Exploration Geophysics - Volume 30, Issue 3-4, 1999

A magnetic susceptibility meter has been used as a mapping tool on outcrops of magnetic metasediments at Broken Hill. On each outcrop, magnetic susceptibility readings were taken on a 0.1 m grid, contoured, and compared with geological features drawn at the same scale. The area chosen, known as the Monuments, coincides with substantial aeromagnetic anomalies generated by magnetite-bearing metasediments of the Sundown Group, part of the Early Proterozoic Willyama Supergroup. The magnetic susceptibility mapping shows that concentrations of magnetite follow bedding, and do not follow high grade or retrograde schistosity. The magnetite concentrations follow around folds with high grade axial plane schistosity (S2), and are truncated by high grade shears that are parallel to the same undeformed high grade schistosity. The magnetite owes its existence to sedimentary or diagenetic processes, which formed concentrations of magnetite or another ferric mineral precursor. The characteristics of many other aeromagnetic anomalies in metasediments at Broken Hill indicate that they have the same origin.

Time varying magnetic fields can influence the accuracy of aeromagnetic surveys. The effect is more pronounced at lower latitudes. It can be quite small at higher latitudes if the induction vector points towards the equator. In any latitude, east–west variations have little effect.

On the Australian continent, inhomogeneous regolith acts as a shallow source of gravity anomalies, and hence provides a significant interpretational problem for mineral explorationists. Published results from Western Australia, identify the geometry of the basement topography as being reflected in the gravity profiles. Exploration beneath this inhomogeneous regolith, using gravity without the effect of the regolith cover itself, would provide a distinct improvement of gravity’s diagnostic capability.

Initial analysis suggested that EM was a feasible method to define basement topography, from which a gravity correction could be attempted. The primary question needed to assess the feasibility was: ‘Does a mappable conductivity boundary correspond to each major boundary of density contrast?’ Testing of this hypothesis was completed at Elura, New South Wales (N.S.W.) by constructing an EM model and a gravity model for the purpose of interpretation. In surprising contrast to petrophysical evidence and Western Australia experience, gravity profiles at Elura reflected lithology as being a more important source of shallow anomalies than the depth of weathering. An attempt to apply the geometry of the modelled EM basement/regolith contact as a means of creating a model for gravity correction saw no improvement in the residual gravity profile. The regolith structure at Elura was geophysically delineated as having a progressively increasing resistivity with depth, but one where gravity highs are not reflected in basement highs.

Riomin Exploraciones SA, a subsidiary of Rio Tinto pic, discovered the Las Cruces volcanic hosted massive sulphide deposit in 1994. The Las Cruces deposit lies at the extreme eastern end of the Pyrite Belt in southwestern Spain.

Interpretation of the regional gravity data indicated that Pyrite Belt lithologies could extend further east than previously interpreted. This possible extension of the Pyrite Belt, however, lays beneath a variable thickness of post-mineral mainly Tertiary aged sediments. Rio Tinto identified the Las Cruces gravity anomaly from a survey conducted across the Laralaes licence block in 1994. The residual Bouguer gravity anomaly is approximately 29 μm/s²: (2.9 mGal).

The deposit lies at a depth of approximately 120 m beneath the base of the Tertiary cover and is hosted by typical Pyrite Belt lithologies consisting of highly altered felsic to intermediate volcanics with minor sediments. Las Cruces is unusual as it contains a substantial zone of enriched secondary copper sulphides lying beneath a gold rich gossan cap. At the end of 1998 a measured and indicated secondary copper resource of approximately 14.51 Mt grading 6.13% Cu had been outlined by drilling. Las Cruces also contains primary copper and zinc mineralisation as well as a gossan gold resource lying above the other orebodies at the base of the Tertiary.

A number of geophysical techniques including electrical soundings, IP, TEM, CSAMT, DHEM and mise-a-la masse surveys have been completed across the deposit. The area is difficult for electrical surveying as it lies within 18 km of the city of Seville. There are a number of domestic and high-tension power lines crossing the area. There are also roads, fences, a factory and several houses to contend with, as well as a high-speed electric rail link in the vicinity. The IP and TEM techniques both worked well in helping to define the extent of mineralisation at Las Cruces.

