Showing papers in "Exploration Geophysics in 2003"

Full-waveform P-SV reflectivity inversion of surface waves for shallow engineering applications

Abstract: Existing surface wave modelling methods fail to correctly interpret some velocity structures that are critical from a geotechnical perspective, for example those with large velocity contrasts and reversals. An inversion scheme based on the observation of an 'effective' phase velocity has proven more successful than conventional methods in such situations. Our new approach incorporates dominant higher modes into the dispersion curve inversion by using a forward calculation with full-waveform P-SV reflectivity synthetic shot gathers to reproduce all wavefields. We then extract the theoretical surface wave dispersion from the plane-wave transformed synthetic gathers, removing the need for mode identification. The measured field dispersion is then inverted, using the forward modelling scheme and a linearised optimisation, to a flat-layered, shear velocity model. With the new method, velocity reversals are better modelled than with conventional inversion methods. In general, low velocity layers directly under a surface caprock are inverted with more accuracy than those masked by buried high velocity layers. Limitations of surface wave inversion which remain include the need to assume layer thicknesses, and the rapid loss of resolution with depth.

Large-scale TEM investigation for groundwater

Abstract: This abstract outlines the use of the transient electromagnetic (TEM) method for hydrogeophysical investigations. The TEM method has proven to be a strong tool for the delineation of water bearing sand and gravel layers. We discuss newly developed TEM systems (PATEM and HiTEM) along with developments in data interpretation. Finally the TEM part of the Aarhus Survey is presented. This survey covers an area of more than 100 km2 and about 6000 TEM soundings have been measured.

Improved downward continuation of potential field data

Abstract: A number of techniques for downward continuation of potential field data, some already established in the literature and some novel, are tested and compared on synthetic and measured potential field profiles. A combination of Wiener filtering and Translation-Invariant denoising gives best results on synthetic data with added white noise. A Multiscale Edge Transform followed by a mild low-pass filter, and the ISVD method, prove to be the two most stable and robust approaches on measured data.

The application of fractional calculus to potential field data

Abstract: Gradients of magnetic and gravity data are used routinely to enhance the edges of anomalies, or as input to interpretation techniques such as analytic signal analysis or Euler deconvolution. The most commonly used gradients are of first and second order, higher orders being used less frequently due to noise problems. This paper discusses the benefits of a generalized approach using fractional gradients, demonstrating their usefulness as an aid to interpretation, both on theoretical models and on aeromagnetic data from the Free State province, South Africa. Fractional horizontal gradients are suggested as a means of avoiding the instability problems present when magnetic data from low latitudes is reduced to the pole. They also allow the use of an improved sunshading algorithm that is less affected by noise than the standard method. Fractional vertical gradients may be used to generate both enhanced analytic signal data and enhanced Euler deconvolution solutions.

Estimating Noise Levels in AEM Data

Abstract: This paper reports the results of analysing AEM data where the same flight line has been flown repeatedly to monitor system performance. When these data are corrected for the effects of variable survey altitude they provide a good measure of the reproducibility of AEM data. The analysis has been conducted for both frequency and time domain systems and shows that, where the area is even moderately conductive, multiplicative errors will provide the dominant source of noise for AEM surveys. Errors typically have standard deviations of 2 % and can easily induce fluctuations of 10%. Moreover, because of their geometric origins, these errors are highly correlated between channels. Frequency domain HEM and Z component time domain systems produce errors that are largely unrelated to time/frequency/conductivity. This is not the case for X component data from asymmetric systems.

Airborne vector magnetics mapping of remanently magnetized banded iron formations at Rocklea, Western Australia

Abstract: Strongly remanent magnetic sources such as those found in the banded iron-formations (BIFs) of the Hamersley Basin impair the interpretation of standard total magnetic intensity survey data. The use of aircraft attitude information, provided by the FALCONTM airborne gravity gradiometer (AGG) system, makes it possible to reference geographically the vector magnetic information collected by the fluxgate triad on board the aircraft. A full aircraft compensation processing method, allowing for permanent, induced and eddy current effects, provides usable vector magnetic data. These data supply valuable extra information for the interpretation of strongly remanent BIFs. An example using data from a FALCONTM survey near the Rocklea Dome in the Hamersley Basin demonstrates improved mapping of the BIFs by the use of the vector-residual magnetic intensity data.

