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  • A method for calibrating seismic stacking velocities against velocities from well measurements has been developed to quantitatively assess the validity of stacking velocities in the vicinity of boreholes and to improve quality of stacking velocities for use in regional depth conversion of interpreted seismic horizons. Accurate depth conversion of seismic interpretation is vital for use as constraints in gravity modelling and in other basin modelling tasks. Examples of this methodology are given for the northern Perth Basin, Australia. The suggested workflow for calibrating seismic stacking velocities against well velocities in a simplified form is as follows: 1. Check each velocity dataset for errors 2. Modify the datum of each dataset to the sea floor 3. Convert all datasets to two-way time and depth domain 4. Resample all velocity datasets to the same two-way time intervals 5. Cross plot stacking velocity depths near a well site with corresponding well depths for equal two-way times 6. Fit a linear polynomial to this cross-plot (higher order polynomials were tried also), and determine calibration coefficient from the gradient of the polynomial. 7. Grid calibration coefficients 8. Multiply depths derived from stacking velocities by calibration coefficient grid An assessment of depth conversion errors relative to wells shows that this methodology improves depth conversion results to within ±50 m down to the maximum well depth analysed (3.5 km below sea floor); this depth uncertainty translates into a modelled gravity anomaly error of about ±20 gu, which is acceptable for regional scale gravity modelling.

  • A Seismic reflection survey was conducted by the Bureau of Mineral Resources during 1955 7 in the La Grange area of Western Australia. The object orthe Survey was to investigate the sedimentary development of the Canning Basin south of the Fenton Fault. A sedimentary section of at least 4,400 feet and possibly as much as 7,000 to 8,000 feet was indicated. Although no evidence of an angular unconformity was obtained, there is probably a change in the type of sedimentation at 2,760 feet.

  • Towa.:ccis the end of 1960 , the Bureau. of Mineral Resources, Geology and Geophysics made a brief seismic survey in the Winton area of Queensland to resolve an apparent contradiction between the interpretations of gravity and aeromagnetic results previously obtained in the area. Gravity and aeromagnetic results both suggested the occurrence of a large fault or fault zone about 20 miles north-west of Winton, but the gravity and aeromagnetic interpretations differed regarding the direction of throw of the fault. A nine-mile seismic reflection traverse was surveyed across the supposed fault. The seismic results indicate the presence of a large fault or monoclinal fold with dowthrown side nouth-wast as suggested by the gravity values and also a smaller fault or monocline about two miles south-east with downthrown side south-east. The variations in thckness of Mesozoic rocks caused by these features were insufficient to explain the observed Bouguer gravity anomaly values, but the seismic results left open the possibilitues that there may be a considerable thickness of pre-Mesozoic sedimemts north-west of the main monocline or fault. It is postulated that the steep gravity gradient observed may be due to a large fault whose main movement took place in pre-Mesozoic times. Indications are that there is 5000 to 6000 ft of Mesozoic sediments in tha area.

  • During 1961 in the southern part of the Surat Basin a seismic party from the Bureau of Mineral Resources surveyed two main traverses by means of seismic reflection and refraction methods; the first was in an east-west direction between Yelarbon and St George and the second was in a north-south direction between Meandarra and Nome. The main purposes of the survey were to find whether the Bowen Basin Permian sediments extend as far south as the latitude of Goondiwindi and whether the Bowen Basin in Queensland and the Sydney Basin in New South Wales formed a continuous region of sedimentation during the Permian period. The east-west seismic traverse indicated a trough of sediments of greatest thickness,tabout 14,800 ft beneath Toobeah; the trough is bounded on the eastorn side at Goondiwindi by a fault down-thrown more than 7000 ft to the west and is bounded on the western side by a series of step.-faults beneath Bungunya and Talweod. The results along the north-south traverse indicated that the trough beneath Meandarra, which represents the southern extension of the Bowen Basin, continues south to Toobeah. The nature of the link, if any, between the Bowen Basin and the Sydney Basin was not established. On the eastern side of the Surat Basin, seismic results indicated that the rocks beneath the Mesozoic sediments are stratified and probably metamorphic. A shelf area between Talweod and St George has about 6000 ft of sediments above a Drobablo metamorphic 'basement'. An anticlinal structure with a dip-reversal of about 1000 ft throw was located between Goondiwindi and Toobeah.

