Expression of terrain and surface geology in high-resolution helicopter-borne gravity gradient (AGG) data: Examples from Great Sand Dunes National Park, Rio Grande Rift, Colorado

Abstract

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 here 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 dunefield 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 dunefield allows buried structures, including a hypothesized buried basement bench, to be mapped beneath the sand dunes.

Geophysical expression of buried range-front embayment structure: Great Sand Dunes National Park, Rio Grande rift, Colorado

“…4B and 4D), interpreted to reflect a concealed structural high (DSH on Fig. 4D) (Drenth, 2013). Another presumed structural high is imaged in the vicinity of Sand Creek, northwest of a structural low between the two highs (SCSH on Fig.…”

“…A density of 1800 kg/m 3 , a value found by Nettleton profiling (Nettleton, 1942) to be appropriate for unsaturated dune sand (Drenth, 2013), was used to compute the terrain-corrected Gzz field (Figs. 4A and 5A).…”

“…The magnetic susceptibilities assigned to the Proterozoic rocks and inferred plutons are only semiquantitative and may not be a good approximation of true susceptibilities. The Proterozoic rocks have an estimated density of 2700 kg/m 3 based on Nettleton profiling (Drenth, 2013), and this density is used for the Proterozoic rocks in all models. The same density is applied to the inferred strongly magnetized plutons.…”

“…Its bulk magnetic susceptibility was estimated at 0.004-0.006 (SI units) from modeling the vertical derivative of the RTP aeromagnetic data over the dune field (modeling not shown). The density of unsaturated dune sand was estimated at 1800 kg/m 3 by Nettleton profiling (Nettleton, 1942;Drenth, 2013). The estimated density of saturated dune sand, 2080 kg/m 3 , is from a shallow density log near the western portion of the dunefield (Grauch et al, 2015).…”

Airborne gravity gradient expression of a buried niobium and rare earth element deposit: Elk Creek carbonatite, Nebraska

“…An integrated geologic interpretation of AGG, aeromagnetic, and borehole datasets has recently been presented (Drenth, 2013a;Drenth, 2014). This presentation focuses on details of the AGG data interpretation workflow, specifically those largely left out or glossed over in the other works.…”