Comparing induced polarization responses from airborne inductive and galvanic ground systems: Tasmania

Geophysics ◽  
2016 ◽  
Vol 81 (6) ◽  
pp. E471-E479 ◽  
Author(s):  
James Macnae ◽  
Kate Hine
Keyword(s):  
Physical Property ◽  
Copper Deposit ◽  
Companion Paper ◽  
Induced Polarization ◽  
Geologic Mapping ◽  
Near Surface ◽  
South Wales ◽  
Airborne Electromagnetic ◽  
Using Data ◽  
Fitting In

We have first analyzed the ability of polarizable and superparamagnetic thin sheets in the near surface to fit airborne electromagnetic (AEM) data using data from western Tasmania. Then we analyzed the results of such fitting in the context of geologic mapping and available ground induced polarization (IP) data. Small to large IP effects were found to considerably improve the fit to the observed AEM data, and the overall fitted IP parameters were spatially consistent. However, the locations of anomalous IP parameters were quite distinct from anomalies in other geophysical data. The airborne chargeability highs were adjacent to or surrounded the ground chargeability highs in the five cases analyzed from Tasmanian data. Modeling using the established Cole-Cole physical property values for sulfides predicts that an inductive airborne system is insensitive to many conventional IP targets, unless the mineral grain size is substantially less than 1 mm. In the cases in which airborne IP responses were adjacent to ground IP targets, we hypothesized that the airborne IP may be finer grained minerals in an alteration halo surrounding the sulfide sources of the large ground IP anomalies. Surficial clays encountered in drillholes did not have significant ground or airborne IP responses. A companion paper comes to a similar conclusion using ground and airborne data from a copper deposit in New South Wales.

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

Geophysics ◽  
2020 ◽  
Vol 85 (5) ◽  
pp. B161-B167 ◽  
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.

Comparing induced polarization responses from airborne inductive and galvanic ground systems: Lewis Ponds, New South Wales

Geophysics ◽  
2016 ◽  
Vol 81 (6) ◽  
pp. B179-B188 ◽  
Author(s):  
Kate Hine ◽  
James Macnae
Keyword(s):  
New South ◽  
Hanging Wall ◽  
New South Wales ◽  
Induced Polarization ◽  
Order Effects ◽  
Galvanic Current ◽  
Electromagnetic System ◽  
Near Surface ◽  
Polarization Effects ◽  
South Wales

We have evaluated the mapping of polarizable material using a Cole-Cole model to fit second-order effects in concentric-loop airborne electromagnetic system responses. At Lewis Ponds in New South Wales, an inverted ground dipole-dipole array data has accurately imaged in 3D disseminated sulfide extending above and around ore grade massive sulfides. The polarizable zone is present in the near-surface, where, from modeling, airborne systems may have sensitivity to the small inductive induced polarization effects. Although the inverted chargeability measured from galvanic current injection into the ground was spatially coincident with the mineralized target, the estimated chargeabilities from induced polarization effects in an airborne versatile time-domain electromagnetic survey were substantially displaced to the east. The airborne induced polarization response may be associated with finer grained mineralization in the hanging wall of the sulfide deposits, or have a quite different source, such as clays associated with faulting.

Electrical Resistivity and Induced Polarization signatures to delineate the near-surface aquifers contaminated with seawater invasion in Digha, West-Bengal, India

CATENA ◽  
2021 ◽  
Vol 207 ◽  
pp. 105596
Author(s):  
Prashant Kumar ◽  
Prarabdh Tiwari ◽  
Anand Singh ◽  
Arkoprovo Biswas ◽  
Tapas Acharya
Keyword(s):  
Electrical Resistivity ◽  
West Bengal ◽  
Induced Polarization ◽  
Near Surface

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

Geophysics ◽  
2010 ◽  
Vol 75 (4) ◽  
pp. WA179-WA188 ◽  
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.

scholarly journals An overview of the spectral induced polarization method for near-surface applications

2012 ◽  
Vol 10 (6) ◽  
pp. 453-468 ◽  
Author(s):  
Andreas Kemna ◽  
Andrew Binley ◽  
Giorgio Cassiani ◽  
Ernst Niederleithinger ◽  
André Revil ◽  
...  
Keyword(s):  
Induced Polarization ◽  
Polarization Method ◽  
Near Surface ◽  
Spectral Induced Polarization

A special circumstance of airborne induced‐polarization measurements

Geophysics ◽  
1996 ◽  
Vol 61 (1) ◽  
pp. 66-73 ◽  
Author(s):  
Richard S. Smith ◽  
Jan Klein
Keyword(s):  
Time Domain ◽  
Eddy Currents ◽  
High Arctic ◽  
Induced Polarization ◽  
Laboratory Measurements ◽  
Dielectric Polarization ◽  
Special Circumstance ◽  
Polarization Measurements ◽  
Airborne Electromagnetic

Airborne induced‐polarization (IP) measurements can be obtained with standard time‐domain airborne electromagnetic (EM) equipment, but only in the limited circumstances when the ground is sufficiently resistive that the normal EM response is small and when the polarizability of the ground is sufficiently large that the IP response can dominate the EM response. Further, the dispersion in conductivity must be within the bandwidth of the EM system. One example of what is hypothesized to be IP effects are the negative transients observed on a GEOTEM® survey in the high arctic of Canada. The dispersion in conductivity required to explain the data is very large, but is not inconsistent with some laboratory measurements. Whether the dispersion is caused by an electrolytic or dielectric polarization is not clear from the limited ground follow‐up, but in either case the polarization can be considered to be induced by eddy currents associated with the EM response of the ground. If IP effects are the cause of the negative transients in the GEOTEM data, then the data can be used to estimate the polarizabilities in the area.

Resistive limit modeling of airborne electromagnetic data

Geophysics ◽  
2002 ◽  
Vol 67 (2) ◽  
pp. 492-500 ◽  
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.

Sign in / Sign up

Export Citation Format

Share Document