Douglas C. Fraser

Resistivity mapping with an airborne multicoil electromagnetic system

Dighem is a helicopter‐borne 900 Hz multicoil electromagnetic survey system. The EM device consists of a 30-ft towed bird containing a transmitter coil in the front and three mutually orthogonal receiver coils in the rear. Resistivity contour maps can be prepared from the EM data using any of several half‐space models. In this paper, two such models are selected and field examples of apparent resistivity derived from them are shown. The multicoil system has encountered areas of widespread conductivity while surveying for metallic minerals. In such areas, EM anomalies can be generated by changes of less than 10 m in survey altitude. EM anomalies of apparent significance, therefore, can reflect decreases in survey altitude as well as increases in conductivity of the earth. Under such conditions, apparent resistivity contour maps can aid the interpretation of the airborne data. The advantage of the contour maps is that anomalies caused by altitude changes are substantially reduced, and the contours reflect m...

The differential parameter method for multifrequency airborne resistivity mapping

Helicopter EM resistivity mapping began to be accepted as a means of geologic mapping in the late 1970s. The data were first displayed as plan maps and images. Some 10 years later, sectional resistivity displays became available using the same “pseudolayer” half‐space resistivity algorithm developed by Fraser and the new centroid depth algorithm developed by Sengpiel. Known as Sengpiel resistivity sections, these resistivity/depth images proved to be popular for the display of helicopter electromagnetic (EM) data in conductive environments.A limitation of the above resistivity and depth algorithms is that the resulting Sengpiel section may imply that the depth of exploration of the EM system is substantially less than is actually the case. For example, a target at depth may be expressed in the raw data, but its appearance on the Sengpiel section may be too shallow (which is a problem with the depth algorithm), or it may not even appear at all (which is a problem with the resistivity algorithm).An algorith...

Inversion of helicopter electromagnetic data to a magnetic conductive layered earth

Inversion of airborne electromagnetic (EM) data for a layered earth has been commonly performed under the assumption that the magnetic permeability of the layers is the same as that of free space. The resistivity inverted from helicopter EM data in this way is not reliable in highly magnetic areas because magnetic polarization currents occur in addition to conduction currents, causing the inverted resistivity to be erroneously high. A new algorithm for inverting for the resistivity, magnetic permeability, and thickness of a layered model has been developed for a magnetic conductive layered earth. It is based on traditional inversion methodologies for solving nonlinear inverse problems and minimizes an objective function subject to fitting the data in a least-squares sense. Studies using synthetic helicopter EM data indicate that the inversion technique is reasonably dependable and provides fast convergence. When six synthetic in-phase and quadrature data from three frequencies are used, the model parameters for two- and three-layer models are estimated to within a few percent of their true values after several iterations. The analysis of partial derivatives with respect to the model parameters contributes to a better understanding of the relative importance of the model parameters and the reliability of their determination. The inversion algorithm is tested on field data obtained with a Dighem helicopter EM system at Mt. Milligan, British Columbia, Canada. The output magnetic susceptibility-depth section compares favorably with that of Zhang and Oldenburg who inverted for the susceptibility on the assumption that the resistivity distribution was known.

The multicoil II airborne electromagnetic system

A helicopter‐towed electromagnetic (EM) system has been developed with two orthogonal transmitter coils. Units currently in service have either two or three orthogonal receiver coils. The associated software yields in‐phase and quadrature channels which are generally free of the conductive response of overburden and of the magnetic induction response of magnetite. These geologic noise sources can mask the response of bedrock conductors for all previously developed airborne EM (AEM) systems. The new technique involves energizing conductors with two orthogonal transmitter coils, both operating at approximately the same frequency (e.g., 900 Hz). The subtracting of the secondary field components of one maximum‐coupled coil pair from the other yields in‐phase and quadrature difference channels. These channels may contain as much as an order‐of‐magnitude increase in the signal/noise (S/N) ratio for bedrock conductors in a geologically noisy environment. The new system can also indicate whether a steeply dipping...

