H.W. Dosso

A review of analogue model studies of the coast effect

Abstract This work reviews electromagnetic analogue model studies of the coast effect, dealing particularly with a vertical interface model, a thin conducting sheet model, a wedge model, and a wedge underlain by a conducting block simulating an upwelling in a conducting zone beneath the coast. The vertical interface model results and the infinitely conducting thin sheet model results show good agreement with calculated values. It is concluded that a sloping sea-land interface alone cannot account for the experimentally observed coast effect, but that a sloping sea-land interface underlain by a conducting step could produce the observed coast effect.

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

Abstract The behaviour of time-varying electromagnetic fields near an island situated in a shallow ocean with a nearby continent is investigated using a scaled analogue model. To study the effect of the proximity of the continent, various island-continent distances are treated. The presence of the continent tends to augment enhancements of the field components at the island coastlines for all channel widths studied, while the island affects the enhancements of the fields over the continental coastline only for very narrow channel widths (half the island width or less), and does not affect the horizontal to vertical magnetic field ratio at the coastline at all. To examine the effect of the shape of the island, square and circular island models are used. For the frequencies studied, the island shape has little effect on the fields over the continental coastline, but over the island, the spatial variation of the fields is considerably less for the circular island than for the square island.

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

Abstract The behavior of electric- and magnetic-field variations over the British Isles region is studied using a scaled laboratory analogue model. The model source frequencies used simulate periods of 20 min to 2 h in the geophysical scale. The results indicate that conductive channelling of induced electric current by the English Channel is important for both E- and H-polarization, while channelling through the North Channel, the Irish Sea, and the North Sea is particularly important for the E-polarization. The funnel shape of the North Sea, leading to the relatively narrow English Channel, results in diffusion of current into the continent and the east coast of the British Isles, contributing to highly frequency-dependent horizontal electric- and vertical magnetic-field coastal anomalies. The model results also give ample evidence of current deflection at the complex coastlines. The model results for simulated 1 2 h period variations indicate vertical magnetic-field values changing by as much as a factor of five or six between certain coastal locations, and at least by a factor of two or three between interior locations. The horizontal electric- and magnetic-field components also show almost equally large changes, with particularly large enhancements near narrow ocean channels and irregular coastlines, responding to channelled and deflected current. With the relatively close proximity of the coasts for essentially all locations on the British Isles, the coast effects associated with the very complex coastlines can be expected to play a major role in the behavior of the electromagnetic fields over the British Isles.

The coast effect response in geomagnetic field measurements

Abstract Two-dimensional (2D) numerical models of electromagnetic (EM) induction for anomalous conductors in continental and island coastal regions are studied to determine constraints on ocean-conductor separation distances that would permit, to within an acceptable approximation, a simple vector subtraction of the coast effect response. The anomalous conductors are in the form of an upwelling or a depression in the asthenospheric conductive substratum at depth. The coast effect response range (YR) is arbitrarily taken to be the distance from the ocean at which the | B z B y n | coast effect response is reduced to a value of 0.2. The response ranges are determined and presented empirically as a function of period for a range of conductive substratum depths, island widths, and ocean depths. The response ranges are tested in 2D numerical models that include an anomalous conductor in the form of either an upwelling or a depression in the conductive substratum under the ocean and host Earth. The induction arrows for the anomalous conductor are shown to be in good agreement with the difference arrows obtained by subtracting the responses of a model without the anomalous conductor from the responses of the model that includes the anomalous conductor. As an application, difference arrows, obtained by subtracting three-dimensional laboratory analogue model coast effect responses from field site responses, are employed in the interpretation of induction arrow responses available for a number of sites in the Bohai Bay region of China and the Kii Peninsula region of Japan.

The Tamar conductivity anomaly

Magneto-variational measurements in eastern Tasmania have shown the presence of a conductivity anomaly coinciding with the Tamar Lineament. Magneto-telluric observations indicate a shallow body (2–5 km deep) with a high conductivity (0.2–1 S m−1. Analogue model measurements made at the university of Victoria have been used to correct for the effect of the oceans surrounding Tasmania. Rather than model the form of the conducting body, the results have been interpreted in terms of current flowing in a NNW-SSE direction in a channel ∼ 40 km wide with a sharp eastern edge and a more gradual tapering to the west. The most likely explanation of the high conductivity is fractured rock saturated with conducting pore fluids.

A comparison of numerical, analogue model, and field-station vertical magnetic-fields for the Vancouver Island region

Abstract Vertical magnetic fields for a three-dimensional numerical model, for a laboratory-analogue model, and from field stations for the Vancouver Island region of British Columbia, Canada, are compared. The numerical results are obtained using a three-dimensional finite-difference numerical technique employing a 25 × 25 × 25 mesh of grid points for a simplified mathematical model of the Vancouver Island region. The calculations are carried out for a source frequency of 0.004 Hz. The analogue model results for four traverses over the Island model and the field station values (obtained from transfer function analysis) for ten locations are those discussed previously by Nienaber et al. (1979a, b). General agreement exists between the numerical, analogue, and field station data, and comparison of results between these methods is important in three-dimensional electromagnetic induction studies of complex geomagnetic induction problems.

