A new vertical continuation procedure for airborne electromagnetic field data from the modified image method

Geophysics ◽  
1999 ◽  
Vol 64 (5) ◽  
pp. 1364-1368 ◽  
Author(s):  
Clyde J. Bergeron ◽  
John R. Brusstar ◽  
Ningke Yi ◽  
Yan Wu ◽  
Juliette W. Ioup
Keyword(s):  
Accurate Method ◽  
Skin Depth ◽  
Flight Path ◽  
Integral Representations ◽  
Algebraic Expression ◽  
Image Method ◽  
Sommerfeld Integral ◽  
Horizontal Variation ◽  
Conductivity Anomalies ◽  
Airborne Electromagnetic

Airborne electromagnetic (AEM) data measured by equipment in a bird tethered to a helicopter have large variations caused by the unavoidable vertical excursions of the helicopter as it traverses its flight path. Such large changes tend to mask the smaller changes in field strength caused by lateral variations in the earth’s electrical conductivity along the flight path, which is the information that is the goal of AEM surveys. Signals produced by conductivity anomalies such as sea‐ice keels and pipelines in marshes or in the shallow ocean are enhanced and may be apparent directly in the continued fields. Furthermore, electronic or environmental noise is more easily detected in the continued fields and reduced by various methods of filtering and signal processing. In the modified image method (MIM) formalism for AEM fields, the algebraic expression for the secondary to primary field ratio [Formula: see text] is given in terms of R, where R is the total complex vertical separation of the primary and image dipoles [Formula: see text] scaled to the coil spacing ρ, [Formula: see text] is the complex effective skin depth, and h is the altitude of the bird. An inverse algebraic relation gives R as a function of [Formula: see text]. In this paper we present a simple and accurate method of continuing the field by way of continuing R. Because R is linear in h, the vertical continuation of R from h to h0 is accomplished by a simple linear translation. This method is applied to a flight line of an AEM survey of Barataria Bay, Louisiana, which includes both the marsh and a near‐shore region of the Gulf of Mexico. The smoothnesss of the continued data over the Gulf implies that the variability of the continued data over the marsh is attributable to horizontal variation in salinity, soil porosity, and water depth rather than noise. To produce more accurate values for R, we have also included details of an extended half‐space renormalization function which, in effect, removes residual differences between the fields calculated from the MIM algebraic and the numerical evaluation of the exact Sommerfeld integral representations of the [Formula: see text] field.

Interpretation of airborne electromagnetic data using the modified image method

Geophysics ◽  
1989 ◽  
Vol 54 (8) ◽  
pp. 1023-1030 ◽  
Author(s):  
Clyde J. Bergeron ◽  
Juliette W. Ioup ◽  
Gus A. Michel
Keyword(s):  
Synthetic Data ◽  
Low Frequency ◽  
Ground Truth ◽  
Skin Depth ◽  
Cape Cod ◽  
Image Method ◽  
Radar Altimeter ◽  
Cape Cod Bay ◽  
Airborne Electromagnetic ◽  
Airborne Electromagnetic Data

The modified image method is used to invert active electromagnetic (AEM) data from a 1984 U. S. Navy survey of Cape Cod Bay. The high‐frequency data (7200 Hz) give a robust value for the altitude of the helicopter‐towed AEM bird and for the first‐layer skin depth and, hence, for the first‐layer conductivity. The inversion of low‐frequency (385 Hz) bottom‐probing signals produces more noise‐sensitive estimates for the water depth and for the conductivity contrast K, the ratio of the bottom to water conductivities. The results show good agreement with “ground‐truth” radar altimeter, sea conductivity, and sonar depth data. To demonstrate the accuracy of the modified image method of inversion, we incorporate ground‐truth measurements along a flight line and the experimental frequencies in a forward Sommerfeld calculation to generate synthetic data which then are inverted using this technique.

