Analogue Model Study Research Articles

Analogue model studies of induction effects at auroral latitudes

In addition to field observations and numerical models, geomagnetic induction effects can be studied by scaled analogue model experiments. We present here results of analogue model studies of the auroral electrojet with an Earth model simulating the Arctic Ocean and inland conductivity structures in northern Fennoscandia. The main elements of the analogue model used were salt water simulating the host rock, an aluminium plate corresponding to the ocean and graphite pieces producing the inland highly conducting anomalies. The electrojet was a time-harmonic line current flowing at a (simulated) height of 100 km above northern Fennoscandia. The period simulated was 9 min. The analogue model results confirmed the well-known rapid increase of the vertical field when the coast is approached from the continent. The increase of the horizontal field due to induced ocean currents was demonstrated above the ocean, as well as the essentially negligible effect of these currents on the horizontal field on the continent. The behaviour of the magnetic field is explained with a simple two-dimensional thin-sheet model. The range, or the adjustment distance, of the ocean effect inland was found to be some hundreds of kilometers, which also agrees with earlier results of the Siebert-Kertz separation of IMAGE magnetometer data. The modelled inland anomalies evidently had too large conductivities, but on the other hand, their influence decayed on scales of only some tens of kilometers. Analogue model results, thin-sheet calculations, and field observations show that the induction effect on the horizontal magnetic field Bx near the electrojet is negligible. On the other hand, the vertical component Bz is clearly affected by induced currents in the ocean. Evidence of this is the shift of the zero point of Bz 0-1° southwards from the maximum of Bx. The importance of these results are discussed, emphasizing the determination of ionospheric currents.

Effective ground resistance of the human feet in high voltage switchyards

The ground resistance of the feet, which is an important factor that determines the current flow in the body of a person exposed to dangerous touch voltage in a high-voltage switchyard, is modified by the proximity of the two feet and the presence of the energized grounding grid below the feet. Based on the Thevenin approach, a general method for determining the effective ground resistance of the feet that takes into account the modifying factors is presented. The method is verified with the help of analog model study. Results show that in most practical situations the effect of the modifying factors can be ignored. The situations in which it is necessary to consider these factors are identified.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Extensional fault systems in sedimentary basins: a review of analogue model studies

Scaled analogue models have provided graphic new insights into the progressive development of extensional fault systems. This paper reviews recent results of an on-going analogue modelling programme and compares the structural styles in the models with examples of natural extensional fault systems. Four fundamental model styles, scaled to simulate deformation in brittle sedimentary rocks in the upper 10 km of the crust, are reviewed. Extension above a basal detachment which undergoes stretching over a limited are produces an asymmetric rift graben bounded by planar faults and in which the internal deformation is accommodated by rotational domino faults. These experimental results are directly comparable to structures found in intracontinental rifts. Extension above a basal detechment which undergoes extension under the whole model and which has no constraints at the ends produces domino-style fault arrays in the pre-rift sequence and listric growth faults in the syn-rift sequence. These results compare with highly extended rift basins and terranes such as the Basin and Range Province, USA. Extension above a simple listric fault produces a characteristic hangingwall roll-over and crestal collapse graben system. At high values of extension, superposed crestal collapse grabens occur and a fan of listric growth faults is developed. These structures bear striking similarities to growth faults in progradational delta systems. Ramp-flat listric extensional fault systems using sand-mica models produce a characteristic family of structures with an upper roll-over crestal collapse system, a ramp syncline and reverse fault/fold zone and a lower roll-over crestal collapse system. An example of a ramp-flat fault system from the Gulf Coast, USA, is presented as a comparison with the analogue model results. Preliminary results of three-dimensional extensional model experiments are presented to illustrate the complexities in such fault systems. In all of the experimental models, it is found that the geometry of the underlying detachment system exerts fundamental control on structures produced in the hangingwall. The analogue models illustrated in this review provide valid geometric and kinematic templates for the progressive evolution of extensional fault systems in sedimentary basins.

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

Abstract 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 Bx, By and Bz 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.

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

Electromagnetic induction in the coastal region of Japan-Korea-China is studied using a scaled laboratory analogue model. The model includes a simulation of the known ocean bathymetry, the complex coastlines and the deep conductive Pacific trench. Model measurements of the electromagnetic field components are carried out for simulated periods of 15–180 min for two mutually perpendicular polarizations of a uniform horizontal inducing source field. In-phase and quadrature B x , B y , and B z model measurements are examined in detail for a series of traverses over Japan, the Korean peninsula, and the coastal region of China and the U.S.S.R. Large anomalous in-phase model magnetic fields are observed over the Korea-Japan strait for E-polarization and over the Bohai, Tsugaru, and La Perouse straits for B-polarization owing to current channelling. The significant responses observed at short periods over the peninsulas in the shallow coastal areas decrease with increasing period. Large gradients in the in-phase B 2 field are observed over Japan for E-polarization for both short and long periods owing to the effects of induced currents in the surrounding oceans. Thus induction arrow responses over all regions of Japan are expected to show the dominant effects of the ocean. A comparison of analogue model and field site induction arrows over the Kii peninsula of Japan indicates that, the ocean effects alone cannot fully account for the observed responses. The differences in the model and field results can be attributed to lateral inhomogeneities in the conductivity structure under Japan.