Detecting Sub-Surface Groundwater Flow in Fractured Rock Using Self- Potential (SP) Methods

Abstract: In the wine district of Clare Valley in South Australia, the natural-voltage SP-signal generated by fluid flow in fractured rock during a pumping test was carefully monitored over time. From ten electrode-locations surrounding the pumping well logged every 15 s, the drawdown cone produced by pumping was determined on basis of the SP measurements together with laboratory measurements of the grain-boundary ? (zeta) potential. Such measurements allowed calculation of the fractured-rock aquifer?s transmissivity and average permeability. Results were confirmed by piezometer measurements to the extent that data were available. The study has revealed that SP-signals generated during pumping tests are of complex nature. However, if the pumping test is sufficiently long to allow the signal to stabilise, and careful field procedures are in place, then SP-measurements have the potential to add significant hydrogeological information. SP-measurements are relatively easy and cheap, and are, contrary to traditional hydrogeological methods, not restricted to the locations of existing piezometers, which is particularly useful in fractured-rock aquifers.

Some comments on potential field tensor data

Abstract: The measurement of potential field tensor (vector gradient) data is rapidly becoming a new tool for geophysical exploration. Data potentially provide significant improvements in resolution, noise suppression, data interpolation, and discrimination of subtle geological basement and regolith features, particularly in areas of high remanence, low latitudes, and in steep terrains. We have constructed algorithms in MATLAB for the three-dimensional inversion of potential field tensor data using Monte Carlo and Downhill Simplex approaches. We use these algorithms to invert simulated magnetic and gravity tensor data generated from simple geological structures, such as linear dykes and faults. The algorithms have a set target misfit (e.g., RMS misfit equal to one), and the final geological models are illustrated in three dimensions.

Approximate 2D inversion of AEM data

Abstract: Airborne electromagnetic (AEM) data are presently inverted with one-dimensional (1D) models, either as Conductivity Depth Images (CDI) or with full non-linear inversion, to build model sections from concatenated 1D models. If lateral conductivity changes are small, 1D models are justified. However, AEM investigations are often carried out specifically to find localized conductors, and in this case, 1D inversion is inadequate and will often produce artefacts in the model section. We have developed an approximate two-dimensional (2D) inversion method that deals with laterally inhomogeneous sections. The method is based on the adaptive Born approximation previously applied by one of the authors (NBC) to the interpretation of central-loop ground EM profiles. The technique produces synthetic models with moderate conductivity contrasts and with some improvement over CDI sections. The computing speed is comparable to that of stitched 1D inversions. An example of processing field data with the approximate 2D inversion method over a massive nickel sulphide deposit shows results that are promising for its routine application on large AEM data sets.

Analytic signal vs. reduction to pole: solutions for low magnetic latitudes

Abstract: As mineral exploration activity shifts to regions at low magnetic latitudes, interpretation skills acquired at high latitudes become harder to apply, thus leading to under-utilisation of expensive magnetic survey data. Changes in anomaly shape, reduction in overall amplitude, and changes in map textures make the ready interpretation of geology from magnetic data difficult. These problems are worst for magnetic inclinations within 20° of the equator. Reduction to pole (RTP) is the best theoretical solution because it normalizes the effect of induced magnetization and strike on the shape of the magnetic anomaly while preserving dip and textural information (the "normal" magnetization and ambient field direction being the vertical). However, in practice, the standard RTP transform is difficult to apply at very low latitudes and produces variable-quality maps, sometimes dominated by declination-parallel artefacts. Additionally, the transform cannot completely reconstruct north-south trending anomalies. The 3D Analytic Signal is a function of magnetic gradients and is easy to compute at all latitudes. The magnitude or amplitude of the 3D Analytic Signal can be computed easily and accurately for any ambient and source magnetization and can be readily imaged. The magnitude is almost, but not entirely, independent of magnetization direction. However, it lacks the resolution that horizontal and vertical gradient maps provide, and lacks the dip (and therefore structural) and textural information that total magnetic intensity and RTP maps contain.

