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 H_x, H_y and H_z 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.The coastal horizontal H_x and H_y 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 (H_z) 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 H_z 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.     

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 H_x, H_y, and H_z 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.     

Laboratory analogue modelling of the Schumann Resonance source field

An oscillating vertical line current source was used in the laboratory to simulate the naturally occurring 8 Hz Schumann Resonance field due to lightning strokes. Vertical to horizontal magnetic field ratios (coast effect), and magnetotelluric ratios, for traverses over a laboratory model of the Assistance Bay region of Cornwallis Island (N.W.T. Canada) for the vertical line current source field are compared with those for an overhead uniform source field. The E- and H-polarizations of the source fields used are defined as the cases of the horizontal magnetic field of the source roughly perpendicular and parallel to the general coastline, respectively.The magnetic field ratios are essentially identical for the two field sources for E-polarization, but differ by a factor of approximately two for H-polarization, with the ratios greater for the overhead source than for the vertical line current source. The magnetotelluric ratios are found to be identical for the two field sources of each of the E- and H-polarizations.     

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

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.     

Electromagnetic induction in the British Isles region: analogue model and field station results

The results of a laboratory electromagnetic analogue model study, which employs a horizontal inducing field over a simple model of the British Isles region, delineate the location and frequency dependence of the major coast effect induction anomalies of the Scotland region. Contours of amplitudes, amplitude ratios, and in-phase and quadrature parts of the model field measurements are presented. The model vertical magnetic fields for two orthogonal source field polarizations and field station values for two hypothetical events for corresponding polarizations are compared.While major discrepancies occur between model and field H_z amplitudes, the H_z gradients across Scotland, which can be attributed to the coast-effect, are comparable in value, although sometimes reversed in sign. Superimposed on this coast-effect, the field data indicate the existence of current concentrations associated with the Great Glen and the Southern Uplands faults and possibly also of currents within the Scottish mainland near the east and west coasts.     

The thermal regime of the San Juan basin since late cretaceous times and its relationship to San Juan mountains thermal sources

Heat-flow and coal-maturation data suggest that the thermal history of the San Juan Basin has been influenced by magmatic and volcanic activity in the San Juan Mountains-San Juan volcanic field located to the north of the Basin. Time-dependent isothermal step models indicate that the observed heat flow may be modelled by a (near) steadystate isothermal step extending from 30–98 km depth whose edge underlies the northern San Juan Basin. The observed maturation levels of the Fruitland formation coals in the northern and central Basin, however, require more heat than can be associated with conduction from a deep thermal source (steady-state step) and from the shallow crustal batholith which underlies the San Juan volcanic field. Magmatic activity within the Basin does not appear to be a reasonable source of additional heat. Increased burial depths of the coals may explain some of the elevated maturation levels observed in the central and northern Basin, but it seems likely that an additional source of heat is still required. Heat advection by groundwater movement may have influenced the coal maturation levels in the Basin as well. Both magmatic activity associated with the emplacement of the San Juan batholith and elevated geothermal gradients associated with the steady-state thermal source at depth may have contributed to heating of the groundwater. An appreciation of heat advection by groundwater flow may therefore be most important to understanding regional patterns of heat flow and hydrocarbon maturation.     

EM induction in the Vancouver Island region: 3D numerical,analogue model,and field site results

Electromagnetic induction in the Vancouver Island region for a uniform inducing source field for 300 s period is investigated with the aid of three-dimensional (3-D) numerical and analogue model results and field site measurements. The thin sheet numerical model, based on the subducting Juan de Fuca plate analogue model ofDosso et al., consists of a 5km thick non-uniform thin sheet (comprising the lateral conductivity contrasts arising from the land, the varying depth ocean, and the sediment) underlain by a four-layer conductive structure. The four-layer conductive structure beneath the non-uniform thin sheet simulates the effect of the Juan de Fuca plate subducting Vancouver Island. To examine the effects of the ocean channel depth between Vancouver Island and the British Columbia (Canada) mainland, numerical results were obtained for two channel depths (0 and 600 m). The results indicate that the channel plays an important role in the geomagnetic response in the central and inner coastal regions of Vancouver Island. The general agreement of the 3-D numerical model induction arrows with the analogue model and field site induction arrows for 300 s supports the premise of a layered conductive substructure dipping at a small angle, at most, beneath Vancouver Island.Lithoprobe Publication No. 311.     

