ASEG Extended Abstracts - ASEG2004 - 17th Geophysical Conference, 2004

A powerful new set of magnetic derivatives is reported, based on the Tilt derivative and its Total Horizontal derivative, They can be used to map geological structures, magnetic fabric, lineaments and depths more effectively than other commonly used derivatives. The methods described are similar to the local phase and local wavenumber but are formulated differently to make them easier to use in profile- and grid-based methods.

This contribution reveals new insights into why the Tilt derivative is better suited to mapping structure, due to its ability to (a) act as an effective AGC (Automatic Gain Control) filter, (b) out-perform the vertical derivative in mapping the spatial extent of bodies; and (c) map edges of bodies when applied to Reduced to the Pole (RTP) or Equator (RTE) magnetic fields by using the anomaly zero crossings.

The Total Horizontal derivative of the Tilt derivative is independent of inclination of the geomagnetic field and generates maximum values over the edges of bodies. Its negative reciprocal provides depth estimates which can be used to estimate depth to sources over large areas, thus allowing a rapid estimation of whether or not mineralised structures are recoverable.

These derivative methods have been applied to re-evaluating the Erindi gold prospect in Namibia using the recently acquired national high-resolution aeromagnetic data. All map images used in this paper have been generated using GETgrid

Recent implementations of Euler deconvolution allow to solve simultaneously for the source position and the structural index. This opens the way to a comparison between this technique and the Continuous Wavelet Transform (CWT) method, that allows the estimation of essentially the same parameters. Direct comparison of Euler deconvolution and CWT methods is possible only by applying the first method to a potential field upward continued to many altitudes. While the two methods give very similar results when the gravity or magnetic field of a one-point source is concerned, they behave different for those sources characterized by fractional structural index, as many real geologic structures are (for example a limited throw fault). In this paper, the variation of the Euler estimated parameters at many altitudes above a magnetized prism is described. Such a variation gives additional information on source geometric parameters and position, which may be recovered from plots of estimated depth and structural index vs. altitude: a) the extended or one-point nature of the source results clearly; b) it is possible to understand to which part of the source the depth estimate is related to; c) it is possible to get indications about the source thickness and lateral dimensions. On the other hand, for sources of finite extent, the CWT analysis may be made only for sets of levels and not at any level, differently from the above outlined Euler deconvolution approach. Nevertheless the results from these two methods are substantially consistent at high or low altitudes.

Deep magnetotelluric (MT) surveys provide information on thickness, electrical resistivity and tectonic history of Precambrian lithosphere that is relevant to diamond exploration. In regions where lithosphere is electrically resistive, its thickness can be inferred from the observed depth of a conductor at 100 km to 300 km taken as defining the top of the electrical asthenosphere. In some locations high conductivity is observed within the lithosphere and, at depths above the diamond stability field, may be attributed to the presence of graphite. The form of crustal and upper mantle conductors, interpreted with deep seismic information, provides information that can be used to constrain the tectonic history of a region, eg, defining the geometry of past subduction.

Surveys in the Slave craton in northern Canada have identified an MT phase anomaly that is coincident with both a diamond-bearing Eocene kimberlite field and an upper mantle geochemical anomaly. The phase anomaly is explained by a conductor, with its upper surface at 80 to 120 km depth and the enhanced conductivity is interpreted as being due to graphite or an interconnected carbon grain-boundary film. In the western Superior Province in central Canada, MT surveys have delineated a mantle conductor at a depth of 130 km. The MT responses define the geoelectric fabric of both the crust and subcrustal lithosphere and indicate that the subcrustal lithosphere is of Archaean age.

In this paper I investigate the level of error in magnetisation direction which might pass undetected in TMI inversions, and estimate the corresponding errors that this will cause in estimation of the location and shape of source bodies. I also show that three-component and gradient tensor data can potentially provide increased sensitivity to source magnetisation direction. These studies are undertaken by forward computing magnetic anomalies for bodies with various magnetisation directions, and then inverting those anomalies with the assumption of induced-only magnetisation. The misfit between input and output fields after the TMI inversions increases with error in source magnetisation direction. Up to 30° to 40° the misfit is likely to be undetectable. Up to 60° to 70° the misfit can be reduced by invoking a more complex magnetisation distribution, but from 70° to 90° the inability to match an anomaly with any feasible distribution of magnetisation is diagnostic of error in the source magnetisation direction. The associated errors in location and dip of a source body increase with undetected error in its magnetisation direction.

