Simple inversion of time‐domain electromagnetic data

Geophysics ◽  
1984 ◽  
Vol 49 (7) ◽  
pp. 925-933 ◽  
Author(s):  
C. T. Barnett
Keyword(s):  
Time Domain ◽  
Eddy Currents ◽  
Early Time ◽  
Late Time ◽  
Current Filament ◽  
Equivalent Current ◽  
Time Domain Electromagnetic ◽  
Inversion Procedure ◽  
Current Filaments ◽  
Simple Inversion

The eddy currents induced in a thin confined conductor by a fixed‐loop time‐domain EM system can be represented by a single equivalent current filament. The equivalent current filament stays in the plane of the conductor at all times during the decay of the secondary field, but tends to migrate from a position of maximum primary field coupling at early time toward the center of the conductor at late time. This filament approximation is used in the design of a least‐squares inversion procedure which fits circular or rectangular current filaments to an observed eddy current distribution. The inversion procedure provides a rapid but precise means of estimating the position, size, and attitude of a conductor which has been detected by a time‐domain EM survey.

scholarly journals Processing and inversion of commercial helicopter time-domain electromagnetic data for environmental assessments and geologic and hydrologic mapping

Geophysics ◽  
2013 ◽  
Vol 78 (4) ◽  
pp. E149-E159 ◽  
Author(s):  
Joel E. Podgorski ◽  
Esben Auken ◽  
Cyril Schamper ◽  
Anders Vest Christiansen ◽  
Thomas Kalscheuer ◽  
...  
Keyword(s):  
Time Domain ◽  
Mineral Exploration ◽  
Effective Means ◽  
Early Time ◽  
Original Data ◽  
Late Time ◽  
Data Set ◽  
Environmental Assessments ◽  
Time Domain Electromagnetic ◽  
And Inversion

Helicopter time-domain electromagnetic (HTEM) surveying has historically been used for mineral exploration, but over the past decade it has started to be used in environmental assessments and geologic and hydrologic mapping. Such surveying is a cost-effective means of rapidly acquiring densely spaced data over large regions. At the same time, the quality of HTEM data can suffer from various inaccuracies. We developed an effective strategy for processing and inverting a commercial HTEM data set affected by uncertainties and systematic errors. The delivered data included early time gates contaminated by transmitter currents, noise in late time gates, and amplitude shifts between adjacent flights that appeared as artificial lineations in maps of the data and horizontal slices extracted from inversion models. Multiple processing steps were required to address these issues. Contaminated early time gates and noisy late time gates were semiautomatically identified and eliminated on a record-by-record basis. Timing errors between the transmitter and receiver electronics and inaccuracies in absolute amplitudes were corrected after calibrating selected HTEM data against data simulated from accurate ground-based TEM measurements. After editing and calibration, application of a quasi-3D spatially constrained inversion scheme significantly reduced the artificial lineations. Residual lineations were effectively eliminated after incorporating the transmitter and receiver altitudes and line-to-line amplitude factors in the inversion process. The final inverted model was very different from that generated from the original data provided by the contractor. For example, the average resistivity of the thick surface layer decreased from [Formula: see text] to [Formula: see text], the depths to the layer boundaries were reduced by 15%–23%, and the artificial lineations were practically eliminated. Our processing and inversion strategy is entirely general, such that with minor system-specific modifications it could be applied to any HTEM data set, including those recorded many years ago.

Field tests of an experimental helicopter time‐domain electromagnetic system for unexploded ordnance detection

Geophysics ◽  
2004 ◽  
Vol 69 (3) ◽  
pp. 664-673 ◽  
Author(s):  
Les P. Beard ◽  
William E. Doll ◽  
J. Scott Holladay ◽  
T. Jeffrey Gamey ◽  
James L.C. Lee ◽  
...  
Keyword(s):  
Time Domain ◽  
Eddy Currents ◽  
Field Tests ◽  
Field Trials ◽  
Unexploded Ordnance ◽  
Signal Attenuation ◽  
Signal To Noise ◽  
Electromagnetic System ◽  
Time Domain Electromagnetic ◽  
Unexploded Ordnance Detection

Field trials of a low‐flying time‐domain helicopter electromagnetic system designed for detection of unexploded ordnance have yielded positive and encouraging results. The system is able to detect ordnance as small as 60‐mm rounds at 1‐m sensor height. We examined several transmitter and receiver configurations. Small loop receivers gave superior signal‐to‐noise ratios in comparison to larger receiver loops at low heights. Base frequencies of 90 Hz and 270 Hz were less affected than other base frequencies by noise produced by proximity to the helicopter and by vibration of the support structure. For small ordnance, a two‐lobed, antisymmetric transmitter loop geometry produced a modest signal‐to‐noise enhancement compared with a large single rectangular loop, presumably because the antisymmetric transmitter produces smaller eddy currents in the helicopter body, thereby reducing this source of noise. In most cases, differencing of vertically offset receivers did not substantially improve signal‐to‐noise ratios at very low sensor altitudes. Signal attenuation from transmitter to target and from target to receiver causes signals from smaller ordnance to quickly become indistinguishable from geological background variations, so that above a sensor height of about 3 m only large ordnance items (e.g., bombs and large caliber artillery rounds) were consistently detected.

