New Zealand to Start Seismic Survey Off North Island
|Friday, December 10, 2004
A significant reconnaissance seismic survey will be acquired off the east coast of the North Island this summer. The survey will be acquired over an area of more than 100,000 square kilometers, between Te Kaka, in the eastern Bay of Plenty, and Castle Point. The survey will extend as far north as 360 km NNE of East Cape. It is expected that acquisition will commence in the first half of January, and take up to 6 weeks to complete.
The research being undertaken by Crown Minerals will significantly increase understanding of the oil and gas potential of the selected frontier basins. The Ministry's planned $15 million work program specifically addresses the exploration industry's key perceived risks in those basins. The data acquired, and the results of subsequent geotechnical studies, will be released to industry free of charge to encourage further exploration in those areas.
Seismic surveying involves the transmission of a sound wave from a seismic source towed behind the survey vessel. The sound wave passes through the water and into the rock layers beneath the sea floor, reflecting a portion of the signal off each rock layer. The reflected sound wave is detected by groups of hydrophones placed at 12.5 m intervals inside the streamer. The reflected signals received by the hydrophone groups are then transmitted along the streamer to 1044-channel recording equipment within the vessel. The 05CM seismic survey off the east coast will be acquired by a multinational seismic acquisition company, who will bring an appropriate specialist vessel to New Zealand waters for the duration of the survey.
Most industry seismic surveys involve a seismic vessel remaining in a relatively small area to acquire data related to specific exploration targets. For example, a 3D seismic survey will typically involve the acquisition of a grid of seismic lines spaced as little as 80 meters apart in the immediate area of a proposed drilling target. The 05CM survey, however, covers a much larger area with seismic lines up to 660 km long, spaced between 10 km and 100 km apart.
Because of the long widely-spaced seismic lines, the vessel conducting the 05CM survey will not remain in any one area. The vessel will traverse through areas, and may return days or weeks later to acquire adjacent seismic lines no closer than 10 km away. The 05CM seismic survey is thus expected to have a significantly lower impact on local fishing activities than previous seismic surveys.
The source employed in the 05CM seismic survey will be an array of compressed-air "sleeve air-guns". This involves the expulsion of air from the piston-like air-guns, creating a low-frequency sound pulse. Operation of the air-guns will be strictly monitored to reduce any impact on marine mammals. The 05CM survey will adopt the seismic operation guidelines that have been recently developed by the Department of Conservation. These guidelines include the stopping of air-guns if whales get within 1 km of the guns, or 1.5 km if a cow/calf pair is sighted.
The starting of air-guns commences with a "soft-start", where a gradual increase in the number of air-guns is fired over a 30 minute period prior to commencement of data acquisition. The soft-start serves to send out a series of warning pulses to deter marine mammals and give them adequate time to leave the vicinity. To put the soft-start into context, the 20 cubic inch air-gun that is fired for the first three minutes is smaller in volume that a standard soft-drink can.
The soft-start will be preceded by a 30 minute period of observation by a dedicated cetacean observer, to ensure that no marine mammals are in the immediate vicinity of the air-guns. Visual observations will be maintained continuously during soft starts to confirm the absence of marine mammals within the safety radius. If marine mammals are sighted within the 1.5 km safety radius during soft-start procedure, the seismic source will be shut down. Re-commencement of soft-start procedures will take place after 30 minutes has lapsed since the last marine mammal sighting within the observation zone.
The vessel will be towing a single acoustic streamer, of up to 12 km length. The polyurethane-covered streamer is 50 mm in diameter, and will be towed at a depth of 6 meters. Computer-controlled depth controllers, approximately 1 meter in width, will be spaced at 300 meter intervals along the streamer. The tail of the streamer, 12 km behind the vessel, will be marked by a 5-meter surface tail-buoy. The vessel, seismic source, streamer and tail-buoy will be moving at approximately 5 knots, which means that the tail-buoy will pass approximately 90 minutes behind the vessel.
Because of the large distance between the vessel and the tail-buoy, and the potential for snagging on the acoustic streamer and depth-controllers, the 05CM seismic survey will also involve the use of a Napier-based chase vessel. The chase vessel will constantly monitor activity between the seismic vessel and the tail-buoy whenever acquisition is occurring in relatively near-shore areas.