Scintrex CG-6 AUTOGRAV gravity meter,

Waiver Rationale Summary

The National Geodetic Survey (NGS) requires the ability to measure terrestrial, relative gravity quickly and accurately. Two current projects employing relative gravity meters include Gravity for the Redefinition of the American Vertical Datum (GRAV-D) and the Geoid Monitoring Service (GeMS). The GRAV-D project is collecting airborne gravity data to produce a one-centimeter geoid in order to modernize the nation’s vertical datum. For each airborne gravity survey, an absolute gravity tie at the airport must be established. This involves measuring with an absolute gravimeter, then using a terrestrial relative gravimeter to calculate a vertical gravity gradient to transfer the value to the height of the relative gravimeter in the aircraft. The gradient is determined through a three tiered measurement. Second, terrestrial gravity measurements are needed to support the GeMS project, by measuring local, high precision gravity data for specific areas as needed: acquiring gravity observations in difficult to reach areas, and/or where portability is a major consideration. Currently NGS has a number of terrestrial relative gravity meters: its five LaCoste and Romberg g-meter instruments obtained in the 1960s and 1970s are now considered obsolete. NGS also owns a very early model Scintrex CG-6, and two newer production model CG-6’s. Compared with the older gmeter instruments, the CG-6 typically obtains the required measurement in one fifth of the total measurement time. The CG-6 has proved to be an excellent instrument, and NGS has changed its data acquisition procedures to take advantage of its unique characteristics (no offsets, small drift, extremely quick set up). NGS requires precise flight data, that is, a measurement and conversion of airborne gravity into the absolute gravity value. In order to provide NGS the ability to efficiently collect high quality terrestrial gravity data, NOAA is purchasing an additional production model CG-6 gravity meter that would allow for both simultaneous high-quality observations, as well as provide a contingency backup for observations in extremely remote locations (such as remote islands). Scintrex/Micro-g LaCoste is the only manufacturer of a quartz-based (non-clamping) relative gravimeter, a system proven to have negligible offsets between measurements. Further, the CG-6 is paired with a corresponding gradient tripod system that allows for a single leveling set up, saving valuable time in the field. In addition, the operation of the gravity is nearly fully automated. No comparable instrument is available from other vendors. The only portable and relative terrestrial gravimeter on the market that meets NGS’s requirements is the Canadian manufactured Scintrex CG-6 Autogravtm Survey Gravity Meter, which due to its patented fused quartz sensor, is capable of measuring relative gravity differences to 5 x 10-8 m/s2 (5 μGal). The Scintrex CG-6 has a precision that is two to four times higher than other similar US manufactured products, providing better quality measurements and reliability.