Marine Vibroseis: shaking up the industry
`
`Bill Pramik* says that after decades of attempts to develop marine Vibroseis, the technology
`has developed to the point that it is now possible to do projects in the field. Use of airguns
`during acquisition in shallow waters is becoming increasingly restricted due to fears over
`their impact on marine mammals. Marine Vibroseis is one technique that could overcome this
`problem.
`
`M ore and more, the oil industry is being asked to
`
`reduce the perceived impact we have on the environ-
`ments in which we work. This is especially true for
`the marine seismic acquisition industry. Recent regu-
`latory changes by the Bureau of Ocean Energy Management
`(BOEM) have expanded the definition of ‘takes’ and imposed
`even more strict requirements for marine mammal observers,
`passive acoustic monitoring and safe working distances. These
`changes have spurred, partially in preparation and partially in
`response, a number of industry and governmental workshops
`dealing with the issue of marine environmental impact and par-
`ticularly on the effects of anthropogenic sound in the oceans. A
`major topic of discussion in most of these workshops has been
`the feasibility and environmental implications of developing
`marine Vibroseis as an exploration technology. Along these
`lines, a joint industry programme on E&P sound and marine
`life was formed and commissioned an environmental assess-
`ment of marine Vibroseis.
`The outcome of these regulatory changes, the numerous
`workshops and the technological advances they inspire will
`have far-reaching implications for many of the areas around
`the world where we acquire, and wish to acquire, marine
`seismic data. Current regulations impact the effective explora-
`tion of areas where whale migrations occur, such as offshore
`California and Australia. Additionally, our ability to explore
`in the presence of dugongs in the Middle East, along the
`environmentally sensitive coastline of Brazil, the littoral oyster
`lakes of Mexico and similar areas, will also be affected by
`these activities.
`Marine Vibroseis is one technology that holds significant
`promise for minimizing environmental impact while maintain-
`ing the quality and efficiency of seismic exploration. One
`conclusion from the Joint Industry Programme Environmental
`Assessment of Marine Vibroseis was that their use, relative to
`airguns, is expected to reduce most types of environmental
`impacts in all environments where it is used.
`
`Marine seismic acquisition and the environment
`Offshore, where seismic companies acquire transition zone,
`ocean bottom and streamer seismic, the source of choice for
`more than 50 years has been the seismic airgun. Before that,
`
`* Geokinetics. E-mail: Bill.Pramik@geokinetics.com
`
`dynamite or other explosives were detonated in the water as
`the seismic energy source. When first introduced, the airgun
`was hailed as a safer and ‘more friendly’ seismic source and
`was quickly adopted by the industry. Now, 50 years later, air-
`guns are beginning to lose their status as an environmentally
`friendly source.
`The principal issue with respect to using airguns to
`acquire seismic data is the amount and nature of the acoustic
`energy being introduced into the marine environment.
`Marine animals may be affected by this acoustic energy. At
`high enough levels, acoustic energy can cause a temporary
`threshold shift (temporary reduction in the ability to hear
`sound), a permanent threshold shift (permanent reduction in
`the ability to hear sound), injury (damage to internal organs
`or structures of the animal) or, at extreme levels, death.
`Schlundt et al., reported on a study that was conducted to
`try and ascertain the levels of sound that would induce a TTS
`(temporary threshold shift) in beluga whales and bottle nose
`dolphins. A graph displaying the results of that study along
`with the acoustic sound levels from a typical 3000 cubic inch
`airgun array at various distances (Figure 1) shows that, even
`at 10 m, the sound levels from the airgun array are below the
`level required to induce a TTS, and outside of the frequency
`band indicated from the study.
