EM responses of an elongated conductor near an ocean — analogue model studies

Chen, J.; Dosso, H. W.

doi:10.1016/s0031-9201(96)03180-9

Cited by 5 publications

(4 citation statements)

References 21 publications

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“…Thus, the PFC (Figs. 3 and 4) relatively constant in-phase responses, as well as the T c values, are very similar to those in the Chen and Dosso (1997) study. It should be emphasised, that this observed similarity is contrary to what should be the case, since the responses at short periods for the central traverse over the 500 km length conductor are those of a 2D structure, while the FDM responses for the near-end (x = 30 km) traverse in the CDK model should be those of a 3D structure.…”

supporting

confidence: 82%

Reply to “Comment on ‘EM induction in elongated conductors normal to a coastline with application to geomagnetic measurements in Nigeria’ by J. Chen, H. W. Dosso, and S. Kang”

Dosso

Agarwal

2014

Earth Planet Sp

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Dr. P. F. Chen (hereafter PFC) has questioned two main differences between the CDK analogue model and his FDM numerical results: 1) The CDK analogue model in-phase V y fall off rapidly with increasing period, whereas his numerical H z /H y do not; 2) The CDK quadrature V y do not show a reversal, whereas his H z /H y do. Upon examining the curves in Fig. 4 carefully, it can be concluded that the CDK quadrature V y do, in fact, also indicate a reversal. It can clearly be seen from the shape of the CDK curves (dashed line), that with decreasing period each response curve is approaching a reversal, though at a period some what below 1 min, being the lowest period included in the analogue model measurements. The CDK in-phase response curves as well, are each approaching a maximum at periods well below 1 min, and should each reach a maximum at roughly the same period as the corresponding quadrature reversal, being characteristic of the specific location (y) between the conductors. These shifts to longer periods with distance indicated in the CDK results, are similar to those in the PFC (solid line) results which for sites at increasing distances from the major conductor (b), show shifts to longer periods to be roughly T c = 2, 3, and 8 min at y = −45, −40, and −30 km respectively. With respect to the more rapid fall-off in-phase response with increasing period, as well as the quadrature reversals at lower periods in the CDK 3D results as compared with the PFC results, these are precisely the characteristics that distinguish 3D from 2D responses. The CDK model for the 1-60 min period range is certainly 3D for all but the shortest periods, since for the near-end (x = 30 km) traverse the conductor length is definitely too small to satisfy the criteron for a 2D conductor in terms of host conductivity skin depths. Thus, the PFC roughly constant in-phase H z /H y over the 1-60 min range appear to show a 2D rather than what should be a 3D response.Unfortunately, PFC does not include any explanation regarding the numerical grid design, the boundary conditions, or the convergence criteria used in his finite difference numerical model for the high conductivity contrast (∼5000), and the large (1-60 min) period range for which the highly resistive host skin depth changes by nearly a factor of 8. Did he use air layers above the surface, or did he use a surface boundary condition? If he used air layers, what was the height extent? Did he use the same uniform grids for all periods, or did he change the grids for different periods where appropriate in order to enhance the accuracy? Without stating any convergence criteria, he does state that his iteration did converge, though slowly, but does this necessarily constitute proof that the numerical results are valid? If the differences between the PFC and the CDK results cannot be attributed to numerical inaccuracies in the PFC results due to possible difficulties with grid design, boundary conditions, convergence, etc., then the departure from the observed CDK 3D results might only be ...

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confidence: 82%

Reply to “Comment on ‘EM induction in elongated conductors normal to a coastline with application to geomagnetic measurements in Nigeria’ by J. Chen, H. W. Dosso, and S. Kang”

Dosso

Agarwal

2014

Earth Planet Sp

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“…Much attention has been paid to the coastline effect and its consequences on the determination of upper‐ and mid‐mantle electrical properties (Rikitake 1961; Bullard & Parker 1970; Parkinson & Jones 1979; Fainberg & Singer 1980; Roberts 1984; Takeda 1993; Chen & Dosso 1997). Several factors contribute to the magnitude of the coastline effect: frequency ω (or period T ), outline and bathymetry of the ocean basins, and the presence of subduction zones or crustal sutures which may serve as conductive pathways through the resisitive crust to deeper conductors (Gough 1989; Wannamaker et al .…”

Section: The Coastline Effectmentioning

confidence: 99%

Geomagnetic induction in a heterogeneous sphere: fully three-dimensional test computations and the response of a realistic distribution of oceans and continents

Weiss¹,

Everett²

1998

Geophys. J. Int.

