No abstract available

In hard rock regions, a large range of stacking velocities is required to correctly stack reflectors of different dips. Typically, horizontal reflectors stack at 6000 m/s, whereas reflectors with dips of 60 degrees stack at 12,000 m/s. For high fold (vibrator) data, correct stack of conflicting dips can be achieved by dip moveout (DMO) correction. However, for lower fold (dynamite) data, the sparse offset distribution complicates application of DMO. An alternative technique involves producing stacks with different stacking velocities and stacking these stacks. This technique was applied to two seismic reflection data sets, low fold dynamite from Broken Hill and high fold vibrator data from the Lachlan Fold Belt. The Lachlan data set was used as both full 60/120 fold and reduced 10/20 fold. Velocity analysis, both analytical and empirical, was carried out to determine the range of stacking velocities. Stacking velocity increases with dip angle (cos-1 theta), but the velocity range across which an event stacks coherently increases more rapidly (approximately cos-3 theta for velocities typical of hard rock)). The most critical area for analysis is the first two seconds of data, due to greater sensitivity of NMO to stacking velocity. The optimum number of stacks is an important consideration, based on the number of stacks in which an event contributes coherently to the sum The Broken Hill stack data showed simultaneous imaging of horizontal and dipping events. For the Lachlan reduced fold data set, horizontal and moderate to steeply dipping events were stacked successfully, although not as well as the post-DMO stack of the full fold data. The technique has some problems at the shallowest levels, where the stack can be degraded due to time shifts of events in the individual stacks.

Legacy product - no abstract available

Legacy product - no abstract available

Shear zones often appear in seismic images of the deep crust as laterally continuous zones of discontinuous reflections with individual reflections often having high amplitudes. Although shear zones are usually modelled in one or two dimensions, they are more likely to be multiple layers that anastomose in three dimensions. Most crustal-scale seismic reflection surveys use single or at the most only a few profiles, and therefore create two dimensional images of three dimensional structures. Multifold common-mid-point data are stacked and migrated assuming that the seismic energy comes entirely from within the plane of the section. However, three dimensional topography on the reflecting surface results in out-of-plane reflected energy coming into the plane of the section, and energy from the plane of the section being lost from the plane of the section. In this paper, we use synthetic data to consider the effects that topography on a reflector surface has on reflection character. For an out-of-plane diffractor in a homogeneous and isotropic medium, the out-of-plane energy will migrate as if it comes from within the plane of the section, but it will appear to come from a greater depth than that of the true diffractor. Therefore, for a sub-horizontal surface with little topographic relief, out-of-plane energy will fall in the plane of the section below the in-plane reflections from the surface. The result will be a zone of reflections in which the top of the zone reproduces the shape of the reflector within the plane of the reflector fairly accurately, and reflections lower within the zone of reflections will be from out of the plane. ...................................

Presentations from the GOMA (Gawler-Officer-Musgrave-Amadeus) Seismic and MT Workshop 2010.

No abstract available

Regional seismic reflection data in hard rock areas contains more shallow information than might first be supposed. Here I use a subset of the 2005 Tanami Seismic Survey data to show that near surface features can be defined, including paleochannels, Palaeozoic basins and structures within the Proterozoic basement. Successful imaging depends on correct determination of refraction statics, including identification of refractor branches, and use of a floating or intermediate datum during seismic reflection processing. Recognition of steep stacking velocity gradients associated with surface referenced processing aids velocity analysis and can further delineate areas of thicker regolith in palaeochannels. The first arrival refraction analysis can also be applied in more detail to estimating thickness of regolith and depth to economic basement in areas of sedimentary cover.

Seismic line 07GA-GC1, described here, forms part of the Isa-Georgetown-Charters Towers seismic survey that was acquired in 2007. The seismic line is oriented approximately northwest-southeast and extends from east of Georgetown in the northwest to south of Charters Towers in the southeast (Figure 1). The acquisition costs for this line were provided jointly by the Geological Survey of Queensland and Geoscience Australia, and field logistics and processing were carried out by the Seismic Acquisition and Processing team from Geoscience Australia. Seven discrete geological provinces have been interpreted on this seismic section (Figure 2). Two of these, the Abingdon and Sausage Creek Provinces, only occur in the subsurface. The upper crustal part of the seismic section is dominated by the Etheridge and Cape River Provinces, but the seismic line also crossed the Broken River Province and the Drummond and Burdekin Basins.

No abstract available