Components of the magnetic field behave similarly but not identically to error in source magnetisation direction, and slight differences between component inversions provide additional sensitivity to source magnetisation for errors above 40°. Unfortunately measurement difficulties for component data are likely to restrict the achievement of this improvement. Magnetic field tensor data also increase sensitivity to source magnetisation direction and enable better-constrained inversions of the magnetic field.

The presence of fractures in a reservoir is a main cause of azimuthal anisotropy of its elastic properties. A shear wave propagating in an azimuthally anisotropic medium splits into two components with different polarizations if the source polarization is not aligned with the principal symmetry axis. If the direction of shear wave propagation is not parallel to the plane of fracturing, shear-wave splitting will depend upon the normal fracture compliance, which in turn depends upon the properties of the filling fluid. If the system of pores and fractures in a fluid-saturated rock is interconnected, then fluid flow between pores and fractures must be taken into account. How shear-wave splitting varies with fluid properties depends upon the assumptions that are made regarding the pressure relationship existing between pores and fractures. In this paper we use the anisotropic Gassmann equations, and existing formulations for the excess compliance experienced due to fracturing, to estimate the splitting of vertically propagating shear waves as a function of the fluid modulus. This is done for a porous medium with a single set of dipping fractures and with two conjugate fracture sets dipping with opposite dips to the vertical. The estimation is achieved using two alternative approaches. The first approach assumes that the fractures and pore space are in full pressure equilibrium with respect to fluid flow. That is, the frequency of the elastic disturbance is low enough to allow enough time for fluid flow between the fractures and the pore space. In the second approach each of the fracture sets are in full pressure equilibrium with the surrounding pore space, but not with the other fracture set. That is, the frequency is low enough to allow fluid flow between a fracture set and the surrounding pore space, but high enough so that there is not enough time during the period of the elastic disturbance for fluid flow between fracture sets to occur. It is found that the second approach yields a much stronger dependency of shear-wave splitting on the fluid modulus than the first one.

CSIRO and Australian exploration companies have developed a new instrument for mineral exploration.

The GETMAG (Glass Earth Tensor Magnetic Airborne Gradiometer) instrument measures the magnetic gradient tensor and the components of the magnetic field. The sensors are high temperature superconducting (HTS) quantum interference devices (SQUIDs) operating at liquid nitrogen temperature. A superconducting transformer coupled to a SQUID magnetometer is rotated to detect all gradients perpendicular to the rotation axis. Three SQUID-loop assemblies are required to measure the full tensor.

Field trials demonstrate that the inversion of a few tensor measurements yields as much information as a high-resolution total magnetic intensity (TMI) survey. Direction to individual sources and their magnetic moments (reduced according to distance) can be determined directly from the tensor. GETMAG has applications in mineral exploration, environmental studies, ordnance detection, ship degaussing, submarine tracking and monitoring of marine currents.

The project was funded by CSIRO, BHP, De Beers, MM, Newmont and WMC.

Airborne electromagnetics (AEM), magnetics and radiometrics flown over the Angas-Bremer alluvial plain southeast of Adelaide, SA, provide information for natural resource management in an area of intense viticulture with a shallow saline water table, and a deeper fresh-water Tertiary aquifer at risk of salinisation.

A specific aim of the surveys was to delineate shallow palaeochannels that could be pumped to lower shallow saline water tables. These are not apparent in the data, either due to absence, or inability to image such features due to lack of contrast in physical properties.

The surveys give a new view of the landscape, geology and regolith framework of the area. There is no evidence for a fault (postulated from resistivity soundings 30 years ago) that has been modelled as a recharge conduit to the deep aquifer. However, several previously unrecognised faults have been interpreted from the data and drill information. These offset the Cenozoic aquifers by up to 80 m. AEM and drill data have helped to determine a new set of structure contours for the base of the aquifer system which will assist more robust groundwater modelling.

