MAPPING EARTH CONDUCTIVITIES USING A MULTIFREQUENCY AIRBORNE ELECTROMAGNETIC SYSTEM

The Tridem vertical coplanar airborne electromagnetic system provides simultaneous in‐phase and quadrature information at frequencies of 500, 2000 and 8000 Hz. The system can map a broad range of earth conductors of simple geometry and provide quantitative estimates of their conductivities and dimensions. Computer programs have been developed to automatically interpret the six channels of Tridem data, plus the output of an accurate radar altimeter, to determine the depth of burial, conductivity and thickness of a near‐surface, flat‐lying conducting horizon. In limiting cases, the interpretation provides the conductance (conductivity‐thickness product) of a thin sheet (ranging from 100 mmhos to 100 mhos) or the conductivity of a homogeneous earth (ranging from 1 mmhos/m to 10 mhos/m). Two actual field examples are presented from Ontario, Canada; one relating to the mapping of overburden conditions (sand, clay and rock, etc) and the other to the mapping of the distribution of a buried lignite deposit. Other areas of potential application of the system to surficial materials would include groundwater mapping, permafrost investigations, and civil engineering studies for roads and pipelines.

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Detection of a perfect conductor with an airborne electromagnetic system: The Gemini Field Test

Geophysics ◽

10.1190/geo2012-0469.1 ◽

2013 ◽

Vol 78 (5) ◽

pp. E249-E259 ◽

Cited By ~ 6

Author(s):

Adam Smiarowski ◽

James Macnae

Keyword(s):

Field Test ◽

Thin Sheet ◽

Perfect Conductor ◽

Primary Field ◽

Electromagnetic System ◽

Phase Measurements ◽

Electrical Conductivities ◽

Airborne Electromagnetic ◽

Gps System ◽

Geometric Changes

The cores of high-grade nickel and copper sulphides appear as “perfect conductors” to most electromagnetic (EM) and airborne electromagnetic (AEM) systems, because they have bulk electrical conductivities of the order of [Formula: see text]. The EM response of these highly conductive cores is essentially undetectable with off-time measurements or when using nonrigid towed-bird systems. Compact AEM systems with accurate primary field bucking and on-time or in-phase measurements are sensitive to perfect conductors, but are incapable of detecting deep targets. Using a GPS system to define geometry, calculations suggest that it should be easy for an AEM system to detect “perfect conductors” provided the receiver was several hundred meters distant from the transmitter. A twin (Gemini) aircraft test was undertaken to test this concept in 2005. The field test successfully demonstrated detection of very conductive targets. Errors associated with geometric changes were better than 0.5% of the primary field at 400 m separation, allowing detection and characterization of the 30 Hz, in-phase response of small and extended conductors. The test shows that a 200 × 100 m very-strongly conductive thin-sheet target would be detectable to depths of 200 m below surface using off-the-shelf technology. Larger conductors would be detectable at greater depths.

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Stripping induced polarization effects from airborne electromagnetics to improve 3D conductivity inversion of a narrow palaeovalley

Geophysics ◽

10.1190/geo2019-0396.1 ◽

2020 ◽

Vol 85 (5) ◽

pp. B161-B167 ◽

Cited By ~ 1

Author(s):

James Macnae ◽

Xiuyan Ren ◽

Tim Munday

Keyword(s):

Thin Sheet ◽

Physical Property ◽

Original Data ◽

Induced Polarization ◽

Near Surface ◽

Conductivity Distribution ◽

Electromagnetic Data ◽

Conductivity Structure ◽

Airborne Electromagnetic ◽

Airborne Electromagnetic Data

The electrical conductivity distribution within wide palaeochannels is usually well-mapped from airborne electromagnetic data using stitched 1D algorithms. Such stitched 1D solutions are, however, inappropriate for narrow valleys. An alternative option is to consider 2D or 3D models to allow for finite lateral extent of conductors. In airborne electromagnetic data within the Musgrave block near the well-studied Valen conductor, strong induced polarization (IP) and superparamagnetic (SPM) effects make physical property and structure estimation even more uncertain for deep channel clays, particularly those whose channel widths are comparable to their depth of burial. We developed a recursive data fitting algorithm based on dispersive thin sheet responses. The separate IP and SPM components of the fit provide near-surface chargeability and SPM distributions, and the associated electromagnetic (EM) fit provides stripped data with monotonic decays compatible with a simple nondispersive conductivity model. The validity of this stripped data prediction was tested through a comparison of 1D conductivity-depth imaging and 3D inversion applied to the original data and the stripped data. Due to the forked geometry of the deep conductivity structure in the region we investigated, we successfully used 3D rather than 2D inversion to predict the conductivity distribution related to the EM data. We recovered from the stripped data a continuous conductivity structure consistent with a branching, clay-filled palaeovalley under cover.