Source mechanism of the 1906 San Francisco earthquake

Surface-wave amplitudes in the period range 50–100 s at eight European and North American stations, horizontal slip profiles along the rupture zone and the timing of certain events along the fault during rupture time are all engaged in unison to reconstruct the motion at the source. A modified source model is used to accommodate a moving rupture with variable dislocation in the direction of propagation.It is inferred that the rupture started at about 13 h 11 m 55 s GMT near San Juan Bautista and propagated unilaterally northwestward along N35°W over 400 km with an average rupture velocity of 3.5 km/s. At 13 h 12 m 12 s, the dynamic shear front, moving with the rupture speed, hit the Lick Observatory. Then, at 13 h 12 m 18 s, the rupture arrived to the vicinity of the epicenter in the Santa Cruz Mountains given by B. Bolt. There the slip changed sharply from an average of 0.5 m to a high value of 3 m causing extensive landslides and avalanches. At 13 h 12 m 32.5 s two railroad clocks at San Rafael were stopped. Finally, at 13 h 12 m 36 s the offset front hit the Naval Observatory at Mare Island and stopped the astronomical clocks there. Conspicuous surface waves, visible on Wiechert seismograms in Europe in the period range 55–65 s, reflect the true rupture time.The seismic data inversion yields an effective radiation source some 240 km long with an average vertical extent of some 34 km over a total fault length of 400 km ($\text{U}\text{d}\text{S ? 29,000}\text{m km}{}^2\text{or}\text{μ}\text{U}\text{d}\text{S ? 9 · 10}{}^{27}\text{dyn cm}$). It began at the Santa Cruz Mountains and ended some 20 km off coast Point Arena. Thus, due to the nonuniform slip profile, only $\text{3}\text{5}$ of the total fracture length contributed to the far radiation field.Although the product of the average source displacement (over the entire fault) and the vertical extent appears to be fairly well determined from the surface-wave spectrums, the separate values of these entities cannot be uniquely determined. If the average surface displacements (～ 3.2 m) are diagnostic of the entire fault, a vertical extent of H = 34 km is required.Finally, a new analysis of surface waves from the Alaska earthquake of July 10, 1958, the Queen Charlotte Islands earthquake of August 22, 1949 and the Kern County shock of July 21, 1952, enables us to draw parallels between the three biggest major events which occurred along the NE Pacific coast during 1906–1958. A common feature of all of these earthquakes is that vertical failure extents of 30–40 km are implied.     

Electromagnetic analogue model measurements and finite-difference numerical calculations of the response of a conducting slab to three different source fields

This work compares experimental analogue model measurements and finite-difference numerical calculations of the electric and magnetic fields for a highly conducting slab embedded in a poorly conducting host earth for three different source field configurations. Measurements and calculations were carried out for a uniform source, a sheet current source with a y exp(?ay) current intensity distribution, and a horizontal magnetic dipole source. The results indicate reasonable agreement with some exceptions between the analogue and numerical methods. The source field is found to have an important effect on the field anomalies at the interface of the highly conducting slab and the poorly conducting host medium.     

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

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 $\text{1}\text{2}$ 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.     