Removal of static shift in two dimensions by regularized inversion

Geophysics ◽  
1991 ◽  
Vol 56 (12) ◽  
pp. 2102-2106 ◽  
Author(s):  
Catherine deGroot‐Hedlin
Keyword(s):  
Electric Fields ◽  
Apparent Resistivity ◽  
Skin Depth ◽  
Two Dimensions ◽  
Small Scale ◽  
Low Frequencies ◽  
Near Surface ◽  
Regularized Inversion ◽  
Conductivity Anomalies ◽  
Local Distortions

A common problem in magnetotelluric (MT) sounding is the presence of static shifts in the data, i.e., a vertical shifting of the log‐apparent‐resistivity versus period curves relative to regional values (Jones, 1988; Jiracek, 1990; Berdichevsky et al., 1989). These static shifts are due to the presence of small‐scale, shallow conductivity anomalies near the measurement site. Electric charge builds up on near‐surface anomalies that are small in comparison to the skin depth of the electromagnetic (EM) fields. The charge buildup produces a perturbation of the measured electric fields from their regional values that persists to arbitrarily low frequencies. Incorrect removal of these local distortions leads to incorrect interpretation of the deeper targets of investigation.

Investigation of dryland salinity using the electrical image method

Soil Research ◽  
1999 ◽  
Vol 37 (4) ◽  
pp. 623 ◽  
Author(s):  
R. I. Acworth
Keyword(s):  
Electrical Conductivity ◽  
Salt Content ◽  
Image Method ◽  
Dryland Salinity ◽  
Conductivity Variation ◽  
South Wales ◽  
Electrical Imaging ◽  
Profile Line ◽  
Conductivity Anomalies ◽  
A Site

Electrical imaging is a 2-dimensional investigation method that can be used to rapidly determine subsurface conductivity variation. In dryland salinity studies, electrical imaging is used to define the vertical extent of high electrical conductivity zones first identified using electromagnetic (EM) profiling equipment. Field techniques are described using 25 or 50 electrodes, connected to a resistance meter by a multi-core cable, to obtain images at a variety of electrode separations. The model of electrical conductivity variation obtained by an inversion of the field data is shown to agree very well with the results of detailed field investigations, including data from soil sampling, 1 : 5 extract analysis, and borehole electrical conductivity logging. Results are described from the Liverpool Plains at Yarramanbah Creek and Round Island, where a thick sequence of smectite clay overlies sands and gravels. The image clearly identifies zones of high salt content in the clay which have been sampled and logged using borehole measurements of electrical conductivity. Results are also described from a dryland salinity area in the upper part of Dicks Creek catchment on the Southern Tablelands of New South Wales. These data show the extent of clay overlying bedrock and correlate very well with the results of 1 : 5 extract analysis from shallow piezometers along the profile line. Electrical imaging is an appropriate follow-up method for the investigation of electrical conductivity anomalies first identified by EM profiling and is advisable before drilling at a site to optimise the location of piezometers.

Electromagnetic diffraction by arbitrary-angle impedance wedges

2007 ◽  
Vol 464 (2089) ◽  
pp. 177-195 ◽  
Author(s):  
Andrey V Osipov ◽  
Thomas B.A Senior
Keyword(s):  
Plane Electromagnetic Wave ◽  
Integral Representations ◽  
Functional Difference ◽  
Algebraic Equations ◽  
Arbitrary Angle ◽  
Impedance Boundary ◽  
Sommerfeld Integral ◽  
Functional Difference Equations ◽  
Tensor Impedance ◽  
Electromagnetic Diffraction

The problem of the diffraction of a plane electromagnetic wave incident at an oblique angle on a wedge of arbitrary angle with general tensor impedance boundary conditions is solved using a semi-analytical approach. Application of Maliuzhinets' method transforms the boundary-value problem into coupled functional difference equations (FDEs) for two unknown Sommerfeld integral spectra in a basic strip. By explicitly separating out the singular parts of the spectra in the strip, followed by a partial inversion of the FDEs, we obtain integral representations of the regular parts of the spectra. The regular parts are then expanded in a Taylor series in terms of a new variable that conformally maps the strip on to a disc. This expansion reduces the integral representations to algebraic equations for the series coefficients and these are solved numerically. We examine the convergence of the procedure, compare the numerical solution with an available reference solution and present solutions of new problems.

scholarly journals The Finite-Conducting Ground’s Effect on the Inductance of a Rectangular Loop

2016 ◽  
Vol 2016 ◽  
pp. 1-11 ◽  
Author(s):  
Xiao Jia ◽  
Lihua Liu ◽  
Guangyou Fang
Keyword(s):  
Time Domain ◽  
Accurate Method ◽  
Perfect Conductor ◽  
Current Waveform ◽  
Electromagnetic System ◽  
Resonance Circuit ◽  
Airborne Electromagnetic