Mapping of the resistivity, susceptibility, and permittivity of the earth using a helicopter‐borne electromagnetic system

Interpretation of helicopter-borne electromagnetic (EM) data is commonly based on the mapping of resistivity (or conductivity) under the assumption that the magnetic permeability is that of free space and dielectric permittivity can be ignored. However, the data obtained from a multifrequency EM system may contain information about the magnetic permeability and dielectric permittivity as well as the conductivity. Our previous work has shown how helicopter EM data may be transformed to yield the resistivity and magnetic permeability or, alternatively, the resistivity and dielectric permittivity. A method has now been developed to recover the resistivity, magnetic permeability, and dielectric permittivity together from the transformation of helicopter EM data based on a half-space model. A field example is presented from an area which exhibits both permeable and dielectric properties. This example shows that the mapping of resistivity, magnetic permeability, and dielectric permittivity together yields more credible results than if the permeability or permittivity is ignored.

Magnetite mapping with a multicoil airborne electromagnetic system

The information content of data from an in‐phase quadrature electromagnetic (EM) system consists of a combination of conductive eddy current response and magnetic polarization response. The secondary field resulting from conductive eddy current flow is frequency‐dependent and consists of both in‐phase and quadrature components of positive sign. Conversely, the field resulting from magnetic polarization is commonly frequency‐independent and consists of only an in‐phase component of negative sign. A magnetite mapping technique was developed for the horizontal coplanar coils of a closely coupled multicoil airborne EM system. The technique yields contours of apparent weight percent magnetite when using a homogeneous half‐space model. The method can be complementary to magnetometer mapping in certain cases. Compared to magnetometry, it is far less sensitive but is more able to resolve closely spaced magnetite zones. The method is also independent of remanent magnetism and magnetic latitude. It is sensitive to ...

Dielectric permittivity and resistivity mapping using high‐frequency, helicopter‐borne EM data

The interpretation of helicopter‐borne electromagnetic (EM) data is commonly based on the transformation of the data to the apparent resistivity under the assumption that the dielectric permittivity is that of free space and so displacement currents may be ignored. While this is an acceptable approach for many applications, it may not yield a reliable value for the apparent resistivity in resistive areas at the high frequencies now available commercially for some helicopter EM systems.We analyze the feasibility of mapping spatial variations in the dielectric permittivity and resistivity using a high‐frequency helicopter‐borne EM system. The effect of the dielectric permittivity on the EM data is to decrease the in‐phase component and increase the quadrature component. This results in an unwarranted increase in the apparent resistivity (when permittivity is neglected) for the pseudolayer half‐space model, or a decrease in the apparent resistivity for the homogeneous half‐space model. To avoid this problem,...

Airborne resistivity and susceptibility mapping in magnetically polarizable areas

The apparent resistivity technique using half‐space models has been employed in helicopter‐borne resistivity mapping for twenty years. These resistivity algorithms yield the apparent resistivity from the measured in‐phase and quadrature response arising from the flow of electrical conduction currents for a given frequency. However, these algorithms, which assume free‐space magnetic permeability, do not yield a reliable value for the apparent resistivity in highly magnetic areas. This is because magnetic polarization also occurs, which modifies the electromagnetic (EM) response, causing the computed resistivity to be erroneously high. Conversely, the susceptibility of a magnetic half‐space can be computed from the measured EM response, assuming an absence of conduction currents. However, the presence of conduction currents will cause the computed susceptibility to be erroneously low. New methods for computing the apparent resistivity and apparent magnetic permeability have been developed for the magnetic c...

Attitude corrections of helicopter EM data using a superposed dipole model

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 birds 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 birds attitude. In contrast, the inductive effect results from the coupling change be...

Airborne resistivity data leveling

Helicopter‐borne frequency‐domain electromagnetic (EM) data are used routinely to produce resistivity maps for geologic mapping, mineral exploration, and environmental investigations. The integrity of the resistivity data depends in large part on the leveling procedures. Poor resistivity leveling procedures may, in fact, generate false features as well as eliminate real ones. Resistivity leveling is performed on gridded data obtained by transformation of the leveled EM channel data. The leveling of EM channel data is often imperfect, which is why the resistivity grids need to be leveled. We present techniques for removing the various types of resistivity leveling errors which may exist. A semi‐automated leveling technique uses pseudo tie‐lines to remove the broad flight‐based leveling errors and any high‐magnitude line‐based errors. An automated leveling technique employs a combination of 1-D and 2-D nonlinear filters to reject the rest of the leveling errors including both long‐and short‐wavelength level...