Geological interpretation of near-surface induced polarization and superparametric effects in 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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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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Induced-polarization effects in airborne electromagnetic data: Estimating chargeability from shape reversals

10.1190/segam2016-13680175.1 ◽

2016 ◽

Author(s):

Richard Smith

Keyword(s):

Induced Polarization ◽

Polarization Effects ◽

Electromagnetic Data ◽

Airborne Electromagnetic ◽

Airborne Electromagnetic Data

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Fitting superparamagnetic and distributed Cole-Cole parameters to airborne electromagnetic data: A case history from Quebec

Geophysics ◽

10.1190/geo2016-0119.1 ◽

2016 ◽

Vol 81 (6) ◽

pp. B211-B220 ◽

Cited By ~ 8

Author(s):

James Macnae

Keyword(s):

Frequency Dependence ◽

Case History ◽

Rock Magnetism ◽

Complex Permeability ◽

Near Surface ◽

Time Constants ◽

Fine Grained ◽

Electromagnetic Data ◽

Airborne Electromagnetic ◽

Airborne Electromagnetic Data

Our aim was to confirm the ability of polarizable and superparamagnetic (SPM) thin sheets in the near surface to improve the model fit of airborne electromagnetic data. Our method was to fit induced polarization (IP) effects with Cole-Cole complex conductivity and fit SPM effects with Chikazumi complex permeability. Surficial conductors were assumed to be the source of the conductivity and IP effects. In this case history from Lac Brûlé, Quebec over an anorthosite intrusion, small to large IP effects were found to be essential to fit most of the observed data. In some areas, it was also possible to separate SPM effects from IP effects in the data. Most IP effects in this unusually polarizable area were adequately fit with a distributed decay characterized with a frequency dependence of [Formula: see text], but some required a sharper response characterized by [Formula: see text]. In general, fitted IP time constants were anticorrelated with fitted frequency dependence, with short time constants fitted to the larger [Formula: see text] values and vice versa. SPM effects were detected in a small but significant fraction of the data, and appear to be spatially related to static magnetic anomalies. The SPM in this case is presumably related to fine-grained rock magnetism, rather than the more common case of weathering products.

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