Integrated navigation and control system

21st February 2013

Paul Newman provides an overview of Gator, the navigation and control software for ocean bottom cable surveys.

Gator is a program developed by Concept Systems of Edinburgh - now part of the ION Geophysical (ION) group, that was designed for the navigation, positioning and operational management of ocean bottom cable (OBC) seismic surveys but is also used for 4D/time lapse surveys and for resistivity surveys using towed electromagnetic transmitters and seabed logging units.

Gator interfaces with the acoustic systems used for the positioning of OBC cables, remotely operated vehicles (ROV), seabed logging units and towed equipment. ION is in the process of developing a 'next generation' Gator that will utilise elements of its Orca Instrument room command and control software to maximise operational efficiency.

Gator runs on computers running Red Hat Linux and is modular in nature, allowing for efficient updating and program management. The typical user requires a basic familiarity and working knowledge of Linux, whereas the configuration of a new installation is necessarily more demanding and often requires a Concept Systems engineer to be present.

Each Gator computer must be connected to an ION interface unit (the latest version is known as a Power Real Time Navigation Unit or PRTNU) that houses a GPS card (used for timing), and both serial and Ethernet connections for external devices such as positioning GPS, heading and motion sensors, echo sounders, acoustic positioning systems, radio modems, air-gun triggers and recording systems as well as output to vessel dynamic positioning (DP) systems. Survey planning, software configuration and operational simulations can be performed before the equipment and crews are sent to the field.

OBC differs from conventional towed marine seismic acquisition in that the hydrophones and/or accelerometers are placed (in a 'cable') on the seabed rather than towed (in a 'streamer') at the sea surface. This reduces the overall speed of the survey but allows for operations to take place in waters too shallow or otherwise restricted such as close to shore or around offshore platforms, and also has benefits when conducting repeat time-lapse (4D and 4C) surveys as the cables can be left in situ or placed on the same locations.

Operations consist of a cable deployment and positioning phase, a shooting/acquisition phase, and then the recovery and re-deployment of cables to new positions. These phases normally involve multiple vessels and crews that are physically separated but that must pass and share timing, navigation and positioning data, and it is this functionality that Gator is able to provide. The GPS cards in the interface units on each vessel provide high-accuracy timing across the fleet and in particular on for the timing of airgun shot and recording system operation. The data management and telemetry system allows transfer of vessel and cable positioning data to be transferred in the background and across multiple redundant links to suit the limitations of the survey (frequencies available, the working range of radios etc), with survey specific tailoring of which items are given priority. The data link runs from a dedicated Gator program module and uses both 900 MHz and 2.4 GHz radio systems.

In the majority of cases, OBC operations are conducted using multiple vessels, each with specific tasks. Those involved in the deployment and recovery/redeployment of cables are known as cable boats, the vessel that hosts the seismic airgun is known as the shooting boat, and the vessel that collects and stores the seismic data from the OBC cables is known as the recording boat. OBC surveys are normally controlled from the recording or 'master" vessel, and the surveyor can send commands to all other 'slave' vessels or individual units as required. It is typical for the surveyor to lock-out higher level functions on slave vessels once the configuration has been set, but the Gator system on each slave can always be remotely configured by the master system as required. Centralising the configuration reduces the chance of costly errors, especially with respect to fundamental parameters such as survey geodesy and line planning.

Vessel equipment

Each vessel in the survey will be equipped with a GPS receiver and radio link so that the positions of the boats can be updated in real time, with each vessel being uniquely identified within the Gator software. Cable boats vary widely in size dependant on the situation, and Gator can run on a ruggedised laptop if required with a miniature, marinised version of the PRTNU that connects to the boats GPS, heading sensors and radio modem, as well as providing remote displays for helmsman and the deck crew.

Installations on shooting and recording vessels will normally use standard 19-inch racks that host the Gator computers and one or more PRTNU as well as radio modems.

During the deployment and positioning phase, the deck crew on the cable boats will attach acoustic positioning beacons (typically the OBC 12 units from Sonardyne) at intervals along each cable, and will physically deploy the cable on receipt of a command from the Gator system onboard as the helmsman moves the boat along the desired survey line. Automated tracking of individual beacons is achieved by using RFID tags that are scanned as the cable passes through a reader on the back of the vessel and logs the GPS position at the actual time of deployment.

Cable positioning

Once on the seabed, the position of the cable is then determined by acoustic systems on the cable or gun boats, and the cables may then be physically dragged into the optimal position using the cable vessel pulling on the buoy lines attached to the end of the cable, or an ROV moving the cable directly on the seabed. If nodal logging units are used instead of cables, there a variety of deployment methods used including free-fall through the water column, lowering from a deck crane, or direct placement by ROV. The final positions of the cables or seabed units are obtained via acoustic range only, or range and direction systems. Gator then applies the obtained distances and directions to the relevant vessel position and generates real world coordinates and depths for the cable based on the model built from multiple observations. The surveyor can edit individual observations and co-ordinates as required.

VectorSeis Ocean

In conventional OBC, the cables will then be physically connected to the recording boat, but an alternative has been developed that utilises 'smart' buoyed recording units for each cable that are co-ordinated and controlled using Gator and radio links. This method allows one vessel to perform all the operations required, resulting in numerous opportunities for cost savings.

The highly automated buoyed OBC system developed by ION and Reservoir Exploration Technology (RXT) of Norway is known as VectorSeis Ocean (VSO). RXT have a number of vessels operating VSO, either on their own or with other vessels as required. Each buoy contains a computer running Gator, hard drives for the recording of seismic data, and a robust data telemetry system.

Shooting phase

Gator is used during the shooting phase of the operation to trigger and adjust the timings of the shots with regard to the survey layout and the gun boats predicted position, and then generate binning statistics at the end of each shot line. The recording systems (on the recording boat or VSL buoys) are also actuated by triggers sent from Gator. Key features of Gator include:

- Allows for centralised management and operational co-ordination, even across a multi-vessel fleet.

- Timing accuracy between vessels of better than 50µs.

- Highly flexible interfaces that can connect with a wide variety of user-provided equipment including acoustic positioning and vessel dynamic positioning systems.

- Can be developed to suit clients demands and to support new methodologies and techniques.

- Data acquired on slave vessels (such as cable positions) automatically transferred to master vessel, but with user-variable priorities on specific data items.

- Allows for multiple, highly configurable displays to assist helmsman, deck crew and client observers.

- Creates a centralised database of all cable positions.

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