Airborne Gravity

The Fugro airborne gravity system arose from a joint effort by three companies, including a leading U.S. defense contractor specializing in advanced GPS applications. The development team incorporated both geophysicists and geodesists, led by Dr. J. (Christopher) Harrison. Dr. Harrison, was a lead scientist, along with Dr. Lucien LaCoste in the world’s first non-military airborne gravity tests, conducted in a B17 in 1959¹. Additionally Drs Harrison and LaCoste pioneered research in the understanding of cross-coupling effects in dynamic gravimetry measurements². Many important aspects of Dr. Harrison’s extensive dynamic gravity knowledge has been incorporated in the design of the Fugro airborne gravity system.

Flight-testing included three separate campaigns, flown over an industry sponsored gravity test range in Texas. In addition, Fugro was contracted to fly a gravity test range in Pennsylvania. Tens of thousands of line kilometers of data, flown in typical real world exploration survey conditions (rather than waiting for ideal weather for flying) were collected, processed and analyzed over a period of four years prior to Fugro conducting its first commercial airborne gravity survey in 1995.

Safety is the paramount factor in Fugro’s airborne gravity operations. Fugro is a founding member of the International Airborne Geophysics Safety Association (IAGSA). In addition to IAGSA guidelines Fugro utilizes it’s own comprehensive safety procedures, each project is analyzed and a full job safety plan activated. Fugro monitors all it’s aircraft worldwide via the Inmarsat-C satellite service.

System Hardware

Fugro uses a LaCoste & Romberg (L&R) dynamic gravity meter upgraded to the ZLS UltraSys™ system. The ZLS model is a modern digital control upgrade of the widely tested and proven LaCoste and Romberg (L&R) dynamic gravity meter sensor. Fugro’s airborne gravity system only uses L&R sensors with the latest generation air dampers (serial number S-88 and higher), which minimize cross-coupling effects.

A unique three-stage vibration isolation platform was developed as a result of extensive vibration testing at the Boyd More Research Laboratory. Fugro’s testing revealed a wide range of significant vibration effects on the gravity meter sensor, at both long and short periods, which have been eliminated by careful attention to platform mounting.

The gravity meter clock is synchronized to GPS time at the beginning of each flight, and kept in synchronization (equivalent of 1 millisecond drift in 32 years) by an extremely stable Rubidium oscillator. The large amplitudes and short periods of aircraft vertical accelerations make tight synchronization of gravity and GPS data a necessity.

GPS data is collected using geodetic quality dual frequency receivers. The system logs full carrier phase (required for maximum accuracy), dual-frequency (required for ionospheric effect removal) raw GPS data for up to nine satellites.