Saurabh K. Verma

Delineation of sediments below flood basalts by joint inversion of seismic and magnetotelluric data

A one-dimensional joint-inversion (JI) scheme considering seismic reflection and refraction, and MT data is developed. Its efficacy to resolve low velocity conducting sediments below high velocity resistive flood basalts is tested for a representative geological model considering noisy, incomplete data. The JI is found to provide improved results in comparison to those obtained by individual seismic and MT inversions.

Inversion of transient electromagnetic profile data using conductive finite plate model

A thin conductive plate in free space is one of the most commonly employed models in the interpretation of transient electromagnetic (TEM) profile data. Despite the fact that such a model ignores the influence of the currents induced in the host medium, it is found to be useful to represent the late-time TEM responses which are not severely affected by the host medium. An iterative linearized inversion scheme is designed that computes the Jacobian matrix from numerical differences by forward computations. Parameter steps are solved using the singular value decomposition and an adaptive damping method. Since the computation of the forward solution for the plate model is fast, the inversion method is suitable for microcomputer environment. The parameters used in the inversion are: strike length, depth extent, x-position, depth of burial, dip angle and conductance. The sensitivity analysis of the Jacobian matrix shows that the parameters that can be resolved from TEM profile data with the greatest accuracy are: the x-position, dip angle, depth of burial and conductance. However, it is possible to obtain reasonably good estimates also for the strike length and depth extent from TEM profile data.

QUASI‐STATIC TIME‐DOMAIN ELECTROMAGNETIC RESPONSE OF A HOMOGENEOUS CONDUCTING INFINITE CYLINDER

Under some simplifying assumptions, the transient electromagnetic response of a homogeneous conducting infinite cylinder is derived. Two types of primary disturbances, represented by step‐function and ramp‐function pulses, are considered. Based on the theoretical results, response and decay constant curves are plotted and their utility in time‐domain electromagnetic surveys for conductive elongated ore deposits is described.

The versatility of digital linear filters used in computing resistivity and em sounding curves

Abstract The applicability of EM filters to compute resistivity sounding curves and that of resistivity filters in computing EM sounding curves is shown. This has been possible mainly because of the flexible nature of the filtering process involved which permits us to redefine an input function in order to suit a particular filter function. The desired output can then be obtained by convolving the modified input with the chosen filter. This flexibility can be used advantageously to compute the electrical and EM response of a layered earth to a number of source-receiver configurations by using the resistivity and the EM filters individually or jointly, as required. The study leads us to establish the relationships among various coil configurations used in EM sounding. A great computational advantage accruing from the present study is that the nine point filter to compute Schlumberger resistivity curves can also be used to compute EM sounding curves for perpendicular and vertical coplanar loop systems on programmable pocket calculators such as the HP-67 or TI-59.

Focused resolution of thin conducting layers by various dipole EM systems

Electromagnetic sounding in the frequency domain can be performed in two ways—either by changing frequency at a location (frequency sounding) or by changing the transmitter‐receiver (T-R) separation using a fixed frequency (geometric sounding). These changes in frequency or separation parameters effect vertical scanning of conductivity distributions below the earth’s surface. In case of thin conducting layers, there could be an optimum range of frequencies or T-R separations that provide maximum resolution of the layer parameters. Thus, for a given buried target layer, it should be possible to find ranges of frequencies or separations that yield the best focusing. The present study deals with the focused resolution of a thin conducting layer in frequency sounding with variable T-R separation for four different dipole configurations. It is observed from the inversion of the data from various dipole electromagnetic (EM) systems that different T-R separations have different resolutions for the same target la...

TRANSIENT ELECTROMAGNETIC RESPONSE OF AN INHOMOGENEOUS CONDUCTING SPHERE

Analytic expressions for the quasi‐static electromagnetic response of a sphere in presence of unit‐step and ramp‐type time varying magnetic fields are derived. The conductivity inside the sphere is assumed to vary linearly with radius, i.e. σ(ρ)=σ0(ρ/a), where ρ is radial coordinate, σ0 is a constant and a is the radius of sphere. Curves showing the decay of the magnetic field for both types of fields are presented. In the case of ramp‐type applied magnetic field, the magnitudes of maxima of the induced magnetic field are found to decrease with increase in the rise time of the applied field and, hence, exciting pulses having small values of rise time should be used. It is believed that the analysis will be useful in the geoelectric exploration for highly conducting mineral deposits.

Response of a layered earth to the Crone pulse electromagnetic system

The response of a layered earth to the Crone pulse electromagnetic (PEM) system, which measures the decay of the time derivative Hz′(t) has been computed. The transient vertical magnetic field component Hz(t) due to a vertical magnetic dipole is obtained by applying a Fourier series summation approach and using digital linear filters to compute the response at individual frequencies. Oscillations in Hz(t) due to Gibb’s phenomena are suppressed with Lanczos weights, and the derivative Hz′(t) is computed numerically by using a linear difference approximation over five points. Decay curves for various half‐spaces are found to cross each other at different values of time. Thus, a single channel response cannot be used to estimate the half‐space resistivity uniquely. This can be achieved, however, by making use of responses at two different channels. Conductivity‐aperture diagrams for half‐space models are plotted for both Hz(t) and Hz′(t). For Hz(t), all the channel amplitudes show well‐defined peaks in the r...

Transient electromagnetic response of a permeable nonuniformly conducting sphere

Quasistatic response of a nonuniformly conducting and magnetically permeable sphere to a step pulse is obtained. the effect of conductivity distribution pattern and permeability of the sphere on the transient response is examined in detail. The results show that the introduction of conductivity inhomogeneity has considerable effect on the observed secondary fields and that the magnetic permeability contrast further

TIME‐DEPENDENT ELECTROMAGNETIC FIELDS OF AN INFINITE, CONDUCTING CYLINDER EXCITED BY A LONG CURRENT‐CARRYING CABLE

Theoretical and numerical computations have been made for the quasi‐static, time‐domain electromagnetic response of an infinite, conducting horizontal cylinder stimulated by long cable‐carrying step and ramp‐function type pulses. The effect of higher‐order induced multipoles on the secondary electric and magnetic field components is analyzed in detail, and the “threshold distances” at which individual multipoles become effective (contributing more than 5 percent of the secondary field) are presented. Also, the field fall‐off directly above the body and the variations in different induced‐field components along a traverse perpendicular to the strike of the ore body are examined.

The effect of induced multipoles on horizontal loop EM response of a permeable conducting sphere

Abstract For a horizontal loop EM (HLEM) system, the effect of permeability on the excitation of multipole moments in the presence of a spherical conductor has been investigated. For a permeable conductor, the higher-order induced multipoles are found to contribute significantly even at very low frequencies. Increase in permeability leads to reduction in the inphase amplitude and finally reverses the polarity of the component. Permeabilities estimated from uniform field formulae are lower than true permeabilities.