By acquiring and analysing seismic data, it is possible to build up an image of the geological structures under the seabed. Once structures that may contain petroleum deposits have been identified, exploration wells can be drilled.
The purpose of seismic surveys is to build up an image of the geological structures under the seabed. Sound pulses are discharged from a vessel on the sea surface and travel down into the rock formations below the seabed. The sound waves are reflected back to sensors that are either placed on the seabed or in seismic “streamers” towed behind the vessel.
Recording equipment on board the vessel registers details of how the sound waves have been reflected. Sound waves move at different speeds in different types of rock, thus providing a picture of the extent, thickness and hardness of the various rock formations. This in turn indicates whether there may be oil or gas in the area.
Seismic data has to be processed before geologists can study and interpret the information on conditions under the seabed.
There are several categories of seismic surveys:
- In 2D surveys, the data are collected by towing a single seismic streamer behind the survey vessel. This provides a general picture of the geology below the seabed, but at low resolution.
- In 3D surveys, several parallel streamers are towed behind the vessel, giving a much more detailed picture of the subsea geology and more rapid data acquisition.
- 4D surveys consist of repeated 3D surveys of the same area in order to detect any changes in a reservoir over time as a result of production or injection. These surveys are usually conducted in fields in production.
The Norwegian Petroleum Directorate is responsible for resource mapping before areas are opened for petroleum activities. Resource mapping includes the acquisition and analysis of seismic data.
The Norwegian authorities have been collecting seismic data in areas of the continental shelf that are not open for oil and gas activities since 1969. This is done by the Petroleum Directorate, which for example has been collecting data in the Barents Sea North (north of 74o 30’ N) in recent years.
All seismic surveys in connection with petroleum activities on the Norwegian continental shelf must be authorised by a production licence or an exploration licence.
Production licences are awarded by the Ministry of Petroleum and Energy through the regular licensing rounds. A production licence applies to a specific area and grants exclusive rights to undertake oil and gas activities in the area.
Exploration licences are awarded by the Petroleum Directorate for areas of the Norwegian continental shelf that are open for petroleum activities but where no production licence has been awarded. They do not grant exclusive rights to activities in an area.
Marketable seismic data means data sets acquired by seismic companies that have been awarded exploration licences to collect data for sale to others. These surveys are generally carried out by companies that do not themselves apply for production licences.
The data are reported to the authorities in accordance with conditions laid down in the licence and the provisions of section 10-4 of the Petroleum Act.
An exploration licence does not grant an exclusive right to collect data. Nor does it grant an exclusive right to make seismic surveys in areas covered by a production licence, unless this is agreed beforehand with the licensees.
The data are managed in accordance with section 85 of the Petroleum Regulations on administrative procedures and the duty of secrecy/release of data.
To ensure that seismic surveys do not come into conflict with other interests, the authorities must be notified of all seismic activity no later than five weeks before it is due to start. Notification is submitted electronically via the Petroleum Directorate’s website. When the notification has been registered, copies are automatically sent to the Directorate of Fisheries, the Institute of Marine Research and the Norwegian Joint Headquarters.
These agencies can provide expert advice on for example spawning, fisheries activity, shipping and military activities in the area. This information is then transmitted to the licensee by the Petroleum Directorate.
The map below shows areas of the Norwegian continental shelf where seismic surveys are in progress at present. Information on planned, ongoing and completed seismic surveys is also published on the Petroleum Directorate’s website
The petroleum legislation requires any vessel engaged in seismic surveying to have a fisheries expert on board in order to ensure that fisheries interests are taken into account and avoid conflict between fisheries activities and seismic activities. The fisheries expert must have completed an approved course and be approved as a fisheries expert by the Petroleum Directorate.