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STACKING OF REFLECTIONS FROM COMPLEX STRUCTURES

Geophysics ◽

10.1190/1.1440440 ◽

1974 ◽

Vol 39 (4) ◽

pp. 427-440 ◽

Cited By ~ 4

Author(s):

Max K. Miller

Keyword(s):

Deep Structure ◽

Control Model ◽

Layer Model ◽

Low Velocity ◽

Seismic Reflection Data ◽

Near Surface ◽

Static Correction ◽

Reflection Data ◽

Actual Field ◽

Interval Velocities

Common‐depth‐point seismic reflection data were generated on a computer using simple ray tracing and analyzed with processing techniques currently used on actual field recordings. Constant velocity layers with curved interfaces were used to simulate complex geologic shapes. Two models were chosen to illustrate problems caused by curved geologic interfaces, i.e., interfaces at depths which vary laterally in a nonlinear fashion and produce large spatial variations in the apparent stacking velocity. A three‐layer model with a deep structure and no weathering was used as a control model. For comparison, a low velocity weathering layer also of variable thickness was inserted near the surface of the control model. The low velocity layer was thicker than the ordinary thin weathering layers where state‐of‐the‐art static correction methods work well. Traveltime, moveout, apparent rms velocities, and interval velocities were calculated for both models. The weathering introduces errors into the rms velocities and traveltimes. A method is described to compensate for these errors. A static correction applied to the traveltimes reduced the fluctuation of apparent rms velocities. Values for the thick weathering layer model were “over corrected” so that synclines (anticlines) replaced false anticlines (synclines) for both near‐surface and deep zones. It is concluded that computer modeling is a useful tool for analyzing specific problems of processing CDP seismic data such as errors in velocity estimates produced by large lateral variations in overburden.

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Breaks in lithology: Interpretation problems when handling 2D structures with a 1D approximation

Geophysics ◽

10.1190/1.3483101 ◽

2010 ◽

Vol 75 (4) ◽

pp. WA179-WA188 ◽

Cited By ~ 12

Author(s):

Alan Yusen Ley-Cooper ◽

James Macnae ◽

Andrea Viezzoli

Keyword(s):

Radial Distance ◽

The Other ◽

Conductive Layer ◽

Near Surface ◽

Depth Images ◽

Modeling Methodology ◽

Wide System ◽

Receiver System ◽

Airborne Electromagnetic ◽

Detection Capabilities

Most airborne electromagnetic (AEM) data are processed using successive 1D approximations to produce stitched conductivity-depth sections. Because the current induced in the near surface by an AEM system preferentially circulates at some radial distance from a horizontal loop transmitter (sometimes called the footprint), the section plotted directly below a concentric transmitter-receiver system actually arises from currents induced in the vicinity rather than directly underneath. Detection of paleochannels as conduits for groundwater flow is a common geophysical exploration goal, where locally 2D approximations may be valid for an extinct riverbed or filled valley. Separate from effects of salinity, these paleochannels may be conductive if clay filled or resistive if sand filled and incised into a clay host. Because of the wide system footprint, using stitched 1D approximations or inversions may lead to misleading conductivity-depth images or sections. Near abrupt edges of an extensive conductive layer, the lateral falloff in AEM amplitudes tends to produce a drooping tail in a conductivity section, sometimes coupled with alocal peak where the AEM system is maximally coupled to currents constrained to flow near the conductor edge. Once the width of a conductive ribbon model is less than the system footprint, small amplitudes result, and the source is imaged too deeply in the stitched 1D section. On the other hand, a narrow resistive gap in a conductive layer is incorrectly imaged as a drooping region within the layered conductor; below, the image falsely contains a blocklike poor conductor extending to depth. Additionally, edge-effect responses often are imaged as deep conductors with an inverted horseshoe shape. Incorporating lateral constraints in 1D AEM inversion (LCI) software, designed to improve resolution of continuous layers, more accurately recovers the depth to extensive conductors. The LCI, however, as with any AEM modeling methodology based on 1D forward responses, has limitations in detecting and imaging in the presence of strong 3D lateral discontinuities of dimensions smaller than the annulus of resolution. The isotropic, horizontally slowly varying layered-earth assumption devalues and limits AEM’s 3D detection capabilities. The need for smart, fast algorithms that account for 3D varying electrical properties remains.

