The Geophysics/Surveying Unit uses two Scintrex CG-3M and one CG-5 Autograv gravimeters to perform gravity surveys. The CG-3M and the CG-5 are a fully automated relative microgravity meter with a reading resolution of 0.001 mgal (milligals). They are lightweight, portable and easy to use.
Unlike absolute gravity meters, which directly measure the acceleration of gravity, relative meters measure only gravity differences between stations. Gravity values are obtained at unknown stations by first measuring a station where the absolute gravity value is known, and then measuring the unknown point. This is called a gravity ‘tie.’ Relative gravity meters are used because they are easier and faster to use in a fieldwork setting than absolute gravity meters.
Microgravity survey at a Phoenix AMA transducer
The Autograv uses a sensing element based on a fused quartz elastic system. An electrostatic restoring force and a spring are used to balance the gravitational force on the proof mass. Changes in gravity alter the position of the mass. DC voltage is applied to capacitor plates to produce an electrostatic force on the mass, restoring it to a null position. This feedback voltage is a measure of the relative value of gravity.