Improved Subsurface Resolution by Controlled Marine Seismic Stimulation

Marine vibrators are non-impulsive sources which can be made to generate energy by oscillating opposite surfaces of an enclosed cavity at specified frequencies in time. The surfaces may be made to oscillate synchronously by changing the enclosed volume for monopole-type of sources, and asynchronously by moving a fixed enclosed volume for dipole-type of sources. Given the complementary nature of the dipole- and monopole-type of sources with respect to the sea surface ghost reflections, we will combine the two sources to generate seismic waves that propagate downwards, free of sea surface ghost reflections. In addition, also the source arrays will be configured such that the source energy is focused at desired subsurface locations and frequency bands. Thus, eliminating the source ghost and better resolving the subsurface structures and the physical properties in areas that are otherwise difficult to image as a result of weak penetration/illumination, for example in Barents Sea. In the beginning of the project, we made use of the reciprocity theorem applied on two states to derive the pressure wavefield generated by the vibrator everywhere inside the domain from the acceleration of the vibrator plates the known wavefields of the reference state. From our derivation we were able to identify two particular cases of the generated wavefields related to the movement of the vibrator plates. One special case is generated by a monopole type of sources, caused by synchronous movement of the plates in opposite direction, and the second special case is generated by a dipole type of source, caused by synchronous movement in the same direction. Ideal monopole and dipole sources have complementary source ghost responses. This property has been demonstrated and analyzed for different source depth and source-receiver distances using our analytical modelling approach. In order to generate synthetic seismic data, the source wavefield needs to be forward propagated into the subsurface using the wave equation and calculated at desired receiver position. In the project we adapted our most general modelling tool, which is based on discretizing the wave equation, to be able to use the moving vibrator sources. The synthetic data showed correctly the impact of the Doppler shift. The synthetic seismic data was also used to adjust available processing parameters and tools and to validate new developments. We have performed a finite element modelling activity of a dipole source. Additionally we have performed concept studies concerning building a dipole source with adequate design parameters. We have successfully calibrated an array of monopoles however, improvements of the sources are necessary for a commercial system. After the calibration we continued the mechanical design research activity developing a finite element model of a critical component. This model facilitated the research studies of how to make a durable system. During the course of the project, we have developed the following methods and tools for marine vibrators: Method and prototype tool for modeling and correcting source and receiver motion in data Method and prototype tools for correcting array directivity Method and prototype tools for modeling source wavefield Method and prototype tools for modeling combined monopole and dipole sources. Five expanded abstracts have prepared and submitted to the annual conferences EAGE and SEG. As a result of the corona pandemic spending was cut on research activities. The scope of the project was consequently altered in 2020 and the project terminated early. In the original plan the intention was to make a geophysical field trial using a combination of mono and dipole arrays at the end of the project. Such geophysical study is expensive and it was decided to perform a lab experiment instead to reduce cost. However, the results from the lab experiment, where an in-air mono and dipole source was designed and operated was used to partly validate the tools and methods developed in earlier stages of the project. The business case for the product is still strong and the developed technology can be brought to the market when conditions are right.

The project has generated knowledge and taken the industry a step closer to a commercial non-impulsive source. Several papers, reports and patent applications has been submitted during the project. The design files and hardware has been stored in a systematic manner. The developed software tools are well documented.

Conventional seismic sources of airgun arrays are set up such that the energy released into the water has a high peak-to-bubble ratio. The released energy propagates as seismic waves outwards, omnidirectional from the ideal center of the source. A part of the energy is reflected by the sea surface causing spurious ghost reflections which causes frequency notches and reduces the subsurface resolution. Some part of the energy is reflected by the seafloor leading to high amount of energy propagating in the water column, which can be hazardous to marine life. The amount of trapped energy in the water layer increases for shallow water and hard sea floors, where seismic wavefields from point sources reach critical angles very fast leading to subsurface penetration problems. We propose a method of controlled seismic subsurface stimulation by downward propagating the source wavefield from tuned marine vibrator arrays of monopole and dipole-type of elements and by focusing the source energy at desired subsurface locations and frequency bands. Hhence, this will remove the source ghost and it will help to better resolve subsurface structures and physical properties in areas that are difficult to image as a result of weak penetration/illumination, for example Barents Sea. At the same time the controlled subsurface stimulation will reduce the environmental impact of marine seismic sources. This technology will be validated both by in-sea and in-air lab experiments. For the in-air lab experiments, the source size and frequency range will be adjusted accordingly. Moreover, we develop and validate key processing and imaging prototype tools needed to handle the new type of source and data generated by it. With increasingly strict environmental policies regarding seismic exploration and market downturn for oil service providers, the most important innovation challenges for the global operators is to develop cost-efficient, environmentally friendly and effective seismic exploration technologies. The proposed technology is capable of opening new market opportunities in mature licensed areas, as well as new and unexplored areas with hard or geologically complex structures, and may be a triggering factor for a huge potential value creation in the market of advanced marine seismic.