Electromagnetic induction in the New Zealand South Island

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

“…They examined the dependence on period, the distance from the ocean, and the depth extent of the horizontal elongated conductors and also showed that the ocean effect could be successfully subtracted to yield the responses of the elongated conductor alone for conductors located as distances as small as 50 km from the coast. These results have particular application to geomagnetic measurements in the New Zealand South Island where a reasonable premise is that of an elongated conductive zone (Chen et al, 1993) being associated with the Alpine Fault system some 50-100 km inland, and roughly parallel to the coastlines.…”

“…If situated near an ocean, the response of such a graben would be expected to be partially masked by the coast effect response. 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).…”

EM Induction in the New Zealand Southern Alps Region.

“…The limitations to such a subtraction of the coast effect have been examined by Wolf (1983), Weaver and Agarwal (1991), and Dosso and Meng (1992) using numerical model calculations, and by Chen (1994) using laboratory measurements for analogue models of conductive faults parallel to a nearby ocean coastline. This latter work demonstrated that except for locations right at the coastline, the coast effect response for a model ocean could be subtracted with reasonable accuracy (to within a few percent) from measured responses of models that included both the ocean and an elongated conductor parallel to the coastline, to yield the responses of the conductor (fault) alone.…”

“…Meng and Dosso (1990) successfully employed laboratory analogue model coast effects to subtract the ocean induction responses from field observations to yield responses of anomalous conductors at depth in the Japan region. More recently, Kang et al (1993) and Chen et al (1993) have also used analogue model measurements to determine the coast effects at 11 sites in southwest Nigeria and at the New Zealand Eyrewell Observatory respectively. In both cases the resulting in-phase and quadrature difference arrows were interpreted as the responses of anomalous conductors (such as conductive faults and sediments) in the survey regions.…”

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

“…In cases where electromagnetic mutual coupling between an anomalous conductor and the conductive ocean is small, the coast effect component in the observed responses can be removed to a reasonable approximation (Dosso and MENG , 1992) by simply subtracting the model induction arrows from the field site induction arrows to provide the response of the anomalous conductor alone. This technique, used in several previous studies for other regions (MENG and Dosso, 1990;Dosso and MENG, 1992;CHEN et al ., 1993) is also used in the present work to interpret the field observations.…”