The detection of lateral anisotropy of the ocean floor by electromagnetic methods

Abstract

SUMMARY In a typical marine controlled-source electromagnetic (CSEM) experiment, one or more self-contained receivers capable of measuring small variations in the electromagnetic field are located in an array on the sea-floor. A remote horizontal electric dipole is towed close to the sea-floor by a surface vessel. Variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers. The variations contain information about the distribution of electrical conductivity of the subsurface rocks. The electrical conductivity is related to such critical physical parameters as porosity, temperature and fluid content. Published studies of the response of the sea-floor to electromagnetic excitation have not included any systematic variation in the sea-floor conductivity with horizontal direction. A horizontally isotropic oceanic lithosphere is not an appropriate assumption particularly in the vicinity of the ridge. Here, for example in the near-surface layers, basalts are extruded onto the sea-floor within the very narrow, fissured, neo-volcanic zone. The fissures may be filled with sea water and sediment which are better conductors than typical basalt. Alternatively, as time progresses, they may become sealed with hydrothermal mineral deposits which can be less conductive than the host rocks. In both instances, the electrical conductivity of the composite material in the direction oriented along the fisures parallel to the ridge will be greater than that in the transverse direction. Measures of anisotropy on sea-floor of varying age are an indication of the history of the tectonic activity at the ridge. The electromagnetic effects of lateral anisotropy are examined through the study of the response of a simple double half-space model. An upper half-space representing sea water is connected to a lower half-space representing anisotropic crustal material. A general theory describing the excitation of this model by any transient electromagnetic source is developed and the step-on responses for systems composed of a horizontal electric dipole transmitter and horizontal magnetic and electric field receivers are computed. The results of previous work indicate that following a change in current in the transmitter, two distinct changes in horizontal electric field strength separated in time are observed at the receiver. The first is caused by the diffusion of the electromagnetic field through the relatively resistive sea-floor, the second is caused by diffusion through the relatively conductive sea water. The times at which the two events occur are robust measures of the isotropic conductivity of the sea-floor and the sea water respectively. The results of this study show that shape and time of the initial change in the electric field varies systematically with both the average conductivity and the anisotropy of the sea-floor. For some orientations of the transmitter and the receiver, the effect of increasing the anisotropy is similar to the effect of increasing the average conductivity. The initial event is delayed. However, there are orientations for which increasing the anisotropy actually results in an earlier arrival. Natural Science and Engineering Research Council in Canada.