A braver approach to seismic velocity analysis in the Taranaki Basin, New Zealand

The Institute of Geological and Nuclear Sciences (GNS) recently re-processed a set of 1991 2-D seismic reflection data from the offshore Taranaki region for the Spectrum Exploration/ Fletcher Challenge Energy Taranaki Joint Venture. A more detailed stacking velocity analysis, incorporating a 40% interval velocity inversion in the Eocene units, resulted in a markedly improved seismic image of Eocene and deeper reflectors. A dense semblance analysis formed the initial part of the velocity analysis sequence. It was followed by interactive constant velocity stack analysis of the velocity inversion, where the weak intra-Eocene reflectors were masked in amplitude by multiples from an overlying strong limestone reflection. A third, automatic, velocity analysis was performed on the final stack using a combination of semblance and digitised horizon times, producing a horizon-based velocity model with data points every 5 traces (63 metres). The combination of these analyses can provide relatively accurate estimates of interval velocities needed in the calculation of overpressures in basin modelling studies.