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.     

Geology of ascension Island, South Atlantic Ocean

Ascension Island is the exposed part of a large volcano located about 80 km west of the Mid-Atlantic Ridge axis in the notrhern South Atlantic Ocean. The volcanic rocks of the island form an alkaline suite with a compositional range of basalt-hawaiite-mugearite-benmoreite-trachyte-rhyolite. Trachyte and rhyolite compose approximately 14% of surface exposures. Petrochemical studies by other investigators have shown that the igneous suite was formed by fractional cyrstallization. The abundance of felsic volcanic rocks and the presence of granite and syenite blocks in pyroclastic deposits suggest that felsic magma chambers have formed and could provide the heat for a convective hydrothermal system. The felsic volcanic rocks form two eruptive centers that may be contemporaneous. The oldest rocks exposed on the surface are rhyolite with K---Ar ages of approximately one million years. Faults mapped on the island have four principal trends and appear to be related to regional structures that are apparent on the bathymetry of the island pedestal.     

Drilling and evaluation of Ascension #1, a geothermal exploration well on Ascension Island, South Atlantic Ocean

A deep (3126 m) geothermal exploration well (Ascension #1) was drilled on Ascension Island in the South Atlantic Ocean as the culmination of an exploration program that began in 1982. Ascension #1 encountered several geothermal fluid entries below a depth of 2400 m, and had a bottomhole temperature approaching 250°C. However, the fluid flow rate was limited. While attempting to improve production by drilling a second leg, a mechanical failure resulted in loss of the well. An analysis of the geologic controls on fluid production suggests that fracture permeability is oriented to the northeast and often associated with felsic dikes. The system may be sealed by a thick sequence of hyaloclastites that are mechanically unable to sustain open fractures. The reservoir intersected by Ascension #1 apparently lacks the permeability required for commercial fluid production.