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Geological interpretation of near-surface induced polarization and superparametric effects in airborne electromagnetic data

10.4133/sageep.33-065 ◽

2021 ◽

Author(s):

J. Macnae

Keyword(s):

Induced Polarization ◽

Near Surface ◽

Geological Interpretation ◽

Electromagnetic Data ◽

Airborne Electromagnetic ◽

Airborne Electromagnetic Data

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Resistive limit modeling of airborne electromagnetic data

Geophysics ◽

10.1190/1.1468609 ◽

2002 ◽

Vol 67 (2) ◽

pp. 492-500 ◽

Cited By ~ 3

Author(s):

James E. Reid ◽

James C. Macnae

Keyword(s):

Current Flow ◽

Integral Equation Method ◽

Near Surface ◽

Electromagnetic Data ◽

Airborne Electromagnetic ◽

Source Current ◽

Numerical Model Experiments ◽

Airborne Electromagnetic Data ◽

Airborne Em ◽

Direct Induction

When a confined conductive target embedded in a conductive host is energized by an electromagnetic (EM) source, current flow in the target comes from both direct induction of vortex currents and current channeling. At the resistive limit, a modified magnetometric resistivity integral equation method can be used to rapidly model the current channeling component of the response of a thin-plate target energized by an airborne EM transmitter. For towed-bird transmitter–receiver geometries, the airborne EM anomalies of near-surface, weakly conductive features of large strike extent may be almost entirely attributable to current channeling. However, many targets in contact with a conductive host respond both inductively and galvanically to an airborne EM system. In such cases, the total resistive-limit response of the target is complicated and is not the superposition of the purely inductive and purely galvanic resistive-limit profiles. Numerical model experiments demonstrate that while current channeling increases the width of the resistive-limit airborne EM anomaly of a wide horizontal plate target, it does not necessarily increase the peak anomaly amplitude.

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A Bayesian inversion framework for yield and height-of-burst/depth-of-burial for near-surface explosions

10.2172/1226968 ◽

2015 ◽

Author(s):

Gardar Johannesson ◽

Vera Bulaevskaya ◽

Abe Ramirez ◽

Sean Ford ◽

Artie Rodgers

Keyword(s):

Bayesian Inversion ◽

Near Surface ◽

Depth Of Burial

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Development And Evaluation Of A Second-Generation Airborne Electromagnetic System For Detection Of Unexploded Ordnance

10.3997/2214-4609-pdb.191.11uxo8 ◽

2002 ◽

Author(s):

William E. Doll ◽

T. Jeffrey Gamey ◽

Les P. Beard ◽

David T. Bell ◽

J.S. Holladay ◽

...

Keyword(s):

Second Generation ◽

Unexploded Ordnance ◽

Electromagnetic System ◽

Airborne Electromagnetic

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Interpretation of 3-D effects in long‐offset transient electromagnetic (LOTEM) soundings in the Münsterland area/Germany

Geophysics ◽

10.1190/1.1443327 ◽

1992 ◽

Vol 57 (9) ◽

pp. 1127-1137 ◽

Cited By ~ 39

Author(s):

Andreas Hördt ◽

Vladimir L. Druskin ◽

Leonid A. Knizhnerman ◽

Kurt‐Martin Strack

Keyword(s):

Integral Equation ◽

Thin Sheet ◽

Three Dimensional ◽

Equation Modeling ◽

Data Sets ◽

Sign Reversal ◽

Near Surface ◽

Transient Electromagnetic ◽

Long Offset ◽

Geological Situation

The interpretation of long‐offset transient electromagnetic (LOTEM) data is usually based on layered earth models. Effects of lateral conductivity variations are commonly explained qualitatively, because three‐dimensional (3-D) numerical modeling is not readily available for complex geology. One of the first quantitative 3-D interpretations of LOTEM data is carried out using measurements from the Münsterland basin in northern Germany. In this survey area, four data sets show effects of lateral variations including a sign reversal in the measured voltage curve at one site. This sign reversal is a clear indicator of two‐dimensional (2-D) or 3-D conductivity structure, and can be caused by current channeling in a near‐surface conductive body. Our interpretation strategy involves three different 3-D forward modeling programs. A thin‐sheet integral equation modeling routine used with inversion gives a first guess about the location and strike of the anomaly. A volume integral equation program allows models that may be considered possible geological explanations for the conductivity anomaly. A new finite‐difference algorithm permits modeling of much more complex conductivity structures for simulating a realistic geological situation. The final model has the zone of anomalous conductivity aligned below a creek system at the surface. Since the creeks flow along weak zones in this area, the interpretation seems geologically reasonable. The interpreted model also yields a good fit to the data.

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