In situ stress field, fault reactivation and seal integrity in the Bight Basin, South Australia

Abstract: We evaluate the in situ stress field and consequent risk of fault reactivation in the Bight Basin in order to assess the risk of fault seal breach at seismically mapped prospects. Borehole breakouts interpreted from dipmeter and image logs in five wells in and around the Bight Basin indicate a 130° maximum horizontal stress orientation. The large variation in water depths across the Bight Basin requires the use of effective stress magnitudes. We use a depth-stress power relationship to define the effective vertical stress based on density log data from 10 wells. The effective minimum horizontal stress gradient is estimated at 6 MPa/km using effective pressures from leak-off tests. We determine an upper bound (18.7 MPa/km) for the effective maximum horizontal stress gradient, using frictional limits to stress. The upper bound to the effective maximum horizontal stress indicates the region is in a strike-slip faulting stress regime. However, a normal faulting stress regime cannot be ruled out. Pore pressure in wells in the region is hydrostatic except in Greenly 1 where mild overpressure occurs below a depth of 3600 m. We use the FAST technique to evaluate the risk of fault reactivation in the Bight Basin. The risk of fault reactivation and consequent seal breach is expressed in terms of the pore pressure increase that would be required to induce failure. We consider three different stress regimes. These include a strike-slip faulting stress regime, a normal faulting stress regime, and a case on the boundary of strike-slip and normal faulting stress regimes. In all three cases, faults striking 40o (±15°) of any dip are the least likely to be reactivated.

Application of the radial basis function neural network to the prediction of log properties from seismic attributes

Abstract: In this paper, the radial basis function neural network (RBFN) is used to predict reservoir log properties from seismic attributes, and is compared to the generalized regression neural network (GRNN), discussed by Hampson et al. (2001). Both of these methods are related to the probabilistic neural network (PNN), which uses a Gaussian kernel estimator based on the distance between points in seismic attribute space. Our goal was to see if there are situations in which the RBFN could improve on the results found using the GRNN. The methodology consisted of first training the neural network at each well location, to find the optimum set of seismic attributes and weighting coefficients, and then applying the trained network to a 3D seismic volume. Each neural network was applied to the Blackfoot 3D seismic volume, which was recorded over a Cretaceous channel sand in central Alberta. Our results showed that, although the training results were quite close for each method, the RBFN approach generally produced higher-frequency results, especially as the number of training values was reduced. By computing the least-squared error between the predicted samples and the known training samples, we were also able to demonstrate the improvement of the results of RBFN over GRNN as the number of training samples decreased.

Resistivity and IP arrays, optimised for data collection and inversion

Abstract: The advent of 3D inversion packages for Resistivity and Induced Polarization has meant that geophysicists are no longer constrained by survey arrays designed to produce data to be plotted manually and interpreted by eye. 3D inversion processing means that there is no longer a need to place receiver and transmitter electrodes in a co-linear array. Electrode arrays can now be designed to optimise target definition and data collection efficiency. The double offset pole?dipole array offers a way to collect large amounts of data efficiently and has superior inversion sensitivity and depth of investigation to standard arrays.

