Exploration Geophysics (EXPLOR GEOPHYS )

Publisher: CSIRO Publishing


Exploration Geophysics publishes excellent research in geophysics, reviews, technical papers and significant case histories in minerals, petroleum, mining and environmental geophysics, and is an official publication of the Australian Society of Exploration Geophysicists (ASEG). Authors and readers are professional earth scientists specialising in the practical application of the principles of physics and mathematics to solve problems in a broad range of geological situations. They are variously in industry, government and academic research institutions. All papers are peer reviewed. Four issues are published each year in both print and online versions and some issues include special sections of particular topics, or collections of papers from the regular ASEG Conferences. We also publish a joint issue as Mulli-Tamsa with the Korean Society of Exploration Geophysicists and as Butsuri-Tansa with the Society of Exploration Geophysicists of Japan; this issue goes to all three societies.

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CSIRO Publishing

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Publications in this journal

  • [Show abstract] [Hide abstract]
    ABSTRACT: Most airborne electromagnetic (EM) processing programs assume a flat ground surface. However, in mountainous areas, the system can be at an angle with regard to the ground. As the system is no longer parallel to the ground surface, the measured magnetic field has to be corrected and the ground induced eddy current has to be modelled in a better way when performing a very fine interpretation of the data. We first recall the theoretical background for the modelling of a magnetic dipole source and study it in regard to the case of an arbitrarily oriented magnetic dipole. We show in particular how transient central loop helicopter borne data are influenced by this inclination. The result shows that the effect of topography on airborne EM is more important at early time windows and for systems using a short cut-off source. In this paper, we suggest that an estimate be made off the locally averaged inclination of the system to the ground and then to correct the data for this before inverting it (whether the inversion assumes a flat 1D, 2D or 3D sub-surface). Both 1D and 2D inversions are applied to synthetic and real data sets with such a correction. The consequence on the ground imaging is small for slopes with an angle less than 25° but the correction factor can be useful for improving the estimation of depths in mountainous areas.
    Exploration Geophysics 10/2014;
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    ABSTRACT: We apply a Hamiltonian particle method, one of the particle methods, to simulate seismic wave propagation in a cracked medium. In the particle method, traction free boundaries can be readily implemented and the spatial resolution can be chosen in an arbitrary manner. Utilisation of the method enables us to simulate seismic wave propagation in a cracked medium and to estimate effective elastic properties derived from the wave phenomena. These features of the particle method bring some advantages of numerical efficiencies (e.g. calculation time, computational memory) and the reduction of time for pre-processing. We describe first our strategy for the introduction of free surfaces inside a rock mass, i.e. cracks, and to refine the spatial resolution in an efficient way. We then model a 2D cracked medium which contains randomly distributed, randomly oriented, rectilinear, dry and non-intersecting cracks, and simulate the seismic wave propagation of P- and SV-plane waves through the region. We change the crack density in the cracked region and determine the effective velocity in the region. Our results show good agreement with the modified self-consistent theory, one of the effective medium theories. Finally, we investigate the influence of the ratio of crack length to particle spacing on the calculated effective velocities. The effective velocity obtained becomes almost constant when the ratio of crack length to particle spacing is more than ~20. Based on this result, we propose to use more than 20 particles per crack length.
    Exploration Geophysics 01/2014;
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    ABSTRACT: Vertical electrical sounding (VES), a resistivity sounding technique, has been applied at two important archaeological sites in the eastern part of the Nile Delta to trace the paleoenvironment, particularly the defunct canals. Like many other archaeological sites in the Nile Delta of Egypt, these two sites have been subjected to urbanisation and agricultural invasion from the local farmers. Therefore, studying the paleoenvironment is an important task for guiding the excavation process and highlighting the importance of these two archaeological sites. The VES stations were arranged to cover the two sites, in the form of traverse profiles for tracing the subsurface sand and gravel facies that intercalated with clay deposits. The acquired VES data were processed based on the available borehole lithological information for the purpose of establishing the resistivity-depth models. Both 1D and 2D processing schemes were applied to the VES data sets to increase the confidence of the obtained results. The clay and silt deposits are characterised by low resistivity values, whereas the sand facies has a relatively high resistivity character. From the constructed cross-sections at the two sites, it was possible to define a consistent character for the clay deposits, which can be inferred as the defunct canals that supplied water to the two sites.
    Exploration Geophysics 09/2013; 44(4):282-288.
