On numerical and data requirements for topographical reduction of airborne gravity in geoid determination and resource exploration

High accuracy of airborne gravity makes these data applicable for precise geoid determination. Moreover, their increasing spatial resolution results in a growing acceptance for their use in geophysical applications such as resource exploration. In both these applications, gravity must be reduced for topographical effects. Different methods for computation of the topographical effects on airborne gravity are applied in the two applications: the Helmert reduction is frequently used for geoid determination while the Bouguer reduction is usually preferred for resource exploration. This contribution considers some practical issues related to evaluation of topographical reduction of airborne gravity in manners that are consistent with accuracy and spatial resolution of airborne gravity data. Values of the two topographical effects at typical flying heights are computed using a high-resolution digital elevation model (DEM) along three profiles across the Canadian Rocky Mountains. Filters and spectral analysis techniques standard in airborne gravimetry are then used to quantify their behaviour as a function of spatial resolution for half wavelengths as short as 1 km. The impact of varying spatial resolution of the used DEM on the accuracy of the topographical effects can be quantified. The results also show the importance of the consistent treatment of airborne gravity and the corresponding topographical effects. To minimize the computation time of numerically demanding algorithms, errors of interpolation of the topographical effects to the flight trajectory are quantified in terms of spatial resolution. The results are used to formulate accurate and efficient methods for the computation of the topographical effects considering the two different applications intended for airborne gravity.