An analogue model study of electromagnetic induction for cape and bay coastlines

The electric and magnetic field variations across bay and cape coastlines are determined from scaled laboratory analogue models. The effect on the field response of such features along an otherwise straight coastline is limited to a distance approximately equal to the dimensions (diameter of the bay or cape) of the irregularity. For the E-polarization of the source field the perturbations in H_z and H_y are more confined, mainly to the immediate region of the bay or cape. The coast effect H_z/H_y for this polarization, and variations with a 4.2 min. period, is approximately a factor of two larger for the cape than for the bay, while the straight coastline H_z/H_y is roughly mid-way between the cape and bay values. For H-polarization the coast effect is again approximately a factor of two larger for the cape than for the bay, but both values are larger than the value for a straight coastline. The response of the various field components to the bay and cape features is decreased at longer periods, and as a measure, the coast effect is approximately a factor of 1.2 greater for the cape than for the bay for a 42 min. period as compared with the factor of 2 for a 4.2 min. period. The bay and cape deflect the induced currents flowing in the ocean and modify the currents diffusing into the land.This study demonstrates that data from coastal sites may be unreliable for correcting magnetic surveys or for analyses of short period geomagnetic source-field variations.     

THE INDUCTIVE RESPONSE OF A HORIZONTAL CONDUCTING CYLINDER BURIED IN A UNIFORM EARTH FOR A UNIFORM INDUCING FIELD*

The inductive response of a conducting horizontal cylinder embedded in a uniform earth is studied using numerical results obtained for an analytical solution for the problem of a conducting cylinder buried in a homogeneous earth for the case of a uniform inducing field. A check of the validity of the numerical results is made by a comparison with analogue model measurements for a number of cases. Numerical results for a range of cylinder radii (a = 1–10 km), depths of burial (d= 0–4 km), conductivity contrasts (σ₂= 10^?2-10 Sm^?1), and source frequencies (f= 10^?1-10^?4 Hz) of interest in the interpretation of magnetotelluric field measurements are presented. The results indicate that for a uniform inducing field the conductivity and depth of burial of a horizontal cylindrical inhomogeneity are best determined through a measurement of the amplitudes H_y, H_z and E_x and the phases φ_y and Ψ_x.     

Magnetic fields due to sea tides in the english channel

The M₂ (12.42-h period) magnetic field variation present at night in the United Kingdom can be interpreted in terms of sea tides, the main contribution being from the Atlantic Ocean. Osgood et al. (1970) observed a difference between the M₂-variations in the total magnetic field (F) variations at Sidmouth (0.8 km from the coast) and Exeter (16 km from the coast) of amplitude 0.8 γ, which must be due to local effects. Analysis of the phase of this difference between Exeter and Sidmouth has shown that it cannot be due entirely to tidally induced electric current systems confined to the English Channel. Measurements with two three-component magnetometers have shown that the night-time variations in the north-south (H) component of 12.42-h period are significantly different at Sidmouth and Exeter. The difference in H is in phase with the expected electric potential difference across Devon due to the difference in the phases of the sea tides in the English Channel and the Bristol Channel. The results of Donato and Rosser (1973) on micropulsations suggest the existence of a region of high conductivity under Exeter. If this connects the English Channel to the Bristol Channel, a current will flow beneath Exeter giving a difference in the M₂-variations in H at Exeter and Sidmouth. This difference in H dominates the M₂-variations in the total magnetic field (F) variations at Exeter and Sidmouth measured by Osgood et al. (1970).     

Comparison of analogue model and field station EM responses on Southern Vancouver Island

Measurements of geomagnetic induction responses for a laboratory analogue model, that includes a simulation of the subducting Juan de Fuca Plate, are compared with those obtained at three sites on Vancouver Island. Good agreement between model and field responses at the central Vancouver Island site is observed over the period range 3–60 min, while at the east and west coastal sites, good agreement is achieved only for periods greater than 20 min. At shorter periods, departures of the observed responses from those of the model are possibly the result of upper crust inhomogeneities not replicated in the simulation, and the complex strait with its numerous small islands and its irregular coastlines inadequately simulated in the course model. Nevertheless, the analogue model results are consistent with the premise of a Juan de Fuca plate, underlain by the conductive asthenosphere, subducting at a shallow depth beneath Vancouver Island.     

