Latest Developments on Alternative Marine Seismic Sources

The April edition of The Leading Edge includes an update on our marine vibrator source development. Although there is less focus on R&D in the current climate, in the future this technology may offer an interesting alternative to traditional compressed air sources.

Read the full article ‘The music of marine seismic: A marine vibrator system based on folded surfaces’.

The promised benefits of marine vibrator sources include:

  • Efficiency | all-electric marine vibrator sources lose less energy to the environment through heat dissipation than conventional compressed air sources
  • Targeted range | marine vibrator sources offer better control of the amplitude and bandwidth of the emitted acoustic energy so signals can be limited to reduce the noise in the marine environment
  • Precise signatures | controlled signal output allows new and flexible source geometries
  • Efficiency | marine vibrators tailor frequency content, phase characteristics, and output level limiting acoustic energy generation to that used for imaging the subsurface
  • Quality | ultra-low-frequency (1–6 Hz) output for full-waveform inversion (FWI)

 

Sea Trial of Prototype Modules

PGS has developed two marine vibrator systems that operate at the low- (covering 1–10 Hz) and high-frequency (covering 10–125 Hz) ends of the seismic spectrum. The marine vibrator design overcomes the historical challenge of efficiency and reliability by exploiting a combination of a large surface area and small displacements as well as resonance tuning to produce sufficient output, especially at low frequencies, without shaking itself apart.

Prototype modules have undergone multiple sea trials at different operating depths and locations and at various power levels (see the image below). The low-frequency module unit was tested at 15 and 60 m depth, while the high-frequency module unit was tested at 7.5 and 15 m depth.

PGS_Marine_Vibrator_Sea_Test
The low-frequency module (A) and the high-frequency module (B) at the test site. The graph shows measured output levels of the two units at different depths.

 

Key Conclusions

With the aid of data examples, the authors demonstrate that the full-source system is stable and can employ active harmonic distortion suppression if required, and that the seismic data acquired using marine vibrators in either intermittent or continuous mode can be processed. This is the most efficient acquisition method as continuous emission mode removes the spatial sampling limitations that arise from intermittent acquisition.

The authors also demonstrate the environmental sound generation advantages of the marine vibrator in comparison to various airgun sources. The output of each, used in different acquisition modes, is modeled. In terms of the peak sound pressure level (pSPL), an environmental metric, the folded-surface marine vibrator (FSMV) has by far the lowest output, compared with airguns. These calculations, limited to the bandwidth used for seismic imaging (0–100 Hz), indicate that output signals are reduced without compromising on efficiency and image quality.