Laboratory and field studies of the application of electromagnetic prospecting for groundwater on Marajó Island, Brazil

On Marajó Island, Pará, Brazil, many cattle die due to lack of potable water, since a majority of the shallow groundwater is brackish. Two types of fresh‐water aquifers were delineated using resistivity methods: (1) a deep aquifer at about 100 m depth and (2) shallow buried‐stream channels filled with sand and gravel (ρ = 40 Ω ⋅ m) and embedded in clay and sand beds with brackish water (ρ = 0.8 Ω ⋅ m — 5 Ω ⋅ m). For the demarcation of the shallow buried‐stream channel aquifers, inductive electromagnetic moving source‐receiver methods were tested to replace the cumbersome galvanic resistivity methods. In order to verify the applicability of EM survey techniques for groundwater exploration in hydrogeologic conditions typical of Marajó Island, analog model studies were carried out. The groundwater‐bearing channels buried in the clays or in the sediments with brackish water represent geoelectrically a less conducting body in a more conducting environment. The buried‐channel “Guajará Mirim” on Marajó Island was chosen for test studies. A 500 times reduced model of this channel was constructed using two aluminium sheets of varying thicknesses to simulate host sediments and the sediments in the buried channel. EM profiles were measured in the laboratory across the channel model using horizontal‐coplanar vertical‐coplanar, and vertical‐coaxial coil systems in the broadside and inline modes of operation. Excellent EM model responses were obtained with all six coil configurations. Finally, EM profiles in the field were carried out using horizontal‐coplanar, vertical‐coplanar, and vertical‐coaxial coils in an inline mode of operation. Field and laboratory results are in very good qualitative agreement.

An analogue model study of EM induction for an island near bay and cape coastlines

Laboratory electric and magnetic model measurements were carried out to examine how the addition of a nearby elliptical island, to an otherwise complex continental coastline involving bays and capes, affects electromagnetic induction in coastal regions. Irregularities in a coastline in the form of bays and capes lead to significant spatial field variations in coastal regions due to variations in density of induced current flowing in the ocean caused by current deflection or channelling by the coastal irregularities. The addition of a nearby island, which for the appropriate field polarization can result in current channelling through the strait, greatly influences these field spatial variations. For E-polarization, the case of the model electric field of the inducing source parallel to the strait, and for short period (4.2 min studied here) variations, the effect on the fields, of the model elliptical island in the shallow coastal ocean, at points on the nearby continental coastline, was to increase by factors as large as two to three the H y amplitudes at the centres of the bay and the cape, and the E x amplitude at each bay, cape and straight coastline in the region of the strait, as compared with the values for coastline models without an offshore island. For H-polarization, the case of the electric field of the source perpendicular to the strait, the effect of the island was to increase by a factor of two to three the H z coastline values at the edge of the bay, and to decrease by a similar factor the value at the edge of the cape. Further, the spatial variations in the fields for both polarizations, over the island and over the strait were strongly influenced by current channelling through the strait. Thus any interpretation of field station measurements on such an island, on the sea floor in the strait, or on the continent in coastal areas near an island, should take into account the effects due to current concentrations in the strait particularly at short periods.

A simple parametric model for the electromagnetic response of an anomalous body in a host medium

A simple approximate representation of the spectral response of an arbitrary kind of electromagnetic (EM) prospecting system to a small conductive target in a conductive environment has been derived. The representation contains the direct response from the layered host medium and the first‐order effects of eddy current induction, current channeling, magnetic induction, and the coupling between eddy current and magnetic inductions in the anomalous body, as modified by the host medium. The only significant computational task in the representation is evaluation of a few Green’s functions for the host medium. As a guide to establishing proper approximations, a fundamental study of integral equations is presented. Very simple solutions for the secondary or scattering sources which represent the EM effect of the body are obtained for a few basic cases. Equations for more general cases are complicated by additional terms in the Green’s functions which represent ac interaction between scattering sources and the host medium, the effects of layering in the host, or interactions between the different types of scattering sources. Through a supplementary study of the response of a conductive disk embedded in a conductive host to an axisymmetric field, the ac interaction between the host medium and scattering sources in the disk is shown to be relatively unimportant. Hence, interaction with layering in the host is also minimal. Green’s functions in the integral equation can consequently be simplified. The representation as a whole has been tested by comparing its predictions of the spectral form of a response with two published data sets, responses of a plate model in a conductive host. The fits for targets of moderate and large size, relative to the scale of the particular experiment, range from excellent to good qualitative resemblance. The representation should find application in making expensive numerical or analog model studies more useful and understandable to the field geophysicist, in converting real and model data between the time‐ and frequency‐domain formats, in constructing practical inverse algorithms, and in predicting the form of EM effects in induced polarization and magnetometric resistivity surveys.