Disturbance daily variation of electrojet current at Indian longitude sector

Using the hourly mean data of the horizontal (H) and vertical (Z) components of the geomagnetic field at the set of nine observatories in India, it has been found that the Disturbance Daily Variation (SD) of H shows a prominent midday depression over the magnetic equator of the H field and a midday increase of Z field at stations near the northern fringe of the equatorial electrojet belt. The magnetic disturbance introduces a westward electric field over the equatorial region, causing a band of westward ionospheric current over the magnetic equator during the day time hours. The latitudinal extent of the disturbance time counter electrojet current seems to be larger than that of quiet time normal electrojet current. This suggests a systematic westward electric field superposed on the normal Sq field at low latitude ionosphere during disturbed periods, the source of which has to be clearly defined. Further correlative data analysis is required to isolate these sources of the disturbed equatorial electric fields.     

Site effect study in central Mexico using H/V and SSR techniques: Independence of seismic site effects on source characteristics

We study the influence of different source characteristics (depth, distance, type and azimuth) on the site effect in Acapulco and the Valley of Mexico. Site amplification was estimated by means of spectral ratios (both horizontal-to-vertical spectral ratio and standard spectral ratio techniques were applied) from earthquake recordings at soft and hard sites. In Acapulco, 125 Mexican earthquakes covering a hypocentral range of 7–290 km and a depth range (H) of 3–61 km were analyzed in three different groups of hypocentral distances. In the Valley of Mexico, we estimate site effect at five locations using recordings from shallow-coastal interplate (200?Δ?570 km; H?35 km) and normal-faulting, intermediate-depth inslab (132?Δ?738 km; 32?H?178 km) earthquakes, as well as from teleseismic events. Our results seem to point to negligible dependence of site effects on the source location and type.     

An analogue model study of EM induction in the North China-Korea coastal region

The behaviour of the magnetic field variations over the North China-Korea coastal region is studied with the aid of a scaled laboratory analogue model. The model source frequencies simulate naturally occurring geomagnetic variations of 3–60 min periods. In-phase and quadrature magnetic B_x, B_y and B_z field measurements for the modelled region are presented for E- and B-polarizations. Large anomalous in-phase and quadrature model magnetic fields are observed over the Korea-Japan strait for E-polarization and over the Bohai strait for B-polarization due to current channelling through the straits. Large responses of the peninsulas in the shallow coastal areas occur at short periods but decrease abruptly with increasing period. In general, the effects of peninsulas, straits, bays and irregularities in the coastlines play an important role in the magnetic field responses both on-shore and off-shore for this complex North China-Korea coastal region. Model and field site induction arrows are compared for three sites west of Bohai Bay.     

Harmful algal bloom species and phosphate-processing effluent: field and laboratory studies

In 2002, the Florida Department of Environmental Protection began discharging phosphate-processing effluent into Bishop Harbor, an estuary within Tampa Bay. Because of concerns that the effluent would serve as a nutrient source for blooms of the toxic dinoflagellate Karenia brevis, a field monitoring program was established and laboratory bioassays were conducted. Several harmful algal bloom (HAB) species, including Prorocentrum minimum and Heterosigma akashiwo, were observed in bloom concentrations adjacent to the effluent discharge site. Blooms of diatoms were widespread throughout Bishop Harbor. K. brevis was observed with cell concentrations decreasing with increasing proximity to the effluent discharge site. Bioassays using effluent as a nutrient source for K. brevis resulted in decreased cell yields, increased growth rates, and increased time to log-phase growth. The responses of HAB species within Bishop Harbor and of K. brevis to effluent in bioassays suggested that HAB species differ in their response to phosphate-processing effluent.     

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

Using an electromagnetic analogue model, the behavior of time-varying electromagnetic fields for an island near a continental coastline is examined for the H-polarization case, in which the electric field of the inducing source is perpendicular to the continental coastline. A study of the effect of the shape of the island, using square- and circular-island models, indicates that over the island, anomalies in the field components are confined to a smaller area for the circular island than for the square island. A study of the effect of ocean-channel width between the island and continent shows that, as the channel width decreases, anomalies in the magnetic field components over the island decrease. The anomaly in the electric field at the continental coastline first increases and then decreases with decreasing channel width.