The coast effect response in geomagnetic field measurements

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

“…If the effect of the one common major response, that of the ocean, can be removed from the field site measurements, then the remaining responses should be those of the conductive anomalies not included in the laboratory simulation. Dosso and MENG (1992) have studied cases where removal of the ocean response (coast effect) can be accomplished to within a reasonable approximation by simply subtracting the model coast effect induction arrows from the field site induction arrows. Such a simple subtraction has validity when the mutual electromagnetic coupling between the ocean and the anomalous conductive structures is sufficiently small.…”

“…The coast effect in the magnetic response, of paramount importance in such coastal locations, can effectively be delineated by laboratory model measurements. 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.…”

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

“…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).…”

“…The modelling facility, including the measuring equipment and the overhead uniform field source, used in a wide range of previous EM induction studies at the University of Victoria, and is not described here. The details on the modelling facility are available in the early work of Dosso (1973). In the present work, the length and frequency scaling factors were chosen so that for example, 1 mm and 100 kHz in the laboratory model simulated 1 km and 5 min period respectively in the geophysical system.…”

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.…”