Main Features Of Seismic Vessel

In previous articles of this three-part series, we learned about the methods used for seismic operations at sea and a general overview of people working on seismic vessels. In this article, we will learn about the main parts of a seismic vessel and the important tools used on these ships.

Seismic survey ships are sophisticated vessels built with features such as special living areas to accommodate both ship and seismic crew, helideck, instrument room, quiet engines, additional propulsion for non-DP vessels, huge winches or reels to store streamer cables, working deck to harbour gun strings, and additional steering mode. These vessels are generally slower and usually measure less than 100 meters long.

The seismic crew is usually engaged in the instrument room, back deck and compressor room.

Main features of Seismic vessels

Instrument Room: This is the place for recording data and monitoring operations. It has computers with big screens, which help in all major operations, from controlling the streamer depths to shooting. The main navigation system in this area is linked to satellites, radio systems, GPS, echo sounders, gyro repeaters, etc.

Back deck: This is where streamers and gunstrings are deployed. It accommodates huge winches and reels where the streamer cables are stowed, along with hydraulic mechanisms for deploying both cables and air guns. The cable and gun repair area is also located here.

Compressor room: Seismic vessels have a separate compressor room to monitor pressure feeding and compressor engines. The compressor room is usually located in the forward part of the engine room and has dedicated generators to power the compressors.

Bridge: It is generally located at the forward of the ship. It is sophisticated and housed with modern equipment. Some vessels are equipped with an additional steering system (Non-DP vessels) and autopilots, called auto-tracking, which is operated by a synchronized computer from the navigation room. It also has controls for additional propulsion, just like an aqua master, that helps in manoeuvring. A camera display unit is provided to monitor the activities on the back deck and waters close to the ship side.

Engine Room: Seismic survey vessels have an unusually large engine room with a lot of machinery for vessels of this size.

The engine room is mainly divided into three compartments, namely

1.) The Main Engine compartment also has auxiliary and harbour generators.

2.) Compressors compartment, which contains the main compressors supplying air to the air guns or seismic source; and

3.) Generators for the compressors and cat room. (This place also houses engines for additional propulsion, and as the engines are usually manufactured by Caterpillar, it is commonly called the cat room.)

Main Seismic Equipment

Some of the main seismic equipment used on such ships are:

Tail buoy: Tail buoy, as the name suggests, is a type of buoy fitted with GPS and a flashing light and attached to the far end of each streamer cable. It keeps the streamers afloat and prevents the cable from entangling. It gives position information and also illuminates during nighttime.

Streamer cable or streamer: It is an optical cable that contains hydrophones to pick up seismic signals, electronic modules to transmit the data, a stress membrane or Kevlar to add rigidity to the cable, and an electrical transmission system to supply power to the cable.

The streamers are divided into 100-meter sections or even fewer – this helps easily replace damaged items. The streamer cables are filled with fluids with a specific gravity of less than 1 to make the streamer buoyant. Nowadays, the fluids are replaced with foams, which are comparatively more rigid, have fewer chances for leaks, and are easy to maintain and get good acquisition.

Bird: They are depth control units attached to the streamer and placed at regular intervals (not more than 300 meters). They control the depth of the streamer cable and are fitted with motors and wings that adjust and maintain the depth of streamers accordingly. They are operated from the instrument room.

Acoustics: Like birds, acoustics are attached to the streamers, with built-in transmitters placed at equal distances. Acoustics emits signals picked up by transducers located on the ship’s side, giving the relative positions of the cables.

Air Gun: This equipment is also called a seismic source. Air guns comprise two chambers: the upper control chamber, the lower discharge or firing chamber, and a solenoid valve. High-pressure air at 2000 psi is fed into the control chamber from the compressor, which passes into the lower chamber.

The firing is then triggered by sending an electrical impulse to the solenoid valve, which allows air to discharge from the lower chamber through ports.

The solenoid valve’s opening and air filling in chambers are very rapid and take a few milliseconds to complete. This ensures continuous firing, and the time interval between each fringing depends on the vessel’s speed. The cycle as mentioned above is the ideal set-up for firing without losing the firing points.

Usually, two sets of gun strings (port and starboard) are suspended from the floating device. Each gun string has an array of six pairs of guns, with two guns for every loop of the string.

The guns are towed at a distance of 500 meters from the vessel, clear off the propeller, and the depth of guns is between 5 to 10 meters for vessels operating in areas where the depth is a maximum of 25 meters. (This can vary depending on the depth and mode of operation)

The above is not an exhaustive list but a set of important parts and equipment used on seismic vessels.

Do you have more information to be added to this article?

Let us know in the comments below.

You might also like to read:

• What are Seismic Vessels?
• Working on Seismic Vessels: A General Overview of Ship’s Crew

About Author

Dilipan Thomas is an aspiring seafarer, who has sailed on both cargo and offshore vessels. He has a good knowledge of the deck department, and is presently preparing and training himself to be a deck officer. He loves working in challenging environments and prefers writing about his sailing experiences during his free time.

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