Numerical and analogue model results for electromagnetic induction for an island situated near a coastline

Abstract The behaviour of time-varying electromagnetic fields near an island situated in a shallow ocean is investigated using both a three-dimensional finite-difference numerical method and a scaled analogue model method. The effect of a coastline located at some distance from the island is included in the study. The numerical model results and the scale model results are compared for various traverses across the island. The results indicate a high degree of compatibility between the two methods for studying problems involving three-dimensional conductivity structures.

A laboratory electromagnetic model study of the Juan de Fuca Plate region

The behaviour of the magnetic field variations over the Juan de Fuca Plate region is studied using a scaled laboratory analogue model. The model includes a simulation of the complex Juan de Fuca Plate subducting the Vancouver Island region. The subducting plate is modelled with a profile of increasing inclination from east to west; horizontal offshore, dipping at 10° under Vancouver Island, and bending further under Georgia Strait to subduct the continent at 30° for the B.C. region and 45° for the Washington-Oregon region. The strike of the bending plate follows the general strike of the continental coastline with an abrupt change in direction (42°) in the Puget Sound area. The model substructure simulates a subducting plate, overplated by a sediment layer several kilometres thick, and underlain by a 30 km thick highly conducting upper asthenosphere. The model source frequencies used simulate periods 5–120 min in the geophysical scale. In-phase and quadrature Hx, Hy, and Hz magnetic field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for E- and H-polarizations (electric field of the source approximately parallel and perpendicular, respectively, to the west coast of Vancouver Island). The fields for three regions of the model; over Vancouver Island, over the Olympic Peninsula and over a linear portion of the U.S. coastline, are examined in detail. The general conclusion is that the effect of the dipping subducting plates is to significantly attenuate, at short periods, the maxima in the anomalies at the coastlines underlain by the 10° dipping plate, while leading to anomalous vertical and horizontal fields over ranges as large as 500 km inland over a wide period range. Anomalous fields are observed over the offshore and inland knee-bends of the subducting plates at all periods for both E- and H-polarizations. For locations on land, the in-phase induction arrows point seaward and perpendicular to the strikes of the dipping plates for all periods, while the quadrature arrows at short periods point landward and rotate to point seaward for periods greater than 20 min.

An analogue model study of electromagnetic induction in the eastern coastal region of North America

Abstract The behavior of electric and magnetic field variations over the eastern coastal region of North America is studied using a scaled laboratory electromagnetic analogue model. The model source frequency used simulates a period of 1 h in the geophysical scale. The results indicate that deflection and conductive channelling of induced electric current is important for both the E-polarization (northeast-southwest direction of the electric field of the source) and the H-polarization (northwest-southeast) of the source field. In the model, conductive channelling occurs through the Strait of Belle Isle, Cabot Strait, and in the St. Lawrence River. Current deflection is particularly prevalent around the southeast coast of Newfoundland for both E- and H-polarization, and around the northeast coastline of Nova Scotia for E-polarization. The model results also show current deflection by cape and bay coastal features, as well as by ocean depth contours. A comparison of model measurements for the cases of a uniform source field and a line current source indicate that the nature of the source field has a measurable but surprisingly small effect on the vertical to horizontal magnetic field ratio for both E- and H-polarizations, and negligible effect on the magnetotelluric ratio for coastal regions. The model fields in coastal regions were found to be strongly influenced by induced currents, deflected and channelled by the coastline and ocean bathymetry, and were dependent on the nature and particularly the polarization of the source field. Thus, along the complex coastline of eastern North America, a wide range of electric and magnetic field values should be expected. In some regions the coast effect, measured by the vertical to horizontal magnetic field ratio at the coast, could be expected to be extremely small or absent, while in other regions the ratio could approach a value as large as unity for variations of 1 h period.

On the behaviour of the induction arrows over a buried conductive plate — a numerical model study

Abstract Two-dimensional numerical model calculations, employing a finite difference technique, are used to study the behaviour of the induction arrows, for a range of periods, for a conductive plate of (i) semi-infinite and (ii) finite width in uniform and layered resistive hosts. The results for the conductive plate at the surface of the host have application to a uniform-depth ocean, while the results for the plate buried at some depth in the resistive host have application to a conductive sill in a resistive Earth. The numerical results indicate that for a profile over the plate-host vertical interface the in-phase arrows for all periods and locations point towards the conductive plate, while the quadrature arrows at periods near the characteristic period of the model are oppositely directed on either side of the interface so as to point towards each other and towards the interface for nearby locations, both over the conductive plate and the resistive host. Further, the quadrature arrow undergoes a second reversal over the resistive host at a distance from the interface that is somewhat dependent on the period. Thus, at either side of the location of this second reversal, the quadrature induction arrows are again oppositely directed, but pointing away from each other, with the arrows near the interface pointing towards, and the more distant arrows pointing away from the conductive plate. The period range for the quadrature-arrow reversal is characteristic of conductivities and layer depths. The features of the quadrature-arrow sign reversals at and near the interface are in accordance with the earlier laboratory analogue model results of Hebert et al. for the Newfoundland coastal region and Nienaber et al. for a conductive plate in a resistive host. It is suggested that in practice the sign reversal of the quadrature arrow may aid in locating a conductor-host interface, and that if the conductivity of the host is known, the maximum in the anomalous vertical magnetic field response may permit an approximate determination of the conductive-layer depth.