Groundwater exploration with AEM in the Boteti Area, Botswana

Abstract: AEM data acquired with the TEMPEST system in the Boteti area, Botswana, were used to map lithologies and structures for potential fresh groundwater. The survey area is characterised by the Makgadikgadi paleolake system bordered by elevated terrain to the south. Lower-lying areas are characterised by a thick layer of Kalahari Beds with saline groundwater generally situated less than 20 m below surface. Elevated areas are generally characterised by Karoo sediments, consisting of sandstones and mudstones, at shallow depths. Within the paleolacustrine terrain, target aquifers are freshwater zones associated with recharge pans above a saline water table. The correlation of AEM-derived conductivity-depth profiles with borehole records shows that the conductivity of Kalahari Beds is primarily a function of clay content, water saturation, and water salinity. The shallow conductivity structure from modelled AEM data outlines several resistive zones, some of them located within Boteti River alluvium beneath the present and past river channels. In the elevated terrain, target aquifers are sandstones lying between dry alluvium or basalt, and mudstone. Basalts are characterised by very low conductivities and a strong magnetic response. The underlying sandstones and mudstones have low and high conductivities, respectively, and are offset by horst and graben structures, which are mapped in both the magnetic data and conductivity-depth sections. Resistivity iso-surfaces generated from the 3D conductivity data facilitate the spatial appreciation of the geometry of modelled structures. Favourable locations for freshwater exploitation interpreted from TEMPEST data include faults, dykes, a paleochannel incised into mudstone, and shallow fractured sandstone especially when overlain by basalt.

Topographic effects in frequency-domain helicopter-borne electromagnetics

Abstract: Many helicopter-borne electromagnetic (HEM) surveys have been carried out in mountainous areas. However, there have been few studies of the effects of topography on the HEM response reported in the literature. We simulate the response to topography using a staggered-grid finite-difference method. Modelling shows that a hill produces a high-resistivity anomaly over its top, and a low-resistivity anomaly over its foot, when the magnetic field response is transformed into the apparent resistivity. In order to reduce these effects, a simple correction procedure is presented and tested on synthetic data. Results indicate that the corrected data do not reproduce the effects of the actual resistivity structure accurately enough to permit quantitative interpretation assuming a flat-earth model. The reason for this is that the geometrical relationship between the coil system and the subsurface structure changes. The most rigorous and accurate approach to interpreting HEM data with topographic effects is to incorporate a forwardsolution scheme, capable of modelling topography, into inversions. A 3D inversion method is successfully tested on synthetic data.

The Olympic Dam copper-uranium-gold-silver-rare earth element deposit, South Australia: A geophysical case history

Abstract: The Olympic Dam polymetallic deposit is located 520 km northnorthwest of Adelaide, South Australia. The deposit has an inferred resource of 2660 Mt at 1.2% Cu, 1.4 kg/t U3O8, 0.5 g/t Au. The deposit occurs in a hydrothermal breccia complex within Mesoproterozoic crystalline basement of the Stuart Shelf. It is overlain by 260 m of flat-lying sedimentary rocks and has no surface expression. The mineralisation is irregularly distributed within the breccia complex, occurring predominately in haematite-rich matrix material. The deposit was located in 1975 by Western Mining Corporation by the drilling of coincident gravity, magnetic and tectonic targets, identified with respect to a geological conceptual model. The Olympic Dam deposit coincides with a 17 mGal gravity anomaly, caused by haematite-rich polymict breccias. The source of a broad 1600 nT magnetic anomaly, which is closely associated with the deposit, is interpreted as being due to several geological entities, including magnetite in the deposit and the host granite. Electrical geophysical surveys have been tried over the Olympic Dam deposit since discovery. Due to the disseminated nature of the sulphide mineralogy, emphasis has been placed on IP techniques. Initial experimentation was not encouraging, due to the low signal-to-noise ratio and the dominance of electromagnetic coupling. However, in 1980 a high-power transmitter and digital IP receiver were employed to combat the above two factors. The deposit was covered with 400 m dipole-dipole surveys with an 800 m line spacing. These data detected an apparent resistivity low, restricted to the southern and central sections of the deposit. The cause of this low is interpreted as being porous, matrix-rich, haematitic breccias, in which the dominant form of haematite is a black crystalline variety. Pore fluids are highly saline, with an apparent resistivity of between 0.1 and 0.05 Ohm.m. The source of a positive phase-angle response is not well understood, but it has a close spatial relationship with the extent of the haematite-rich breccias, whether these breccias are mineralised or not. This interpretation is not consistent with the results of downhole logging using relatively small geometric survey parameters. This inconsistency reflects the difficulty in trying to reconcile small-scale physical property measurements with the larger scale surface measurements, for such a complex inhomogeneous mineralised environment.

