Marine Seismic Investigations: A Perspective from France and Europe

Abstract

Abstract Research institutions in European countries conduct seismic experiments to investigate the geology of all the provinces of the ocean floor. Some objectives of deep reflection-refraction studies of continental margins are of mutual interest to academia and the oil industry. An inventory of existing facilities in Europe for carrying out these studies suggests that, chartering of commercial ships aside, the research potential could be enhanced by combining forces (MCS equipment, ocean floor seismographs, ships) and, in due course, by the operation of a dedicated or semi-dedicated seismic vessel for large experiments. Introduction In the early days of marine seismic exploration "oceanographers tried to get support from the oil companies in adapting geophysical methods for use at sea, but it was said that no one would ever want to look for oil underwater"1. Today, hydrocarbons are being produced in water depths down to 1.5 km, petroleum reconnaissance extends into water depths greater than 3 km and 3D multichannel seismic (MCS) reflection investigations using multiple streamers have become the norm. The new "triangular" exploration vessels, starting with the Ramform Explorer in 1996, about 40-m wide at the stern, tow two arrays of guns, shot alternatively, and up to twelve 6-km long streamers spaced every 25 m, with 16 streamers already envisaged. These ships currently conduct 3D surveys of areas of up to 10,000 km2. Oceanographic research institutions cannot compete with service companies in 3D MCS data acquisition but, for them, seismic exploration of the ocean floor covers a far broader range of geological objectives than those of the petroleum industry. Here, we refer to different kinds of seismic experiments of interest to both academia and industry in terms of depth of targets and resolution. We focus in particular on low frequency, deep penetration, investigations and consider the potential of European research institutions to conduct them. Background The first seismic experiments at sea were made on the New Jersey continental shelf in the late 1930's 2 and in the English Channel 3. Renewed interest in the ocean floor after World War II, coupled with the availability of surplus explosives and more sensitive hydrophones, gave an impetus to the extension of seismic exploration into deep water. Vertical incidence reflections were recorded during the 1947-1948 Swedish deep-sea expedition 4. Numerous reflection profiles were shot in the North Atlantic and deep-water refraction data collected there 5–8 and in the Pacific. By about 1960, the principal features of the seismic layering of the oceanic crust had been determined by refraction measurements 9,10 and the first order thickness and distribution of continental margin, midoceanic ridge and abyssal floor sediments had been established. Although dynamite provides high explosive energy, logistics were difficult and low firing repetition rates were a hindrance to the correlation of sub-sea floor echoes. A number of other sound sources for shallow penetration were built in the 1950's, including the electrical sparker, the boomer, gas exploders and low-frequency echo-sounders. For deep penetration, dynamite and other explosives continued to be used well into the 1960's.