W. Nienaber

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 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.

An analogue model study of electromagnetic induction in the Tasmania region

Laboratory analogue model magnetic measurements are carried out for a model of the region including Tasmania, Bass Strait with its highly conductive deep sedimentary basins, and the south coast of mainland Australia. The model source frequencies used simulate naturally occurring geomagnetic variations of periods 5–120 min. In-phase and quadrature magnetic Hx, Hy and Hz field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for E-polarization (electric field of the source in the N-S direction) and for H-polarization (electric field of the source in the E-W direction). Large anomalous in-phase and quadrature model magnetic fields are observed over Bass Strait and the coastal regions at short periods for both E- and H-polarization, but with increasing period, the field anomalies decrease more rapidly for E-polarization, than for H-polarization. The difference in response with polarization for the Bass Strait region is attributed to current induced in the deep ocean, for all periods, being channelled through Bass Strait for H-polarization but not for E-polarization. The persistent large coastal field anomalies elsewhere, for H-polarization, can be accounted for by the coastal current concentrations due to currents induced in the deep ocean for all periods deflected to the south and to the north by the shelving sea-floor and channelled through Bass Strait and around the southern coast of Tasmania. The phenomena of current deflection and channelling for H-polarization for the geometry of the southern Australia coastline and associated ocean bathymetry is particularly effective in producing field anomalies for a large period range. n nThe coastal horizontal Hx and Hy field anomalies, present for E-polarization at short periods and for H-polarization at all periods, do not extend far inland, and thus, for inland station sites somewhat removed from the coast, should not present serious problems for magnetic soundings in field work. The sharp vertical field (Hz) gradient over Tasmania at short periods, which is predominantly in the E-W direction for E-polarization and the N-S direction for H-polarization, is strongly frequency dependent, becoming almost undetectable at 60 min. The behaviour of the Hz field gradients, however, are very similar from traverse to traverse over inland Tasmania, and thus, the effects of the ocean should not present too serious a problem in the interpretation of field station studies. The discrepancies between model and field station results should be useful in mapping geological boundaries in the region.

Laboratory electromagnetic modelling of the subducting Juan de Fuca Plate

Abstract The effect of the depth of the dipping Juan de Fuca Plate subducting the west coast of the Vancouver Island-Washington-Oregon region is studied using a scaled laboratory analogue model. The subducting plate is modelled horizontal offshore, then dipping at 10° under Vancouver Island, and dipping further at 30° as it subducts the continent for the British Columbia region and 45° under the Washington-Oregon region. The strike of the bending plate follows the general strike of the continental coastline with an abrupt 42° change in direction in the Puget Sound area. The model results show that the coast effect, described by the vertical to horizontal magnetic field ratio, is highly dependent upon the period and the depth of the dipping substructure, while the conducting boundaries of the subducting plate significantly reduce the apparent resistivities for coastal locations. Enhanced induction arrows exist for a large range inland, owing to the steeply dipping subducting plate.

Analogue model study of electromagnetic induction in the Newfoundland region

Abstract The behaviour of magnetic field variations over the Newfoundland, Canada region is studied using a scaled laboratory analogue model. The model source frequencies used simulate periods of 100 s, 300 s, 900 s, and 1800 s in the geophysical scale. Contour views of in-phase and quadrature-phase parts of the three magnetic field components for two orthogonal source field polarizations for 100 s and 1800 s period variations are shown. Large anomalous in-phase and quadrature model magnetic fields are observed over the Strait of Belle Isle and Cabot Strait for all four periods studied, with the anomalous fields reaching maxima when the skin depth is of the order of six times the depth of the strait. The “cape effect”, that is, the large field enhancements due to deflection of induced current at capes, is observed for all four periods. Enhanced fields are observed at the continental shelf edge for 1800 s variations, but not for 100 s variations. A general shift in the anomalous field from the in-phase to the quadrature part for all three magnetic components is found for increasing period. The magnitudes and directions of inter-station induction arrows and single station induction arrows are studied. The two types of induction arrows differ when an anomalous horizontal magnetic field is present. This is especially evident near the coastline for a period of 100 s. For all periods studied, the in-phase induction arrows point towards the nearest and largest current concentrations. In traversing the coastline, the single station quadrature arrows reverse direction for the 100 s period, but not for the 1800 s period. The model results indicate that there is no preference for using either single station or inter-station induction arrows with respect to minimizing the effects due to coastal variations for inland locations. Further, the model results show that for periods studied, 100 s–1800 s the coastal effects would not produce serious perturbations to the field measured over crustal anomalies within the central regions of Newfoundland.