An analogue model study of electromagnetic induction in the British Isles region

Nienaber

et al. 1983

Self Cite

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

Section: Introductionmentioning

confidence: 99%

The electromagnetic response of the Assistance Bay region.

Heard

Nienaber

et al. 1983

Self Cite

“…Examples of laboratory analogue model studies for various islands near continental coastlines are provided by NIENABER et al (1979) for the Vancouver Island region, Dosso et al (1980a) for the British Isles region, Dosso et al (1980b) for the eastern coastal region of Canada, CHAN et al (1981a) for the Queen Charlotte Islands region and HEBERT et al (1983) for the Newfoundland region. These studies examined the channelling of telluric currents in the conductive sea channel between the island and continent, the effect of the nearby deep ocean on the induced electric currents and the associated electric and magnetic fields.…”

Section: Introductionmentioning

confidence: 99%

“…On the basis of ocean sediment information given by LuD-WIC et al (1979) and NINO and EMERY (1961), it was appropriate to add the equivalent of 150m of seawater to account for the sediments in the coastal region extending roughly to the 1km ocean depth contour. The method of constructing the model ocean and suspending it in the salt water tank was the same as that discussed previously in earlier work (e. g. Dosso et al, 1980aDosso et al, , 1980bCHAN et al, 1981a;HEBERT et al, 1983). The atoll (Zhongsha Islands) is one of the important features in the present model.…”

mentioning

confidence: 99%

Laboratory model magnetic fields for the Hainan Island region.

Nienaber

1983

Self Cite

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.

“…A suitable combination of these materials may yield the desired conductivity contrast, but the conductivity will have an abrupt variation at the boundary. Several geophysical problems, such as sea-land or sea-island boundary which exhibit a sudden conductivity discontinuity at the interfaces have been successfully studied in scale models (e.g., Dosso, 1973;Dosso et al, 1980;NIENABER et al, 1979). On the other hand certain geological situations, such as sedimentary layers which may indicate more or less a 1 The experimental work discussed in this paper was carried out while the author was at the University of Victoria as a post-doctoral fellow.…”

Section: Introductionmentioning

confidence: 99%

An electromagnetic scale model study of the Rhinegraben anomaly.

Ramaswamy

Winter

1983

The behavior of time-varying geomagnetic fields in the Rhinegraben region is investigated using a laboratory scale model. The two dimensional geophysical model proposed by WINTER (1973WINTER ( , 1974, which consists of a thin sedimentary layer with nonuniform conductivity and a series of subsurface layers, is modelled in the laboratory. The study indicates good agreement between the scale model results and the field station observations. The model results have further established that the behavior of the surface geomagnetic fields observed in the Rhinegraben region is strongly influenced both by the near-surface sedimentary layer as well as by a highly conducting layer in the upper mantle.