Upendra Raval

NEGATIVE ELECTROMAGNETIC SCREENING BY A CYLINDRICAL CONDUCTING COVER

The paradoxical augmentation of the electromagnetic response of an inner core by a conducting cover is reexamined by investigating the behavior of a cylindrical model to establish the geometrical independence of the negative screening phenomenon. The theory is developed for the coaxial cylindrical system in analogy with the spherical case. Results of numerical evaluation of the analytical expressions for the screening factor prove the geometrical independence of the phenomenon of negative screening, as indicated in earlier studies.

Seismicity of the Koyna Region and Regional Tectonomagmatism of the Western Margin (India)

Recent findings on the Meso-Cenozoic tectonomagmatism and deep-seated anomalous geophysical structures suggest a close linkage between the seismicity of the Koyna region, the Westernghat uplift (WG-U) and associated thermomechanical and fluid activities. The WG-U seems to be the result of late Cretaceous thermal mobilization, erosion of the Deccan trap cover and superposition of compressional stress. The association of seismicity with uplift seems to result from movement of deep-seated heat and fluids/volatiles along the edges (or boundary faults) of the uplift; because the force required for crustal deformation depends on the relief. Observed gradients in relief may be attributed to the differential erosion-rates and heat inputs, due to the time gap of ∼ 50 Ma in the break-ups and plume activities on the eastern and western sides and consequence magmatism. Further, the geology and tectonics strongly indicate that the western margin (WM) is a relic of a mobile arm (MA), that included Madagascar, and which formed a part of the Proterozoic mobile belt of ‘greater India’ (fort>85 Ma). The mobile nature of the WM facilitates mantle upwellings and transient elevation of isotherms at depth, raising the possibility of intermittent metamorphism and greater deformation.Superposition of the ongoing compression and uplift-induced forces make local permeability and pore-fluid pressure vital in triggering the seismic slip over the Peninsular shield. Certain representative model calculations have been carried out to estimate change in the e.m. induction characteristics caused by an intermittent hydraulic connectivity. The results show a drop in the resistivity which could be a useful monitoring index. The close connection of uplift and fluid activity as discussed here seems applicable for other active parts of the South Indian Shield (SIS) also.

Electromagnetic response of a permeable inhomogeneous conducting sphere

Abstract Electromagnetic response of a two-layered spherical model to a dipolar field has been calculated considering the simultaneous influences of a permeable conducting cover, magnetic permeability contrast with the surrounding medium, and radial conductivity variation inside the core. Computations have been made and representative curves are given to provide an insight of the contribution of these physical parameters. For certain conductivities of the cover, the response of the composite system has been found to decrease with increasing core-conductivity. The increase in the induction number of the covering shell makes it difficult to interpret the response data. The multifrequency response of the system exhibits two peaks which are diagnostic of the two layers.

Turam response of a permeable inhomogeneous cylindrical conductor surrounded by a thin shell

SummaryElectromagnetic response characteristics of an inhomogeneous permeable cylinder surrounded by a thin insulated concentric dissipative shell is obtained in presence of a line-source of current. The 3-layer boundary value problem is solved by assuming the thickness of the shell to be much smaller than the skin-depth in the shell. Numerically computed results are presented in the form of argand diagrams to examine the influence of the shell parameters (conductivity and its proximity with the core) and magnetic contrast. The multi-frequency response of the system is also given which resolves the two conducting layers and analyzes the frequency dependence of screening behavior of the shell.

Electromagnetic scattering due to deformed inhomogeneous bodies; Part II, Cylinder

SummaryThe potential technique of the perturbation has been employed to find the analytical expressions for the response of conducting bodies of irregular shapes. In Part I spherical bodies and in Part II cylindrical ones are considered. The conductivities are assumed to be finite and radially varying. The deformation of the boundaries in both the cases are considered to be of the typekp =r0[1++ηf1(θ, φ)] where η is the perturbation parameter. Calculations have been made for uniform as well as non-uniform excitation fields. Since the models represent the geological ore-formation more closely, the results will find application in preparing type curves for induction prospecting.

Electromagnetic response of a covered inhomogeneous permeable conductor in time domain

SummaryAn initial value induction problem has been solved to examine the response of a massive sulphide ore body under conditions of common occurence. The generalized expression for the induced field due to a step-function dipolar field in the presence of a non-uniformly conducting permeable sphere surrounded by a finitely resistive overburden has been derived. Two cases, when the concentric shell is (i) insulated from and (ii) in galvanic contact with the inner core, have been considered. The analytical results are in computable form to yield the decay-characteristics for application in geoelectromagnetics. Particular cases of the problem treated by the other investigators are discussed.

Quasi-stationary electromagnetic response of covered permeable conductors to some pulses of spatially uniform magnetic field

SummaryThe quasi-stationary electromagnetic response of permeable conductors surrounded by conducting halo and excited by a uniform magnetic field having a step function time dependence has been obtained theoretically for both spherical and cylindrical conductors. For the magnetic conductors the results give the long period limit of the response and define a critical time complementary to critical frequency (Ward [1]2)) which may be used to estimate the magnetic content. Further utilizing the principle of superposition the responses to ramp, half-sine, saw tooth and triangular pulse excitations are also given.

Induced Current Distribution In A Conductive Two-layer Spherical Body

Under complex geologic situations the analysis of induced current distribution, rather than of the scattered field, may provide a useful physical insight into the electromagnetic response characteristics of a conducting ore body. Influence of a conducting cover and inhomogeneity in the conductivity on the currents induced in aspherical conductor excited by a uniform alternating magnetic field has, therefore, been investigated. The analysis shows (a) reduction of the current‐density amplitudes in the core by the cover, (b) occurrence of current‐density maxima inside the conducting system for some characteristic frequencies, (c) enhancement of the in‐phase component in some cases, and (d) reduction of current‐density in the shell with increase in the core conductivity and significant influence of the inhomogeneity in conductivity at low frequencies. Since there is a direct correspondence between the electromagnetic response of a target and the currents induced in it, these results are expected to aid in the...

Quasi-static transient response of a covered permeable inhomogeneous cylinder to a line current source

SummaryTransient electromagnetic response of a cylindrical conductor covered by (a) an insulated and (b) a galvanically connected coaxial shell has been obtained. The exciting pulse is produced by switching off a long alternating current source. The study incorporates the influence of magnetic permeability contrast of the cylinder with the surrounding medium and also that of the inhomogeneity in the conductivity. The generalised initial value problem is significant in the study of certain geomagnetic anomalies. The results will also aid to the interpretation of induction prospecting data for elongated porphyry conducting mineral deposits.