Braver and better velocities in the Taranaki Basin, New Zealand

The Institute of Geological and Nuclear Sciences (GNS) recently reprocessed a set of 1991 2D seismic reflection data from the offshore Taranaki Basin 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 by 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 enhanced the reflectors throughout the section, as well as providing, relatively accurate interval velocities which are critical in overpressure calculations within basin modelling studies.