  • Exploration Geophysics 09/2013;
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    ABSTRACT: This paper presents the results of 34 new heat flow estimates taken in 2004 from 16 water bores and 18 petroleum exploration wells in the western Otway Basin. The average estimated heat flow measured across the study area is 65.6+/-9.4mW/m2, with a range of 42-90mW/m2. There are three recognisable sectors within the study area where heat flow is slightly elevated relative to the background levels. These sectors can be broadly classified as Mount Schank (73.5+/-0.5mW/m2), Mount Burr (71.2+/-7.6mW/m2) and Beachport (78.3+/-10.4mW/m2). Thermal conductivity values for each unit involved in heat flow estimation were determined from laboratory measurements on representative core using a divided bar apparatus. Borehole thermal conductivity profiles were then developed in this study by assigning a constant value of conductivity to each geological formation. The process of collecting temperature data involved measuring temperature profiles for 16 water bores using a cable, winch and thermistor, and compiling well completion temperature data from 18 petroleum wells. The precision of temperature data was higher in the water bores (continuous logs) than in the petroleum wells (largely bottom-of-hole temperature estimates). Inversion heat flow modelling suggests heterogeneous heat flow at 6000m depth, with two zones where vertical heat flow might exceed 90mW/m2, and several zones where vertical heat flow might be as low as 40mW/m2. Therefore, while slightly higher surface heat flow does coincide with some of the volcanic centres, heterogeneous basement heat production is a more likely explanation, as there are no heat flow anomalies greater than 5-10mW/m2 associated with the Pleistocene-Recent Newer Volcanics Province. The distribution of heat flow in south-east South Australia is most simply explained by non-volcanic phenomena.
    Exploration Geophysics 04/2013; 44(2):133-144.
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    ABSTRACT: Acquisition of magnetic gradient tensor data is anticipated to become routine in the near future. In the meantime, modern ultrahigh resolution conventional magnetic data can be used, with certain important caveats, to calculate magnetic vector components and gradient tensor elements from total magnetic intensity (TMI) or TMI gradient surveys. An accompanying paper presented new methods for inverting gradient tensor data to obtain source parameters for several elementary, but useful, models. These include point dipole (sphere), vertical line of dipoles (narrow vertical pipe), line of dipoles (horizontal cylinder), thin dipping sheet, and contact models. A key simplification is the use of eigenvalues and associated eigenvectors of the tensor. The normalised source strength (NSS), calculated from the eigenvalues, is a particularly useful rotational invariant that peaks directly over 3D compact sources, 2D compact sources, thin sheets, and contacts, independent of magnetisation direction. Source locations can be inverted directly from the NSS and its vector gradient. Some of these new methods have been applied to analysis of the magnetic signature of the Early Permian Mount Leyshon gold-mineralised system, Queensland. The Mount Leyshon magnetic anomaly is a prominent TMI low that is produced by rock units with strong reversed remanence acquired during the Late Palaeozoic Reverse Superchron. The inferred magnetic moment for the source zone of the Mount Leyshon magnetic anomaly is ~1010Am2. Its direction is consistent with petrophysical measurements. Given estimated magnetisation from samples and geological information, this suggests a volume of ~1.5km×1.5km×2km (vertical). The inferred depth of the centre of magnetisation is ~900m below surface, suggesting that the depth extent of the magnetic zone is ~1800m. Some of the deeper, undrilled portion of the magnetic zone could be a mafic intrusion similar to the nearby coeval Fenian Diorite, representing part of the parent magma chamber beneath the Mount Leyshon Intrusive Complex.
    Exploration Geophysics 04/2013; 44(2):114-127.
