Numerical results obtained from an extension of D’Yakonov’s (1959) exact solution for the case of a buried horizontal circular cylinder, excited by a horizontal line source of current above the surface of a uniform half‐space and parallel to the axis of the buried cylinder, are analyzed. Results show the field impedance [Formula: see text], the ratio of the vertical magnetic component to the horizontal magnetic component [Formula: see text] as well as the corresponding phase differences at the surface for a range of periods (T = 0.25 to 2 hours), varying cylinder conductivity [Formula: see text] and cylinder radii (a = 0.5 and 25 km). The results have possible application in the interpretation of long period magnetotelluric and geomagnetic measurements in auroral and equatorial electrojet regions.
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.
D'YAKONOV'S (1959) general solution for the problem of induction in a conducting sphere embedded in a uniform conducting half-space by a spherically symmetric source has been evaluated numerically for the particular case of an overhead vertical magnetic dipole source. The validity of the numerical results are examined by comparing the results for several cases with analogue model measurements. Numerical results for a range of source frequencies, sphere radii, depths of burial, and conductivity contrasts of geophysical interest are presented.
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