AEM response in the aquifers correlates loosely with measured water salinity, but the relationship is not tight enough to allow imaging of high quality groundwater from the AEM. The DEMs constructed from the surveys give better accuracy to landscape modelling in the area, and the radiometrics have the potential to form the basis for upgraded soil maps. Ten broad land management units across the area have been defined by reference to geophysical properties and drill hole information.

The Bedout High in the Roebuck (formerly offshore Canning) Basin at the NW Australian Margin (NWAM) has recently been re-interpreted as a massive impact structure that appears to be associated with the global Permian/Triassic extinction event. This impact may have significantly modified the crustal structure in the region. Depth conversion of reflection seismic data indicates that depth to basement at the top of the Bedout High is ~ 3.9 km, and that the High stands more than 4 km above the surrounding sedimentary basins. The basement and crust in the Roebuck Basin have a number of features that distinguish it from other basins at the NWAM. Rapid crustal thinning outboard of the Bedout High and the presence of a thick layer of magmatic underplating in the lower crust are among these features. The meteorite impact may have been one of the possible causes to have triggered upper mantle melting and generation of a voluminous layer of underplated material. On a finer scale, OBS-derived seismic velocity variation along the basement is speculatively interpreted to be consistent with impact-related effects. However, existing seismic and potential field data do not allow accurate estimates of the extent of the crust affected by the meteorite impact, and effects that it may have had on the subsequent rifting, thermal, sedimentation and hydrocarbon maturation regimes in the area. Further multidisciplinary research is necessary to answer these questions.

The gravity field of the geologically contrasting Central and Southern Andes has been investigated with regard to iso static state, crustal density structure and rigidity of the lithosphere. The gravity database, together with other geophysical data, is a product of several years of field data acquisition in the Central and Southern Andes as part of the German Collaborative Research Centre SFB267 ("Deformation Processes in the Andes").

All gravity data are tied to the IGSN71 gravity datum and are terrain-corrected. The gravity effect of the subducting Nazca Plate was removed from both Bouguer and isostatic residual anomalies and then correlated with mean topographic heights to identify areas of disturbed isostatic equilibrium.

The abundance of geophysical information obtained by the SFB267 and its partners (e.g. geological, structural, seismic and magnetotelluric data) allows the development of well-constrained 3D forward gravity models. These models allow estimation of subsurface loading and, therefore, permit more rigorous estimation of lithospheric rigidity than if topographic loading alone were considered. To date we have used estimates of surface and subsurface loading from such 3D models with a 2D coherency method to estimate lithospheric rigidity in the central Andes.

Although gravity gradiometry was among the first geophysical methods used successfully in applied geophysics (Eotvos torsion balance), the technology fell from favour after the 1930s. Here we present a new analysis of torsion balance measurements that were made at this time to detect salt domes in the Northwest German Basin.

The study was aimed at assessing methods for interpretation of torsion balance observations. The approach is to synthesize classical procedures with computer based numerical methods, taking horizontal gradients Wxz and Wyz as input parameters. First we give an overview of torsion balance operational details and then discuss the interpretation of torsion balance data.

A total of 39 maps at 1:25,000 scale covering the Northwest German Basin have been digitised. These maps provide some 40,000 torsion balance measurements, but no field books or field forms are available. The gradients were digitised from old paper maps, then reprocessed and recalculated. 3D modelling of Wxz, Wyz and other components of the Eotvos tensor provides detailed insight into the geometry of salt domes and provides additional constraints for modelling the Bouguer gravity field.

3D forward modelling is conducted using the IGMAS program package. This package allows the calculation of potential, gravity and its components, and the Eotvos tensor components. Results show that knowledge of the second derivatives of the potential could fundamentally change the role of gravity field measurements in underground investigations, not only for resource exploration, but also for investigations of large fault systems like the Dead Sea Transform.

In this work, the effect of natural fractures on the azimuthal response from structured reservoirs encased within carbonate rocks is investigated through forward modeling. Two fractured carbonate reservoirs are examined: the first one is composed by sediments nearly flat; the second one is in an area with reflectors having dip up to twenty degrees.

