GEOPHYSICAL OPERATIONS IN THE NORTH SEA

Abstract

During the last three years the discovery of the world’s second largest natural gas field at Groningen in the Netherlands has touched off in the North Sea one of the greatest competitive offshore geophysical operations in history. Before 1962, only minor amounts of geophysical work had been done there. Severe weather conditions were expected, but seismic operations have not been as much affected by weather as was originally anticipated. However, strong currents, making cable location uncertain, hampered reflection stacking and refraction operations. Location by radio was also a serious problem due to lack of sufficient available frequencies for a lane identification system. Seismic work was carried out safely in areas mapped as mine fields. Few problems with the fishing industry have so far been encountered. As a preliminary, the geophysics of the Groningen area are discussed. A gravity compilation of the North Sea shows that there are three major basins—the Northwest German Zechstein Basin, the British North Sea Basin, and the Norwegian North Sea Basin. The British Basin which contains Tertiary, Cretaceous, Jurassic, Triassic, Permian, Carboniferous, and older sediments shows considerable salt movement with salt domes, walls, and pillows being in evidence to within 30 miles of the eastern coast of England. Some examples of seismic record sections show the quality of data and the kind of structures encountered. Water reverberations were satisfactorily reduced by means of anti‐ringing procedures. Multiple reflections were frequently observed and often interfered with or obscured simple reflections. Refraction studies indicate that two main refractors, the Upper Cretaceous Chalk and the Upper Magnesian Limestone of the Permian, are present over most of the British Basin. Mapping the key basal Permian reflector is made difficult by deterioration of the reflection under areas of salt growth. Stacking sometimes enhances this reflection. Also intrusions of Permian salt into the Mesozoic beds give rise to large and rapid changes in thickness of the overlying low‐velocity Tertiary and high‐velocity Cretaceous chalk sections. A correction system for these large lateral velocity changes is described.