`Nonetheless, it is prudent that we begin looking at
`alternate seismic energy sources for the marine environment
`and the current leading contender for that role is the marine
`vibrator. At present, there are several research programmes in
`the industry dedicated to the development of a viable marine
`vibrator. One is being co-ordinated by a joint industry project
`which has the goal of developing three competing designs
`within the next few years. This project relies heavily on the
`results of the previously mentioned Environmental Assessment
`of Marine Vibroseis as a precursor to the actual vibrator
`development. The Environmental Assessment addresses a
`wide variety of marine life for multiple potential impacts and
`attempts to relate the impact of marine Vibroseis to the impact
`of airguns. In all cases, for nearly all impacts, the conclusions
`of the study are that marine Vibroseis poses an impact no
`worse than airguns, and in many cases, impacts significantly
`less than those of airguns.
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`first break volume 31, November 2013
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`special topic
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`Figure 1 A graph showing the amplitude spectra
`of a typical 3000 cubic inch airgun array at dis-
`tances of 1 m, 10 m, 100 m and 1000 m and the
`acoustic energy levels and frequencies required to
`induce TTS (Temporary Threshold Shift) in Bottle
`Nose Dolphins and Beluga Whales. At distances
`greater than 10 m from the airgun array, the level
`of acoustic energy falls below those required to
`induce TTS.
`
`Marine Vibroseis vs. airguns
`When discussing marine Vibroseis, it is useful to establish
`a frame of reference to a known quantity. A typical 3000
`cubic inch airgun array can provide a starting point for com-
`parison. This comparison is not as straight forward as simply
`discussing data quality, though that is certainly an important
`aspect to be considered. There are two basic characteristics
`that must be evaluated when making this comparison; the
`total amount of sound energy transmitted into the water, and
`the portion of that energy which is considered ‘useful’ for
`seismic exploration. Further complicating this comparison is
`the fact that we are trying to compare an impulsive source to
`a ‘long duration’ source of acoustic energy, and these types
`of sounds are quantified differently.
`For seismic purposes, we are concerned with the acoustic
`energy generated between about 5 Hz and 120 Hz. This rep-
`resents the bandwidth of the energy that is typically used for
`seismic exploration and, accordingly, we use this bandwidth
`to compare and contrast the various seismic source options
`available to us. This measurement of the usable energy is
`identical for impulsive and vibratory seismic sources.
`Acoustic energy measurements used in environmental
`comparisons are very different from those used for seismic
`purposes. The entire acoustic bandwidth must be considered,
`including frequencies not useful to the seismic industry.
`Additionally, impulsive, intermittent and continuous sources
`of acoustic energy are quantified differently and thus far,
`have different regulatory considerations.
`
`Seismic purposes
`When an airgun array is fired, an acoustic impulse with sig-
`nificant sound pressure level (SPL) is generated. For analysis,
`
`a typical 3000 cubic inch airgun array was modelled using
`the Marine Source Modelling (v 1.6.0) function within
`the Nucleus+ (v 2.2.0) geophysical analysis package. The
`full-spectrum (0  Hz to 1000  Hz), peak-to-peak sound pres-
`sure level of the modelled output is about 136 bar-metres
`(Figure  2) which is not uncommon for this type of source.
`Examination of the amplitude spectra of the source signature
`shows that the peak amplitude is at about 70 Hz and has an
`amplitude of 211 dB (re 1 mPa at 1 m / Hz) (Figure 3). The
`amplitude spectrum also shows that, within the bandwidth
`of frequencies useful to seismic exploration, between about
`5 and 120 Hz, there is good acoustic energy generation. For
`comparison, an array of three marine vibrators, each having
`peak-to-peak amplitude of 2 bar-metres and sweeping for six
`seconds from 5 Hz to 120 Hz was modelled. For seismic pur-
`poses, this source outputs the same usable acoustic energy as
`the airgun array (Figure 3) while having only 4% of the sound
`pressure level. With respect to the usable energy for seismic
`acquisition, these two sources would be considered equivalent.
`
`Environmental purposes
`Evaluating the same two acoustic energy sources from an envi-
`ronmental perspective yields a different result. The airgun array
`generates a high sound pressure level, but only momentarily
`while the marine vibrator generates a lower sound pressure
`level over a longer period of time. If this comparison is taken
`into a seismic acquisition model where the airgun array fires
`every 12 seconds and the vibrator vibrates for 6 seconds and
`then listens for 6 seconds, over a course of a day of acquisition,
`both sources will put out the same total amount of acoustic
`energy into the water, at least within the seismic bandwidth.
