oa Azimuthal Imaging Challenges in Coal-Scale 3D Multi-Component Seismic Reflection

In certain geological situations, improved characterisation of the subsurface can be achieved through the integration of conventional P-wave seismic reflection and PS-wave (converted, or C-wave) reflection. Initial noteworthy successes for this technology related to offshore petroleum targets, which were difficult to image with standard P-wave methods (e.g. Barkved et al., 1999; Hanson et al., 1999). More recently, integrated interpretation of P and PS imagery has been shown to yield improved geological interpretation of coal targets, including detection of new fault structures, improved determination of fault geometries, and superior imaging of top-of-coal for shallow open-cut targets (e.g. Hearn, 2004; Velseis, 2003, 2007). Application of PS-wave techniques is arguably more challenging in the onshore situation, due partly to severe S-wave receiver statics. These statics are more problematic with shallow high-resolution data, where maintenance of high frequencies is critical.

In the P-wave arena, there is an increasing trend toward 3D surveys since these produce a better spatial interpretation. It would be reasonable to assume that 3D PS-wave surveys would also lead to an improved geological interpretation. However, 2D PS-wave surveys have identified a number of issues that will make 3D PS exploration more challenging than the conventional case. For example, different geological interpretations may result from images created with positive (forward shooting) or negative (back shooting) offsets. Additionally, the asymmetry of PS-wave paths means that it is more difficult to achieve regular azimuthal and offset distributions when designing 3D grids. Associated with this is the fact that PS offsets may be more restricted due to phase effects. PS path asymmetry is generally more pronounced at the shallower depths appropriate to coal exploration (50-500m). There is also a greater tendency to use rays having higher incidence angles. Hence we believe that the problems noted here are likely to be more severe at coal depths than at petroleum depths.

In this paper, the discussion of these issues is largely based on prior 2D experience, and numerical modelling of 3D effects. It represents the design phase of a 3D converted-wave field experiment to be carried out in late 2008 (ACARP, 2008). Although we emphasise the peculiarities of the shallow coal exploration environment, the discussion is of more general relevance.