Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Terrain correction and its effect on 3D inversion of
airborne gravity gradiometry data
Yaoguo Li 1 , M. Andy Kass 2 , Kristofer Davis 3 , Marco Braga 4 , and Cericia Martinez 5
1 Gravity and Magnetics Research Consortium, Department of Geophysics, Colorado School
of Mines (ygli@mines.edu)
2 Gravity and Magnetics Research Consortium, Department of Geophysics, Colorado School
of Mines
3 Gravity and Magnetics Research Consortium, Department of Geophysics, Colorado School
of Mines
4 Iron Ore Division, Vale
5 Gravity and Magnetics Research Consortium, Department of Geophysics, Colorado School
of Mines
Abstract
The terrain effect generally dominates airborne gravity gradiometer measurements, and this issue
must be dealt with as part of the interpretation process. Terrain corrections must be calculated in an
accurate and consistent manner. To accomplish this, the required terrain resolution and the effect of
low-pass filtering of the data in acquisition and processing must be understood. Furthermore, efficient
computation of terrain effect within a reasonable time frame is required to process and interpret largescale
data sets. In this paper, we will examine the issues and develop practical algorithms for
addressing them. We will illustrate the discussion using both synthetic and field examples, including a
set of airborne gravity gradiometry data acquired for iron exploration.
Introduction
The terrain effect is usually the dominant component in airborne gravity gradiometer (AGG) data. It
manifests itself as a high amplitude signal over a wide range of scales or wavenumber bands. A
reliable method to remove the terrain effect is a pre-requisite for any interpretation technique in AGG
work. As a result, the quality of terrain correction and the efficiency when calculating the correction
directly determines the applicability and quality of each of the plethora of available interpretation tools.
Two particular aspects of terrain correction are crucial. We examine these and demonstrate their effect
on quantitative interpretation using a generalized 3D inversion algorithm.
The first aspect is the practical issue of the efficiency of the terrain calculation process itself. We have
developed a space-domain based fast algorithm that uses an adaptive mesh representation of a digital
elevation model (DEM) and incorporates a user-specified error tolerance for the calculated terrain
effect. The algorithm can achieve significant computational speed-up over conventional algorithms.
This efficiency enables routine calculation of, and reliable correction for, the terrain effect using the
highest DEM resolution dictated by the terrain clearance and the acquisition system filters.
The second aspect is the importance of frequency matching in terrain correction and the
corresponding requirements on the resolution of DEM. This issue stems from the fact that acquisition
systems necessarily employ a set of low-pass filters in order to reduce high-frequency noise. A
consequence of applying a low pass filter is the modification of the geological and terrain response in
the same high frequency band. We examine the importance of matching the frequency content of the
terrain corrections to an estimate of the low-pass filtering, and then quantify the optimal DEM
resolution that meets the requirement for reliable terrain correction whilst allowing an efficient
calculation process to be applied.
In the following, we will first discuss an efficient terrain correction algorithm that is based on adaptive
quadtree disretization of a DEM. This approach can accelerate the calculation by up to two orders of
magnitude in comparison with a brute force approach. We then discuss terrain resolution and the
effect of low-pass filtering used in acquisition systems. We conclude with an example of quantitative
131