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Long‐period geomagnetic data can resolve large‐scale 3‐D mantle electrical conductivity heterogeneities which are indicators of physiochemical variations found in the Earth’s dynamic mantle. A prerequisite for mapping such heterogeneity is the ability to model accurately electromagnetic induction in a heterogeneous sphere. A previously developed finite element method solution to the geomagnetic induction problem is validated against an analytic solution for a fully 3‐D geometry: an off‐axis spherical inclusion embedded in a uniform sphere. Geomagnetic induction is then modelled in a uniform spherical mantle overlain by a realistic distribution of oceanic and continental conductances. Our results indicate that the contrast in electrical conductivity between oceans and continents is not primarily responsible for the observed geographic variability of long‐period geomagnetic data. In the absence of persistent high‐wavenumber magnetospheric disturbances, this argues strongly for the existence of large‐scale, high‐contrast electrical conductivity heterogeneities in the mid‐mantle. Lastly, for several periods the geomagnetic anomaly associated with a mid‐mantle spherical inclusion is calculated. A high‐contrast inclusion can be readily detected beneath the outer shell of oceans and continents. A comparison between observed and computed c responses suggests that the mid‐mantle contains more than one order of magnitude of lateral variability in electrical conductivity, while the upper mantle contains at least two orders of magnitude of lateral variability in electrical conductivity.

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“…If situated near an ocean, the response of such a graben would be expected to be partially masked by the coast effect response. Removing the unwanted ocean induction components by subtracting numerically calculated, or analogue modelled coast effect induction arrows before interpretation has been studied by Wolf (1983), Weaver and Agarwal (1991), Dosso and Meng (1992), Chen (1994), Kang (1995), and Chen and Dosso (1997). The numerical model method, using thin sheet modelling, has been used by Bapat et al (1993) and Chamalaun and McKnight (1993) in geomagnetic surveys in Japan and New Zealand respectively, while the analogue model method of accounting for the coast effect has been applied to measurements at sites in coastal regions of North China , Japan , New Zealand (Chen et al, 1993, Dosso et al, 1996a, and Northwest Nigeria (Kang et al, 1993).…”

Section: Introductionmentioning

confidence: 99%

“…In a recent work, Chen and Dosso (1997) have used analogue models to study the response of elongated conductors with strikes parallel to an ocean coastline. They examined the dependence on period, the distance from the ocean, and the depth extent of the horizontal elongated conductors and also showed that the ocean effect could be successfully subtracted to yield the responses of the elongated conductor alone for conductors located as distances as small as 50 km from the coast.…”

Section: Introductionmentioning

confidence: 99%

EM Induction in Elongated Conductors Normal to a Coastline with Application to Geomagnetic Measurements in Nigeria

Chen

Dosso

Kang

1997

J. geomagn. geoelec

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Magnetic induction responses of elongated pairs and single conductors with strikes perpendicular to a local coastline of a deep ocean are studied with the aid of laboratory model measurements. The dependence on period, the width of the conductor, and the distance from the end of the conductors are examined for traverses over the conductors. It is shown that for the model geometry studied, the ocean induction effects can be subtracted to yield the Parkinson induction arrows for the conductor alone. At sites between pairs of parallel conductors the quadrature arrows compared with the in-phase arrows show a very differing period dependence. These results have particular application to the interpretation of geomagnetic measurements at sites between major elongated conductive structures such as sediment filled grabens.

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EM responses of an elongated conductor near an ocean — analogue model studies

Cited by 5 publications

References 21 publications

Reply to “Comment on ‘EM induction in elongated conductors normal to a coastline with application to geomagnetic measurements in Nigeria’ by J. Chen, H. W. Dosso, and S. Kang”

Reply to “Comment on ‘EM induction in elongated conductors normal to a coastline with application to geomagnetic measurements in Nigeria’ by J. Chen, H. W. Dosso, and S. Kang”

Geomagnetic induction in a heterogeneous sphere: fully three-dimensional test computations and the response of a realistic distribution of oceans and continents

EM Induction in Elongated Conductors Normal to a Coastline with Application to Geomagnetic Measurements in Nigeria

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