3 Volcano morphology 2-10 30-500 Volcano dynamics 0.15-1 30-100 1 Marine GeologylGeophysics Topography prediction from geoid 0.1 3,000 Gravity/Magnetics Admittances-gravity 10 10,000 Terrain correction-gravity 1 200 Satellite gravity 3 1,000 Magnetics 10 200 Polar Science Basic Inventory Large-scale features rice domes, divides, streams, ice shelves, drainage basins)
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From page 24... ...
VERY HIGH RESOLUTION STUDIES WITH AIRBORNE TECHNIQUES Land-based gravity measurements have long been used to identify the salt domes and other structural traps for of} along the Gulf Coast and the large mineral deposits in the western United States. As the easily identified reservoirs and mineral deposits become depleted, however, exploration industries will require increasingly detailed information on the subsurface of the Earth.
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From page 25... ...
Free Air Gravity 0 10 20 30 40 50 km 25 FIGI}RE 1.6 Evidence for active volcanism beneath the West Antarctic Ice Sheet from precise surface altimetry measurements and other airborne geophysical observations. The measurements were made as part of a major study of the stability mechanisms of the West Antarctic ice sheet and were collected along a north-south profile.
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\\ ~ ~N 1~ 0 400 FIGURE 1.7(a) Map showing waste sites and buildings in the survey area, Oak Ridge National Laboratory.
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%4967 27 >10.0 4 0-10.0 3.0-4.0 20-3.0 1.0-2.0 0.0-1.0 s.o_o.o 2s700 FIGURE 1.7(b) Short-wavelength magnetic anomalies (vertical component)
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From page 28... ...
These high-resolution applications can be addressed by airborne geophysical technology and precise positioning. The ability of airborne techniques to characterize waste sites safely and rapidly was recently demonstrated at Oak Ridge National Laboratory (Doll et al., 1993~.
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