Electromagnetic induction in the south-west region of Nigeria is studied by comparing laboratory analogue model measurements with observations at eleven field sites. The scaled laboratory model simulates the ocean bathymetry and the coastlines in the region. Model magnetic fields for simulated periods of 20, 30, and 60 min for two mutually perpendicular polarizations of a uniform horizontal source field were used to obtain the in-phase and quadrature induction arrows. The model results show the typical coast effect as expected. The observed arrows show a complex and difficult to interpret behaviour. However, when the model arrows are vectorially subtracted from the observed arrows, the behaviour of the resultant difference arrows, can be related to induction effects associated with the Benue Trough and several of the north west trending shear zones.

Magnetic induction responses of elongated pairs and single conductors with strikes perpendicular to a local coastline of a deep ocean are studied with the aid of laboratory model measurements. The dependence on period, the width of the conductor, and the distance from the end of the conductors are examined for traverses over the conductors. It is shown that for the model geometry studied, the ocean induction effects can be subtracted to yield the Parkinson induction arrows for the conductor alone. At sites between pairs of parallel conductors the quadrature arrows compared with the in-phase arrows show a very differing period dependence. These results have particular application to the interpretation of geomagnetic measurements at sites between major elongated conductive structures such as sediment filled grabens.