Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
A Turnkey Airborne Gravity System – Concept to
reality
Introduction
Nigel Brady 1
1 Micro-g Lacoste Inc. (nigel@microglacoste.com)
Airborne gravity surveying has been performed with widely varying degrees of success since early
experimentation with the Lacoste and Romberg dynamic meter in the 1950s (Nettleton et al., 1960).
There are a number of different survey systems currently in operation including relative gravity meters
and gradiometers. However, there are presently a number of roadblocks to achieving high quality
results at a low cost, including market availability for those who wish to own and operate a survey
system.
Airborne gravity survey operations have historically had difficulty in producing the desired data quality
at the significant cost advantage claimed per line kilometre when compared with a ground based
survey. Airborne surveys represent an expensive investment in equipment and skilled personnel, and
survey operators must provide a reasonable return on that investment to stay viable. There are a
number of factors involved in realizing the potential that airborne gravity offers and if any one of them
is deficient the survey can quickly turn the cost benefit ratio negative.
Airborne operations are logistically complex as they require an airport, hangar and FBO (Fixed Base of
Operations) support. Survey operations normally involve at least, pilots, equipment operators, aircraft
maintenance and a geologist/geophysicist for planning and data quality control. Equipment
maintenance and repair downtime can add significant time and cost to the survey. The data quality
itself can be highly reliant on the operator and data processor’s experience and skill levels as well as
the flight characteristics of the aircraft. Finally the data is of much lower value without a very accurate
positioning and timing system.
In 2004, Lacoste and Romberg (now Micro-g Lacoste) decided to build on their success with the newly
developed AirSea II dynamic meter and use that system as the basis for a dedicated airborne
gravity instrument. The Air Sea II was designed as a marine gravity meter but can be adapted to run
in an airborne configuration. Advances in electronics, timing and positioning technology created the
opportunity to refine both the hardware and software, and to develop a truly turnkey system that would
work well for users with little or no airborne gravity experience as well as those with more extensive
experience.
The following goals were set for the new system to ensure that it would meet or exceed the current
requirements of airborne gravity surveying:
� portability and ease of install and setup,
� reliability and ease of operation,
� a repeat line accuracy of 1 milligal or better with an anomaly wavelength resolution of 5 km or
less,
� develop a complete understanding of the system behaviour that allows corrections rather than
filtering to be used for data reduction, and
� provide for fast data processing in the field by an operator for quality control of daily survey
data.
Hardware Solutions
The AirSea II gravity meter (Figure 1) has been shown to have an accuracy at sea of better than
1 milligal in reasonable sea conditions (i.e., 30,000 – 50,000 milligals of vertical acceleration or less).
However, an airborne installation has an inertial acceleration environment that is significantly different
to the low frequency and symmetric accelerations experienced at sea. Air turbulence experienced in
28