A Least-Squares Minimisation Approach to Depth, Index Parameter, and Amplitude Coefficient Determination from Magnetic Anomalies Due to Thin Dykes

Abstract: We have developed a least-squares approach to determine, successively, the depth, index parameter, and amplitude coefficient of a buried thin dyke, using moving-average residual anomalies obtained from magnetic data using filters of successive graticule spacings. By defining the moving-average residual anomaly value at the origin on the profile, the problem of depth determination is transformed into the problem of solving a nonlinear equation, f(z) = 0. Knowing the depth and applying the least-squares method, the index parameter is determined by solving a nonlinear equation of the form l(q) = 0. Finally, knowing the depth and the index parameter, the amplitude coefficient is determined in a least-squares sense using a simple linear equation. In this way, the depth, index parameter, and amplitude coefficient are determined individually from all observed magnetic data. We have developed a procedure for automated interpretation of magnetic anomalies attributable to thin dykes. We apply the method to synthetic data with random errors, complicated regionals, and interference from neighbouring magnetic rocks, and we test it on two field examples from Brazil and Canada.

On the use of magnetics and gravity to discriminate between gabbro and iron-rich ore-forming systems

Abstract: If the major rock-forming minerals are divided into three categories, and, if the density, magnetic susceptibility, and the proportions of each category are known, then the density and susceptibility of a mixture of the three categories can be determined. Conversely, if the physical properties of a mixture are known along with the physical properties of each of the three components, then the proportions of each component can be calculated. This reasoning can be used to superimpose category-percentage contours onto a density-susceptibility scatter diagram. Such a diagram can be used to relate petrophysical measurements on a rock specimen to its mineral category content, the veracity of which can be appraised by a geologist with a hand lens. Several hundred qualitative tests suggest that the approach is valid. The analysis is independent of scale, and can be applied to the bodies of a density-susceptibility model developed to simulate exploration data. Gabbro with magnetite plots along a specific locus on the combined phase/scatter diagram and can often be distinguished from denser accumulations of hematite and/or sulphides. Case histories for a number of economic and non-economic density-susceptibility anomalies give support for the usefulness of the method.

Feature Detection Using Sunshading

Abstract: Horizontal derivatives of potential field data are routinely used to sharpen the edges of linear features such as dykes and faults. They are also used as input to the sunshading filter. Given the azimuth and elevation of a source of illumination, this filter calculates the reflectance from a surface composed of the data to be interpreted. Linear features orthogonal to the illumination azimuth are enhanced, while those lieing parallel to it become less apparent. Since the sunshading filter is relatively insensitive to features of different azimuth from that being searched for, this paper first describes how to tune the angular sensitivity to any desired degree. Secondly, the field gradients are calculated in a cylindrical polar coordinate system rather than the usual Cartesian one. This allows both the enhancement of circular anomalies of any radii, and of linear features of any orientation. The cylindrical gradients may also be used with the sunshading reflectance algorithm. The new filters are demonstrated on various gravity and magnetic datasets.