In an airborne electromagnetic system, which transmits time-domain half-sine current waves generated by a resonance circuit, the inductance of the transmitting loop is of great significance and directly related to parameters of the half-sine current waveform. However, in general, the effect of a finite-conducting ground on the inductance of the transmitting loop was neglected, or the ground was handled as a perfect conductor. In other words, there was no accurate method to evaluate ground’s effect on the inductance of the transmitting loop. Therefore, a new and convenient algorithm, calculating ground’s effect on the inductance of a rectangular loop, is proposed in this paper. An experiment was constructed afield, showing that the inductance increased gradually when the loop was lifted up from 0 m to 30 m, which supported the algorithm positively.

A grounded vertical long wire source system for plane wave magnetotelluric analog modeling

Geophysics ◽  
1980 ◽  
Vol 45 (10) ◽  
pp. 1523-1529 ◽  
Author(s):  
R. N. Edwards
Keyword(s):  
Plane Wave ◽  
Current Source ◽  
Skin Depth ◽  
Uniform Field ◽  
Induced Current ◽  
Horizontal Magnetic Field ◽  
Helmholtz Coils ◽  
Analog Modeling ◽  
Conductivity Anomalies ◽  
External Connections

A typical electromagnetic (EM) analog modeling apparatus consists of an electrolytic tank with embedded graphite blocks, representing conductivity anomalies. A plane wave magnetotelluric (MT) source is generated by alternating currents in a set of parallel horizontal overhead wires. A uniform horizontal magnetic field is produced over the surface of the electrolyte. A similar uniform field may also be generated by grounded semiinfinite vertical wires. Four such wires, two carrying current upward and two downward, when arranged at the corners of a rectangle of defined dimensions, generate a more uniform field than a corresponding pair of Helmholtz coils. If the size of the rectangle is large compared with a skin depth in the electrolyte, Cagniard’s MT relationships are obeyed both on and beneath the electrolyte. The vertical current source has the advantage over the horizontal current source since it requires no ancillary external connections between the ends of the modeling tank to complete the induced current circuit.

Sensitivity functions of frequency-domain magnetic dipole-dipole systems

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. F45-F56 ◽  
Author(s):  
Rasmus Juhl Tølbøll ◽  
Niels Bøie Christensen
Keyword(s):  
Frequency Domain ◽  
Magnetic Dipole ◽  
Central Zone ◽  
Practical Investigations ◽  
Flight Path ◽  
Lateral Resolution ◽  
Sensitivity Distribution ◽  
Airborne Electromagnetic ◽  
Lateral Extent ◽  
Target Characteristics

The resolution capabilities of airborne electromagnetic (AEM) frequency-domain systems are traditionally analyzed in terms of the footprint, which provides a simple measure of the lateral extent of the earth volume involved in a given measurement. However, considerably more detailed insight into the system resolution capabilities can be obtained by studying the 3D sensitivity distribution as defined by the Fréchet derivatives. A qualitative analysis of the 3D sensitivity distributions for six typical magnetic dipole-dipole configurations demonstrates that the spatial resolution characteristics differ widely and that the optimal coil configuration for practical investigations depends on the expected target characteristics. For all six coil configurations, the 3D sensitivity distributions reveal significant sign changesdownwards and outwards from the center, stressing the necessity of reliable starting models for successful inversion of frequency-domain AEM data. Likewise, the central zone of sensitivity for the in-phase component is always larger than for the quadrature, indicating an inferior lateral resolution of the former. A new sensitivity footprint is defined, based on the at-surface behavior of the sensitivity distribution, simply as the area where the sensitivity is at least 10% of its maximum value. For the vertical coaxial (VCA) coil configuration, the size of the sensitivity footprint in the [Formula: see text]-direction (perpendicular to the flight path) is approximately a factor of two larger than in the [Formula: see text]-direction (along the flight path), while there is virtually no difference for the horizontal coplanar (HCP) coil configuration. The ratio of the HCP to VCA sensitivity footprint exceeds one in both [Formula: see text]- and [Formula: see text]-directions, suggesting that the VCA coil configuration has the best lateral resolution.

