Abstract
Modern technological advances notwithstanding, volcano geodesy rests on a solid foundation of time-tested and proven techniques inherited from the exacting and relatively staid discipline of classical geodesy. From time immemorial, wanderers on land and sea have needed to know where they were and where they were going. The early tools they developed to locate and navigate themselves evolved into the surveyor’s chain, transit, and various derivatives that still have a place in the volcanologist’s tool kit. A noteworthy difference is that, whereas traditional geodesists generally concern themselves with fixing the locations of stable points on the ground, volcano geodesists are energized by where and how fast their points are moving!
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References
Jean-Dominique Cassini s given name was Giovanni Domenico, but he also used Gian Domenico and, later, Jean-Dominique. He was the first of the famous Cassini family of astronomers, and as such he is also known as Cassini I.
Traverse is a method of surveying in which lengths and directions of lines between points on the Earth are obtained by or from field measurements, and used in determining positions of the points. A first-order traverse is one that extends between adjusted positions of other first-order control surveys and conforms to the current specifications of first-order traverse. For surveys in the U.S.A., see Federal Geodetic Control Committee (1984); similar documents exist to guide geodetic surveys in other countries (e.g., New Zealand, Bevin (2003); Australia, ICSM (2000); Canada, Surveys and Mapping Branch (1978)).
Network configurations comprising a set of closed geometric figures with common sides (e.g., a chain of triangles, braced quadrilaterals, centered figures, double-centered figures, or some combination thereof) are favored, because they provide the redundancy required for geodetic adjustment (Bomford, 1980, pp. 3–6). Such configurations are seldom realized at volcanoes, however, for reasons of difficult access or lack of station intervisibility. The latter difficulty can be avoided by observing the network with GPS (Chapter 4).
The term dry tilt was coined at the USGS HVO in the late 1960s to distinguish the technique from the wettilt method, in which water and air hoses were used to connect brass containers (pots) attached to separate concrete piers, thus forming a long-base watertube tiltmeter (Eaton, 1959; Yamashita, 1992). The air hoses assured equal pressure between pots. Micrometers were used to measure the height of water in the pots simultaneously, so elevation differences between piers could be calculated precisely. Wet tilt measurements are no longer made so the term dry tilt is obsolete and its origin is becoming obscure, but it still appears in the volcanological literature.
Davis (1986) showed that an ellipsoidal source produces a better fit to surface deformation data at Kīlauea than a spherical source. Subsequent studies support the idea that Kīlauea’s summit reservoir is elongated vertically, with an eccentricity of about 2. The difference in Δg/Δh for spherical and ellipsoidal models is small, and the spherical model is mathematically simpler and more generally useful. For these reasons, the spherical model is used here.
Jachens and Eaton (1980) and Dzurisin et al. (1980) envisioned that incomplete collapse occurred when magma drained out of fractures comprising the summit magma reservoir, thus creating voids and lowering the reservoir’s bulk density. Subsequently, Johnson et al. (2000) called attention to the importance of magma compressibility, which suggests a more plausible scenario in which reservoir magma decompressed as a result of partial draining, thus producing the same effect. The remainder of this discussion is couched in these terms for brevity, but Johnson etal. (2000) are credited with revising my thinking about the physics of the process.
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© 2007 Praxis Publishing Ltd, Chichester, UK
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Dzurisin, D. (2007). Classical surveying techniques. In: Volcano Deformation. Springer Praxis Books. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-49302-0_2
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DOI: https://doi.org/10.1007/978-3-540-49302-0_2
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