Abstract
A highly accurate two-color interferometer with automatic correction of the refractive index of air was developed for crustal strain observation. The two-color interferometer, which can measure a geometrical distance of approximately 70 m, with a relative resolution of 2 × 10−9, clearly detected a change in strain due to earth tides in spite of optical measurement in air. Moreover, a large strain quake due to an earthquake could be observed without disturbing the measurement. We demonstrated the advantages of the two-color interferometer in air for geodetic observation.
Highlights
For geophysical observations, various methods are used to measure crustal deformation, such as global positioning system (GPS), interferometric synthetic aperture radar (InSAR), satellite laser ranging (SLR), electronic distance meter (EDM), and very long baseline interferometry (VLBI) [1,2,3,4,5].In addition to these methods, long-baseline strainmeters have been used in more sensitive geodetic and seismic observations to measure the distance between two Technologies 2014, 2 reference points on the basis of a stable rod, fiber-optic strain sensor or a wavelength of laser light as a measurement reference [6,7,8,9]
As the absolute value of the refractive index of air and its relative change with respect to the environmental parameters depend on the wavelength of light, an interferometer using two light sources of different wavelengths can measure a geometrical distance without measuring the environmental parameters of the air
The wavelength of the light and the temperature, pressure, humidity and carbon dioxide density of the air are denoted by λ, T, P, h, and xc, respectively, and the refractive index of air is denoted by n(λ, T, P, h, xc), which is expressed by the following functional formula: n λ, T, P, h, xc 1 naxs λ 1 F T, P, h, xc nws λ 1 W T, P, h, xc where naxs(λ) is a function of the refractive index for standard air, whose parameters are T = 15 °C, P = 101,325 Pa, h = 0% and xc = 450 ppm
Summary
Various methods are used to measure crustal deformation, such as global positioning system (GPS), interferometric synthetic aperture radar (InSAR), satellite laser ranging (SLR), electronic distance meter (EDM), and very long baseline interferometry (VLBI) [1,2,3,4,5]. As the absolute value of the refractive index of air and its relative change with respect to the environmental parameters depend on the wavelength of light, an interferometer using two light sources of different wavelengths can measure a geometrical distance without measuring the environmental parameters of the air. This interferometer is called a “two-color interferometer”, and many studies have been performed on such interferometers [13,14].
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