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Seismic Velocity Variations in the Northern Amadeus Basin, Central Australia, from an Expanding Spread Reflection Profile
Abstract
An expanding reflection spread with a maximum shot-receiver offset of 25 km was recorded in the northern part of the Amadeus Basin, Central Australia, to obtain seismic velocity estimates throughout the crust. Arrival times of reflections indicate significant lateral variations in P-wave velocities superimposed on the small northerly dip of the sedimentary sequence. Nevertheless, a useful velocity profile that contains two pronounced low velocity zones has been derived for the basin sedimentary rocks to depths of 8.6 km. Below depths of 3 km, these interval velocities are a considerable improvement on those obtainable by standard velocity analysis techniques on the coincident near-vertical incidence reflection data; the results thus enable the determination of more reliable estimates of the thicknesses of the deeper sedimentary formations. The refracted arrivals from the expanding spread yield well-constrained P-wave velocities for shallow depths, but below about 1.5 km depth, the refraction interpretation is problematic owing to the absence of a refracted arrival through the high velocity lid in the interval velocities at depths between 1.8 and 2.7 km. The determination of useful velocity estimates for basement rocks is hampered by the presence of peg-leg multiples produced within the overlying sedimentary formations.
... A positive gravity anomaly in the Sydney Basin was interpreted as a mafic body in basement extending to 13 km depth. Wright et al. (1989) described the recording of seismic expanding-spread reflections to a maximum offset of 25 km, in the northern part of the Amadeus Basin. Although some of the outer shots could not be used because of lack of coherence, a good vertical velocity profile was derived for the sediments to a depth of 8.6 km. ...
The first twenty years of ASEG publications have been reviewed. The review has taken the form of summaries of developments over the period in a number of topics; these include petroleum exploration, coal exploration, potential fields, electrical and electromagnetic methods, and regional and deep crustal geophysics. The survey gives a good indication of the development of ideas and methods in these topics, and shows that the ASEG journals and conferences have played a significant role in stimulating and promulgating the practice of geophysics in Australia.
The Australian Bureau of Mineral Resources has recorded expanding reflection spreads coincident with regional deep Seismic reflection profiles on land, both in sedimentary basins and in areas of exposed basement rocks. Arrival times of refracted, reflected and diffracted signals have been matched with synthetic times computed by ray tracing to give one- or two-dimensional velocity models for the regions immediately below the expanding spreads. Significant lateral variations in Seismic velocity in both sedimentary and basement rocks over distances of about 20 km are widespread, making it difficult to fit refraction, near-vertical incidence and moderate- to wide-angle reflection data with reasonably simple velocity models. One expanding spread recorded across the Nebine Ridge in southern Queensland shows P-to-SV and SV-to-P converted reflections from a bright spot in the coincident near-vertical incidence reflection data at a depth of about 17 km. Another expanding spread recorded in the Amadeus Basin has provided a detailed velocity model for the 9 km thick sedimentary section. The resolution of deep basement velocities is best below the Surat Basin, although the absence of persistent, continuous reflections and the presence of reflections from outside the vertical plane through sources and receivers limits the reliability of estimates of basement velocities.
Deep Seismic profiling undertaken in central Australia comprised expanding spread reflection profiling, long-range refraction work and a small-scale three-dimensional refraction survey, as well as the more usual near-vertical incidence reflection profiling. Scismic reflection sections within the Arunta Block in central Australia show abundant northerly dipping events that are interpreted as reflections from dipping faults, many of which are evident in surface geological mapping. The Redbank Deformed Zone, a major thrust feature, has been imaged to depths of at least 30 km and defines a marked change in reflection character. South of the Redbank Zone, steep, northerly dipping reflections are absent; sub-horizontal basement reflections are prevalent below the Southern Arunta Province and Amadeus Basin. Seismic refraction profiles indicate that the crust is more than 50 km thick below both the boundary between the Southern and Central Arunta Provinces and the northern part of the Amadeus Basin. Expanding reflection spreads and a three-dimensional refraction survey have added fine details to the results in regions of special geological interest and include the resolution of complicated velocity variations in the sedimentary rocks of the northern part of the Amadeus Basin and the measurement of anisotropy in the granulites of the Central Arunta Province adjacent to the Redbank Zone.
