Exploration Geophysics - Volume 28, Issue 1-2, 1997

The usual interpretation model for resistivity soundings is a one-dimensional Earth which consists of multiple horizontal layers, each isotropic and homogeneous. The standard interpretation method is least-squares inversion. This paper introduces a software package called RINVERT for Windows which has been developed in Australia for the routine interpretation of resistivity soundings in a Windows^TM environment. It has been designed to handle the Schlumberger, Wenner and Bipole-Bipole electrode arrays. For feasibility studies, training, or testing the closeness of an initial model, RINVERT for Windows provides a forward modelling facility as an adjunct to its least-squares inverse modelling procedure. An important feature of RINVERT for Windows is the incorporation of a Monte Carlo equivalence analysis which allows an automatic assessment of the extent of equivalence in each layer. Most of the effort required in preparing the resistivity survey report has been obviated by an automatic report generator, which generates and captions high quality figures and tables ready for incorporation in a report. An extensive Help facility, which is useful for novices, includes a hypertext glossary and an explanation of resistivity method fundamentals. The software is supported by a 110-page Users Guide with a step-by-step tutorial.

Estimation of effective porosity in shaly sand formations is the prime parameter for reserve calculation and for good understanding of the behaviour of a formation during production. The Late Carboniferous to Early Permian, fluvio-deltaic Tirrawarra Sandstone is a kaolinite-bearing sandstone and an important hydrocarbon reservoir in the Cooper Basin. Petrographie point count and image analysis data from 130 samples, together with data from about 650 core samples and wireline log data from 14 wells of the Tirrawarra Sandstone in the Moorari and Fly Lake Fields, have been used to estimate effective porosity from sonic log.

Based on integration of all data and with the knowledge of the volume fraction of kaolinite and associated microporosity (20% for kaolinite masses in the Tirrawarra Sandstone in the studied samples) effective porosity can be expressed as:

where ϕe is effective porosity and ϕsonic is sonic porosity, and Vk is volume fraction of kaolinite.

On average, estimated effective porosity is 2 porosity units less than the total porosity and it varies for depositional environments which have different kaolinite contents.

Since the late 1970s the Tasmanian Geological Survey has been involved in a number of small geophysical projects which were either targetted at specific problems or intended to provide an incentive for exploration. In the East Coast Coal Project, gravity and magnetic data were used to optimise drilling for coal under as much as 600 metres of dolerite, and later to provide ground information for mine planning.

One of the incentives to mineral exploration was the Mt Read Volcanics Project, which included an evaluation of the geophysical signatures of all types of western Tasmanian metallic mineralisation and the applicability of geophysical techniques in the search for these deposits. A later project in northeastern Tasmania involved the acquisition and interpretation of high resolution data to promote an under-explored gold province, with the subsequent pegging of 95% of the area offered.

A thorough understanding of the the influence of geological factors on electrical petrophysical properties of sulphide orebodies requires careful integration of geophysical and geological measurements at a variety of scales. Two new methods for studying the influence of microscopic and mesoscopic ore textures on electrical properties of sulphide rocks have been developed. Voltage contrast microscopy is a scanning electron microscope technique which enables direct imaging of the effects of an applied potential on a polished rock surface. It has been used to investigate the detailed mechanism of electrical conduction in fine-grained base metal sulphide ores. Initial results suggest that conductive paths may exist along grain boundaries within otherwise resistive minerals, such as sphalerite. Continuity mapping uses a modified flat-bed plotter to directly image the electrical continuity of drill core or hand specimens. A spring-loaded copper electrode traverses the polished sample surface on a grid to produce an image. Electrode movement and potential measurement are fully automated. This technique has been used to study the influence of mesoscopic ore textures on the electrical continuity of massive and disseminated sulphide ore samples. Both new methods are still being developed and refined but initial results suggest that they will usefully complement existing laboratory-based petrophysical techniques.

The application of borehole geophysical logging in three different geological environments is presented. The three case studies discussed are: the evaluation of the Sendelings Drift alluvial diamond deposit near Oranjemund in Namibia; the Sadiola gold orebody in western Mali; and a kimberlite pipe in central Botswana. It is shown that the usefulness of borehole geophysical logging is not confined to the oil industry, and that quantitative methods such as those used in the oil industry may be effectively applied to any ore deposit via lateral thinking and a sound understanding of the relevant geology. The methods described are considered especially useful in the case of marginal ore bodies where a rapid three-dimensional understanding of the deposit may become critical.