Efficient 3D electromagnetic modelling in the presence of anisotropic conductive media, using integral equations

Abstract: We present a novel technique to simulate numerically the measurements performed by a borehole induction-logging tool in 3D anisotropic rock formations. The simulations are based on an integral equation formulation. Previously, such a formulation was considered impractical for solving large-scale problems due to the resulting large full matrix. To overcome this difficulty, we assume a uniform background model and make use of a uniform grid whereupon there is no need to construct explicitly all of the entries of the full Green?s function matrix. Using a uniform background model, the entries of the corresponding electric and magnetic Green?s tensors are relatively easy to calculate. In the presence of a uniform grid (not necessarily cubic), it is only necessary to calculate the first row of the resulting electric Green?s function matrix. Further, because the matrix is block Toeplitz, it can be rewritten into a block circulant form, and therefore matrix-vector multiplication can be efficiently performed with two FFTs and one inverse FFT. This strategy reduces the computation cost from O(N ´ N) to O(N ´ log2N). In addition to the substantial computer savings, the FFT technique also substantially reduces memory storage requirements because only the first row and the first column in the block Toeplitz matrix are needed to perform the computations of the remaining entries of the matrix. Numerical simulations of the measurements performed with an induction tool in dipping and anisotropic rock formations are benchmarked against accurate 3D finite-difference and 1D codes. These benchmark exercises show that the newly developed integral-equation algorithm produces accurate and efficient simulations for a variety of borehole and formation conditions.

Geophysics of the Prominent Hill prospect, South Australia

Abstract: In November 2001, Minotaur Resources announced the results of discovery drill hole URN001, at the Prominent Hill prospect. The drill hole intersected 107 m averaging 1.94% Cu and 0.66 g/t Au from 200 m depth, and a further 152 m averaging 1.20% Cu and 0.61 g/t Au from 429 m, hosted in a haematite breccia. URN001 was drilled to test a gravity anomaly, located to the south of an intense magnetic anomaly. Following the announcement of the assay results from URN001, potential field and electrical surveys were completed over the prospect area. Downhole induced polarisation logging of URN001 showed that the high-grade copper mineralisation was associated with chargeability and resistivity anomalies. In an attempt to highlight zones of high-grade copper mineralisation, a dipole-dipole IP/resistivity survey was performed. A strong phase anomaly, coincident with the gravity anomaly, was highlighted by this survey. Drill testing of this anomalous source resulted in the intersection of massive haematite, containing little to no copper mineralisation. Downhole IP logging and petrophysical work have confirmed that the haematite is chargeable and has a low resistivity. The electrical properties of the "barren haematite" are comparable to the high-grade copper mineralisation. The barren haematite is also a significant contributor to the gravity anomaly. As a consequence, it is difficult to distinguish between mineralised and unmineralised breccias using geophysical methods. Additional geophysical survey methods, such as TEM and CSAMT have been trialled at Prominent Hill, but with limited success. Interpretation of electrical datasets is hampered by the presence of conductive black shale in the cover sequence.

Improving the accuracy of shallow depth determinations in AEM sounding

Abstract: Conductivity-depth images (CDIs) are finding application in salinity, groundwater, and environmental mapping. Hydrological modelling demands are for a much higher vertical resolution than the 10+m accuracy that was adequate in CDIs used for mineral exploration. Contractors are increasingly confident of system waveform, geometry, and some provide corrections for factors such as pitch, roll, and yaw. This increased system accuracy is the trigger for efforts in increasing the accuracy of processing. The CDI process makes a number of approximations in order to increase the speed of processing. One of the most critical in program EMFlow is an assumption that the transmitter and receiver are entirely within the current system induced in the ground at all delay times. This assumption equates to all components of the secondary field decaying monotonically with time. For typical fixed-wing AEM geometries, this assumption is poor for the z component of the response, and in fact on a CDI, z component data may predict the top of a surficial conductor to be several metres in the air. Allowing for part of the induced currents to lie between the transmitter and receiver, when coupled with an Inductive Limit constraint, leads to an accurate prediction of surficial conductors to lie at or below surface.