Electromagnetic mapping of saline contamination at an active brine pit

1996 ◽  
Vol 33 (2) ◽  
pp. 309-323 ◽  
Author(s):  
I J Ferguson ◽  
W J Taylor ◽  
K Schmigel
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Power Lines ◽  
Late Time ◽  
Signal Distortion ◽  
Electromagnetic Methods ◽  
Time Domain Electromagnetic ◽  
Survey Results ◽  
Limited Degree ◽  
Cultural Noise

Frequency-domain and time-domain electromagnetic methods were used to investigate groundwater contamination at an active brine pit in southwestern Manitoba, Canada. The objectives of the survey were to delineate contamination suspected to be occuring at the site and to compare frequency-domain electromagnetic (FDEM) and time-domain electromagnetic (TDEM) measurements in a survey area containing pipelines, fences, and power lines. The survey successfully delineated a region of high conductivity around brine pit, confirming that leakage is occurring from the pit. Modelling of the FDEM results suggests the contamination is spreading within a series of shallow sand units. Comparison of FDEM and TDEM survey results indicate that small-separation FDEM systems are much more useful for mapping in a developed area containing sources of cultural noise. The FDEM systems permit rapid mapping of spatial variations in conductivity, are affected to only a limited degree by cultural features, and provide some resolution of the depth variation of conductivity at shallow depth. It was not possible to obtain useful TDEM measurements anywhere near the active brine pit because of the signal distortion in the late-time response. Key words: geophysics, electromagnetic, brine pit, saline contamination.

scholarly journals Comparison of Matrix Pencil Extracted Features in Time Domain and in Frequency Domain for Radar Target Classification

2014 ◽  
Vol 2014 ◽  
pp. 1-9 ◽  
Author(s):  
Mahmoud Khodjet-Kesba ◽  
Khalil El Khamlichi Drissi ◽  
Sukhan Lee ◽  
Kamal Kerroum ◽  
Claire Faure ◽  
...  
Keyword(s):  
Feature Extraction ◽  
Frequency Domain ◽  
Time Domain ◽  
Early Time ◽  
Matrix Pencil ◽  
Support Vector ◽  
Radar Target ◽  
Late Time ◽  
K Nearest Neighbor ◽  
Matrix Pencil Method

Feature extraction is a challenging problem in radar target identification. In this paper, we propose a new approach of feature extraction by using Matrix Pencil Method in Frequency Domain (MPMFD). The proposed method takes into account not only the magnitude of the signal, but also its phase, so that all the physical characteristics of the target will be considered. With this method, the separation between the early time and the late time is not necessary. The proposed method is compared to Matrix Pencil Method in Time Domain (MPMTD). The methods are applied on UWB backscattered signal from three canonical targets (thin wire, sphere, and cylinder). MPMFD is applied on a complex field (real and imaginary parts of the signal). To the best of our knowledge, this comparison and the reconstruction of the complex electromagnetic field by MPMFD have not been done before. We show the effect of the two extraction methods on the accuracy of three different classifiers: Naïve bayes (NB), K-Nearest Neighbor (K-NN), and Support Vector Machine (SVM). The results show that the accuracy of classification is better when using extracted features by MPMFD with SVM.

An Eddy Current Phenomenon

1978 ◽  
Vol 15 (2) ◽  
pp. 125-129
Author(s):  
J. Greig ◽  
David Thompson
Keyword(s):  
Magnetic Field ◽  
Eddy Current ◽  
Eddy Currents ◽  
Current Filament ◽  
Current Filaments ◽  
The Magnetic Field

An experiment which shows negligible reaction from eddy currents in iron on their inducing field is described. The observations are shown to agree with the extension of Searle's results for the magnetic field of a long current filament in iron to an aggregation of current filaments such as eddy currents.

An experimental study of the time-domain electromagnetic response of a buried conductive plate

Geophysics ◽  
2005 ◽  
Vol 70 (1) ◽  
pp. G1-G7 ◽  
Author(s):  
Mark E. Everett ◽  
Alfonso Benavides ◽  
Carl J. Pierce
Keyword(s):  
Experimental Study ◽  
Time Domain ◽  
Metal Plate ◽  
Electromagnetic Response ◽  
Late Time ◽  
Conductive Plate ◽  
Spatial Response ◽  
Time Domain Electromagnetic ◽  
The Time Domain ◽  
Cultural Noise

It is important to understand the effects of a buried metal object on electromagnetic data, whether the object is a source of cultural noise or a target of interest. The time-domain electromagnetic response of a buried metal plate exhibits several remarkable properties. An experimental study has been undertaken to confirm these properties. The spatial response of a shallow-buried plate is temporally self-similar and exhibits a late-time dipolelike response. Clutter-generated noise can be significant if the plate is poorly coupled to the primary transmitter flux. A vertical plate exhibits a transition from a horizontal to a vertical mode of eddy current induction.

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