In 1989, as part of the Lithoprobe East program, high-resolution 60-fold seismic reflection profiles were recorded using Vibroseis sources in the vicinity of the Buchans mine. The area is considered to be a fold and thrust belt in which the massive sulphide-barite orebodies occurring within volcanic and sedimentary units of the Buchans Group are repeated in a large number of thrust systems. The main objective was to image the individual faults, generally narrow zones of brittle shear, some of which have been intruded by diabase sills. In 1991, Memorial University of Newfoundland recorded a 24-fold line using explosives as sources, coincident with one of the Lithoprobe lines, in order to compare the resolution of shallow structures of economic importance with that obtained using Vibroseis. The explosive sources provided reflections richer in high frequencies than the Vibroseis sources for depths less than 1 km. The resolution of the reflections is greatly improved by two processes: (1) the application of refraction static corrections, and (2) spectral balancing of the NMO-corrected CMP gathers over two octaves prior to stacking. Compared with Vibroseis, the approach using small explosive sources is considered preferable for future work in mineral exploration and mine development because of lower costs and better resolution of shallow moderately-dipping faults.
To investigate the tectonic evolution of the Arunta Block and the Ngalia and Amadeus Basins, a regional north-south seismic reflection line 420km long from the Northern Arunta Province to the southern part of the Amadeus Basin, and an east-west refraction profile over 400km within the Arunta Block, were recorded by the Bureau of Mineral Resources in 1985. The most significant basement features observed in the reflection data are prominent bands of northerly dipping reflections originating from beneath the Northern Arunta Province and the Ngalia Basin at times to between 4 and 10s. In this region, reflected energy results imply that the rocks have high Q to depths of at least 18km. For the Arunta Province the refraction data suggest an average crustal thickness of about 55km. Below the Southern Arunta Province there is a zone of northerly dipping reflectors at depths between 21 and 30km, which suggests deeply buried rocks of sedimentary origin. -Authors
The Amadeous basin is an isolated intracratonic basin at the center of the Australian continent which, because of its location and geometry, provides an ideal opportunity to investigate depositional controls. To this end, more than 6000 km of seismic data, in conjunction with a field and well-log program, have been used. -from Author
Techniques have been developed for the usefulness of the expanding reflection profile whose basic elements were described by Dix in 1955. Procedures have been established which make the shooting and interpretation of these expanding reflection spreads simpler and more reliable. Special presentations have been developed for the displaying of these profiles on record sections, and nonlinear paper has been designed for plotting the time‐squared versus distance‐squared graphs. The expanding spread is a valuable seismic tool and has numerous applications. It may be used for the identification of reflections and multiples. From the true reflections, calculations can be made to present the normal velocity survey information, i.e., time, average velocity, and interval velocity versus depth. Among other uses is the determination of the normal moveout curve. Various types of presentations are used to display expanding spreads in record section form. Also, various noise and multiple problems are exhibited.
The Reciprocal Method of shallow seismic refraction investigation provides a simple reliable method of surveys where the absolute depths to important refractors and an analysis of the velocities in the refractors are required in detail. The depth computation is based on an early (1931) concept, termed here the time-depth, which differs slightly from the delay-time. The calculation of the individual time-depths and of the corrected travel times within the refractor require only simple numerical computation. Hence, the method is very suitable for routine survey procedures in which use is made of composite velocity terms for multilayered overburdens. The basis and procedures of the Reciprocal Method are presented together with examples illustrating the application of the method and the technique of velocity analysis.
