Magnetite mapping with a multicoil airborne electromagnetic system

Geophysics ◽  
1981 ◽  
Vol 46 (11) ◽  
pp. 1579-1593 ◽  
Author(s):  
Douglas C. Fraser
Keyword(s):  
Half Space ◽  
Eddy Current ◽  
Weight Percent ◽  
Positive Sign ◽  
Mapping Technique ◽  
Current Response ◽  
Magnetic Polarization ◽  
Electromagnetic System ◽  
Homogeneous Half Space ◽  
Frequency Independent

The information content of data from an in‐phase quadrature electromagnetic (EM) system consists of a combination of conductive eddy current response and magnetic polarization response. The secondary field resulting from conductive eddy current flow is frequency‐dependent and consists of both in‐phase and quadrature components of positive sign. Conversely, the field resulting from magnetic polarization is commonly frequency‐independent and consists of only an in‐phase component of negative sign. A magnetite mapping technique was developed for the horizontal coplanar coils of a closely coupled multicoil airborne EM system. The technique yields contours of apparent weight percent magnetite when using a homogeneous half‐space model. The method can be complementary to magnetometer mapping in certain cases. Compared to magnetometry, it is far less sensitive but is more able to resolve closely spaced magnetite zones. The method is also independent of remanent magnetism and magnetic latitude. It is sensitive to .25 percent magnetite by weight when the sensor is at a height of 30 m above a magnetitic half‐space. It can individually resolve steeply dipping narrow magnetite‐rich bands which are separated by 60 m.

Depth sounding by means of a coincident coil frequency‐domain electromagnetic system

Geophysics ◽  
1997 ◽  
Vol 62 (1) ◽  
pp. 49-55 ◽  
Author(s):  
Kenneth Duckworth ◽  
Edward S. Krebes
Keyword(s):  
Frequency Domain ◽  
Half Space ◽  
Free Space ◽  
Thin Sheet ◽  
Electromagnetic System ◽  
Homogeneous Half Space ◽  
Scale Modeling ◽  
Physical Scale ◽  
Depth Sounding ◽  
Theoretical Developments

The concept of electromagnetic depth sounding by means of a coincident‐coil frequency‐domain electromagnetic system is developed in theory and demonstrated by means of physical scale modeling. The concept is based on the use of distance from the target as the sounding variable. The theoretical developments are confined to soundings conducted in free‐space with respect to either a homogeneous half‐space or a thin sheet conductor in conditions that approach the resistive limit. The use of distance from the target as the sounding variable becomes practical when the sounding system is a single compact unit of the type that a coincident coil concept inherently provides. In this method of sounding, the distance from the target is determined by taking the ratios of the fields measured at a variety of distances from the target conductor. This permits not only the distance to the target to be determined but also the direction to that target as may be of interest in soundings conducted in mines.

QUANTITATIVE INTERPRETATION OF INPUT AEM MEASUREMENTS

Geophysics ◽  
1973 ◽  
Vol 38 (6) ◽  
pp. 1145-1158 ◽  
Author(s):  
G. J. Palacky ◽  
G. F. West
Keyword(s):  
Decay Rate ◽  
Half Space ◽  
Half Plane ◽  
Quantitative Approach ◽  
Quantitative Interpretation ◽  
Electromagnetic System ◽  
Quantitative Models ◽  
Apparent Conductivity ◽  
Homogeneous Half Space ◽  
Airborne Electromagnetic

Recent improvements of the INPUT airborne electromagnetic system have made possible a more quantitative approach to interpretation. The necessary interpretational aids can be obtained in two ways: either by correlating the system and ground EM measurements, or by devising computational or analog quantitative models. Both approaches have been explored. In the former, the system decay rate can be correlated with the apparent conductivity‐thickness (σt) estimated by ground surveys. In the latter, four quantitative models were investigated, vertical half‐plane, vertical ribbon, dipping half‐plane, and homogeneous half‐space. Nomograms have been constructed which make it possible to determine σt, conductor depth, and dip for sheet‐like conductors, and conductivity for a homogeneous half‐space. Field examples show that this procedure can be used satisfactorily in the routine interpretation of records obtained by this system.

