An analogue model study of electromagnetic induction for island-continent ocean channels

1983

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The behaviour of the electromagnetic fields over the Hainan Island region of the South China Sea is studied using a scaled laboratory analogue model. The model source frequencies used simulate periods of 5 min. to 30 min. in the geophysical scale. To examine the electromagnetic field variations due to the irregular coastlines, the deep ocean, a narrow ocean channel, and an atoll (modelled as a seamount), field components for individual traverses over the model are presented.The coast effect for the bay coastline is visible only for short periods at which the depth of the bay is at least .1 skin depths. The response of the seamount, overlain by a 500m depth ocean, differs greatly from the response of an island. Electric current flow over the seamount, confined to this thin layer, produces a quadrature field directly opposite to that for an island. Buried features such as the seamount and depth profile of the deep ocean show the largest in-phase response at the lower frequency where the overlying ocean is much less than .1 skin depth thick.

Section: Discussion Of Resultssupporting

confidence: 85%

mentioning

confidence: 99%

Laboratory model magnetic fields for the Hainan Island region.

1983

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“…These authors have used field stations along profiles extending westward from the continental coast in order to study the combined effect of the coast and inland crustal anomalies. As well, AULD (1968) andNIENABER et al (1973) have reported substantially anisotropic apparent resistivities for magnetotelluric stations at Victoria, B.C.…”

Section: Introductionmentioning

confidence: 90%

“…RAMASWAMY et al (1975RAMASWAMY et al ( , 1977 showed good agreement between three-dimensional finite difference numerical calculations and scaled analogue model measurements for a square island at some distance from the con-tinental coastline. Extending the study to include the more realistic irregular and curved island shapes, NIENABER et al (1976NIENABER et al ( , 1977 used the analogue model to illustrate two effects for an island near a continent, namely the effect of the channel width between the island and the continent using various channel widths, and the effect of the island shape using square and circular island models. The study of NIENABER et al (1976NIENABER et al ( , 1977 was carried out for two polarizations of a uniform horizontal inducing field.…”

Section: Introductionmentioning

confidence: 99%

An analogue model study of electromagnetic induction in the Vancouver Island region.

Law³

et al. 1979

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The behaviour of the time-varying electromagnetic fields over the Vancouver Island region is investigated using a scaled analogue model. The results show that conductive channelling in the shallow seawater channel around the island is important for both E and H polarization of the inducing field and for all frequencies studied. It appears that channelled telluric currents flow along either side of Vancouver Island with some diffusion of current into the continent to the southeast of the island. For the H polarization case there is some evidence of current flow around the SE tip of Vancouver Island as well. The results also show the effects of local confinement of current whereby the direction of telluric flow is modified by the geometry of the seawater channel.

“…The inducing overhead source field is provided by a pair of parallel wires separated by twice their height above the salt water surface. This current system, described earlier by RAMASWAMY et al (1975) andNIENABER et al (1976) A 25km by 35km portion of Barrow Strait and the southern coast of Cornwallis Island was simulated in the laboratory model. Figure 1 shows only the region in the neighbourhood of the field stations, and does not include the 125m depth contour.…”

Section: Analogue Model Descriptionmentioning

confidence: 99%

The electromagnetic response of the Assistance Bay region.

Heard

et al. 1983

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The electromagnetic response of the Assistance Bay region (N.W.T., Canada) is studied using laboratory analogue model and field station measurements. Magnetic field components for the frequency range 0.1-15Hz, which includes the well known lightning induced Schumann Resonances, were measured at 8 sites along a 6 km line through the central region of Assistance Bay and roughly perpendicular to the Cornwallis Island coastline. Two of the stations were on land and six were off-shore on the ice. Although the field data were examined in the three bandwidths 0.1-6Hz, 6-10Hz, and 10-15Hz, the present work deals only with the 8Hz Schumann Resonance component within the 6-10Hz range. Calculated average amplitudes and phases of the three magnetic field components show the relative response of the region at stations along the 6km line. The response is discussed in terms of the effect of the on shore topography, the coastline, and the bathymetry.A laboratory analogue model of the Assistance Bay region was constructed and model measurements carried out for a simulated frequency of 8 Hz for E-Polarization (electric field in the east-west direction) and H -Polarization (electric field in the north-south direction) of an overhead uniform horizontal inducing field. The magnetic field amplitudes Hx, Hy, Hz for both polarizations for 10 traverses perpendicular to the coastline are presented and the anomalous fields attributed to the response of the coastline bay, capes, bathymetry and source field polarization.The analogue model and the field station coast effects (vertical to horizontal field ratio) are compared for the model traverse coinciding with the 6km line of field stations. The analogue model results show a maximum enhancement of approximately 1.3 directly over the coastline with rapidly decreasing values seaward followed by a second enhancement over the sharp ocean depth gradient beyond the 55 m depth contour. The field station results do not show maximum at the coastline, but rather an increasing value seaward, reaching a value of 1.7 at the station over the sharp ocean depth gradient near the 55 m depth contour, indicating the effective coastline to be at the deep water edge. This difference may in part be attributed to the shallow water conductivity being smaller than simulated in the laboratory model. The effect of the 2m layer of ice, as well as the accumulation of fresh water from streams in the Assistance Bay region, may lead to a significantly lower effective conductivity than assumed for the