Application of the modified image method to inversion of synthetic active electromagnetic bathymetric data

Geophysics ◽  
1987 ◽  
Vol 52 (6) ◽  
pp. 794-801 ◽  
Author(s):  
Clyde J. Bergeron ◽  
Juliette W. Ioup ◽  
Gus A. Michel
Keyword(s):  
Correction Factor ◽  
Image Plane ◽  
Skin Depth ◽  
Correction Function ◽  
Wave Form ◽  
Image Method ◽  
Correction Factors ◽  
Electromagnetic Data ◽  
Good Agreement ◽  
Electromagnetic Source

A modified image method is used to invert synthetic active electromagnetic data generated from shallow ocean models by means of the exact Sommerfeld theory. The modified image method offers approximate solutions to the Sommerfeld problem: the response of a layered structure of ohmic conductors to an active electromagnetic source. The image plane for the active electromagnetic source is assumed to be at a complex depth given by [Formula: see text], where [Formula: see text] is the skin depth of the first layer and [Formula: see text] is the multilayer correction factor. Two ad hoc correction factors have been introduced into the modified image method. They bring the image field [Formula: see text] of the single image at a complex depth into very good agreement with the secondary field [Formula: see text] of the exact Sommerfeld theory of a two‐layer bathymetric model. We demonstrate that these correction factors can be calculated from synthetic data. This allows for an accurate, algebraic, and fast inversion of active electromagnetic data in terms of the parameters of a two‐layer model: the conductivities of the sea and sea bottom, and the depth and ohmic skin depth of the sea. The β factor, which takes into account the departure of the primary active electromagnetic field from a plane wave form at the air‐sea interface, is incorporated into the complex two‐layer correction function [Formula: see text]. A rescaling factor F brings [Formula: see text] into good agreement with [Formula: see text] in the depth regime where the β correction factor is ineffective, i.e., where the first layer depth is greater than two skin depths.

Application of Lattice Imaging Techniques to Amorphous Films

1975 ◽  
Vol 33 ◽  
pp. 8-9
Author(s):  
S. R. Herd ◽  
P. Chaudhari
Keyword(s):  
Nearest Neighbor ◽  
Random Network ◽  
Imaging Techniques ◽  
Network Models ◽  
Accurate Method ◽  
Direct Transmission ◽  
Lattice Imaging ◽  
Transmission Electron ◽  
Lattice Planes ◽  
Nearest Neighbor Distances

Electron diffraction and direct transmission have been used extensively to study the local atomic arrangement in amorphous solids and in particular Ge. Nearest neighbor distances had been calculated from E.D. profiles and the results have been interpreted in terms of the microcrystalline or the random network models. Direct transmission electron microscopy appears the most direct and accurate method to resolve this issue since the spacial resolution of the better instruments are of the order of 3Å. In particular the tilted beam interference method is used regularly to show fringes corresponding to 1.5 to 3Å lattice planes in crystals as resolution tests.

The effect of temperature on high-resolution TEM image contrast

1995 ◽  
Vol 53 ◽  
pp. 640-641
Author(s):  
T. Geipel ◽  
W. Mader ◽  
P. Pirouz
Keyword(s):  
Form Factor ◽  
Inelastic Scattering ◽  
Accurate Method ◽  
Waller Factor ◽  
Effect Of Temperature ◽  
Specimen Thickness ◽  
Influence Of Temperature ◽  
Debye Waller Factor ◽  
Influence Of Temperature On ◽  
Atomic Form Factor

Temperature affects both elastic and inelastic scattering of electrons in a crystal. The Debye-Waller factor, B, describes the influence of temperature on the elastic scattering of electrons, whereas the imaginary part of the (complex) atomic form factor, fc = fr + ifi, describes the influence of temperature on the inelastic scattering of electrons (i.e. absorption). In HRTEM simulations, two possible ways to include absorption are: (i) an approximate method in which absorption is described by a phenomenological constant, μ, i.e. fi; - μfr, with the real part of the atomic form factor, fr, obtained from Hartree-Fock calculations, (ii) a more accurate method in which the absorptive components, fi of the atomic form factor are explicitly calculated. In this contribution, the inclusion of both the Debye-Waller factor and absorption on HRTEM images of a (Oll)-oriented GaAs crystal are presented (using the EMS software.Fig. 1 shows the the amplitudes and phases of the dominant 111 beams as a function of the specimen thickness, t, for the cases when μ = 0 (i.e. no absorption, solid line) and μ = 0.1 (with absorption, dashed line).

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