`The JIP Environmental Assessment indicates that, for the same
`
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`received energy level, the impulsive nature of the airgun array
`will have greater auditory effects when compared to marine
`vibrators. Another consideration when comparing the different
`sources from an environmental perspective is the fact that the
`airgun array generates energy at frequencies above the usable
`seismic bandwidth while the vibrator array does not (Figure 4).
`An important element in the design of a marine Vibroseis
`source is its ability to control or ‘not generate’ frequencies out-
`side of the bandwidth useful for seismic exploration. According
`to the JIP Environmental Assessment, ‘This single factor could,
`for MarVib systems, be the most significant design feature with
`potential for reducing environmental impact’.
`
`Wasted energy
`Not all of the energy that goes into the 136 bar-metre
`peak-to-peak sound pressure level of the airgun array is
`useful for seismic acquisition. A significant portion of that
`energy is above 120  Hz, which is the nominal highest fre-
`quency commonly used in analyzing seismic data. If all of
`that energy were removed from the airgun array signature,
`the peak-to-peak SPL is reduced to 96 bar-metres (Figure 5),
`meaning that 40 bar-metres of SPL are wasted for the
`intended purpose of the array. Unfortunately, we have yet to
`discover a means of preventing this energy from being gener-
`ated although there are some efforts in that direction.
`
`Figure 2 A modelled far-field signature of a
`typical 3000 cubic inch airgun array with no filters
`applied showing a peak-to-peak amplitude of 136
`bar-metres.
`
`Figure 3 The amplitude spectra from 0 Hz to
`200 Hz of a modelled far-field signature of a typi-
`cal 3000 cubic inch airgun array and a modelled
`array of three marine vibrators, each having a
`peak-to-peak amplitude of 2 bar-metres sweep-
`ing for 6 seconds between 5 Hz and 120 Hz. Each
`source provides good acoustic output within the
`desired bandwidth for seismic exploration.
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`Because the marine vibrator has much better control over
`the frequencies generated, wasted energy is not a problem.
`This is true even in the presence of harmonics, a feature all
`mechanical systems have, since the control systems of the
`vibrator should ideally be able to suppress harmonics up to
`several kilohertz.
`
`A scalable source
`There are times when seismic acquisition does not require the
`full strength of a 3000 cubic inch airgun array and there are
`times when more energy is required. A drawback of the seismic
`
`airgun array is that it is not easily scaled up or down. Airgun
`arrays are typically made up of a number of individual airguns
`having a range of volumes from 40 cubic inches to 300 cubic
`inches or more. The combination of different airgun volumes is
`essential to maintain the desired shape of the airgun signature
`and altering this combination by adding or removing guns
`can produce undesired effects. Another, less common mode of
`changing the output level of an airgun array is to increase or
`decrease the firing pressure used in the guns. This too has limi-
`tations because the guns and air compressors are designed to
`work within a narrow pressure range, typically between 2000
`
`Figure 4 The amplitude spectra from 0 Hz to
`1000 Hz of a modelled far-field signature of a typi-
`cal 3000 cubic inch airgun array and a modelled
`array of three marine vibrators, each having a
`peak-to-peak amplitude of 2 bar-metres sweeping
`for 6 seconds between 5 Hz and 120 Hz. Energy
`generated by the airgun array above 120 Hz is
`typically not useful for seismic exploration and
`presents a significant potential impact for marine
`life within the area of seismic operations.
`
`Figure 5 The modelled far-field signature of a typi-
`cal 3000 cubic inch airgun array with no filters (red
`line) and a filter restricting the frequencies to less
`than 120 Hz (blue line) showing that 40 bar-metres
`of the generated signal energy falls outside of the
`desired bandwidth for seismic exploration.