When modeling fracture behavior, even if fractures are sub-vertical, it is usual to suppose that they are vertical. For dipping reflectors, even if fractures are supposed to be vertical, they are not normal to vertical versus the dipping reflectors. Thus, the reflections from a dipping reflector above vertical fractures are quite similar to reflections from a flat reflector with inclined fractures below.

We compare synthetic seismic data from two different models having respectively vertical fractures and sub-vertical fractures (with an inclination of 70°). Fracture inclination perturbations give azimuthal variations especially on results of converted waves.

Indeed, reflections from dipping interfaces present azimuthal variations of amplitudes even for isotropic media. For a model composed by several dipping layers, we investigate the combined effects of fractures and dipping reflectors.

In particular, we discuss Amplitudes versus Offset and Azimuth (AVOA) of converted waves that, at present, have been used just for their splitting and we show how AVOA can help fracture characterization.

Developments in the application of statistical signal processing techniques to mineral exploration have shown that, in addition to providing a ranked set of targets, it is possible to make useful statements about the probability of successfully detecting a target and the probability of producing a false alarm. These probabilities can then be use to assist and assess exploration decision-making. This paper outlines a method to analyse the cost-effectiveness of a given geophysical technique as part of an exploration programme and demonstrate the increase in NPV that geophysics can provide.

I consider a simple model for the exploration process. The model accepts the probabilities produced from the target selection process and uses them with simple, generalised prospectivity information and exploration financial constraints to estimate the probability of exploration success. Then, using standard methods from financial analysis, this scheme has been generalised to a more complete study of the profitability of the exploration process. Thus, instead of modelling a single exploration scenario, we can model in a way that accommodates wide ranges of possible inputs and NPV outcomes.

Like most financial analysis, this type of analysis can't be rigorous in the same way we think of geophysical data analysis. However, it is an objective way of examining and justifying exploration expenditure that meshes well with many other types of financial analyses that are used to make decisions in most other aspects of the mining industry. In that sense it provides exploration management a quantitative way of evaluating exploration options and justifying exploration expenditure.

RESOLVE frequency domain helicopter electromagnetic (HEM) data have been used to map the distribution of near-surface clay-rich sediments in and around the Riverland irrigation districts of South Australia.

The results of the inversion also allow us to reconstruct the strandline-dominated palaeo-topography left when the sea retreated from the Murray Basin in the Early-Mid Pliocene. The survey also revealed a hitherto unsuspected, deeper variability in conductivity following the Pliocene strand line pattern. The cause of this pattern is not clear, but could be due to variations in the porosity of Loxton Sands or to strandline-correlated variability in the elevation of the contact between the Bookpurnong Beds and the Lower Loxton sands.

Target selection and ranking is a critical aspect of mineral exploration that is complicated when the host geology either masks or mimics the responses from targets of interest. The approach described here uses statistical signal processing techniques, specifically matched filtering, to help recognise targets in the presence of such confusing effects.

The essence of this approach is to assume that the data are composed of noise, generated by the background geology, and signals generated by targets of exploration interest. It is this recognition of the geology as a noise source, which allows traditional signal processing strategies to be applied to the problem. The critical step, absolutely necessary to success, involves pre-whitening the data so that those characteristics of the target that are as different as possible from this "geological noise" are emphasised.

Matched filtering is a well-established technique in fields other than mineral exploration. Work here has shown that, in many instances, it can also be applied successfully to AEM data. It provides a useful way of highlighting targets for further investigation by simplifying the interpretation process and filtering out many complex anomalies that are not feasible targets. This paper presents some of the necessary theory and the results of a number of case studies where the technique has been applied.

A 3-D laboratory tank model system was constructed and used to investigate the crosshole DC electric imaging possibilities of a conductive target using a bipole-bipole array, with one current electrode located in each simulated borehole. Both scanning and profiling measurements were made, for five different classes of cylindrical model, depending on the cylinder orientation relative to the plane of the boreholes and whether the target was continuous, terminating, or broken between the points of measurement, and whether the conductor was intersected by one of both boreholes. Each class of model produces a characteristic response, which can be used to identify the conductor and locate its upper and lower boundaries. Data were collected for different bipole separations. Very clear anomalies are produced for the scanning arrangement (both bipoles moved together) for models involving conductor intersection. Electric profiling, which entails one fixed bipole and one mobile bipole, produces a more subtle but nonetheless diagnostic pattern.