Impulsive moments at work

Abstract: The moments of the impulse response are a new tool for the interpretation of transient electromagnetic data. The nth moment is the integral of the impulse response weighted by time to the nth power. The zeroth- and first-order moment are equivalent to the inductive and resistive limits and the higher-order moments place emphasis on the late time data. A good approximation to the moments can be calculated relatively easily from the measured data. Also, for some simple models, the formulae for the moments are relatively simple expressions. Hence, it is comparatively easy to invert these expressions to derive source parameters from the measured moments. For example, the conductance of a thin sheet or the conductivity of a halfspace can be derived from the low-order moments. There are no analytic expressions for the high-order moments of a half-space, so the concept of realizable moments has been introduced to allow the higher-order realizable moments to be converted to a conductivity or conductance estimate. The moment data have been shown numerically to be additive. This has two ramifications. 1) The earth can be approximated by a multiplicity of small spheres and the properties of these spheres can be inverted for. A prototype-imaging scheme which does this gives promising results. 2) The regional or background response can be subtracted from the data and the residual anomaly can be modelled. Because the modeling algorithms are fast, they can be incorporated into inversion schemes linked to database packages such as Geosoft montaj.

Frequency dependent anisotropy of porous rocks with aligned fractures

Abstract: One of the main issues in the characterization of any reservoir is the ability to predict the effect of fluid properties on seismic characteristics. This effect is studied by modelling fractures as very thin and highly porous layers in a porous background. Elastic moduli of a porous rock permeated by a system of such fractures distributed periodically are obtained using the result of Norris for elastic properties of layered poroelastic media. When both pores and fractures are dry, such material is equivalent to a transversely isotropic elastic porous material with linear-slip interfaces. When saturated with a liquid this material exhibits significant attenuation and velocity dispersion due to wave induced fluid flow between pores and fractures. At low frequencies the material properties are equal to those obtained by anisotropic Gassmann theory applied to a porous material with linear-slip interfaces. At high frequencies the results are equivalent to those for fractures in a solid (non-porous) background. The characteristic frequency of the attenuation and dispersion depends on the background permeability, fluid viscosity, as well as fracture density and spacing.

The relationship between the magnetotelluric tensor invariants and the phase tensor of Caldwell, Bibby, and Brown

Abstract: We examine the relationship between the seven invariants of the complex MT tensor, which we previously proposed as a vehicle for testing the dimensionality of the regional conductivity structure pr...

Surface seismic imaging by multi-frequency amplitude inversion

Abstract: We have developed a new full-waveform inversion scheme for crosshole seismic imaging. In this paper, we adapt it to surface seismic imaging in which only the spectral amplitudes are used. We perform numerical experiments to test the known source signature algorithm (KSS) against two other inversion schemes?one which estimates the source signature from the shot gathers (ESS) and the other which avoids knowledge of the source by using normalized data (NDI). All three approaches rely on a finite element method for the 2D/2.5D acoustic modelling, and operate in the frequency domain. Three shallow subsurface models were considered, representing buried karst topography, dipping blocks, and isolated waste ponds. The synthetic experiments involved only a limited number of 'surveys' with just 8, 3, or 1 shots into variable-length geophone arrays. In all cases, conventional seismic data processing fails to recover the structure. The spectral inversions, which use frequencies over the range 50?400 Hz, involve the entire seismograms, including primaries, multiples, direct waves, etc. The features in the models typically have dimensions of 5?20 m, which can be compared with the geophone spread length of 40 m (8 shots) to 100 m (3 or 1 shot). The 8 shot inversion performed best for all the models. In fact, it was not possible to recover the karst structure with just 1 or 3 shots, but the other models were recovered, albeit with less accuracy, using such a small number of shots. ESI and NDI perform remarkably well on the dipping-block and waste-pond models. KSS yielded superior results to the other two schemes. The estimated source spectra were close to but did not exactly match the known wavelets. Imaging results are most impressive for such a small number of shots.

