Field absolute gravimeter for high-accuracy measurement of free fall acceleration

Seroglazov P.V., Bunin I.A., Nosov D.A., Sizikov I.S.

Institute of Automation and Electrometry sb ras, Novosibirsk, Russia

flameeffect.nsk@gmail.com

This report presents the description of a new absolute laser ballistic gravimeter GABL-P, designed in Institute of Automation and Electrometry SB RAS. The instrument allows true field operation in harsh field conditions on open outdoor sites and differs the small size and weight. As source of the light in interferometer of the gravimeters aplying optical standard with wavelength of the radiation =532 nm, consisting of Nd:YAG - a laser of the runninging wave with intracavity by reduplication of the frequency of the radiation and systems to stabilizations on resonance of the saturated absorption in molecular iodine. Instrumental mean square error of the measurement of the gravimeter of absolute value of the gravity does not exceed  5∙10^-8 m/s² (5 µGal).

The operating principle of the ballistic gravimeter is based on measuring the times Ti during which a free falling body travels specified paths S_i (i = 1,...,N). The value of g calculated from the measurements of S_i and T_i is the acceleration due to gravity. To determine g with a small error (about a few mikrogals, 1 Gal = 10⁻² m/s²), the paths traversed by a freely falling body need to be measured, at least, to within nanometers, and the corresponding times to within a few tenths of a nanosecond. The problem of precision measurement of path is solved by interferometric methods. The use of an interferometer with an optical length standard based on a stabilized laser allows one to solve this problem since in this case the laser wavelength serves as a measure of path. The relative stability and reproducibility of the frequency of the length standard used here and considered in detail below is not worse than 2∙10⁻¹⁰. The measure of time intervals are signals of a precision (e.g., rubidium) standard. An AR-40A commercial rubidium standard provides stable reproduction of frequency of up to 5∙10⁻¹⁰. Thus, the device incorporates length and time standards, which ensures its high metrological characteristics.

The field tests of the GABL-P gravimeter were performed in 2009 in the Gornyi Altai (Altai Mountains) and the Zapolyar’e (Subarctic) under substantially different environmental conditions. The measurements in summer in the Altai were carried out at high air temperature of +29 to +31˚ C and a reduced atmospheric pressure of 650–655 mm Hg. In the Arctic, the air temperature varied within 0 to +10˚ C and the atmospheric pressure within 760–770 mm Hg. In the tests, the device was powered from a 12 V car battery through an inverter, from a 220 V gasoline-powered generator, and an external power supply of 220 V (if present at the measurement point). The device was housed in a common tourist tent. During the tests, the remote control of all with gravimeter units with a portable computer was tested in two modes: across the wire and wireless.