Aeromagnetic survey and interpretation, Ascention Island, South Atlantic Ocean

A detailed aeromagnetic survey of Ascension Island, which was completed in February and March of 1983 as part of an evaluation of the geothermal potential of the island, is described. The aeromagnetic map represents a basic data set useful for the interpretation of subsurface geology. An in situ magnetic susceptibility survey was also carried out to assist in understanding the magnetic properties of Ascension rocks and to aid in the interpretation of the aeromagnetic data. The aeromagnetic survey was interpreted using a three-dimensional numerical modeling program that computes the net magnetic field of a large number of vertically sided prisms. Multiple source bodies of complex geometry were modeled and modified until a general agreement was achieved between the observed data and the computed results. The interpretation indicates northeast- and east-trending elongate bodies of much higher apparent susceptibility than adjacent rocks. The relationship to mapped geologic features such as volcanic vents, dikes and faults suggests that these magnetic sources are zones of increased dike density and of other mafic intrusives emplaced along structures that fed the many volcanic centers. A large magnetic source on the northeastern portion of the island may be the intrusive equivalent of trachyte lavas present at the surface. A low-magnetization area, mainly north and west of Green Mountain, appears to be the most likely area for the presence of a geothermal system at moderate (1–3 km) depth.