Exploration Geophysics - Volume 25, Issue 1, 1994

An interpretation of geological and geophysical data is presented to suggest that the Precambrian rocks of Tasmania comprise a series of Palaeozoic thrust slices embedded in the western part of the Lachlan Fold Belt and the eastern part of the Kanmantoo Fold Belt. This interpretation contrasts with previous interpretations which regarded the Precambrian rocks as the basement on which the Palaeozoic rocks formed.

In western Tasmania, east- or west-facing thrusts and involving detached Precambrian blocks occur in discrete zones. West-facing thrusts only occur northwest of an axis from Elliott Bay in southwest Tasmania to Penguin in northern Tasmania. This thrust system overlaps an older east-facing system which becomes more intense eastward but which may be recognised across the entire island.

We report here two new applications of the Poisson relation useful when a joint magnetic and gravity gradient survey has been made. In the first, an apparent susceptibility contrast to density contrast ratio map (which we call the “pseudo-lithology” map) can be estimated. In the second, selected values of the above ratio are used in a “pseudo-magnetic” filter to subtract from the magnetic field measurements the effect of a particular rock type. This would be especially useful as a filter of “geological noise” due perhaps to weathered magnetite products, particularly if the latter had an effect over a large area.

As a test of these techniques we have applied them to a simulated joint airborne survey over a model which we designate “Mini-Elura”, in which the magnetic magnitude and vertical gravity gradient anomalies are not visible above the noise due to surficiai variations.

The computed pseudo-lithology map clearly distinguishes the regions of the map dominated by the alluvium source from those due to the “Mini-Elura” orebody, while the pseudo-magnetic filter does very effectively remove the “noise” due to the alluvium from the magnetic map.

Spectral measurements of apparent resistivity and phase shift using a two electrode array or a Wenner electrode array over a cement-graphite sheet model immersed in an electrolyte (i.e. volume polarisation), reveal that the frequency of maximum phase shift, Fm, varies with the array geometry and its disposition with respect to the target model. In the authors’ opinion, this suggests that practical mineral discrimination by measurement of spectral IP may only be a remote possibility. However, the Fm, for a given electrode array, is much smaller for a metallic model of the same dimensions, immersed in the same electrolyte. This means that in order to do accurate resistivity modelling, the excitation current frequency needed is much less than that is required in the case of the metallic model under the same conditions. The model tank apparent resistivity responses over the cement-graphite model sheet measured with time domain IP equipment very much resemble the corresponding resistivity responses obtained over a metallic model of the same dimensions under similar conditions. Practising geophysicists should keep this result in mind when interpreting IP field data.

Soil salinity profiles frequently display monotonically increasing or decreasing salt concentrations with depth, z. This salt concentration is strongly correlated with the conductivity, σ, of the ground and frequently can be represented by an equation σ(z) = σ1 exp(± |b|z), where σ1 and b are the parameters that define the conductivity profile.

For exponentially decreasing conductivity with depth, the late time coincident loop response is identical to that for a slab of thickness 1/b and conductivity σ1.

For exponentially increasing conductivity with depth, expressions can be found for the voltages recorded by a modified INPUT or an airship system. The INPUT system must be modified by having the measurement of the voltages made over delay times longer than 1 ms. Calculations show that these voltages are sensitive to variations in the parameter b and they are sufficiently large to be measured by an airborne system. The parameter σ1 is difficult to resolve for late times. The particular numerical example chosen shows that the normalized voltage Vr is not very sensitive to σ1 but is more sensitive to the parameter b, while the reverse is true for Vz.

The parameter b is the more important of the two, as it shows the rate at which salt concentration varies with depth. Accordingly some idea of the salinity of the soil may be obtained from these EM methods.

An alternative approach to rays for determining the wave motion in horizontally stratified layered medium is presented. Rays obey Snell’s law and are bending towards or away from the normal when crossing an interface. We show how by squaring the propagation velocities and scaling interface depth the rays are stretched so as to move along straight lines when crossing the interface. We chose to call it corpsular straight trajectory as it is closely related to Newton’s Corpuscular Theory of wave motion. We show that this new concept is useful in transforming two or fully three spatial dimensions wave fields as function of time to many simple essentially one dimensional problems. It may provide a new look on the dichotomy of particles versus waves. Its potential lies on the massive parallelism that can be obtained by solving the various corpsular trajectories emitted from the source in a plurality of directions as separate quasi one dimensional problems. The summation of these individual solutions yields the three dimensional solution of the wave equation.