Yves Lamontagne

Imaging quasi‐layered conductive structures by simple processing of transient electromagnetic data

An “imaged” conductivity section of a layered earth can be obtained by simple transformation of step‐response electromagnetic data measured in the quasi‐static zone. This method of data transformation is presented as an alternative to conventional apparent conductivity transformations. At each delay time, the variation of the step response as a function of geometry (transmitter and receiver location) is transformed to an equivalent reference depth h, which can be related to the depth of electromagnetic field diffusion. The behavior of h as a function of delay time is nearly independent of the source‐receiver geometry. The slowness dt/dh divided by the magnetic permeability is almost exactly proportional to the cumulative conductance measured from the surface down to a depth h. Thus we can estimate an apparent conductivity, which we call the “imaged conductivity,” at depth to be d2t/μ0dh2. The cost of this transformation is a fraction of the cost of conventional data inversion, and it does not require an a...

Conductivity‐depth imaging of airborne electromagnetic step‐response data

An adaptation of the Macnae‐Lamontagne method allows transform of airborne step‐response electromagnetic (EM) data to a conductivity‐depth image. The algorithm is based on a nonlinear transformation of the amplitude of the measured response at each delay time to an apparent mirror image depth. Using matrix algebra, the set of mirror image depth‐delay time data pairs can then be used to derive a conductivity section. Data can be efficiently processed on a personal computer at rates faster or comparable to the rate required for collection. Stable conductivity fitting as a function of depth is obtained by damping the matrix inversion by specification of the first‐ and second‐derivative smoothness weights of the fitted conductivity‐depth sounding. Damping parameters may be either fixed or varied along the profile; their choice can be constrained by geologic control. Stability of the process is enhanced by accounting for the transmitter and receiver tilts. The mirror image depth‐delay time data can also be use...

A time‐domain EM system measuring the step response of the ground

A wide-band time-domain EM system, known as UTEM, which uses a large fixed transmitter and a moving receiver has been developed and used extensively in a variety of geologic environments. The essential characteristics that distinguish it from other systems are that its system function closely approximates a step-function response measurement and that it can measure both electric and magnetic fields. Measurement of step rather than impulse response simplifies interpretation of data amplitudes, and improves the detection of good conductors in the presence of poorer ones. Measurement of electric fields provides information about lateral conductivity contrasts somewhat similar to that obtained by the gradient array resistivity method.

Noise processing techniques for time‐domain EM systems

A variety of signal processing techniques can be used to minimize the effects of noise on linear, wideband, electromagnetic (EM) systems operating in the time‐domain. All systems use repetitive waveforms with polarity reversal in alternate half‐cycles. Exponential averaging or digital integration (stacking) is employed to increase signal‐to‐noise (S/N) ratios by limiting the noise acceptance to narrow frequency bands centered on odd harmonics of the repetition frequency, the width of the acceptance bands being inversely proportional to stacking time. For certain types of nonstationary noise (e.g., occasional transients) or coherent noise (e.g., powerlines) it is possible to increase S/N ratios above those obtained by simple stacking for an equal time by use of techniques such as pruning, tapered stacking or randomized stacking. With some system waveforms and when the noise spectrum is not “white”, use of preemphasis filtering in the transmitter and a corresponding de‐emphasis filter in the receiver may si...

Em Response Of A Rectangular Thin Plate

By formulating the fundamental electromagnetic (EM) equations in terms of the stream potential of the surface current density, we can express the EM response of a rectangular thin plate as the solution of a single equation subject to simple boundary conditions. A finite difference approximation of this equation reduces the problem to that of solving a large set of linear algebraic equations. The solution of these equations by a modified Gauss‐Seidel iterative method yields the stream potential and thus permits visualization of the eddy currents circulating inside the plate conductors. The secondary field calculated from the stream potential compares well with that given by scale model measurements provided that the grid spacings used in the finite differences are small enough. Using a further approximation, we can also simulate inductively thick conductors if the conductors are not geometrically thick.

Multiple conductor modeling using program MultiLOOP

We have developed a model that calculates the Electromagnetic (EM) response for multiple, planar conductors located in a resistive host. The program, called MultiLOOP, will handle any system waveform in time or frequency domain and will model fixed, moving and coincident loop receiver geometries. The assumption made to simplify the solution to allow for implementation on a microcomputer is that the current in the plates can be constrained to flow in ribbons, and that no currents may galvanically cross from one ribbon to another. Provided the tramnsmitter and receiver are reasonably distant from the conductors, the computed EM response agrees well with that computed by other modelling programs.

Geophysical application of a new surface integral equation method for EM modeling

We describe some 3-D electromagnetic (EM) geophysical applications of a new frequency-domain iterative surface integral equation (SIE) method. The new method is formulated by applying the scalar Greens theorem to the three Cartesian components of the magnetic field individually. The boundary conditions are implemented through iterations. The convergence behavior of the iteration is demonstrated explicitly through an example. Frequency-domain results for a number of simple geophysical models are given. The transient response of one model, obtained via inverse Fourier transform, is shown to be consistent with previously published results. The method is found to be stable for a wide range of conductivity contrasts and capable of giving reasonable descriptions of the EM response from the resistive limit up to the inductive limit.

The effect of discrete conductivity isotropy on electromagnetic surveys

The effect of discrete conductivity anisotropy on electromagnetic survey interpretation is examined with a numerical modeling study. Discretely anisotropic media are shown to mimic the characteristics of horizontally layered media, alter the apparent decay constants of conductors, and profoundly alter the amplitudes of the anomalous response. If unrecognized, the effects of discrete conductivity anisotropy on (mis)interpretation can be significant. The illustrations we present are representative of cases encountered in the Athabasca Basin in Canada.