High-resolution crosshole imaging techniques are proving to be useful tools for mapping out mineralisation between exploration boreholes. Radio Tomography (RT) and electrical resistivity tomography (RESTOM) surveys were conducted at a disseminated sulphide test site in South Africa. The RT and RESTOM results correlate well, but contradict the existing interpretation as inferred from geological logs. It is concluded that the integrated application of high-resolution crosshole imaging techniques could play a major role in second-phase base metal exploration.

The geoelectrical imaging technique has been widely used for environmental and engineering studies in recent years as this technique gives a graphic representation of the distribution of electrical properties in the subsurface. The evaluation of subsurface objects and detection of buried utilities, such as pipes, is of particular interest for a variety of engineering and environmental applications.

The geoelectrical imaging technique as a non-destructive and inexpensive tool for mapping buried objects was successfully used to locate an underground metal pipe with a diameter of 1.20 m buried at a depth of 0.9 m in the Universiti Sains Malaysia campus. The 2D and 3D geoelectrical imaging surveys using the Wenner, Pole-Pole, Pole-Dipole and Wenner-Schlumberger electrode arrays successfully mapped the shape and location of the underground pipe.

The 2D resistivity sections as well as the 3D vertical slice sections from these surveys demonstrate the effective of the improvements made in field data acquisition that has partially suppressed the effects of a variety of electrical noise sources surrounding the surveyed area. It has reduced the effects of such electrical noise on the field measurements, and has enabled the field results to be more representative of the subsurface target. Moreover it has strengthened the current signal and thus improved the voltage- to - noise ratio for the different electrode arrays used in these surveys.

A rapid numerical method for evaluating Total Field Magnetometric Resistivity (TFMMR) responses of arbitrary threedimensional structures located between two point current sources has been developed. Finite-difference equations are derived from the Poisson equation which governs electric potential distribution within a given conductivity model. Solution of the resulting matrix equation is made using the Successive Over-Relaxation (SOR) technique combined with matrix pre-conditioning. Computation of current density from the potential and conductivity models is then made. A new, rapid Fourier domain method for evaluating the magnetic field arising from the sub-surface current flow is developed. The new method evaluates the X, Y and Z components of the magnetic field in the full three dimensions of the model space. TFMMR measurements may then be computed as a linear combination of these components. The modelling procedures developed are shown to be extensible to modelling Total Field Magnetometric Induced Polarisation (TFMMIP) responses resulting from electrically polarisable structures.

Comparison of TFMMR computer models generated by analytical and the derived numerical means shows good agreement for a hemispherical sink and two dyke models. An underestimate of the response for a dipping interface model was observed because of the inability of the scheme to accurately model boundary conditions when conductivity inhomogeneities extend to artificial model boundaries.

An analysis of strike length on MMR response is made using conductive, rectangular prism models of varying lengths. This analysis showed a dependence of the MMR response approximately proportional to strike length of the prism. Investigation of the effect of conductive overburden on MMR response showed consequences of the layer were negligible compared to masking that occurs in conventional resistivity methods.

Occasionally, airborne magnetic anomalies can have amplitudes as large as 70 000 nT and, for shallow dyke-like bodies, it is possible for the amplitude to vary by more than 70 000 nT over a distance of about 200 m. In such cases, the horizontal spatial gradients can be as large as 700 nT/m. Moving an induction coil sensor through a zone where the spatial gradient is large will, by Faraday’s law, result in a voltage being induced in the coil. Because airborne transient electromagnetic (ATEM) systems normally use induction coils, it is possible for voltages as large as a few volts to be induced when a system crosses a very large magnetic gradient. For a sensor height of 70 m, the wavelength of this type of voltage anomaly is typically about 240 m, which corresponds to a frequency of 0.25 Hz for a flying speed of 60 ms^-1. The stacking algorithms employed in ATEM systems are specifically designed to reject signals with frequencies this low.

