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Geodesy in the Year 2000 (1990) / Chapter Skim
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Seafloor Geodesy by the Year 2000
Pages 100-113

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From page 100...
... Seafloor geodynamic problems are such that first generation systems would be useful if they could produce accuracies of a few centimeters for measurements of horizontal or vertical position change, strain measurements of a few parts in a million, and tilt determinations in the 100
From page 101...
... At least five differences between subaerial and subsea circumstances drive the design and development of seafloor geodetic systems (NAS, 1983~: Electromagnetic radiation at frequencies high enough to be "useful is highly absorbed in the sea because of the electrical conductivity of seawater. The only useful range is the optical one, but even there, one's capabilities are limited to tens, or perhaps a few hundred meters.
From page 102...
... Borehole oriented approaches using strain or tilt measuring devices installed in deep-sea drilling program holes should have no problem. Similarly, well-logging methods used to detect changes in the crosssectional shapes of holes or to document orientation of breakouts in order to infer the nature of stresses in the uppermost parts of the crust should operate as well at sea as on land once there are established systems for doing wireline re-entry into deep-sea drill holes (Langseth & Spiess, 1987~.
From page 103...
... Since the sound velocity field must be determined experimentally by moving one, or perhaps a few j point measuring instruments through it, there can be dynamic situations in which the spatial variations of water temperature and salinity can vary rapidly enough with time that the sound velocity averaged over long travel paths may not, in a practical sense, be knowable to the same accuracy as the individual measurements. Particularly in shallow coastal waters the effective accuracy may not be better than a part in 103, while in the open ocean, paths including near surface water may only be good to a part in 104.
From page 104...
... A large number of observations are then used to determine the internal geometry of the transponder array, with the advantage of averaging over acoustic paths traversing differing portions of the area and thus averaging over the time-varying aspects of the sound velocity field. Computer simulations of this type of system, using sets of 300 observations distributed through a four transponder array having a 2 km radius resulted in baseline length errors of 1 to 2 cm-when errors having a Gaussian distribution with 10 cm standard deviation were inserted into the range data (Spiels, 1985a)
From page 105...
... Conventional commercially available transponders capable of operating over km ranges operate by having a circuit that recognizes an incoming pulse and then transmits a pulse at some other frequency. This process, for systems operating in the 5 to 20 kHz regime, introduces timing uncertainties equivalent to a meter or more uncertainty in range (NAS, 1983~.
From page 106...
... Both of these are pushing a major technological advance - the use of fiber optic information transmission links built into the long main strain cables that support the ''garage" unit from which the neutrally buoyant work vehicle operates. The high data rates expected from these telemetry systems will allow operation of full bandwidth television systems for viewing the vehicle's surroundings and the tasks they are carrying out.
From page 107...
... Localized networks utilizing seafloor strain measurements and short range direct path acoustic systems could have their initial trials within the borderland itself. Some sites should be shallow enough that decreasing temperature with depth would counter increasing pressure to produce sound velocity gradients leading to downward, rather than upward, refraction.
From page 108...
... coast and the East Pacific Rise at 13° N If it were not for a restrictive weather window, the Juan de Fuca site would be a very desirable one; however, operations there with our normal research ships in support of development of new approaches would be limited to the summer months with resulting constraints on coordinating ship schedules with other research activities.
From page 109...
... The gradual development of our awareness of the many facets of plate tectonics, and the fact that nearly all plate boundaries lie in the ocean, give considerable emphasis to the desirability of including research in ocean floor geodesy within the NSF Ocean Sciences purview. A problem in this is that, while de facto NSF provides continuous support for various aspects of ocean science, the general pattern for administering its ocean floor research grants (except deep-sea drilling)
From page 110...
... llQ The technology discussed in Section II is quite diverse and its development has been carried out by a larger number of contributors than even the list of references implies. Ihe authors geodedy-oriented activities have been supported primarily by NASA, but with small inputs from the Office of Naval Research, NQAA, and NSF.
From page 111...
... H Jordan, Vector Constraints on Quaternary Deformation of the Western United States East and West of the San Andreas Fault in Tectonics and Sedimentation Along the California Margin edited by J
From page 112...
... R., Definition of the Plate Boundary Along the East Pacific Rise Off Mexico Marine Geodesy 4 29-43 1980.
From page 113...
... G Larson, Measurements of the_Sensitivities and Drift of Digiquartz Pressure Sensors, Deep-Sea Research, 29, No.


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