Exploration Geophysics - Volume 13, Issue 3, 1982
Volume 13, Issue 3, 1982
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Effects of Geological Structure and Geophone Field Configuration on Seismic Velocity Accuracies
More LessAuthors Gregory J BlackburnAbstractIn the seismic reflection method the determination of vertical signal velocities is of basic importance, but timing perturbations which result from raypath distortion may cause discrepancies in the process of calculating true velocities from the velocities obtained by a stacking process. In particular, if different configurations of geophones (‘field configurations’) are employed over an area, the ‘stacking velocities’ obtained can vary greatly with different shot-to-receiver distances, even though a constant separation ('spread length') of the geophones is maintained. In this paper the modelling of raypaths in some simple geological sections shows that the combined effects of both velocity errors and timing errors may be so significant as to obscure the resolution needed in the method for the detection of subtle hydrocarbon traps.
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Gross Crustal Structure Beneath Northeastern Queensland from Long-Period P Seismic Data
More LessAuthors Shane F. Ingate and David W. KingCrustal thickness and average seismic velocity can be determined by fitting the spectral ratio of the vertical and horizontal radial components of teleseismic long-period seismic P-waves to the theoretical spectral ratio of feasible crustal models. The vertical and horizontal radial transfer functions are calculated by the Haskell-Thomson matrix formulation pointwise in the frequency domain, by estimating the transmission coefficient of a given layered system for each of a sequence of input frequencies at the base. The character of the spectral ratios is affected mainly by the total crustal thickness and average velocity. Knowledge of these parameters is particularly valuable in combination with complementary data from deep crustal seismic reflections at near vertical incidence. Comparison tests using data from the ASRO/HGLP installation at Charters Towers (near the Queensland continental margin, Australia) reveal that the mean crustal thickness at Charters Towers is about 40 km and the mean Moho P-wave velocity contrast is 0.8. These values are consistent with independently derived models of ‘normal’ continental crust.
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Correlative Geophysical Data in the Australian Region for Use in the MAGSAT Project
More LessAuthors J. C. Dooley and P. M. McGregorAbstractMagnetic data available for use in correlation with MAGSAT data in the Australian region include observatory recordings, first-order and third-order vector regional magnetic surveys, long airborne scalar profiles, and approximately 75% coverage of the continent with detailed aeromagnetic mapping of total intensity. Reconnaissance gravity coverage is virtually complete, and deep seismic sounding refraction and reflection surveys have been carried out at widely scattered locations. There are scattered data from geothermal investigations. Magnetic and gravity coverage is also available over much of the surrounding marine area, and in selected areas in Antarctica. It is planned to use this material in a comprehensive study of the broader litho-spheric features of the area.
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Geomagnetic Field Fluctuations Over Australia in Relation to Magnetic Surveys
More LessAuthors F. E. M. LilleyFluctuations of the earth’s magnetic field with time can be a source of error of up to tens of nanotesla in magnetic surveying or prospecting. The best defence against possible difficulties such fluctuations may cause lies in understanding their origin, and their geographical uniformity or non-uniformity. The geographical uniformity of magnetic fluctuations has now been measured over several different areas of the Australian continent as part of research projects to examine natural electromagnetic induction taking place in the earth. The present paper shows examples of the results and gives some general description of magnetic fluctuations characteristics at coastlines, inland, and offshore. A major zone of non-uniform magnetic fluctuations has been detected in two places and may run north-south right across the continent.
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A Playback System for Portable Seismic Recorders
More LessAuthors Y. S. B. Liu and K. J. SeersAbstractSeismic data acquired by the Bureau of Mineral Resources (B.M.R.) for crustal studies are recorded on magnetic tape as frequency modulated (FM) signals accompanied by an amplitude modulated (AM) serial time code. A playback system is described which permits accurate recovery of seismic signal amplitudes from FM tape recordings: an advantage over AM systems which, because of the effects of many unknowns such as tape quality, require continuous calibration for absolute amplitude recovery. By using the recorded time-code carrier frequency to sense tape speed, the playback system corrects for speed changes in the original recording and compensates for variations in its own speed characteristics. The novel speed control system, combining both frequency and phase control, ensures accurate tracking, rapid response, and a wide acquisition range. An adaptable detection threshold ensures that the recovered and decoded time signal is sufficiently reliable to be used for computer controlled event searching. The data processing task is further aided by overall computer control of the playback system, and programmable digitizing modes.
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