Geosciences

• Western Geophysical's new Commando OBC 4D survey vessel.
  
• PGS's Ramform Valiant under construction.

New seismic vessels

Now, newbuild vessels are beginning to take on major acquisitions around the world. Led by the advent of PGS's state-of-the-art Ramform Explorer and last year's even further improved Ramform Challenger, PGS is soon to launch two more Ramform vessels, the Ramform Valiant and the Ramform Viking.

Western Geophysical, on the other hand, has just launched the industry's first purpose-built ocean-bottom cable survey vessels, the Commando and the Crusader. And Veritas Digicon Geophysical has a yet-to-be-named 3D seismic vessel due to enter service in early 1998.

The Ramform Valiant and Ramform Viking are very similar to their predecessors, except that the hull volume is higher, allowing for additional fuel capacity, which increases their operational time at sea to130-140 days. Their power has been increased by 3,000 hp to a total of 20,000 hp. Both vessels are being equipped with 20 full streamer winches. They're being built in Norway.

Western's Commando and Crusader are advanced acquisition vessels specifically designed for time-lapse 4D and four component surveys. Dynamically positioned and incorporating proprietary automated cable deployment technology and vessel steerage/cable retrieval software, the vessels feature large back decks for rapid deployment of energy source, cable, and recording equipment - all from one vessel. Designed for small-to-medium-sized targeted time lapse 4D, four-component surveys, the Commando is currently operating in the North Sea and the Crusader has just been launched.

Veritas DGC's unnamed vessel, to be owned and operated by Eidesvik Survey, is designed to deliver more propulsion power than any seismic vessel currently afloat, to allow it to tow a wider spread of streamers, up to 12 of the 24-bit reduced diameter array streamers, although eight will be used in its initial operations. At 305 ft long and 72 ft wide, it will be the largest of Veritas DGC's fleet. It is scheduled to begin operation with a 3D on the Atlantic Margin to add to Digicon's data library.

EAGE highlights

The 59th EAGE Conference, held this year in Geneva, brought together geoscientists and engineers from across Europe and the world to bridge their various disciplines in an interchange of ideas and technology. Dr. Fanz X. F?hrer, past president and member of the board of directors, stated to Offshore that this year's convocation was a remarkable witness to the cooperation between the EAGE and the Society of Exploration Geophysicists and American Association of Petroleum Geologists.

Numerous papers were presented on current technological issues, including streamers vs stationary receivers, stripping, application of various configurations of streamers, crossline offset variations, vertical cable, Digiseis's dual hydrophones, and OBC.

One of the more interesting papers attended was "Non-Conventional Marine Acquisition Techniques - Theoretical Aspects and Applications," presented by Giovanni La Bella of Agip. It proposed three variations on conventional shooting: cross-shooting, zig-zag shooting, and skipped acquisition.

In the first, a conventional towed streamer acquisition covers twice the survey area with an enlarged cross-line bin size, with the two passes at 90 degree angles to each other. As a result, the data are collected by the tight bin dimension either in the in-line or cross-line direction, without an increase in the fold or acquisition costs. Number of cables and sources deployed do not affect the method. The advantage is capability of dramatically increasing the full fold of coverage of the near offset and achievement of a better homogenization of the information contained in each single bin. Furthermore, it allows acquisition of a bimodal distribution of azimuth families.

In the second technique, zig-zag, two vessels are employed, with one acting as shooting and recording unit and the other as the source unit. The first vessel acquires in a conventional way by parallel sailing, while the second vessel continuously reverses at 45-degree angles. As a consequence, the cross line bin size is no more determined by the source/near cable distance but is a function of the shooting rate. The benefit is the obtaining of a pseudo multiazimuthal volume of data that provides the most effective method in multiples attenuation. (Agip has used this method successfully to image subsalt targets with low seismic response and masking multiples.)

The last technique, skipped acquisition, is achieved by repositioning sources and cables in terms of cross line spacing to acquire a suite of CDP rows with a few theoretically completely empty (skipped). Combinations between covered and empty CDPs are obtainable and can be tailored to each survey's requirements. Thus, it is possible to acquire data with tighter parameters (bin size) and take advantage of them after an aimed rebinning. This provides a significant cost reduction over conventional surveys and achieves the same acquisition parameters and improved data quality. There were other significant presentations.