Transient electromagnetic waves applied to mineral exploration
Creators: name: Lee, Terence John
Publisher: name: University Of Tasmania
Year: 2023

description: The effective use of transient electromagnetic waves for mineral exploration depends on a proper understanding of how these waves propagate in and are effected by different geological materials and structures. of these effects forms the basis of the work presented here. Transient electromagnetic waves propagate in the ground at a rate that is determined by the electrical conductivity and the magnetic permeability of the ground. The direction in which the maximum current density propagates is determined by the type of source that is used to generate the waves (15, 18,19). For realistic host rock conductivities and measurement times, the velocity of propagation is quite fast. For this reason the effect of loop height of the transmitter is unimportant except for the earliest of times (16). However, some care should be exercised in interpreting the depths to conductors by means of the direction of the observed magnetic fields since the currents, even in a uniform ground, can behave as though they derive from sources at depth (17). For the situations that are generally met with in practice, displacement currents have a negligible effect. Drastic changes to the appearance of the transient decay curves can be caused by geological materials whose conductivity or magnetic permeability varies with frequency. For a ground with a frequency dependent conductivity that may be described by means of the Cole-Cole Model, changes in sign of the transient may be observed (11,12). These effects may also be seen in the case where the conductivity of a structure, within an otherwise uniform ground, varies in the manner of the Cole-Cole Model (4). These studies show that the transient electromagnetic method of prospecting is sensitive to induced polarization effects, and this may account for some of the anomalous transients that have been encountered by the field geophysicist (4,12). A magnetic permeability that varies with frequency is sufficient to explain all the observations that have been made in regard to surveys carried out over areas where superparamagnetic minerals are present on the earth's surface (1,2). A weakly magnetic ground, however, produces a transient that is quite similar to those observed over a non magnetic ground (2). The effect of a conducting host enclosing a conducting structure is, for some stage of the transient, to produce a transient that can not be described by a simple exponential function (4,5,6,9,13). For the case of a conducting host the final form of the transient may be modelled by assuming that the electric field within the conductor can be approximated by the inducing electric field (6). One study showed that the resonant responses of the conductor rapidly decay and that the final form of the transient is determined by the conducting host rock enclosing the conductor (5). It has been usual to model the transients by Fourier transformation of the frequency response of the structure. A consequence of this is that it is frequently desirable to have closed form expressions for a number of frequently occurring integrals. Some of these integrals have now been evaluated (7,10). An alternative approach is to view the equation, for the transient, as being made up of a number of terms that are related to the singularities in the function describing the spectrum of the transient (14). This approach readily separates out the resonant from the non resonant response of the structure (5,13). It frequently leads to quite simple expressions, that are suitable for practical purposes (1,2,5,6,8). Also, the alternative perspective leads to quite different questions being asked about the transient (14). Finally, it can be demonstrated that the effect of the host rock is to make the inversion of transient electromagnetic data quite difficult and that supplementary information is needed. This is true for layered and non layered structures (3,8).