Drenth reported in 2013 that airborne gravity gradient (AGG) data are rapidly becoming standard components of geophysical mapping programs because of their advantages in cost, access, and resolution over measurements of the gravity field on the ground. Unlike conventional techniques that measure the
gravity field, AGG methods measure derivatives of the gravity field. This means that effects of terrain and near-surface geology are amplified in AGG data, and that proper terrain corrections are critically important for AGG data processing. However, terrain corrections require reasonable estimates of density for the rocks and sediments that make up the terrain.
A recommended philosophical approach is to use the terrain and surface geology, with their strong expression in AGG data, to the interpreter’s advantage. An example of such an approach is presented in this paper for an area with difficult ground access and little ground gravity data. Nettleton-style profiling
is used with AGG data to estimate the densities of the sand dune field and adjacent Precambrian rocks from the area of Great Sand Dunes National Park in southern Colorado. Processing of the AGG data using the density estimate for the dune field allows buried structures, including a hypothesized buried basement bench, to be mapped beneath the sand dunes.
Drenth concluded that airborne gravity gradient (AGG) data are especially well suited to mapping the effects of near-surface terrain and geology. Processing of AGG data is similar to processing of gravity data in that the density effects of near-surface rocks and/or sediments must be properly identified and removed. Where significant terrain exists, the effects of the terrain can be exploited to interpret the densities of near-surface geology.
An example of such an approach is Nettleton-style profiling to estimate terrain density, a method long used with gravity field data but even better suited for AGG data. Application of the method to high-resolution AGG data at GRSA, where ground-based gravity is impractical because of difficult access, yields an estimated density of dry dune-field sand of 1800 kg/m3, and an estimated density of 2700 kg/m3 for Precambrian rocks at the Sangre de Cristo Mountains range front. Further processing of the AGG data using the 1800 kg/m3 density of the dune field allows mapping of structures beneath the dune field. There, an AGG high reflects relatively dense rocks, with the simplest interpretation being a previously unknown buried bench of Precambrian rocks. From a
philosophical standpoint, careful consideration of both terrain and density (i.e., geology) is viewed as the best approach to AGG data interpretation in areas of high relief.
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