The paper discusses constrains imposed by the energy conservation law on a set of possible solutions to Maxwell’s equations. An integral equation having a contraction kernel is derived for the electromagnetic field induced by an external source in a dissipative medium. The equation can always be solved using simple iterations. The Iterative-Dissipative Method (IDM) based on this integral equation has been successfully applied to a wide range of practical 3-D problems encountered in surface and borehole electromagnetic exploration.

The approach imposes no restrictions on the frequency of the field variations, gradients of the electric conductivity or dielectric permittivity. It can also be applied to an anisotropic medium. The IDM integral equation keeps its contraction properties on a numerical grid. This makes the convergence of the iterative process practically independent of the grid parameters.

The paper presents a brief theory of the method and an example of the electromagnetic field calculation for a geoelectric model of the Australian continent and the surrounding oceanic area.

Physical modelling of seismic wave propagation can provide insights into complex structural, stratigraphic and reservoir characterisation problems. The detection and delineation of fracture zones is very important in development drilling. Determination of fracture orientation within a reservoir may play an important role in EOR programs. The fundamental principle used in AVO studies is the extraction of shear wave information present in the reflection amplitude variation with source-receiver distance. The objective of this study was to determine if the shear information in 3-D AVO surveys can be utilised for fracture detection and delineation.

A 3-D physical modelling experiment was conducted over a simulated fracture system at the Allied Geophysical Laboratories at the University of Houston. The ‘fracture system’ consisted of three isotropic homogeneous layers with a transversely isotropic Phenolite disc embedded in the central portion of the middle layer. The three layers are: an upper layer of black resin 2741LV; a middle layer of blue Stycast resin 2850MT (containing the anisotropic disc); and a lower layer of Evercoat casting resin. Two 3-D seismic surveys were acquired over the model, one with acquisition lines oriented parallel to, and a second survey with the acquisition lines perpendicular to, the fracture direction. Multi-offset and multi-azimuth seismic experiments were also conducted over the simulated fracture system. After applying corrections for spherical divergence and source-receiver directivity the reflection amplitudes from the black/Stycast resin interface were analysed.

The results show that the ‘fractured’ disc is characterised by low AVO gradients in both surveys. In addition, the 3-D AVO gradient estimates from the survey perpendicular to the fracture orientations are 30% to 40% lower than the AVO gradient estimates from the survey with line orientation parallel to the fracture direction. Also, AVO effects along different line orientations suggest that at near offsets the amplitude behavior is similar for all azimuths but at far offsets an anomalous amplitude is seen along an azimuth perpendicular to the fracture zone.

Hill 50 Decline (part of the Hill 50 Gold Mine group of deposits) is located at Mount Magnet, 560 km north-northeast of Perth, in the Murchison Province of the Yilgarn Craton. The Hill 50 orebody is hosted by banded iron formation (BIF) "bars" of the Sirdar Formation, generally having a width greater than 20 m. Gold mineralisation is localised along the north-northeast to northeast striking faults, known as Boogardie Breaks, and accompanying sub-horizontal fractures. Sulphide replacement of the adjacent oxide facies BIF has resulted in steeply plunging stratabound shoots ("pencils") of massive gold-bearing pyrite-pyrrhotite ore, with pyrrhotite being the dominant sulphide at depth and at the centre of the ore zones. Previously, pyrrhotite-rich BIFs have been shown to be characterised by significantly higher conductivities than their unmineralised counterparts.

Downhole electromagnetic (DHEM) surveying carried out in the Hill 50 underground was aimed at detecting the massive, pyrrhotite conductors. A 65 m x 80 m transmitter loop was laid out underground and drillholes were surveyed using the three-component, Crone pulse EM system. Logistical problems experienced during data collection were largely related to firing times and data quality was greatly influenced by the numerous noise sources encountered in an operating underground mine (eg, drill rigs, trucks, etc.).

