The 3861-tonne 'Nordic Explorer' this week started the two-month survey over the Maui mining licence, essentially covering the same ground as the $35 million 3D survey STOS had done in 1991, which covered most of PML 381012.

The survey is covering exactly the same lines again and, when compared with the 1991 results, should give some idea of changes in the field and aid with future reservoir management of this country's largest, but rapidly dwindling, energy resource.

STOS hopes previously bypassed pockets of gas will again be identified by the latest survey and perhaps drilled if considered worthwhile.

The 1991 "quad-quad" seismic system used two vessels, the 'Geco Kappa' and 'Geco Delta', and covered about 1000sqkm of the 4324sqkm Maui licence.

It included the "undershooting " of the Maui A platform, which had not been done before, and resulted in a 15th well for the field being planned for late that year. During that survey, seismic lines were laid every 25m or so, instead of the more usual 100m, meaning every square kilometre surveyed was covered by about 40km of subsurface lines.

The survey has started just weeks after the 2002 New Zealand Petroleum Conference in Auckland, where one of the main topics of conversation was Shell New Zealand's predicted possible tripling of the producer price of gas, to a "global norm" of $US3 per Gigajoule, after Maui faltered, perhaps as early as mid-2007.

This 4D seismic operation holds some hope for the Maui partners of possibly extending the life of the field through finding and exploiting previously untapped gas.