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    ABSTRACT: Applications of seismic inversions strongly depend on inversion methods, data quality, and reservoir complexity. An advanced inversion scheme to integrate seismic data, well data, and geological knowledge is employed by combining statistical Caianiello convolutional networks with a hierarchical seismic convolutional model for impedance estimation and with nonlinear petrophysical models for porosity and clay-content inversions. The method used to measure the reliability of seismic inversions for different geological complexities is important for reservoir characterisation. The widely used cross-validation may not be the best for the evaluation of the reliability of seismic inversions because of different geological conditions away from wells. As a supplementary means and also to understand failed cross-validations, we propose a systematic methodology to measure the reliability of seismic inversions through prior seismic-to-well correlation analyses for the fidelity of seismic data. The resulting correlation coefficients at the main frequencies of seismic data may express what degree the seismic data reflect the subsurface reliably in both amplitude and phase. First, the low-cut filtered borehole impedance logs are correlated with the seismic relative impedance traces computed by trace integration of seismic traces at wells. The resulting correlation coefficients within the seismic frequency band could be an index with which to evaluate the reliability of seismic inversions for impedance estimation. Second, the correlation between borehole impedance and porosity/clay-content is analysed by measuring the overall trend across the cloud of data points in the logging-databased cross-plot. The resulting correlation coefficients could be used to evaluate the reliability of mapping seismic impedance to porosity/clay content. Case studies from several oilfields across China show that the prior seismic-to-well correlation analyses are an excellent way to test the reliability of seismic inversions before the implementation of inversions.
    Exploration Geophysics 03/2013; 44(2):87-103.
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    ABSTRACT: With advanced computational power, prestack reverse-time migration (RTM) is being used increasingly in seismic imaging. The accuracy and efficiency of RTM strongly depends on the algorithms used for numerical solutions of wave equations. Hence, how to solve the wave equation accurately and rapidly is very important in the process of RTM. In this paper, in order to improve the accuracy of the numerical solution, we use a time-space domain staggered-grid finite-difference (SFD) method to solve the acoustic wave equation, and develop a new acoustic prestack RTM scheme based on this time-space domain high-order SFD. Synthetic and real data tests demonstrate that the RTM scheme improves the imaging quality significantly compared with the conventional SFD RTM. Meanwhile, in the process of wavefield extrapolation, we apply adaptive variable-length spatial operators to compute spatial derivatives to decrease computational costs effectively with little reduction of the accuracy of the numerical solutions.
    Exploration Geophysics 03/2013; 44(2):77-86.
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    ABSTRACT: Shallow warm water resources associated with low enthalpy geothermal systems are often difficult to explore using geophysical techniques, mainly because the warm water creates an insufficient physical change from the host rocks to be easily detectable. In addition, often the system also has a limited or narrow size. However, appropriate use of geophysical techniques can still help the exploration and further investigation of low enthalpy geothermal resources. We present case studies on the use of geophysical techniques for shallow warm water explorations over a variety of settings in New Zealand (mostly in the North Island) with variable degrees of success. A simple and direct method for the exploration of warm water systems is shallow temperature measurements. In some New Zealand examples, measurements of near surface temperatures helped to trace the extent of deeper thermal water. The gravity method was utilised as a structural technique for the exploration of some warm water systems in New Zealand. Our case studies show the technique can be useful in identifying basement depths and tracing fault systems associated with the occurrence of hot springs. Direct current (DC) ground resistivity measurements using a variety of electrode arrays have been the most common method for the exploration of low enthalpy geothermal resources in New Zealand. The technique can be used to detect the extent of shallow warm waters that are more electrically conductive than the surrounding cold groundwater. Ground resistivity investigations using the electromagnetic (EM) techniques of audio magnetotellurics (AMT or shallow MT), controlled source audio magnetotellurics (CSAMT) and transient electromagnetic (TEM) methods have also been used. Highly conductive clays of thermal or sedimentary origin often limit the penetration depth of the resistivity techniques and can create some interpretation difficulties. Interpretation of resistivity anomalies needs to be treated in a site specific manner. (auth)
    Exploration Geophysics 01/2013; 44(3):215-227; doi: 10.1071/EG13036.
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    ABSTRACT: This paper describes the use of magnetic susceptibility measurements on a set of nearly 3000 soil samples (one sample per km2) collected for geochemical analyses within the framework of a geological mapping program in Ghana. The result is a map of soil magnetic susceptibility which has been compared with other maps. There is a good consistency with geological domains and lithologies, as well as with some of the geochemical soil analyses and also partly with the aeromagnetic data. In the tropical, deeply weathered lateritic context of the study area, soil magnetic susceptibility reveals similarities with magnetic and/or geochemical survey results, suggesting this rapid and easy to use technique can be an effective tool for exploration and geological mapping programs.
    Exploration Geophysics 01/2013; 44(1):48-55.
