Dighem Resistivity Techniques in Airborne Electromagnetic Mapping

Douglas, Craig

doi:10.4095/122346

Cited by 7 publications

(5 citation statements)

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“…Flight lines were oriented northwest-southeast and spaced 400 m (1/4 mile) apart except in the vicinity of known gold deposits where the spacing was 200 m (1/8 mile) apart. Apparent resistivity data were derived from the AEM data using algorithms described by Fraser (1986) and form the data sets presented in this workshop. The data were projected into UTM coordinates (base latitude 0°; central meridian 117°W) and interpolated onto a grid having equally spaced intervals of 0.1 km.…”

Section: Airborne Electromagnetic Datamentioning

confidence: 99%

Materials provided at the workshop Geophysical map interpretation on the PC, convened April 21-22, 1993

Grauch¹,

Phillips²,

Hoover³

et al. 1993

Open-File Report

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Open-File Report 93-560 93-560A Text (Paper Copy) 93-560B Version 2.11 update to potential-field software (4 diskettes) 93-560C Version 1.03 REMAPP Software (2 diskettes) 93-560D Getchell grids and image files (8 diskettes) This report is preliminary and has not been reviewed for conformity with U. S. Geological Survey standards or stratigraphic nomenclature. Any use of trade names is for descriptive purposes only and does not imply endorsement by the U. S. Geological Survey. Although these programs and data have been used by the U. S. Geological Survey, no warranty, expressed or implied, is made by the USGS as to the accuracy and completeness of the data nor the accuracy and functioning of the programs and related program material, nor shall the fact of distribution constitute any such warranty, and no responsibility is assumed by the USGS in connection therewith.

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Section: Airborne Electromagnetic Datamentioning

confidence: 99%

Materials provided at the workshop Geophysical map interpretation on the PC, convened April 21-22, 1993

Grauch¹,

Phillips²,

Hoover³

et al. 1993

Open-File Report

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“…Because of the wide frequency range and the multiple coil configurations, modern helicopter electromagnetics (HEM) systems are capable of mapping magnetic permeability (Fraser, 1981;Huang and Fraser, 2000), dielectric permittivity (Huang and Fraser, 2002b), and conductivity anisotropy (Yin and Fraser, 2002;Yin and Hodges, 2003). DIGHEM systems (a specific type of HEM system) (Fraser 1979(Fraser , 1986Huang and Fraser, 2001), in particular, are well suited for such applications. Manuscript The data from HEM systems commonly are interpreted using 1D isotropic models of a homogeneous half-space or horizontally layered earth.…”

Section: Introductionmentioning

confidence: 99%

Attitude corrections of helicopter EM data using a superposed dipole model

Yin

Fraser²

2004

GEOPHYSICS

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The analysis of helicopter-borne electromagnetic (EM) survey data commonly assumes that the EM sensor (bird) has flown straight and levelly, whereupon the coils are either horizontal or vertical. The EM data typically are transformed, under this assumption, to yield the electrical properties (conductivity, permeability, permittivity) of a homogeneous earth, or they are inverted to a horizontally layered earth. In actual fact, the bird exhibits some roll, pitch, and yaw rotation, which will generate changes to the data that we call bird attitude effect.On the basis of a superposed dipole model, we distinguish the geometric part of the bird attitude effect from the inductive part. Theoretical investigation shows that the geometric effect results from the reorientation of the bird's coil axes related to the earth. It is independent of the system frequency and the electrical properties of the earth but dependent only on the bird's attitude. In contrast, the inductive effect results from the coupling change between the coils and the conductive earth. It depends not only on the attitude of the bird but also on the system frequency and the earth's electrical properties.We calculate ratios of the EM responses when the bird is rotated to those when the bird flies straight and levelly. These ratios, reflecting the purely geometric effect of the bird's rotation, are simple trigonometric functions of the rotation angles and thus can be easily incorporated into our processing of the EM data. We show that, in the general case of helicopter EM systems, the majority (>95%) of the attitude error results from the geometric effect rather than from the inductive effect. Sufficient accuracy is obtained by using the simple geometric technique for attitude correction, without requiring information about the earth's electrical properties. We demonstrate the usefulness of the method, using an example from a test survey.

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“…As a by-product, the algorithm also returns an estimate of apparent distance (D a in Figure 1) to the ground surface. Since sensor altitude (h) is also measured, an apparent depth Fraser (1978Fraser ( , 1986 suggests that the parameter has no physical meaning other than to indicate that the data and model are inconsistent. Alternatively, when d a < 0, the existence of a conductive layer above a more resistive substratum is often inferred (Sengpiel, 1988;Siemon, 2001).…”

Section: Aem Resistivity Modelingmentioning

confidence: 99%

The canopy effect in airborne EM

Beamish

2002

GEOPHYSICS

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Airborne electromagnetic (AEM) surveys undertaken at low altitude and small flight separations may be used to provide environmental subsurface assessments. Some of the land quality issues require quite detailed scales (< 1 km) of information. Trial, fixed-wing AEM surveys were conducted in the central English Midlands for such purposes. This paper investigates a specific issue, that of the canopy effect, which needs to be addressed when high-resolution AEM data are to be interpreted accurately. Any elevated feature (typically tree cover) that gives rise to underestimated altimeter readings causes the canopy effect. The inaccurate measurement of sensor height above ground level influences the ground resistivity models that may be obtained from the data.1D half-space models obtained from conventional, nomogram/look-up procedures together with formal numerical inversion techniques form the basis of the study. Both theory and survey data are used to assess the significance of the canopy thickness, here ranging from zero to 20 m, on the resistivity models obtained. The study uses a small survey area (1.5 km  1.5 km) which provides a number of cultural and environmental influences and contains three distinctive canopy zones (forestry, plantation and copse).The conventional pseudo-layer half-space method has a stated immunity to altitude errors. The method is found to be highly effective in returning resistivity estimates unbiased by altimeter errors. The associated positive apparent depths provide realistic estimates of canopy thickness while, elsewhere, negative values may be returned. Published numerical inversion schemes, used for AEM modeling, do not discuss any corresponding requirement to reduce canopy effect bias. Underestimated altitude measurements introduce false, high resistivity zones with high wavenumber content unless an equivalent pseudo-layer concept is used. The study indicates a requirement for a formal, pseudo-layer (an atsurface perfect resistor of variable depth) to be included in the model when canopy zones are present. The procedure returns stable, zero estimates of canopy thickness in the absence of tree cover and realistic heights of the canopy. In the example considered, resistivity models obtained from a conventional lookup procedure appear more conservative (i.e. of a lower resolution) than their equivalent formal inversion counterparts.3

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Dighem Resistivity Techniques in Airborne Electromagnetic Mapping

Cited by 7 publications

References 0 publications

Materials provided at the workshop Geophysical map interpretation on the PC, convened April 21-22, 1993

Materials provided at the workshop Geophysical map interpretation on the PC, convened April 21-22, 1993

Attitude corrections of helicopter EM data using a superposed dipole model

The canopy effect in airborne EM

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