Overcoming Arctic Conditions using Dual-sensor Streamer Technology
Per Eivind Dhelie1, Robert Sorley2, Stephen Pharez4, Jerry Witney2, and Steven Cochran3
1MultiClient, PGS, Lysaker, Norway.
2MultiClient, PGS, Houston, TX.
3Data Processing, PGS, Houston, TX.
4Technical Marketing, PGS, London, United Kingdom.
Seismic towed streamer acquisition in Arctic waters has historically been very challenging. The open water season is short, the weather conditions are harsh and the overall imaging conditions have proved likewise just as difficult. This paper presents results from several acquisition and processing projects conducted over the last three seasons utilizing dual-sensor streamer technology. Results have so far been very encouraging. In terms of managing a short season, high efficiency of the acquisition is required. The dual-sensor technology allows the streamer to be towed very deep (15-25 m) without compromising the high frequency bandwidth of the data. The deep tow means that the sensors are less prone to sea-surface generated swell noise and therefore the operation can continue in weather otherwise too rough for conventional seismic acquisition. Historic efficiency comparisons to conventional acquisition projects reveal that the new technology increases the survey efficiency by more than 30%. Effectively, this has allowed every dual-sensor seismic program offshore Canada and Greenland to complete on time within a single season only.
In addition to increasing the survey efficiency, the dual-sensor technology has proven to give very high quality seismic data in a region notorious for lack of signal penetration. The increased towing depth allows the sensors to capture more of the low frequencies. The nature of the sea surface ghost’s act such that the deeper you tow the more low frequency information you capture. Compared to conventional streamers towed at 6-8 m the dual-sensor technology enhances the low frequency signal by approximately 10dB at 10Hz. After three seasons operating in ice infested waters offshore Greenland, Labrador and Sydney Basin the confidence of the acquisition platform is confirmed.
Preliminary data processing done on-board followed by onshore processing, proved to be equally challenging due to the very hard water bottom which sets up water column as well as peg leg, interbed and diffracted multiple trains. New demultiple techniques utilizing both the up-going and the down-going wavefields (P-up and V-down) available from the dual-sensor data, show great improvement over conventional seismic data. In addition, the volcanic basalt flows act as shields to the seismic waves, thereby limiting the reflected signal. The limited S/N-ratio in conventional acquisition is greatly improved by the deep tow of the dual-sensor streamer.
AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.�����������������������������������������������������������������������������������������������������������������