Modelling and interpretation of the resulting DHEM data has identified several conductors. All the interpreted conductors are positioned within the Hill 50 BIF Bar, except for one conductor, lying in the Outer BIF Bar 1. Most conductors are modelled as steeply dipping, while those modelled as shallow-dipping are considered to be related to mineralisation within the sub-horizontal fractures. Of the models tested so far, all have been shown to be directly related to massive sulphide mineralisation, with the majority being associated with economic gold grades, demonstrating that the technique has been effective in delineating the pyrrhotite-rich BIF target underground.

A new dip-moveout (DMO) processing technique is proposed. The method is based on an integral DMO method in the Radon domain called Radon DMO, which is especially applicable to irregularly sampled 3-D datasets. Radon DMO offers several advantages for processing surveys with irregularly sampled design or acquisition characteristics, common problems for land and Ocean Bottom Cable 3-D surveys in particular. First, the Radon DMO operator is nonaliased and dealiasing. Missing traces do not cause spatial aliasing, precursor noise, or unbearable distortions of phase and amplitude. Second, Radon DMO does not require that input traces belong to one offset bin. Input traces can be organised from multiple offset bins in the same azimuth grouping to perform Radon DMO. Third, the DMO-corrected output can be either stacked or unstacked, which enables the full range of post-DMO processing including post-DMO velocity analysis.

The Radon DMO operator directly maps data from the NMO-corrected time domain to the DMO wavefield in the Radon domain. The impulse responses of Radon DMO are hyperbolas. The method is built upon a process that transforms a single, NMO-corrected trace into multiple traces spread along hyperbolas in the Radon domain. Most integral methods include the application of a 45° phase shift, as well as offset, time, and frequency-dependent gain factors when spreading the traces along ellipses. Such compensations are generally unnecessary with Radon DMO, which greatly simplifies program development and reduces the number of critical elements to control. By eliminating costs associated with gain factor application, the added costs for inverse Radon transform are alleviated. Total costs compare well with conventional integral DMO methods.

A range of sets of electromagnetic observational data (geomagnetic depth sounding and magnetotelluric sounding) now exist for the Australian continent. These data show regions of the continent where the conductivity structure is approximately one-dimensional on a gross scale, relative to zones, called ‘conductivity anomalies’, where induced electric current flows preferentially.

Progress is being made in the development of a numerical method which inverts such observational data automatically, to give an image of the conductivity distribution of the whole continent, set in its surrounding seas. The thin-sheet algorithms developed by Weaver and Weidelt and colleagues are employed, and conductance values are found for grid units in a sheet representation of the continent which extends from the surface down to depth 10 km. The grid units are typically 100 km in horizontal dimension. The model is solved by numerical inverse methods.

This paper presents results obtained from inverting data in the form of Parkinson or Induction arrows, at a single period, for sites spread widely across Australia. The major continental conductivity anomalies are given quantitative expression in the model resulting from the inversion.

A core function of the New South Wales Department of Mineral Resources is the ongoing geological mapping of the State of New South Wales. Until recently, airborne geophysical data were not used in field mapping. Now that Second Edition geological mapping has begun, and with assistance from the National Geoscience Mapping Accord, large areas of the State have been flown with high resolution airborne geophysical surveys. With the State Government now requiring more to be done with less, the integration of these airborne datasets with geology in a collaborative process is necessary to maximise efficiency of geological mapping. Integration is achieved by initially using standard data presentations in a pre-mapping interpretation, followed by a suite of data enhancements during the field mapping phase. On-going interpretation is further refined by the use of ground geophysics and potential field modelling to resolve specific problems. Post-mapping synthesis of the data in a GIS environment enables mismatch between datasets to be highlighted. The end result is more professionally produced, high quality geological maps.

The ability of a number of processing techniques to accommodate a complex near-surface velocity field resulting from a highly irregular water bottom has been evaluated on seismic data from the Timor Sea. Conventional processing is not appropriate where such large lateral velocity variations exist. One approach is to effectively remove the influence of the irregular water bottom by replacing the water layer with higher velocity material and recomputing seismic travel times. Conventional processing can then proceed more successfully. The water replacement statics technique is a simple but efficient method for computing corrective time shifts. A more accurate algorithm to compensate for propagation effects of the near-surface velocity field is utilised in the wave-equation layer replacement technique. Both water-layer replacement techniques produce sections with increased horizon continuity and less structural distortion when compared to those generated using only conventional processing techniques. Although results from the two methods are similar in the present case, the wave-equation method is generally preferred because of its more realistic computation of travel-time corrections.

