It's no secret that offshore oil and gas exploration and production processes are expensive, time-consuming, and often carried out in the most demanding working environments imaginable. The technical challenges can be just as demanding as the natural ones. Take, for example, seismic surveying, where absolute precision and reliability of data are essential. Meeting those standards in complex operations can typically cost more than $100,000 a day.

Modern three-dimensional seismic imaging techniques are essential for accurately characterizing reservoirs, as well as for achieving good recovery efficiencies. Operational methods have improved dramatically over the past 20 or so years due mainly to the increased number of multiple cables, or seismic streamers, vessels can now tow.

The first 3D datasets were derived from closely spaced 2D lines acquired with a single streamer. In the transition from 2D to 3D, we should note, there have been ten-fold increases in volumes of seismic data, while corresponding volumes of supporting positional data have increased some thirty-fold.

Nowadays, newer seismic vessels can tow as many as 20 streamers, each between 3km and 5km in length and around 25m apart. Coupled with advanced lateral deflector systems, this increased capability can effectively provide a "carpet" of seismic sensors covering an area of up to 10-square-km for every shot point of sail lines.

These capabilities depend on extremely accurate positioning methods, if only to ensure that a given exploration area has been fully covered. At the same time, efficient acquisition of high-resolution seismic data requires very precise coordination of source or air gun position, as well as hydrophone groups along streamers. To achieve that precision, a number of complementary positioning methods are usually adopted, involving, to name a few examples, high-frequency acoustics, laser or fluxgate compasses for hydrophones, and, for the most critical of all seismic survey processes, differential GPS.

It's not surprising, then, that positioning for 3D survey operations often accounts for a big proportion of total survey costs. The types of positioning data can be many and complex, straining resource requirements for both real-time and post-processing data functions.

The Right Stuff
The real effectiveness of any seismic survey, then, ultimately depends on the most accurate possible positioning data that differential GPS can provide. With that in mind, let's consider the main criteria for selecting the appropriate equipment.

First, of course, it should be versatile enough to accommodate fast processing and relay of data along with high levels of availability. Second, multi-channel GPS receivers are essential, providing as they do enhanced tracking capabilities, faster acquisition and re-acquisition times, superior performance during high dynamic maneuvers, and an update rate of better than one second per cycle.

Finally, good, adaptable data links are necessary for reference stations, relay transmitters and mobile receivers to communicate calculations at each location. Those links help to build a reservoir of data that can be crosschecked for precision at each point. Since these data links are crucial for acquisition and transfer of reliable GPS data, they should also allow for optimum system integration as well as hardware and software compatibility.

For seismic survey vessels, determining the location of hydrocarbon resources over typical depth ranges down to 16,400 feet (5,000m) generally requires a continuous differential GPS positional accuracy of 5m or better.

Other techniques, like Relative GPS (RGPS), are used to give the position of tail buoys relative to the absolute position of the vessel, preferably to even better accuracies of 1m and involving relay of data via high-speed, bi-directional UHF data links. The RGPS method is commonly used for soil mapping.

To meet these and other offshore positioning needs, Thales Navigation has developed a new range of its established Aquarius series of DGPS multi-channel systems. They provide centimetric accuracies in real time over maximum ranges of between 12 and 40km, depending on operating mode. With fully configurable radio links offering a choice of UHF, HF-MF frequency bands for reliability of reception, they effectively provide carrier phase-based differential correction services using either RTK or LRK modes.

Kinematic Applications in Real Time
RTK, or real-time kinematic, is a process in which GPS signal corrections are transmitted in real time from a reference receiver at a known location to one or more mobile units. Corrections automatically compensate for any atmospheric delay, orbital errors and other variables of GPS geometry in order to produce centimeter-level accuracies.

Thales Navigation, however, has further refined this process with the development of Kinematic Applications in Real Time (KART) for single-frequency receivers.

Unlike other ways of resolving positional or statistical ambiguities which might require multiple software resources, KART is based on a single algorithm for validating solutions. It enables real-time kinematic operation with single-frequency receivers in applications otherwise impossible without dual-frequency ones.

Like the KART process, LRK extends the normal operating capability of a dual-frequency receiver from about 10km to upwards of 40km by effectively reducing initialization times to just a few seconds at all stages of operation - even when there are a reduced number of visible satellites.

The Aquarius2 features an additional precise heading function supported by very fast processing and even greater operational flexibility. With the addition of a second set of GPS/GNSS channels, for example, it provides up to 56 independent, parallel single or dual-frequency channels. Use of a dual-frequency core module ensures very fast initialization and allows for a long antenna baseline to reach a high accuracy for heading (typically 0.01 degree for a 20m baseline). As such, the technology provides gyro-type accuracy with almost instant start-up. There is, for instance, no waiting for settling time as with a gyro, and little or no servicing costs, either.

Proven Offshore
The new Aquarius series already has a good track record for seismic surveying operations, with systems having been used extensively by Compagnie GŽnŽrale de GŽophysique (CGG) in support of its worldwide fleet of multi-streamer vessels. Recent operations in offshore areas of North Africa, for example, involved the Aquarius2 LRK-type receivers supported by land-based UHF reference station correction facilities with receiver interfaces, enabling relay of information direct to CGG's own data acquisition and processing system.

Similarly, some 18 dual-frequency Aquarius2 UHF/HF systems linked to base stations on platforms have recently been used for a series of ocean bottom cable (OBC) surveys by CGG in Indonesia. OBC surveys, generally conducted in relatively shallow waters down to about 656 feet (200m), involve the laying of seismic cables on seabeds rather than towing near the surface. Advantages of the method are improved flexibility of acquisition geometry, greater surface consistency involving more combinations of source and detector at different azimuths for a given midpoint, and better opportunities for working in and around obstructed areas as well as environmentally sensitive coastlines.

Eleven support vessels were used for the Indonesian operations - an air gun supply boat, three survey launches for acquisition of data, and seven cableships - all equipped with Aquarius units similarly interfaced to CGG's own data acquisition and processing facilities. In addition to acquiring real-time positional data, operators of cableships have also used the Aquarius2 integral TRM 100 navigation and control terminal for steering along pre-programmed routes.

Additional Aquarius units have also been used as DGPS and RTK base stations when others were operated as signal integrity monitors. Apart from providing automatic profile and homing modes, the TRM 100 terminal's other main navigation features include a series of internal quality control functions governing precision of GPS and radio signals. The terminal effectively extends the overall versatility of the Aquarius concept of positioning, versatility which not only provides three types of adaptable data links to ensure absolute reliability and integrity of information, but also comprehensively reduces the possibility of errors in position of underwater equipment that would otherwise degrade seismic images.