We present an example of a constrained inversion method that uses a categorical property, lithology, as the primary model parameter. A 3D geological model is supplied as the starting model. The topology of this model is used as prior information, together with parameters that define the probability density function of a secondary property, density, and observations of the gravity field.

The 3D geological model is a prediction of the geometry of geological interfaces given points where these interfaces have been observed, structural observations of the orientation of these interfaces and a table describing the relationships between the various geological units. The interfaces are modelled as equipotential surfaces, and cokriging is used to interpolate between the supplied points.

The inversion method returns a number of models which are consistent with the supplied prior information. This ensemble of acceptable models can be analysed statistically to derive conclusions.

A 3D geological model of Broken Hill was constructed from a range of geological inputs. Constrained inversion of ground gravity was then carried out. Prompted by the results of initial inversions, a number of adjustments were made to both the geological model and estimates of the density for each of the units. The final inversion was used to demonstrate a high degree of internal consistency amongst these amended forms of the prior information. The use of airborne gravity gradiometry (AGG) data was deferred pending modification of the forward modelling algorithm to incorporate the band-limited character of these observations.

Flexural modelling studies have previously been used to infer the rigidity of the Central Andean lithosphere. The results of these studies suggest important along-strike and orogen-perpendicular variations in crustal structure. The along-strike variations probably reflect differences in bulk crustal composition. These compositional differences can partly explain the contrasting geological evolution of different parts of the Andes. The Central Andes (~15-35°S), are dominated by the high and broad Altiplano-Puna plateau, but the Southern Andes are characterised by a narrower and more subdued mountain range.

Lithospheric rigidity can also be determined from the admittance technique, a statistical approach that examines the correlation between topography and gravity anomalies. The results of a simple ID admittance analysis applied to the Central and Southern Andes show some general similarities to those derived from flexural studies, but important differences remain. The admittance results predict higher rigidity for the non-plateau part of the Andes and a Moho depth under the plateau that is extremely shallow compared with observations. These discrepancies reflect either the simplicity of the admittance method applied or the complications induced by the unusually thick crust associated with the Altiplano-Puna plateau.

Traditional methods of regional-residual gravity field separation typically involve high-pass filtering, low-order polynomial fitting or computations using isostatic models. All of these methods are susceptible to ambiguity and subjectivity in the selection of filter parameters, polynomial degree and isostatic model. Global Geopotential Models (GGMs), on the other hand, provide long-wavelength gravity data that are based on observations. Therefore, they have the potential to be used to more objectively define the regional gravity field and to give residual anomalies with reduced ambiguity.

We have examined residual gravity anomalies in the Central Andes computed using a regional field defined by the recent EIGEN-GRACEO1S satellite-only GGM. The resulting residual anomalies show significant correlations with the active volcanic arc and the geometry of the subducting Nazca plate. These results suggest that GGMs have a place in regional gravity interpretation, but the advantages over existing techniques remain to be thoroughly tested.

Overview scale interpretation of the Murray-Riverina airborne magnetic and radiometric survey has revealed many bedrock features in a region covered by significant overburden. Regional Bouguer gravity data suggest basins and granitic bodies at depth.

Pattern contrasts in the magnetic data contribute to a greater understanding of event timing and define numerous covered features, including granitic bodies, two areas of intrusive pipe-like clusters, possible extension basins of the Oaklands Basin and numerous mineral sand strand lines and their re-deposition as "lunettes".

Radiometric data in this area mainly reflect the lack of outcrop and define soil types and drainage patterns.

Opportunities exist for a number of deposit types of economic potential:

• petroleum near a Bouguer gravity low at Booligal;
• biogenic Gas and Coal in two grabens extending north from Hay;
• orogenic Gold associated with an extension of the Bendigo Terrane from Victoria into the Murray Basin;
• porphyry gold associated with previously unmapped granites; and
• diamonds associated with the clusters of intrusive pipes in the central west of the survey area.