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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    Exploration Geophysics website
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    Periodical, Internet resource
  • Document type
    Internet Resource, Computer File, Journal / Magazine / Newspaper

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CSIRO Publishing

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    • On author's personal repository or institutional repository
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Publications in this journal

  • [Show abstract] [Hide abstract]
    ABSTRACT: Magnetotelluric (MT) time-series are often contaminated with noise from natural or man-made processes. A substantial improvement is possible when the time-series are presented as clean as possible for further processing. A combinatorial method is described for filtering of MT time-series based on the Hilbert-Huang transform that requires a minimum of human intervention and leaves good data sections unchanged. Good data sections are preserved because after empirical mode decomposition the data are analysed through hierarchies, morphological filtering, adaptive threshold and multi-point smoothing, allowing separation of noise from signals. The combinatorial method can be carried out without any assumption about the data distribution. Simulated data and the real measured MT time-series from three different regions, with noise caused by baseline drift, high frequency noise and power-line contribution, are processed to demonstrate the application of the proposed method. Results highlight the ability of the combinatorial method to pick out useful signals, and the noise is suppressed greatly so that their deleterious influence is eliminated for the MT transfer function estimation.
    Exploration Geophysics 10/2014; 45(2).
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    ABSTRACT: Discrete conductor models like sphere and plate were introduced in the 1950s as modelling tools in airborne electromagnetic (AEM) survey interpretation. In the last 20 years, with the development of inversion techniques, they have been integrated into parametric inversion programs. The recent advent of powerful workstations makes them useful tools for interactive AEM interpretation. Different problems have been encountered in the implementation and application of discrete objects as modelling and inversion tools. The sphere response is modelled using a sum of spherical functions. Assuming that the radius of the sphere is small compared to the distance between the transmitter and receiver to the centre of the sphere, the response can be approximated by using only the first term of the solution. This approach is reviewed for modelling the response of a conductive sphere in free space or buried in a layered earth. Plate modelling is based on spectral methods or the integral equation method, which provide different techniques for estimating the response of a plate in free space. A comparison of the results of these techniques show differences attributed to the different discretisation methods. A case history from Abitibi, Canada, shows that plate inversion using two different inversion methods provides useful information when the target is a plate-like conductor in a resistive environment.
    Exploration Geophysics 10/2014; online.
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    ABSTRACT: Unmanned aerial vehicles (UAVs) have recently received attention in various research fields for their ability to perform measurements, surveillance, and operations in hazardous areas. Our application is volcano surveillance, in which we used an unmanned autonomous helicopter to conduct a dense low-altitude aeromagnetic survey over Tarumae Volcano, northern Japan. In autonomous flight, we demonstrated positioning control with an accuracy of ~10 m, which would be difficult for an ordinary crewed vehicle. In contrast to ground-based magnetic measurement, which is highly susceptible to local anomalies, the field gradient in the air with a terrain clearance of 100 to 300 m was fairly small at 1 nT/m. This result suggests that detection of temporal changes of an order of 10 nT may be feasible through a direct comparison of magnetic data between separate surveys by means of such a system, rather than that obtained by upward continuation to a common reduction surface. We assessed the temporal magnetic changes in the air, assuming the same remagnetising source within the volcano that was recently determined through ground surveys. We conclude that these expected temporal changes would reach a detection level in several years through a future survey in the air with the same autonomous vehicle.
    Exploration Geophysics 08/2014; 45(1).
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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: 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: In order to obtain an accurate EM response with modelling software, most people assume that it is necessary to know or specify the excitation current waveform (or its derivative) precisely. A mathematical analysis shows that accurate model results can be obtained during the off time if the amplitude of the waveform is specified precisely in the latter parts of the waveform; however, in the earlier parts of the waveform, the amplitudes can be approximate as long as the area under the waveform is specified accurately. This means that the discretization should be fine in the latter parts of the waveform, but can be coarse in the early parts of the waveform. Coarse sampling of the waveform means that the convolution integrals can be calculated more efficiently. An example shows that the exponential rise and linear ramp assumed by some modelling software to approximate a waveform can give poor results with errors close to 10%. Another approximate waveform that is precise in the final parts of the waveform and has an accurate area under the waveform curve gives errors less than 0.15%.
    Exploration Geophysics 01/2013; 44(1):1-5.