Eddy-current imaging of defects in a conductive half-space

1992 ◽  
Author(s):  
Rodrigo de Oliveira Bohbot ◽  
Dominique Lesselier ◽  
Bernard Duchene ◽  
Nathalie Coutanceau
Keyword(s):  
Half Space ◽  
Eddy Current ◽  
Eddy Current Imaging

Inversion of slingram electromagnetic induction data using a Born approximation

Geophysics ◽  
2013 ◽  
Vol 78 (4) ◽  
pp. E201-E212 ◽  
Author(s):  
Jochen Kamm ◽  
Michael Becken ◽  
Laust B. Pedersen
Keyword(s):  
Half Space ◽  
Born Approximation ◽  
High Efficiency ◽  
Three Dimensions ◽  
Background Model ◽  
Desktop Computer ◽  
Near Surface ◽  
Homogeneous Half Space ◽  
Integral Equation Formulation ◽  
Forward Operator

We present an efficient approximate inversion scheme for near-surface loop-loop EM induction data (slingram) that can be applied to obtain 2D or 3D models on a normal desktop computer. Our approach is derived from a volume integral equation formulation with an arbitrarily conductive homogeneous half-space as a background model. The measurements are not required to fulfill the low induction number condition (low frequency and conductivity). The high efficiency of the method is achieved by invoking the Born approximation around a half-space background. The Born approximation renders the forward operator linear. The choice of a homogeneous half-space yields closed form expressions for the required electromagnetic normal fields. It also yields a translationally invariant forward operator, i.e., a highly redundant Jacobian. In connection with the application of a matrix-free conjugate gradient method, this allows for very low memory requirements during the inversion, even in three dimensions. As a consequence of the Born approximation, strong conductive deviations from the background model are underestimated. Highly resistive anomalies are in principle overestimated, but at the same time difficult to resolve with induction methods. In the case of extreme contrasts, our forward model may fail in simultaneously explaining all the data collected. We applied the method to EM34 data from a profile that has been extensively studied with other electromagnetic methods and compare the results. Then, we invert three conductivity maps from the same area in a 3D inversion.

Numerical seismograms obtained using ω-κ integrals, for a point P source in a vertically inhomogeneous anelastic model

1996 ◽  
Vol 86 (3) ◽  
pp. 750-760
Author(s):  
F. Abramovici ◽  
L. H. T. Le ◽  
E. R. Kanasewich
Keyword(s):  
Half Space ◽  
Homogeneous Layer ◽  
Variable Step Size ◽  
Step Size ◽  
Adaptive Process ◽  
Cpu Time ◽  
Homogeneous Half Space ◽  
Linear Layer ◽  
Variable Step ◽  
Viscoelastic Layers

Abstract This article presents some numerical experiments in using a computer program for calculating the displacements due to a P source in a vertically inhomogeneous structure, based on the Fourier-Bessel representation. The structure may contain homogeneous, inhomogeneous, elastic, or viscoelastic layers. The source may act in any type of sublayer or in the half-space. Synthetic results for the simple case of a homogeneous layer overlaying a homogeneous half-space compare favorably with computations based on the Cagniard method. Numerical seismograms for an elastic layer having velocities and density varying linearly with depth were computed by integrating numerically the governing differential systems and compared with results based on the Haskell model of splitting the linear layer in homogeneous sublayers. Even an adaptive process with a variable step size based on the Haskell model has a poorer performance on the accuracy-cpu time scale than numerical integration.

Surface-wave dispersion computations

1970 ◽  
Vol 60 (2) ◽  
pp. 321-344 ◽  
Author(s):  
Fred Schwab ◽  
Leon Knopoff
Keyword(s):  
Half Space ◽  
Rayleigh Waves ◽  
Wave Dispersion ◽  
Reduction Technique ◽  
Surface Wave Dispersion ◽  
Single Precision ◽  
Homogeneous Half Space ◽  
Full Control ◽  
Significant Figures ◽  
Rayleigh Wave Dispersion

abstract Fundamental-mode Love- and Rayleigh-wave dispersion computations for multilayered, perfectly-elastic media were studied. The speed of these computations was improved, and the accuracy brought under full control. With sixteen decimal digits employed in these computations, fifteen significant-figure accuracy was found possible with Love waves and twelve to thirteen figure accuracy with Rayleigh waves. In order to ensure that the computed dispersion is correct to a specified accuracy, say σ significant figures, (σ + 1)/4 wavelengths of layered structure must be retained above a homogeneous half-space. To this accuracy, the homogeneous half-space is a sufficient model of the true layering it replaces. Using this result, it was possible to refine the usual layer-reduction technique so as to ensure retention of the specified accuracy while employing reduction. With this reduction technique in effect, and with σ specified below single-precision accuracy, the program can be run entirely in single precision; the specified accuracy is maintained without overflow or loss-of-precision problems being encountered during calculations.

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