Broadbanded Environmentally friendly Seismic Source

In the early days of marine seismic, the source impulse was generated by throwing dynamite off the back deck of the seismic recording vessels. Later it developed into today's airgun arrays. The airgun array generates an acoustic impulse by abruptly releasing pressurized air into the water. The impulse contains all frequencies, also frequencies outside the band of interest for seismic. Although having served the seismic business well for many years the airgun system have some fundamental shortcomings. The amount of energy at the very low frequencies is limited. More energy is needed in order to be able to look deeper into the structure and to improve the accuracy of the geological image. Further the source amplitude- and frequency content signal cannot be controlled. The latter aspect, in combination with the high sound pressure, puts environmental restrictions onto the use of airguns sources in many areas of the world. The main challenge with the project is to develop a technology able to displace the huge amount of water required for the low frequencies while maintaining mechanical robustness. In the first year of the project, 2015, we focused on developing the fundamental mathematical models behind a frequency- and amplitude controlled very broad banded non-impulsive seismic source. The source where to emit more energy than airguns at the very low frequencies addressing the need for better imaging and deeper penetration. Further it will operate in the time domain, so the sound pressure will be moderate and unlikely to be harmful to mammals. The source will be electrically controllable so that no energy will be emitted outside the frequency band of interest to seismic. In the second year, during the fall of 2016, two scaled prototypes were tested. The characteristics were examined by geophysicist at PGS. The last year of the project, in late 2017 a non-impulsive source concept was tested. It covered the full seismic band with a limited power output. The sub sources covering the higher frequency bands were tested for the first time in this project. They performed well but the technology is still not mature. The project group believe it is feasible to be able to offer the market a non-impulsive source, but more research and development effort is needed. This new type of controlled signal output offers new methods and requires further research in signal processing, device efficiency and reliability.

The primary objective of this project is to develop the fundamental technology behind a controllable very broad banded non-impulsive seismic source addressing current geophysical challenges with penetration of sub-basalt layers as well as environmental political challenges associated with the use of impulsive sources. In other words, develop a source used in marine seismic acquisition that can look deep into the geological structure, return signal with high degree of details, and that has a minimal impact in marine life. The project is addressing three of the main goals with PETROMAKS2: increased recovery, new discoveries, environmental friendly technology. Today's airgun sources emit its signal impulsively, where the impulse contains all frequencies. The proposed source applies a time signal with moderate sound pressure. Further the frequencies can be controlled so as to significantly reduce emitting signals at mammal sensitive frequencies. The fundamental challenge for the R&D project is to develop new technologies for the low frequencies needed for deep penetration. To obtain the required sound pressure level at the very low frequencies a substantial amount of water need to be displaced. Many attempts has been made the last 30 years to reach this target but failed due to robustness issues and poor response at low frequencies. Robustness, size/weight and cost will be the boundary conditions, and those factors need to be balanced throughout the R&D cycle. Further there are many octaves between the lowest and highest end of our targeted frequency range and it may well be that different technologies may be applied throughout the frequency band. Further, as the source will differ from current sources in that it will be non-impulsive and controllable, geophysical R&D is needed on source modeling, signal design and processing flow.