3D High-Resolution Geophysical Surveys
Towed System Designed and Developed to Fill in Gaps In 2D Data for Surveying Offshore Structures
By Peter Sack Managing Partner and Tor Haugland Principal Partner Sound Oceanics LLC Houston, Texas
igh-resolution geophysical surveys are used to identify potential marine subsurface hazards early and reduce risk from the placement of fixed and anchored installations on the seabed. In the geophysical industry, they are used to identify potentially harmful anomalies, such as shallow gas pockets. These features are either too small or too shal- low to be effectively imaged by exploration seismic surveys. While their resolution is sufficient, conventional 2D high- resolution surveys can potentially miss small features due to the data missing between acquisition grid lines. Recently, high-resolution surveys have been proposed as a method to monitor the overburden of producing oil and gas reservoirs. Offshore developments, such as wind farms, have presented other applications for this technique.
3D Design Considerations While the value of the 3D method is understood, high- resolution 3D surveys (HR3D) have been somewhat rare for practical and economic reasons. Acquiring a 3D data set involves complex towing equipment and vessel rigging, not
(Top) HR3D navigation network. (Bottom) High-resolution streamer spread.
to mention a vessel capable of towing a large spread of in- sea equipment. The cost of this equipment is proportional to that used for a 2D survey but at a much larger scale. There- fore, the primary objective was to create a 3D acquisition system that meets the spatial-accuracy requirements of high resolution while maintaining a practical commercial model. Further objectives were to implement modern survey tools into the equipment package and provide a design that could be used repeatedly. The approach to these objectives was to utilize off-the- shelf components, including mechanical and electric con- nectors, ropes, positioning aids and floatation, and adapt them to the design. The methods and techniques for these commercial items have been developed operationally and with respect to safety, which reduces the need to train opera- tors on specialty equipment.
www.sea-technology.com OCTOBER 2012 / st 23