The mapping of induced currents around the Kenya Rift: a comparison of techniques

Summary. Single-station and inter-station transfer functions are derived from data recorded by a magnetometer array in and around the Kenya Rift Valley. Different methods of presentation of the transfer function estimates are compared in terms of their ability to define lateral variations in electrical conductivity. When, as in East Africa, the conductivity structure is complex and three-dimensional, by far the most useful method of presentation is to use the transfer functions to simulate the anomalous internal fields associated with regional current flow at a particular azimuth. The use of inter-station rather than single-station transfer functions is almost essential where the amplitudes of anomalous horizontal fields are of the same size or greater than the normal fields. Horizontal field transfer functions prove to be just as useful as those usually calculated for the vertical component, and provide strong additional constraints on the internal current system.
Maps of simulated vertical and horizontal fields of internal origin indicate the importance of current channelling around the Rift Valley. A conductor just to the east of Nairobi apparently funnels regionally induced currents into two conductors associated with the Rift and dome; one at shallow depths beneath the Rift floor, and a deeper body to the east.