Reflections from layers in the underground are recorded by sensors placed in seismic cables that are towed after the ship.
The methods for searching for oil are based on seismological principles. The Seismological Observatory has from 1960 played a central role both in connection with the investigation of the Norwegian continental shelf, and in educating candidates for Norwegian and foreign oil industries.
Hydrocarbons (oil and gas) are formed by the breakdown and transformation of organic materials under high pressure and temperatures in sedimentary rocks (oil bearing rocks). Oil and gas have then migrated from the oil bearing layer to a porous layer and have finally been captured in a trap consisting of an overlaying impenetrable sediment.
In seismic investigations, the sedimentary layers are mapped in search of traps where oil and gas can be found. In the early days, explosives were used in order to generate seismic waves. This, however, lead to large-scale protests from fishermen claiming that it was greatly harming the fish stocks. This resulted in an extensive search for alternative energy sources. In the late 1960s, the use of compressed air was introduced as a replacement, and today the so-called airguns are dominating.
Reflections from layers in the underground are recorded by sensors placed in seismic cables that are towed after the ship.
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Modern seismic ship. |
Air guns being fired |
An example of a seismic section going from the area west of Bergen and crossing Norskerennen.
The layers under the seabed are visible as lines in the seismic section.
It was early known that the southern parts of the North Sea were covered by sedimentary rocks. However, many geologists believed that these were confined to the south by a line between Shetland and southern Norway where old granite rocks were exposed. It therefore came as a surprise to many experts when the Seismological Observatory demonstrated, in seismic investigations, that massive sedimentary layers (several kilometers thick) existed along the entire continental shelf, including the Barents Sea and the area around Svalbard. As a result, candidates from the Seismological Observatory were highly in demand and central to the establishment of the Norwegian oil industry in the early 1970s.
This map shows oil fields on the Norwegian continental shelf.
The colored areas indicate fields where oil is being explored.
The first discovery of oil on the Norwegian shelf was at Ekofisk in the North Sea. This happened in 1969, and the production started in 1971. The Ekofisk oil field has been sinking for many years due to the oil extraction, and water is being pumped into the reservoir to compensate for this. Due to a damaged well, water was pumped into the layer above the oil reservoir, resulting in a manmade earthquake of magnitude 5 in May, 2000. NNSN recordings were essential in explaining what had happened.
The Ekofisk field.
