Many prospective passive ocean margins are covered by large areas of basalts. These basalts are often extremely heterogeneous and scatter the seismic energy of the conventional seismic reflection system so that it becomes difficult to obtain information on deeper reflectors. Since high frequencies are scattered more than low frequencies, we argue that the acquisition system for sub‐basalt targets should be modified to emphasize the low frequencies, using much larger airguns, and towing the source and receivers at about 20 m depth. In the summer of 2001 we obtained seismic reflection data over basalt in the northeast Atlantic using a system modified to enhance the low‐frequency energy. These new data show deep reflections that are not visible on lines shot in the same places with a conventional system.

Many prospective passive ocean margins are covered by large areas of basalts. These basalts are often extremely heterogeneous and scatter the seismic energy of the conventional seismic reflection system so that it becomes difficult to obtain information on deeper reflectors. Since high frequencies are scattered more than low frequencies, we argue that the acquisition system for sub‐basalt targets should be modified to emphasize the low frequencies, using much larger airguns, and towing the source and receivers at about 20 m depth. In the summer of 2001 we obtained seismic reflection data over basalt in the northeast Atlantic using a system modified to enhance the low‐frequency energy. These new data show deep reflections that are not visible on lines shot in the same places with a conventional system.

No abstract

Offshore electromagnetic methods have proven to be a useful addition to seismic exploration. But next to the need to be imbedded in a seismic analysis, the main restriction was its limited penetration depth. To change this limit and its restriction to certain geological settings, we set out to improve the existing system a hundred-fold. To achieve this goal the positioning of the multi-component seabed receivers and their noise floor was improved 10-fold. Additionally, the source system was completely redesigned to allow for amplitudes up to 10 000 amperes. The system is designed to record beyond 3 km water depth and can reach targets down to 4.5 km beneath the seabed. We have tested the new system together with the standard CSEM equipment in several field tests in the Norwegian Sea and the results shown here confirm the envisioned improvements, which open most areas worldwide for CSEM exploration.

The idea of imaging beneath a high‐velocity layer using converted waves has been popular since 1990. Because these wave types have their maximum amplitudes at mid‐ to far‐offsets, the search for pure P‐waves at the highly multiple‐contaminated near‐offsets can be avoided. For the Atlantic Margin, with buried thin‐layered basalts, our quantitative study shows that the initial single‐layered approach is not viable. Even in an unrealistic ideal geological setting, the amplitude of the symmetrical PSP‐mode is far too weak to be recognized on towed streamer data. Furthermore, in the far‐offset window, where locally converted waves have their strongest amplitudes, there is a multitude of other reflections, refractions and interbedded multiples, which have similar moveouts and, often, higher amplitudes. Without the removal of these events, a reliable image of the subsurface cannot be produced. We show that even if this problem were solved, it would be far easier to use the P‐wave reflection from beneath the basalt at near‐offsets. Our study shows that this wave type is by far the strongest response. A borehole‐derived model using a thin‐layered basalt sequence reveals that the strongest locally converted wave has an asymmetrical path and is 10 times weaker. All our results indicate that the pure P‐modes provide the best chance of imaging sub‐basalt sedimentary interfaces.