Measuring gravity gradient in rugged terrain: HeliFalcon survey in Aso-Oguni, Japan
Since the first Falcon AGG survey in 1999 (van Leuwen et al., 2000), fixed-wing airborne gravity gradiometry (AGG) has proved to be a valuable tool for mining and oil and gas explora¬tion (Dransfield, 2007). The more compact digital Falcon AGG allowed the system to be installed in a helicopter and helicopter AGG surveys began in 2006 (Boggs et al., 2007). Compared with fixed-wing aircraft, the helicopter platform can fly surveys low and slow, offering superior spatial resolution as well as an improved signal-to-noise ratio (Dransfield, 2007). The superior spatial resolution and improved signal-to-noise ratio provided by helicopter AGG enhances its capability to detect smaller targets and better delineate subtler features. Dransfield and Christensen (2013) reported a HeliFalcon performance of 6 Eö RMS at 45 m resolution in vertical gravity gradient, by far the finest spatial resolution of any airborne AGG system. Another advantage afforded by helicopter AGG is its capa¬bility to follow terrains more closely especially in areas of high relief. Christensen and Hodges (2013) compared the vertical grav¬ity gradient from HeliFalcon with the simulated fixed-wing result over the Iron Range survey in the Canadian Rocky Mountains where the terrain variation reaches 1900 m. They showed that in this terrain, a fixed-wing survey would have to fly at ground clearances of more than 1000 m for much of the survey area. This high ground clearance would have greatly suppressed the signal at short wavelengths and degraded the spatial resolution. In this paper, we look at the HeliFalcon AGG survey conducted in the Aso-Oguni area exploring for geothermal fields, examin the technical challenges presented by the rugged terrains, describe some aspects of the data acquisition and processing, and present the survey results.