  • This report outlines the field geophysical work carried out in the vicinity of Wilkes Base (Lat. 660 15' 8, Long. 1100 31' E), Australian Antarctic Territory, during the Australian National Antarctic Research Expedition of 1965-66. The work can be broadly divided into four parts. 1. An autumn traverse where seismic reflection stations were established every 10 miles around a triangle with corners at Cape Folger, the Dome Centre and Cape Poinsett. 2. A mid winter traverse to the inland glaciology station S-2 for a programme of seismic ice velocity studies. 3. A spring traverse where a rectangular grid network of seismic, gravity, and elevation stations were installed in a region between S-2 and 80 miles south of S-2. 4. Two attempts at recording reflections off the Mohorovicic discontinuity shot in the vicinity of Wilkes.

  • Some experimental geophysical work was undertaken by the Bureau of Mineral Resources at the Moura Coalfield in Queensland in order to investigate the structure of coal seams which lie within 1000 feet of the surface. The aim of the survey was to evaluate the use of geophysical techniques for locating faults of small displacement. Most of the effort was concentrated on shallow seismic reflection techniques but additional techniques (magnetic, gravity, resistivity, electromagnetic and induced polarization) were used. The whole area of the survey was covered with a close-spaced grid of gravity and magnetic stations. A lesser effort was devoted to resistivity, electromagnetic, and induced polarization methods, the object being to take advantage of the opportunity to tryout these relatively inexpensive methods in the hope that they might give some indications of fault locations. These methods will be evaluated in a separate Record; this one deals- with the results of the seismic work. Encouraging results were obtained from seismic reflection work. The uppermost economic coal seam was mapped to within 300 feet of the surface and areas of faulting could be deduced from time differences and lack of continuity in reflections.

  • Current geological surface mapping of the Officer Basin, W.A., by BMR has given an incomplete picture because the area is largely covered by flat-lying Permian fluvioglacials or lateritized Cretaceous rocks and the outcrops give no indication of the structure and composition of the sediments in most of the basin. The geological boundaries within the basin and at its margins are ill-defined, and the only reliable shallow subsurface information available from geophysical and well data is along the northern part of the basin near Warburton Mission. A seismic survey in the Officer BaSin, W.A., is planned to operate from mid-July to December 19720 The survey is to be conducted along a NE-SW line roughly following the road between Lake Throssell and Warburton Mission at the two margins of the basin. It will consist of a series of combined refraction and reflection probes located along the road. The operations will start with two probes near the centre of the basin, and the location of subsequent probes will depend on the progressive assessment of results. The results will be tied to those from the earlier seismic surveys in the northern part of the basin.

  • In November 1964, the Bureau of Mineral Resources Seismic Party No. 1 carried out a velocity survey in CBMR 12 (Cockroach) Well, Northern Territory, which had been drilled to 4000 feet and logged using sonic and other logging methods. The time/depth values obtained in the velocity survey were in good agreement with those obtained by integration of the sonic log. A curve showing the variation with time-of'the coefficient of reflection calculated from the sonic log showed an outstanding feature, which appeared to correlate with the best reflection recorded on an experimental seismic traverse nearby. According to the geological interpretation of the well log, this reflection arises from velocity changes near the boundary of the ArrinthrungaTormation and the Marqua Beds at a depth of 2,721 feet.

  • The seismic reflection survey was carried out in the Parish of Darriman, Victoria. The survey was planned to investigate a gravity anomaly, which may be an indication of a structure within the sedimentary section favourable to the accumulation of oil or natural gas. For the most part reflections were exceptionally good, and it was possible to trace one reflecting layer over most of the area. Contouring showed the presence of an anticlinal structure, plunging to the east, but rising and broadening to the west. On the flat crest of the tructure, there is probable closure in two places. The thickness of the Tertiary sediments may be a maximum of 6,000 feet, but may be only 3,000 feet on the crest of the structure and 4,000 feet on the flanks.

  • We report four lessons from experience gained in applying the multiple-mode spatially-averaged coherency method (MMSPAC) at 25 sites in Newcastle (NSW) for the purpose of establishing shear-wave velocity profiles as part of an earthquake hazard study. The MMSPAC technique is logistically viable for use in urban and suburban areas, both on grass sports fields and parks, and on footpaths and roads. A set of seven earthquake-type recording systems and team of three personnel is sufficient to survey three sites per day. The uncertainties of local noise sources from adjacent road traffic or from service pipes contribute to loss of low-frequency SPAC data in a way which is difficult to predict in survey design. Coherencies between individual pairs of sensors should be studied as a quality-control measure with a view to excluding noise-affected sensors prior to interpretation; useful data can still be obtained at a site where one sensor is excluded. The combined use of both SPAC data and HVSR data in inversion and interpretation is a requirement in order to make effective use of low frequency data (typically 0.5 to 2 Hz at these sites) and thus resolve shear-wave velocities in basement rock below 20 to 50 m of soft transported sediments.