Modelling of airborne EM anomalies with magnetic and electric dipoles buried in a layered earth

Abstract: An automated algorithm is described that selects and models spatially discrete anomalies in airborne EM (AEM) data sets. For anomalies to be selected they have to be wide enough to have their origin in the subsurface and narrow enough to be caused by a discrete conductor. After determining background conductivity models with layered-earth inversions from the EM data, identified EM anomalies are modelled with magnetic and electric dipoles buried inside a layered earth. Magnetic dipoles are appropriate models for discrete, sheet-like conductors inside a resistive host, i.e., in scenarios where vortex currents dominate, whereas electric dipoles are expected to model well elongated structures excited by current channelling. The model parameters determined from each data segment include, for magnetic dipole solutions: the target conductor position, depth, dip, size, and conductance; and for electric dipoles: the position and depth. The method is fully automated with the dipole start models being determined by curve matching from digital look-up tables. Results from synthetic data indicate the efficiency and reliability of the method. The technique was applied to TEMPEST and GEOTEM data acquired across the Bull Creek prospect, Queensland and Harmony deposit, W.A., respectively. The algorithm provides a sensible description of both mineralisations. Other anomalies are interpreted as being caused by shallow structures channelling current, discrete conductors beneath the overburden, and the lateral heterogeneity of the overburden.

Exploration for unconformity uranium in Arnhem Land (NT)

Abstract: Since the mid nineties Cameco Australia Pty Ltd (Cameco) has been involved in exploration for unconformity-related uranium deposits in Arnhem Land, Australia. During this time the exploration model has evolved from the initial Athabasca Basin (Canadian) unconformity model. Physical property measurements and orientation programs have led to the current integrated exploration strategy that incorporates the disciplines of geology, geochemistry, and geophysics. Airborne radiometric surveys continue to be the primary tool for identifying near-surface uranium anomalies. However, other geophysical techniques are utilised to aid in mapping basement lithologies, alteration, and the depth of sandstone cover, which are all keys to Cameco's exploration objectives. With these aims in mind, hyperspectral, magnetic, and electromagnetic airborne geophysical techniques have been extensively utilised as efficient methods for quickly evaluating large areas, where rugged topography prevents effective use of ground techniques. Examples from Cameco's King River project, located in northwest Arnhem Land, show that integration of airborne electromagnetics, and hyperspectral surveys, is a significant factor in improving the exploration process.

Improvement to long-wavelength Australian gravity anomalies expected from the CHAMP, GRACE and GOCE dedicated satellite gravimetry missions

Abstract: The concepts and mission parameters of the CHAMP, GRACE and GOCE dedicated satellite gravity missions are summarised, followed by the improvements that they are likely to make upon previous methods of determining the Earth's long-wavelength gravity field. An example of an error in the EGM96 global geopotential model, caused by the use of an incorrect digital elevation model over Australia, is used to exemplify the deficiencies in current global gravity models. Summarised results of a preliminary study to quantify long-wavelength errors in Australian gravity anomalies using data from the GRACE-derived EIGEN-2 global geopotential model indicate the presence of long-wavelength (>1113 km) errors of over 10 mGal in Australian terrestrial gravity anomalies. Finally, the likely prospects for improved geophysical studies are speculated upon.

Electrical imaging of peridotite weathering mantles as a complementary tool for nickel ore exploration in New Caledonia

Abstract: The first 2-D electrical resistivity tomography (ERT) survey on the well explored (for Nickel ore) lateritic mantle of a new Caledonian ultramafic massif shows a good fit between the geoelectrical sections and the core loggings. Data collected along several 1 km long profiles, with 10 m electrode spacing to reach about 100 m penetration depth, are reliable and the preliminary interpretation of resistivity models indicates: well-defined geoelectrical layers with significant resistivity contrast; a very good fit of the above layers with the various units of a weathering mantle, i.e. ferricrete, saprolite and bedrock; the suspected geometry of horizons, deduced from core logs, is better defined by the continuous ERT imaging; a transverse (section to section) continuity of saprolite troughs and bedrock ridges along N140° strike, which is the main structural feature of Tiebaghi ultramafic massif. 2D Electrical Resistivity Tomography appears to be an appropriate geophysical method to investigate the structure of weathering mantles of ultramafic massifs in New Caledonia. It should become a useful complementary tool to locate favourable zones, i.e. the saprolite horizon where Ni accumulation can be found, or, at least, to economize on drillings.