As an alternative to the water-layer replacement methods, prestack depth migration (PSDM) can be used to image seismic data in areas characterised by irregular water-bottom topography. However, PSDM is sensitive to the interval velocity model used for the migration. To achieve results which justify the computational expense of PSDM, considerable effort must be spent deriving an interval velocity model. In this study, an iterative migration/linear inversion scheme is used to construct an accurate model. PSDM based on this model produces a seismic section superior to those generated from the replacement techniques. Incorporation of high-resolution reflection tomography into the model-building process is suggested as a means of further improving PSDM results in areas of complex near-surface velocity fields.

It has been widely accepted for many years that constant velocity 3-D DMO cannot be accurately applied to multi-azimuth data acquired with typical 3-D recording geometries. This problem is particularly severe for many land and OBC geometries. Recent work by G.J.O. Vermeer and co-workers has provided a clear theoretical basis for the application of 3-D DMO to one class of data with varying shot-receiver azimuths. For the case of cross-spread geometry, the locus of contributing midpoints for a given output point is an hyperbola in the (x,y) plane passing through the output point. In practice, the input traces will not generally be located exactly on the appropriate hyperbola, implying that the DMO operator will not be correctly sampled at the output point. This gives rise to poor integration of DMO operators, leading to distortion of signal amplitudes, loss of frequency and a reduction in the signal-to-noise ratio.

Conventional ‘input-oriented’ 3-D DMO techniques operate by ‘smiling’ an input trace along the direction of the shot-receiver azimuth. The DMO operator is discretely sampled at regular intervals along the azimuth and the ‘smile traces’ collected into their respective CMP bins.

Since the input points do not, in general, lie along the required hyperbola in the (x,y) plane the operator at each output point is not well sampled and this results in DMO-generated noise and distortion of signal amplitudes.

Vermeer has pointed out that the ideal solution to this problem would be a fully output-oriented DMO implementation, computing input traces with regular and adequate spacing along the appropriate (x,y) hyperbola for each output point. The output-oriented approach ensures that input traces contribute exactly to a given output location (not merely to the output bin, as would be the case with the input-oriented approach). Also, the operator at the output point is well sampled. The drawback of this approach is that every output point requires a different hyperbola and therefore different input points so that a near continuum of recordings is required. This is an extremely expensive method.

This problem has been addressed by pre-conditioning the input data so that it more closely matches the correct hyperbolic loci. By recognising the pattern of the geometry in this way, an accurate 3-D DMO can be achieved for many multi-azimuth geometries. Significant improvements have been achieved for single-fold synthetic data and for multifold land and OBC field data.

The generalised reciprocal method (GRM) is a convenient and substantial enhancement to the capability of the refraction method in dealing with irregular refractor interfaces with lateral changes in refractor velocity. Processing approaches have been developed which implement a fast automated GRM-based approach and are capable of handling multifold refraction data such as that from long seismic reflection survey lines. A method is proposed for determining the resolution achievable with the velocity analysis technique as a function of the detector separation. This provides a means of optimising the resolution with spatially under-sampled data for most shallow exploration applications.

There are three major stages in this approach: the recognition through stacking coherency of individual layers within the multifold data, and the production of a single composite travel-time graph; the determination of the seismic velocities within each refractor using an automatic curve recognition technique, using the velocity analysis function to obtain the optimum geophone migration separation and to locate the lateral velocity variations within a refractor; and depth inversion in which a modified depth conversion factor is proposed to control the trade off between the vertical resolution of velocity layers and changes in layer thickness. This factor reduces spurious details in inverted depths layers and is stable when a layer appears or disappears in the section.

The approach has been tested successfully on both synthetic data and real data from the Kalgoorlie deep seismic reflection traverse. Results show that application of the techniques achieves better resolution than conventional manual interpretation, and provides a fast and direct inversion that appears to discount spurious detail.