The Bureau of Mineral Resources has recently recorded expanding spread reflection profiles coincident with its regional near-vertical incidence seismic reflection lines primarily to obtain velocity estimates throughout the crust. Accurate shot corrections are required in the first stage of the processing of these expanding spread profiles. A simple least-squares procedure that uses the time differences between reversed shot-receiver paths has been developed for the calculation of shot static corrections. Numerical examples from an expanding spread recorded across the Nebine Ridge in south-east Queensland have been given to show that the standard errors in these corrections for high quality records should generally not exceed 1.0 ms.
“Over the years, ray theory has furnished the exploration geophysicist with most of the working tools for understanding and interpreting events observed on reflection seismic sections. Even today, notwithstanding the pace at which the more powerful acoustic wave theory is introducing its new tools, ray theory, in the hands of the authors, retains its preeminence for providing insights into fundamental problems in reflection seismology. Professor Krey's earlier contributions are part of ray theory's rich heritage. Alongside C. Hewitt Dix and Hans Durbaum, he elucidated relationships between interval velocity and observed reflection moveout.”
The major objective of the Central Australian seismic experiment is to investigate the structural evolution of the Arunta Block and the Ngalia and Amadeus Basins. A regional north-south reflection line of 420 km length from the Northern Arunta Province to the southern part of the Amadeus Basin was recorded in 1985. The most significant basement features are prominent bands of reflectors from beneath the Northern Arunta Province and the Ngalia Basin at times of between 4 and 10 s that dip towards the north. Deep crustal features south of the Ngalia Basin are less clear except in the Redbank Zone. Bands of deep reflectors similar to those observed in the north occur at times of between 5 and 10 s beneath the southern part of the Amadeus Basin. Additional seismic profiling included a reflection line of 40 km length recorded across the northern margin of the Redbank Zone, three expanding spread reflection profiles and a tomographic experiment. An east-west seismic refraction profile of 400 km length was recorded within the Arunta Block, and suggests an average crustal thickness of 55 km.
An algorithm SUMMOOTH for smoothing raw observations that are unequally spaced is explained. The process generalizes the method of summary values introduced into geophysics by H. Jeffreys. Smoothed data points, defined as the intersection of the local linear and parabolic least-squares fits, are computed for overlapping intervals rather than fixed sequential intervals as in previous work. A new feature is the parallel computation of (smoothed) summary gradients, essential for the Herglotz velocity integration. The interval selection is objective because the position of the smoothed values at each stage depends only on the spacing of the raw sample points. Selection of the starting interval can be made objective by addition of a principle, such as symmetry, or a rule, such as the fitted local curvature must never exceed a fixed value. In practice, selection should employ a trade-off curve between resolution and variance. An advantage of the process is that uncertainties at the summary points are independent. A comparison is given, for scarce data of Mossbauer spectra, with the smoothing method of Talmi and Gilat. Application to ragged time series for observations in earthquake prediction studies and to the construction of seismological travel-time curves illustrates the value of the method in geophysics.
Over six hundred values of the P wave travel-time gradient (dT/dΔ) have been derived from 224 events recorded at the Warramunga Seismic Array over the distance range 28° to 99°. These dT/dΔ measurements have been corrected for local structure beneath the array, and analysed on a regional basis to try and establish the presence of velocity anomalies in the lower mantle. The smoothed dT/dΔ data have been inverted by the Herglotz-Wiechert method to obtain a velocity model over the depth range 700 to 2700 km. The anomalies in the dT/dΔ curve correspond first to fairly low velocity gradients between depths of 800 and 850 km, 1070 and 1110 km, 1260 and 1330 km, 1750 and 1850 km, and 2460 and 2600 km; and secondly to high velocity gradients between 1160 and 1220 km, 2180 and 2370 km, and possibly between 2700 and 2750 km. These results have been shown to agree satisfactorily with recent travel-time and array dT/dΔ investigations, once the sources of systematic error in such studies have been isolated and compared.
Late Proterozoic and Early Palaeozoic depositional facies of the Northern Amadeus Basin, Central Australia’, Field excursion 25 B, 12th International Sedimentological Congress
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