Precision electromagnetic distance-measuring instrument for determining secular strain and fault movement

Abstract

A three-wavelength EDM instrument has been developed with accuracies better than 1 · 10 −7. With this instrument there is no need for the usual meteorological observations and corrections. Most current instruments are used in conjunction with a low-flying aircraft which flies near the line of sight at the time of the range measurement taking temperature and humidity readings for the determination of integrated air and water densities along the line. This EDM instrument simultaneously measures the optical distance at three wavelengths, two optical and one microwave. It then computes the correct distance, eliminating the first order effects of temperature, pressure and relative humidity fluctuations along the measured line. The optical sources are a red He-Ne laser (632.8 nm) and a blue He-Cd laser (441.6 nm). A 9.6-GHz microwave source is used for the third wavelength, and a small, general-purpose minicomputer for the control and computation. Field tests with this instrument show a standard deviation of less than 1 · 10 −7 on a 10.1-km line and less than 5 · 10 −8 on a 5.3-km line. Signal levels observed on the 5.3-km line indicate that a range exceeding 20 km is feasible with favorable atmospheric conditions.