In order to assess the efficacy of ATEM stacking algorithms, the response of a thin shallow highly magnetic body has been simulated and processed using the signal processing algorithms implemented in the GEOTEM ATEM system. The stacking algorithm essentially removes the low frequency signal. The minuscule residual signal that is not removed is at least an order of magnitude smaller than the current noise levels. Hence, flying an EM system through a large horizontal spatial gradient will not have an observable effect on the stacked EM response that is measured by the GEOTEM system.

In their ascent to the aircraft detector system, the gamma-rays recorded in airborne radiometric surveys are attenuated, first by the surface materials wherein they originate, then by the intervening atmosphere. Increased ground-clearance thus entails reduced count-rates. It also implies diminished spatial resolution, because the same cone of incident radiation derives from progressively larger ‘footprints’ that, for a given sampling rate, increasingly overlap. But suitable post-processing of the gridded, twodimensional imagery can be used to correct these types of height-dependent degradation and hence produce sharper, quantitatively useful maps of radioactive isotope distribution. An essential prerequisite for such inverse filtering is noise-suppression, achieved here through maximum-noise-fraction (MNF) transformation of multi-channel data. High-frequency noise amplified by the deconvolution step is brought under control by a variant of Wiener filtering. The combined de-noising and deconvolution process is illustrated by application to an airborne gamma-ray survey from the Marble Bar area, Western Australia.

During the last 10 years gamma-ray spectrometric surveys by governments and mineral companies have generated increasingly accurate and detailed data on K, Th and U surface concentrations of much of the mineral rich land currently being explored. Interpretations of these data commonly involve no more than subjective visual classification of colour images, either images of the concentrations of the individual elements, or images of the three elements combined in a RGB composite. This interpretation method does not extract the maximum information from the data.

In areas of exposed and consolidated rocks, potassium, thorium and uranium contents generally correlate within each major rock type. For display and interpretation purposes there are advantages in separating the major rock types using K, Th and U values, then, for each major rock type, calculating the data in terms of components. One component measures the distance along the average geochemical variation path, and other components measure deviations of Th and U from this path. Maps of these components allow the more detailed quantification of original lithology, mapping of any sharp breaks in lithology, and mapping of later chemical modification. Compared with using principal components, the use of deviations of Th and U from the mean path as the second and third components, allows for the big difference of the three elements in fractional errors, and makes it easier to relate the components to geology.

Three widely used frequency domain algorithms for post-stack migration are the omega-x, PSPI and explicit algorithms. They differ in speed and accuracy with omega-x being the least accurate and fastest. Phase shift plus interpolation (PSPI) is the slowest and less accurate than the explicit algorithm. The explicit algorithm is the most accurate of the three and its run time falls between the omega-x and PSPI algorithm.

Polarisation analysis quantitatively describes the particle motion of a seismic wavefield. It is the fundamental vector processing technique applied to multi-component seismic data. Synthetic and real VSP examples illustrate the application of polarisation analysis via particle-motion hodograms, and an automated single-trace time-domain method.

Polarisation analysis can facilitate the extraction of pure P and S-wave sections, removal of unwanted noise events, and recovery of information relating to fracturing, porosity and lithology from multi-component data. Two applications of polarisation analysis are demonstrated, namely the extraction of pure P and S-wave sections, and S-wave splitting analysis.

The success of polarisation analysis depends on the accuracy with which the recorded seismic wavefield represents the true vector wavefield. Care must be taken to ensure particle motion is not distorted during acquisition and preprocessing. Sensor response and coupling must be matched across components. Amplitude equalisation must be identical on all components. Frequency and/or velocity filtering should be designed to remove only noise, so as not to modify the underlying signal.

The effect of the analysis window length is demonstrated, with the best compromise between resolution and stability being provided by a window approximately equal to the dominant period of the recorded signal. If more than one seismic event exists within the analysis window, single-trace polarisation analysis cannot accurately recover infoimation on the individual wave types. A comparison of polarisation analysis in the t-x and T-p domains highlights the value of utilising both slowness and polarisation information to enhance the accuracy of discriminating between interfering wave types.