Abstract
Extremely conductive bodies, such as those containing valuable nickel sulfides, have a secondary response that is dominated by an in-phase component, so this secondary response is very difficult to distinguish from the primary field emanating from the transmitter (because by definition they are identical in temporal shape and phase). Hence, an airborne electromagnetic (AEM) system able to identify the response from the extremely conductive bodies in the ground must be able to predict the primary field to identify and measure the secondary response of the extremely conductive body. This is normally done by having a rigid system and bucking out the predicted primary (which will not change significantly due to the rigidity). Unfortunately, these rigid systems must be small and are not capable of detecting extremely conductive bodies buried deeper than approximately 100 m. Another approach is to measure the transmitter current and geometry and subtract the primary mathematically, but these measurements must be extremely accurate and this is difficult or expensive, so it has not been done successfully for an AEM system. I exploit the geometric relationship of the primary fields from a three-component (3C) dipole transmitter. If the transmitter is mathematically rotated so that one axis points to the receiver, then linear combinations of the fields measured by a 3C receiver can be combined in such a way that the primary fields from the transmitter sum to zero and cancel. Alternatively, the measured transmitter current and response could be used to estimate the transmitter-receiver geometry and then to predict and remove the primary field. Any residual must be the secondary coming from a conductive body in the ground. Hence, extremely conductive bodies containing valuable minerals can be found. An AEM system with a 3C transmitter and a 3C receiver should not be too difficult to build.
Highlights
Electromagnetic (EM) methods are a successful tool used to explore for mineral deposits (Grant and West, 1965; Nabighian, 1991; Fountain, 1998)
Ampere’s law tells us that every current has an associated magnetic field radiating in all directions; in the case of the EM transmitter, the field penetrates below the surface, where mineral deposits are located. This field is called the primary field, and it is made to vary as a function of time, so Faraday’s law of induction tells us that an electric field will circulate around the primary magnetic field
The airborne EM system described in this paper is capable of identifying the response from extremely conductive bodies
Summary
Electromagnetic (EM) methods are a successful tool used to explore for mineral deposits (Grant and West, 1965; Nabighian, 1991; Fountain, 1998). The linear transformation (comprising a set of trigonometric rotations) that would rotate the 3C transmitter in its frame of reference so that the axial vector passes through the receiver was determined A second approach could be to use the nine components measured to estimate the orientation of the receiver and possibly the transmitter and to rotate the measured data to some nominal geometry, for example, where the transmitters and receivers have their dipoles aligned with a specific coordinate system (e.g., along line, crossline and up; north, east and up; or aligned with the specific geology). There is clearly considerable scope for a great deal of future research into processing and interpreting data from 3C transmitter systems
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