Current channelling in time domain airborne electromagnetic data

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The Cavendish Test Site: A UTEM Survey Plus a Compilation of Other Ground Geophysical Data

This paper appeared as Research in Applied Geophysics, No. 12, Geophysics Laboratory, University of Toronto.This report presents data from three recent University of Toronto surveys at the Cavendish geophysical test site, namely UTEM, horizontal loop (HLEM) and ground magnetic surveys. Interpretation of the UTEM survey has clearly defined the presence of a short strike-length pod of high conductance within the regional zone A conductor, present on line C. The pod is interpreted to have a conductance in excess of 400 Siemens, compared to about 10 Siemens for the regional zone A conductor extending across the grid. HLEM data interpretation strongly confirm this conclusion. Zone B is interpreted to have the top of its main conductivity at some depth. This conclusion, if true would indicate that the crosscutting westerly dip for zone B postulated by Williams et al. (on the basis of one drill hole that did not intersect sulphides) may no longer be necessary to explain the drill data. The detailed magnetic field data collected, when contoured, show a very different picture to that of earlier data. On the basis of UTEM late-time limit interpretation it was possible here to determine the nature of the source of the near surface static magnetic anomalies i.e. whether induced or remnant magnetization is predominant. The second part of the report presents a compilation of ground geophysical data at Cavendish obtained from a variety of sources. This data set is by no means complete, and due to the constraints of the original surveys, no valid inter-system comparisons can be made. A reference list of other, easily available, data on the Cavendish test site is also included. In an appendix the vector diffusion process of EM induction is illustrated in the UTEM E field case for the Cavendish test site

Geophysical Prospecting with Electric Fields from an Inductive EM Source

It is possible when carrying out EM surveys for mining exploration to measure the electric (E) as well as the magnetic (H) components. Such measurements have been made at several sites with the UTEM time-domain, fixed transmitter EM system. The object of this thesis is to provide a comprehensive basis for the interpretation of such data. It is shown that three distinct EM phenomena are involved, which have clearly separable effects on data recorded with a time-domain, step-response system such as UTEM. At low enough frequencies (in frequency domain), or at sufficiently long times after a step change in primary E field, the observed electric field consists of the divergence-free primary E field and a curl-free secondary E field that arises from charges induced in regions where the conductivity changes. The response at the limit, called the resistive limit E (RLE) field response has been calculated for a number of geologically reasonable bodies. RLE model responses can be found which fit late-time UTEM E field data in many cases. Additional geological information, particularly about resistive features is obtained from the RLE response that is not interpretable from the magnetic EM components alone. EM prospecting conventionally uses a magnetic receiver to detect the secondary fields of induced current flow, and interpretation of the conductivity structure of the ground is per-, formed using the time or frequency dependent nature of the response. The secondary induced currents create a secondary vi divergence-free electric field which through boundary interactions also contributes to the secondary curl-free E field although this latter contribution is usually small. Divergence-free E and H field anomalies (when normalized to their respective primary fields) are not of identical magnitude or form, but are related to the geometry of the induced current flow. Simple circuit estimates, together with limited scale modelling were used to predict the E field inductive anomalies for some simple models. These results were used to interpret UTEM survey data, and it was found that by including the E fields that estimates of the geometry of secondary current flow could be improved over those using H field data alone in some cases. Conventional induced polarization (IP) surveys show that many earth materials have a frequency dependent conductivity. The effects of polarizable bodies of simple shape on E fields were calculated for an inductive EM source field. The predicted effects are able to explain observed anomalous late-time UTEM E field data quantitatively. As in the case of MIP (IP surveys using a grounded transmitter and magnetic receiver) no IP anomaly is generated in the resistive limit by horizontal layers of polarizable material. Due to geometrical complications, and because the UTEM sampling times are designed to define EM time decays and do not cover a very wide time range with respect to IP effects, characterization of fundamental IP parameters can only be done in special cases.Ph.D

An Atlas of Primary Fields Due to fixed Transmitter Loop EM Sources + Erratum

It has long been known in exploration geophysics that any fixed transmitter EM system has "blind zones" for thin, plate-like conductors. These occur whenever the primary field is nearly parallel to the plane of the conductor, with the result that very little current is induced. The body may then escape detection, particularly if noise or other conductors are present. Recent work in borehole EM has revived interest in this topic. As well as this interest in the shape of primary field, knowledge of its amplitude is also very important in order to asses the relative strength of secondary fields cause by induction, and in the determination of optimum loop sizes ..

Resistive limit modeling of airborne electromagnetic data

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The feasibility of EM gradiometer measurements

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