Exploration Geophysics - Volume 14, Issue 2, 1983

In October and November 1981 a short experimental high resolution seismic survey was undertaken at Wambo colliery, in the Hunter Valley, N.S.W. to determine if an area southwest of the current colliery is dissected by a fault. The seismic survey recorded good reflections from the coal seam in question (the Whybrow Seam) over a line 2.1 km long across the location of the postulated fault. The seismic data showed that the Whybrow Seam was not faulted at least within the estimated 6.5 m of seismic ‘resolution’. The seismic data also provided information on the configuration of the underlying coal seams and provided a comparison between the Mini-SOSIE (trade mark of Societe Nationale Elf Aquitaine, Production) technique and the conventional dynamite seismic technique. The comparison demonstrated that both methods can be effectively used in this area and that selection of one particular technique would depend on the survey objectives, near surface geology and exploration budget.

A gravity survey was carried out across the ‘Agate Pocket’ to help investigate the relationships between various units of the Agate Creek Volcanics, and the structure of the underlying basement rocks. The volcanics are the source of high quality agates and other nodules. A key factor operationally was the careful placement of the traverse across elongate topographic features to enable two-dimensional computer processing of topographic effects. Interpretation of the data hinges on the analysis of the effect on the density profiling process of the anomalies caused by geological bodies. A large ridge of rhyolitic rocks provides key information in the form of a bulk density value of 2.45 t.m.^-3, deduced by iterative analysis of Bouguer density profiles. The agate-bearing andesitic volcanics are interpreted to be about 400 m thick. They overlie a homogeneous basement, are fault-bounded on the southwestern side, and abut against the roots of rhyolitic volcanics on the northeastern side.

Measurements of the magnetic properties of rocks constrain magnetic interpretation and resolve much of the ambiguity which afflicts the magnetic method. Magnetic petrophysical studies invariably indicate the importance of remanence as a source of magnetic anomalies. The susceptibility, remanent intensity and Koenigsberger ratio exhibited by a rock containing magnetic mineral grains is not only a function of the volume fraction of magnetic material with a given composition, but is also sensitively dependent on the domain state of the magnetic grains. Superparamag-netic, stable single domain and multidomain size ranges are given for magnetite, titanomagnetite, maghaemite, haematite and monoclinic pyrrhotite. The susceptibilities, remanent intensities and Koenigsberger ratios of super-paramagnetic, single domain and multidomain grains of these minerals are plotted. Charts of typically observed magnetic parameters for various rock types are presented. Rocks often bear a multicomponent remanent magnetisation. The various components are often carried by grains with different coercivity or blocking temperature spectra and can be resolved using palaeomagnetic cleaning techniques. Surface samples typically contain palaeomagnetic noise which must be identified and removed if representative remanence values for the rock unit are to be determined. Under favourable conditions probable remanence directions of rock units can be inferred from a knowledge of the age of the rock and its locality and of apparent polar wander with respect to the crustal block within which the rock unit is located. Formulae are given for inversion of palaeomagnetic data to obtain magnetisation directions, and a table of primary remanence directions throughout the Phanerozoic is given for Australia. Apart from providing input to magnetic interpretation, many other applications of magnetic petrophysics are apparent. Palaeomagnetic dating of mineralisation, magnetic fabric studies for structural interpretation, magnetostratigraphy, detection of redox chemical remanent magnetisation effects and magnetic techniques of mineral identification and quantitative analysis, are some of the promising applications in the mineral exploration and processing industries.

The analysis of geophysical data by computer often includes post-processing with digital filters. Of particular interest are those digital filters which approximate ideal low pass, high pass, band pass or band reject responses. Design data are presented for digital filters which approximate a Butter-worth response. Low pass, high pass, band pass and band reject filters are discussed and it is shown that in general the nth order low pass Butterworth transfer function may be factorised with the denominator consisting of linear and/or quadratic factors. Simple frequency transformations permit the realisation of the associated high pass, band pass and band reject transfer functions for these factors. Application of the bilinear transformation yields the z transforms for these factors which, when used recursively, provide the desired filter response. Fortran computer programs are included and the performance limitations of the digital filters are illustrated by comparison with some ideal Butterworth responses.

Model results for a dipole-dipole transient electromagnetic technique applied over sheet conductors, indicate that sign changes in the response depend not only on target depth but also on target inclination for a given depth. Maximum response is obtained when the transmitter-receiver separation is chosen as 1.33 to 1.54 times the target depth.