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    ABSTRACT: The Coorong is a shallow (typically 1.5m) narrow coastal lagoon extending ~110km parallel to the coastline, and forms an extensive wetland area of international significance. It is divided into two lagoons, the North and South lagoons. The northern lagoon section opens into the mouth of the Murray River and the southern lagoon section is essentially closed, being connected to the North Lagoon via a choke point. During periods of extended drought where there is no flooding to flush the lagoon system, hypersalinisation gradually increases, especially in the southern lagoon section where salinity may be in excess of four times that of seawater. A helicopter time-domain EM (TEM) system was flown along the Coorong, as extensive flood waters from Queensland (2010) were reaching the North Lagoon lowering the salinity. The derived bathymetry from TEM data was shown to be in fair agreement with known bathymetry in areas of high salinity. The conductivities of waters ranging from saline to hypersaline in the North Lagoon and upper half of the South Lagoon, and underlying sediment, was estimated from inversion of TEM data using the known water depth as a fixed parameter. The derived conductivity varied from ~1.6S/m in the north of the North Lagoon to ~8-10S/m at its southern end and in the South Lagoon. These values underestimate the known strong salinity gradient (~0.6 to ~13S/m respectively) observed from a sparse distribution of fixed conductivity meters located in the Coorong. The application of AEM in this region is challenging because of the very large range of water conductivities and because the average water depths are comparable to the typical residuals between known depths and depths derived from AEM data in previous studies in Australian coastal waters. These results do however show that AEM has the potential to remotely map shallow water depths, and water conductivity gradients using known bathymetry to monitor hypersalinisation in these significant wetland areas where changes in the ecology have been linked to high salinity.
    Exploration Geophysics 01/2013; 44(2):63-69.
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    ABSTRACT: In this paper, some strategies to improve previous edge detection methods are given, and two different forms of improved theta map filters are presented. These filters use the power transform and the exponential transform of the theta map to recognise the edges of the sources. The new filters are demonstrated on synthetic and measured gravity and magnetic anomalies, and the resolving power of the new filters is tested by comparing the results with those obtained by the conventional theta map filter. The advantage of the new filters lies in their ability to delineate the source edges more clearly.
    Exploration Geophysics 01/2013; 44(2):128-132.
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    ABSTRACT: Estimating the strength, especially compressive strength, of rocks is one of the major problems in many civil engineering applications. Compressive strength of a rock is usually measured in a laboratory test of rock specimens obtained in boreholes drilled in the investigation site. If seismic velocity can be used for estimating rock strength, the seismic method can be employed effectively for profiling strength of a large rock mass because it is capable of determining subsurface seismic properties over a large area. Rock strength is often estimated from seismic velocity using correlations between data of each property, measured in the specific rock mass, or by using existing empirical relations. These techniques, however, have problems in accuracy and reliability of their estimates, because the correlations between measured data generally are made with a small number of data, and empirical relations are restricted for applicable rock types. In this study, a rock physics model is studied to estimate the compressive strength from seismic velocity more accurately and reliably. The confined compressive strength-seismic velocity relationship is modelled by combining two effective-medium models for (1) the confined compressive strength v. porosity and (2) seismic velocity v. porosity. The model is applied to S-wave velocity log data in soft sedimentary rocks, and the model predictions are compared with confined compressive strength, measured with the triaxial compression test on rock cores sampled in the same borehole as that used for the velocity logging. The model is also applied to ultrasonic P-wave velocity and confined compressive strength data measured in the laboratory, using core samples of sedimentary rocks collected from various sites in Japan. Good agreement between model-calculated and measured data in the confined compressive strength-seismic velocity relationships in these cases reveals that the confined compressive strength can be estimated from seismic velocity using a rock physics model.
    Exploration Geophysics 01/2013; 44(1):31-35.
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    ABSTRACT: Gravity data is widely used in the search for mineral deposits, and can be collected using ground, borehole, airborne, shipborne and satellite platforms. The boundaries between rocks with differing densities result in boundaries in their corresponding gravity anomalies. These boundaries, or edges, can be located using techniques based on horizontal derivatives of the data. Edge contours that do not relate to the current interpretation project can be considered as noise, and high resolution data can possess a proliferation of such edges, making interpretation difficult. While smoothing can be applied to reduce the number of edges, it has the disadvantage that the remaining edge locations are shifted and the shape of the edge contours is altered. Three methods are discussed here for the removal of unwanted edge contours. The first method removes edges which are not present in the data after its upward continuation to a user-selected height. The second method removes edges whose amplitude is below a given threshold, and the final method removes the smallest edge contours.
    Exploration Geophysics 01/2013; 44(1):42-47.