Although hydrocarbons had been found on land around the North Sea in the first half of the twentieth century, it was not until 1965 that gas was found and late 1969 saw the first oil discoveries. In the early 1970s, giant oilfields such as Forties, Brent and Piper were discovered and hydrocarbon production took off. Exploration and predicting where oil and gas could be expected became big business.
Seismic profiling became widely used as a tool in the search for oil and gas in the North Sea in the 1970s. Reflection seismology involved ships towing a source – usually an air gun producing an underwater explosion – and several kilometres of hydrophones to record seismic reflections. Exploration revealed a great thickness of sedimentary rocks in the North Sea, more than 4 kilometres thick in places. The sediments had accumulated in shallow water, close to sea level, and the distribution of older sediments especially was controlled by normal faults running the length of the North Sea.