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.

Analogue model study of electromagnetic induction in the Newfoundland region

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.

The horizontal propagation of P waves through scattering media: Analog model studies relevant to long-range Pn propagation

abstract We compare seismograms produced through the use of analog models of scattering and nonscattering Earth structure to access the effect of lateral heterogeneities on the horizontal propagation of P waves. Our results are applicable to propagation out to 16° and to periods as short as 5 sec. We find that layers of scatterers within the mantle near the turning points of P waves can cause these phases to have long coda similar in certain respects to the coda observed in long-range Pn. The overall appearance of this coda is similar to that produced in laterally homogeneous models that include low-velocity crustal material near their surfaces. Scatterers at shallower depths in the mantle or crust have a different effect. Those in very uppermost mantle seem to suppress the horizontal propagation of P waves while those in the crust have very little overall effect, at least at the perod range we studied. We also find that the scatterers have an amplifying effect on the horizontal propagation of the very longest periods (&amp;gt;20 sec), an effect that may be related to the scattering-controlled boundary waves discovered by Biot (1968).

Water analogue model achieves optimal design of furnace flue gas collection system

Arc furnaces used for melting and refining steel generate large amounts of smoke and dust that can cause severe pollution in and around the furnace building unless a satisfactory depollution system is provided. This paper discusses a water analogue model study conducted at Neyrtec's Research & Test Centre to design an efficient furnace-pollution collection system in which the gas flow to be processed was optimized. The study showed that this flow could be 25–30 per cent lower than the value recommended by dust collection plant suppliers on the basis of standard design procedures. A fullscale collection system based on the design developed with the model has just been completed. The fullscale system confirmed the excellence of the collection efficiencies and the validity of the model results.

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.

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.

An analogue model study of electromagnetic induction in the Queen Charlotte Islands region.

In order to ease the interpretation of geomagnetic data for resource charting and tectonic studies of the Queen Charlotte Islands region, the behaviour of the electromagnetic variations over the region is studied using a laboratory analogue model. The results indicate that conductive channelling is important in Hecate Strait for both E and H source field polarizations. Current deflection is observed at Rose Point for E-Polarization and at the northern and southern tips of the Queen Charlotte Islands for H-Polarization. Model results for simulated variations of 4, 40, and 120min show a wide range of field values over the Queen Charlotte Islands, for example, Hz varies by a factor of 4 or more between coastal and inland locations for E-Polarization at short periods and by as much as a factor of 5 between certain coastal locations at long periods. The model results show that in decreasing the depth of the conducting layer in the mantle from 200 to 100km, the Hz, Hx, Ex, and Ey anomalies are attenuated by 20%, and the Hy anomalies by roughly 8% over the deep ocean and by 12% over Hecate Strait for 4min period variations.

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.

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 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.

REPLY TO the preceding Discussion by R. N. Athavale of “Electric Analog Model Study of Aquifer in Krishni‐Hindon Interstream Region, U.P., India”

REPLY TO the preceding Discussion by R. N. Athavale of “Electric Analog Model Study of Aquifer in Krishni‐Hindon Interstream Region, U.P., India”

DISCUSSION OF “Electric Analog Model Study of Aquifer in Krishni‐Hindon Interstream Region, U.P., India,” by C. P. Gupta, M. Thangarajan and V.V.S. Gurunadha Rao, May‐June 1979 issue, v. 17, no. 3, pp. 284‐290

DISCUSSION OF “Electric Analog Model Study of Aquifer in Krishni‐Hindon Interstream Region, U.P., India,” by C. P. Gupta, M. Thangarajan and V.V.S. Gurunadha Rao, May‐June 1979 issue, v. 17, no. 3, pp. 284‐290

An Analogue Model Study of Ocean-Wave Induced Magnetic Field Variations Near a Coastline

A laboratory analogue model, described previously by MILES et al. (1977), was used to study the effect of a sloping ocean floor with a shelf, and a step with a shelf, on the magnetic variations induced by ocean waves moving in a static magnetic field. The model measurements indicate that for a shallow ocean, the amplitudes of the magnetic variations are attenuated as the wave travels over the wedge, reach a minimum at the shelf edge, then again increase over the shelf. For the case of the step with a shelf maximum attenuation occurs at the shelf edge followed by an enhancement over the shelf. For both cases, the depth of the fluid over the shelf is the important factor in determining the behaviour of the induced field.