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Feature Article

3D High-Resolution Geophysical Surveys
Towed System Designed and Developed to Fill in Gaps In 2D Data for Surveying Offshore Structures

Feature Author
Peter Sack
Managing Partner
Feature Author
Tor Haugland
Principal Partner
Sound Oceanics LLC
Houston, Texas

HR3D navigation network.
High-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 shallow 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 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. Therefore, 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 connectors, 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 operators on specialty equipment.

The system was designed with concentrations on the physical (towing), navigation, geophysical and commercial components, all of which were tested individually in field trials.

Towing Configuration
The exploration seismic industry is facilitated by a vendor market rich with acquisition tools for the design of a cost-effective HR3D system. Further, the physics of towing an HR3D equipment spread offer some advantages of simplification, the most significant being the reduction of in-sea equipment, thus eliminating many of the connections between this equipment and the vessel.

The technology of towing a 3D equipment spread has been used for more than 20 years by the exploration seismic survey industry. Basically, a number of streamers are spaced perpendicular to the towing direction by fixed tension members. During towing, these are spread by deflectors also towed from the vessel. Conventional seismic surveys utilize individual lead-in cables to connect streamers to the vessel and the recording instrument. These cables are managed on board the towing vessel by individual winches and towing points.

Connection of multiple streamers to individual lead-in cables has been explored, but the use of this method is uncommon due to the large forces exerted on the lead-in cables and the significant expense of the specialized equipment to break out individual streamers, which can be up to 12 kilometers long. Further, a break of a multistreamer lead-in could be catastrophic to the operation; a single streamer sinking to crush depth would have a snowball effect on the adjacent streamers.

In the case of HR3D, the streamer towing burden is much lower, and recovering the streamers, even in the event of a tension member or lead-in cable break, is manageable. The streamer length is less than the equipment crush depth. Thus, sinking and subsequent damage to the spread is highly unlikely. Therefore, a multistreamer lead-in was developed for the HR3D system as the electromechanical connection of the spread to the vessel. This component was designed to separate and isolate the electrical and mechanical connections between streamers. While this requires multiple connections to the component, these connections are made by common off-the-shelf connectors, which allows for relatively simple adjustment to the streamer spacing.

The most direct benefit of multistreamer lead-ins is reduction of drag. This reduces the size of the tension members connecting the spread together, as well as the spread to the towing vessel. Modeling of the in-sea drag initially suggested 10 to 15 percent less drag versus individual connections to the vessel. During field trials in March 2011 offshore Âlesund, Norway, this proved to be conservative, with actual reduction greater than 20 percent. This drag reduction potentially lowers acquisition cost. The towing load brings the towing vessel specification out of the realm of specialty seismic vessels or large tugs to supply vessels, standby vessels and smaller oceanographic vessels. These vessels may already be employed by the client in the area of interest, eliminating the need to hire and outfit an additional vessel.

Apart from the operational and cost advantages, drag reduction from the multistreamer lead-in allows a shorter offset (X/Y distance between the sound source and hydrophone), which enables geophysical methods such as zero or near-zero offset that add value to exploration of specific wells or reservoirs. To continue this article please click here.

Peter Sack is managing partner of Sound Oceanics LLC. He has more than 20 years of experience in the marine engineering and geophysical fields. He holds a degree in ocean engineering from Florida Institute of Technology.

Tor Haugland is co-founder of Sound Oceanics LLC. He has more than 40 years of experience in the geophysical and naval industries. He has authored numerous papers on marine seismic acquisition.

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