24th June 2020

BASS Marine Vibrator – A Game Change Towards an Environmentally Friendly and Superior Productivity Seismic Source

By Sara Amar (Shearwater) and Håkon Aune (Shearwater)

Shearwater, in collaboration with Equinor and the Research Council of Norway, is developing BASS (Broadband Acoustic Seismic Source), a new marine vibrator system that will reduce the source environmental footprint, bring superior acquisition efficiency, and enable novel geophysical processing techniques. Ever more restrictions will be put onto seismic operations worldwide and a reliable and environmental-friendly source is key for O&G companies to maintain their license to operate.

Figure 1: BASS can probe the earth in both vertical and transverse directions

Our oceans have never been quiet, with a wide variety of sounds ranging from natural sounds such as wave motions and marine mammals communicating, to man-made sounds of ship movement, military activities, construction, piling for wind farms, and in the last few decades seismic sounds for oil and gas exploration. Strict guidelines and regulations have been implemented by many regulatory agencies and the geophysical industry responded in order to mitigate the impact of seismic exploration on marine mammals, fish, and other sea life.

Marine vibrators have the environmental advantage of sweeping continuously rather than emitting an airgun impulse. For large marine mammals, the injury threshold is significantly higher for continuous sounds than it is for impulsive sounds. It is this reduced animal sensitivity, rather than the vibrator’s lower peak pressure, that gives the vibrator its environmental advantage over airguns for marine mammals. For fish, the situation is different, and it is the peak pressure that matters. Vibrators are more fish-friendly than airguns because of their lower peak pressure. Vibrators win in both cases.

Marine vibrators open up for geophysical advantages as well. The output acoustic waveform (amplitude and phase) can be independently specified at each frequency, something conventional airgun sources are unable to do. This provides an unprecedented level of flexibility when designing the source for a given survey area and geological setting.

A conventional vibrator sweep emits its energy in an omnidirectional fashion. As an alternative, a pair of vibrators can be run in anti-phase to produce a dipole source and to emit a horizontal transverse gradient sweep. This can be done on a sweep by sweep basis, such that a vibrator pair emits alternating omnidirectional and transverse gradient sweeps.

For Ocean Bottom Node (OBN) surveys, it has been demonstrated that such a sweep pattern can increase the source efficiency and consequently the survey efficiency as it allows gradient based interpolation of source points. The sail-line move-up can be increased, so that the source effort is reduced. Such an acquisition pattern can reduce the typical source bottleneck experienced in OBN surveys.

The total cost of an OBN survey would be significantly reduced. It also has environmental benefits, due to faster acquisition and less overall time spent to conduct an OBN survey. Finally, it facilitates a better cooperation and time sharing between the seismic industry and other marine industries such as the fishing industry.

Figure 2: Example acquisition scenario for a nodal survey using synthetic data

Precise control of the phase offers more advantages over conventional sources.  Elaborate phase sequencing can be used to attenuate residual source noise (RSN).  This allows vibrators to sweep continuously without any quiet listening time, enabling more data to be collected per sail line.

For all pulsating acoustic sources, the fundamentals of physics lead to an inherent reduction of output at low frequencies. The radiated acoustic amplitude is proportional to the second time-derivative of the water volume displacement and is proportional to the square of the frequency. There is therefore a noticeable roll-off at low frequency.

As the name BASS alludes to, Shearwater’s actuator technology generates low frequencies with very high amplitude, despite this roll-off. In fact, BASS provides the highest sound pressure levels known today for marine vibrators. An array of BASS vibrators provides more low-frequency energy than a comparable airgun source array. The quest for ever more low-frequency content is likely to welcome a marine vibrator that outperforms conventional sources.

Marine vibrators have the potential to be a game changer. Previous attempts to introduce vibrators have not been successful. We believe that this third wave of vibrator technology will finally break into the market and change the game forever.

Stay tuned for the next article in the Trilogy series on Marine Vibrators.