Crossing the borders: from plains to rugged terrains

Abstract

We often assume that the Earth is flat and that all geophysical surveys are conducted on a flat survey plane, just as many geophysical text books suggest. However the surface of the Earth is often rugged, and the effect of this rugged surface depends directly on the height of the survey above the ground. Mountainous areas have always provided a challenge for both airborne and ground surveys, and for the processing and interpretation of the survey data. For example, airborne surveys in these terrains suffer variable terrain clearance between adjacent survey lines as the aircraft attempts to 'hug' the terrain. This combined with the undulating survey surface, causes data levelling problems and distorted anomalies that often lead to erroneous interpretations of the survey data. Problems associated with airborne surveys conducted in rugged terrains are, in recent times, becoming evident in the low-level surveys now being conducted with agricultural aircraft over seemingly undulating terrains. The close proximity of the survey aircraft to the gently undulating terrain produces the same terrain induced problems as experienced in mountainous terrains. All ground geophysical methods are strongly affected by rugged terrain. For these surveys, it is often possible to use the rugged terrain to advantage by obtaining multiple 'views' of a buried source by surveying several slopes of the terrain around the target source. In addition, EM loops and galvanic electrodes can also be judiciously located on the terrain to obtain a more accurate definition of a buried anomalous source. Explorers are also demanding more accurate interpretations of data from rugged terrains in order to resolve target responses from the distorted responses measured in these terrains. To this extent modern 3-dimensional computer modelling software can now accommodate survey data from an undulating survey surface and the geophysical responses of geologically complex undulating terrains can now be computed. Rugged terrains present new challenges to survey design, survey operations, data processing and interpretation, and modelling of the survey data. As a consequence of this there is now an urgent requirement for high resolution digital terrain models for survey design and for accurate reduction, interpretation and modelling of all geophysical survey data. Techniques developed for rugged terrain environments can also be applied to underground and open-pit mine environments.