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Free air laser strain meter (a feasibility study of a new method of observing tectonic motions) Cannon, Wayne

Abstract

Theoretical investigation into the feasibility of making laser strain meter measurements over large distances through the uncontrolled atmosphere indicate that such observations are likely possible with present day technology. The observing device, here-in called a free air laser strain meter, would use laser output at three frequencies, appropriately spaced in the spectrum, coupled with the dispersion of the atmosphere to separate "geometrical" fluctuations from "refractive" fluctuations of the optical path length between the end mirrors of the strain meter. The geometrical fluctuations can be averaged to reveal tectonic changes in distance between the end mirrors. Theoretical expressions for averaging times, confidence limits, strain sensitivity, accuracy of observations, fringe count rate, fringe visibility, aperture size and laser power are derived in terms of the relevant physical, geophysical, and atmospheric parameters. Free air laser strain meters, like conventional laser strain meters, appear to be capable of measuring earth strain over distances of several Kms. up to the limit of frequency stability of the laser in the presence of moderate atmospheric turbulence. In the free air laser strain meter the laser frequency stability and not atmospheric effects is the ultimate factor limiting strain sensitivity. Over distances of several kilometers free air laser strain meters are theoretically able to make strain measurements more sensitively, ( δD/D ~ 10⁻¹¹ - 10⁻¹² ) , and more rapidly, ( observation interval Δτ ~ 10 minutes - 1 hour ) , than any other device, known or proposed, operating through the uncontrolled atmosphere. The successful operation of free air laser strain meters would represent an improvement over present methods by a factor of 10³ - 10⁴ in sensitivity of observation. The realization of free air laser strain meters would provide geophysicists in the fields of geodesy and tectonics with a tool of unusual capabilities and as such would be recognized as a major advance.

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