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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.

Magnetic induction arrow responses of elongated conductors embedded in a uniformly resistive host earth and underlain by a conductive substratum are studied with the aid of laboratory analogue model measurements. The dependence on period, the overburden depth, and the underlying conductive basement depth, are examined for three different conductor lengths that were chosen so as to include the transition in the responses from two dimensional (2D) to those of three dimensional (3D) structures. The characteristic period T c (the period at which the in-phase anomalous response of the elongated conductor model is maximum and the quadrature response is minimum) is found to depend on the conductor length, the overburden depth, and the conductive basement depth. A decrease in conductor length or conductive basement depth, each have the effect of decreasing both the response amplitude and the characteristic period, while increasing the overburden depth has the effect of increasing the characteristic period and decreasing the response amplitude. At short periods, say below the characteristic period, the responses appear to be roughly 2D, and to become progressively more 3D-like with increasing period above the characteristic period.

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