Electromagnetic Induction in South-West Nigeria: Analogue Model and Field Results.

Abstract

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.

EM Induction in Elongated Conductors Normal to a Coastline with Application to Geomagnetic Measurements in Nigeria

“…It was demonstrated that even for such major conductors the ocean responses could be subtracted successfully, and that the behaviour of the difference arrows was highly period and location dependent, with the quadrature and in-phase components at given periods differing from each other considerably from site to site. Figure 8 shows the model and the field site in-phase and quadrature induction arrows at 20 min at eleven sites in Nigeria discussed previously by Kang et al (1993). The solid and dashed arrows in Fig.…”

“…The sites that warrant further consideration are BAD, IKD, AGB, EJI, and OKI. In the earlier work (Kang et al, 1993), the behaviour of the arrow at BAD was considered rather erratic with changing period (periods of 20, 30, 60 min), and since some measurements were known to be in error, no attempt was made to interpret the observations. However, noting that BAD is situated between a pair of grabens (GI and G2), the fact that the in-phase response at 20 min is negligible while the quadrature response is large with the arrow pointing towards graben Gl, is quite plausible, particularly when it is realized that the more distant graben G3 would also have an effect at BAD, and at 20 min the in-phase response of Gl to the west could very well be cancelled by the combined responses of G2 and G3 to the east of BAD, while the quadrature response could be showing G2 as providing the major contribution.…”

“…9 the behaviour of the difference arrows at sites near the grabens and the Benue Trough can now be reinterpreted based on the analogue model results that the coast effect responses could be subtracted successfully to yield the responses of major (wide) conductors normal to a local coastline, both Kang et al, 1993).…”

“…In an earlier work Kang et al (1993) discussed geomagnetic field site results and analogue model measurements at 11 sites in south-west Nigeria. Analogue model ocean effect responses were subtracted from the field site responses to yield induction arrow responses of any anomalous conductors in the region.…”

“…Removing the unwanted ocean induction components by subtracting numerically calculated, or analogue modelled coast effect induction arrows before interpretation has been studied by Wolf (1983), Weaver and Agarwal (1991), Dosso and Meng (1992), Chen (1994), Kang (1995), and Chen and Dosso (1997). The numerical model method, using thin sheet modelling, has been used by Bapat et al (1993) and Chamalaun and McKnight (1993) in geomagnetic surveys in Japan and New Zealand respectively, while the analogue model method of accounting for the coast effect has been applied to measurements at sites in coastal regions of North China , Japan , New Zealand (Chen et al, 1993, Dosso et al, 1996a, and Northwest Nigeria (Kang et al, 1993).…”