A critical element in the use of amplitude versus offset (AVO) for gas detection or lithological discrimination is to define a parameter that represents anomalous behaviour. Shuey’s approximation to the Zoeppritz equations for the compressional wave reflection coefficient has been widely used as a basis for defining AVO anomalies. Shuey’s approximation states the AVO response as a linear equation. The intercept and gradient from this equation have frequently been used as key parameters to quantify AVO response. Generally, large gradients are normally assumed to be related to gas saturation. In order to better understand the gradient term, 25 model pairs of shale over gas sand, and shale over brine sand, were used to analyse the gradient term as the sum of two functions. The first function is defined as the non-Poisson’s ratio contrast term, and the second as the Poisson’s ratio contrast term.

Three conclusions have been reached from this analysis.

1. 1. The gradient may be significant even for a small Poisson’s ratio contrast. This effect may be one of the reasons that AVO analysis is misleading in some geological settings.
2. 2. Similar values of Poisson’s ratio contrast may produce quite different gradients in different rocks.
3. 3.In most cases the non-Poisson’s ratio contrast term is constructive to the magnitude of gradient when the normal incidence reflectivity and Poisson’s ratio contrast have opposite signs, while it is destructive to the magnitude of gradient when both have the same signs.

Many permeability models and equations are empirical in log analysis and evaluation. Therefore, their application is restricted to a given set of lithology conditions. It is essential to select the correct transform relationships for each type of lithology in order to determine accurately permeability from well logs. This is especially true in the Mardie Greensand, a lithologically complex formation. There are four porosity and permeability transforms determined based on the core data in the sandstone formations of the Mardie ‘B’ and ‘C intervals in the Barrow Field, Carnarvon Basin, Western Australia. The aim in this study was to determine which porosity and permeability transform is applied at which formation from well logs, using statistical pattern recognition of electrofacies. In this paper a number of electrofacies corresponding to these porosity and permeability transforms were determined in the complex formation. The application of pattern recognition to this complex lithology makes it possible to identify electrofacies and select corresponding transforms to calculate permeabilities from well logs. Results indicate that it can reduce the uncertainty of log-derived permeability and help to simplify the problem of complex lithologies in log interpretation by recognising known lithologies within a formation, thereby improving the precision of permeability determination in this complex formation.

This paper presents results of an integrated appraisal of recent high-resolution aeromagnetic, radiometric and remote sensing data over the Rssing mine area and lower Khan Gorge region of Namibia. The interpretation used stereoscopic aerial photographs to establish structure around the Rssing mine and high-pass filtered aeromagnetic data for the regional structure, followed by analysis of spectral Landsat TM data, total field magnetic intensity data and imaged airborne gamma-ray spectrometer data to delineate lithology. Resulting interpretation maps clearly show an hierarchy of polyphase folding, which had been described by earlier field mapping, and a hitherto-unrecognised system of late (post-F3) sinistral strike-slip faulting and thrusting. Early Palaeozoic alaskitic intrusions of the type which host the world-class Rssing uranium deposit appear to be largely related to late sinistral transtensional ladder veins associated with the north-northeast trending post-F3 sinistral strike-slip faults.

The patterns of source and receiver locations, i.e., the acquisition geometry, used in a seismic survey affect the resulting data in a variety of ways. Herein is a study of the relationships between the acquisition geometry and the resulting subsurface illumination patterns by comparing basic land and Ocean Bottom Cable geometries with modern marine geometries. From the imaging point of view, there are certain advantages to some of the land-type acquisition geometries. By examining a new set of acquisition attributes based on imaging properties, such differences in image quality can be predicted and perhaps corrected in the survey design stage.

Hamersley Iron Pty. Limited operates five iron ore mines and has a number of advanced evaluation projects in the Hamersley Basin, Western Australia. These deposits range in style from bedded iron in banded iron formation host, to detrital iron accumulations. Numerous problems are encountered during mining in these varied geological environments. These include pods of high manganese concentration, self-igniting black shales, definition of detrital iron accumulations, fault-exploiting dolerite dykes, and lithology determination, amongst others.

Each of these problems requires an unique solution. For example, natural gamma ray logging is used to determine stratigraphic level of intersected lithologies. Together with other parameters, this method is now being extended to identify actual lithologies using artificial intelligence technology. Ground penetrating radar and radio imaging methods are employed to outline ore geometries and assist in assessing ore quality. More classic methods, such as high resolution ground magnetometry and resistivity are used to define dykes, map structures and support ore block modelling. Thus, geophysics is now playing an integral role in mine site planning. As a result, production costs should decline, and product quality should improve through better definition of ore/waste geometry.