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`and 2500 psi. Where airguns generally have a single mode of
`increasing or decreasing the output of the array, marine vibra-
`tors have three potential modes of altering the output levels.
`One simple way is to increase or decrease the number of vibra-
`tors being used in the array. Also, the output drive level of a
`marine vibrator has much more flexibility than the output of
`an airgun. From the maximum level, the vibrator output can
`be smoothly decreased by as much as 90%. Finally, the third
`scaling mode of marine vibrators is in the length of the sweep.
`This represents a significant advantage, particularly as it relates
`to environmental issues. Without increasing the drive level of
`the vibrator, a simple doubling of the sweep length generates
`about 41% more acoustic energy within the usable seismic
`bandwidth without increasing the SPL. By using the combined
`effects of the number of vibrators, the drive level of each vibra-
`tor and the length of the swept signal, marine vibrators can
`adjust the amplitude and character of their output to match
`almost any environmental or operational requirement.
`
`Marine vibrator development
`The Vibroseis technique was developed and pioneered by
`Conoco in the late 1950’s and it took about ten years before the
`first attempts were made at developing a marine vibrator. These
`first marine vibrators were considered technical failures in that
`they were not capable of generating seismic data with enough
`consistency and quality for exploration. Then, in the late 1970s,
`another serious attempt was launched to develop a commercial
`version of marine Vibroseis. This was a moderate technical suc-
`cess but a commercial failure. These vibrators were capable of
`producing good acoustic energy in a bandwidth between about
`15 Hz and 120 Hz, but were not able to produce significant
`acoustic energy between 5 Hz and 15 Hz. This deficiency kept
`this generation of marine vibrators from gaining significant
`
`acceptance in the industry. A few companies with inventive
`designs continued their efforts to develop a commercial marine
`vibrator over the next 35 years, and some prototypes were built,
`but none received the level of industry support that would be
`necessary to ensure the success of the technology. As the general
`awareness of how humanity interacts with our environment
`increased, and as the oil industry endeavored to perform opera-
`tions in more environmentally sensitive ways, discussions about
`marine Vibroseis were rekindled and interest levels in the indus-
`try started to climb. This is creating a new wave of development
`in the field of marine Vibroseis and a variety of new design ideas
`are being put forth.
`The Geokinetics marine vibrator project dates back to
`around 1994 with the concept of using a flextensional shell
`as the acoustic generator. Prototypes were built and tested in
`1999. Though the geophysical results of the tests were excellent
`(Figure 6), the vibrators did not have the reliability required to
`stand up to the rigours of seismic operations and the project was
`set aside. In 2007, with a concept for a new drive mechanism
`for the vibrators and ever increasing environmental restrictions,
`there was a realization that the marine vibrators could provide
`an excellent alternate source to airguns, particularly in environ-
`mentally sensitive shallow water and transition zone regions
`and the project was restarted. New, more reliable drive elements
`were designed and constructed along with improvements in
`most of the vibrator’s mechanical and electrical systems.
`Many previous marine vibrator designs somewhat mimic
`traditional land vibrator designs, using large plates or dia-
`phragms to try to induce pressure waves into the water. This
`idea works well for higher frequencies, but fails at lower
`frequencies. The reason this idea fails is related to the fact
`that a vibrating plate or diaphragm is only good at generating
`frequencies inversely proportional to the diameter of the plate.
`
`Figure 6 Comparison seismic lines from the 1999
`test of a flextensional shell marine vibrator (right)
`and a 760 cubic inch airgun array (left) showing
`that the marine vibrator data is comparable to the
`airgun data.
`
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`As frequencies get lower, the wavelength gets longer, and the
`ability of the vibrating plate to generate pressure waves at the
`lower frequency decreases. A partial solution to this problem is
`to have the vibrator operate, at least partially, in a volumetric
`mode. That is to say, instead of just moving a plate back and
`forth in the water, if the vibrator changes volume as well, this
`partially avoids the problem of the relationship between the
`size of a moving plate and the lowest frequencies that can
`be efficiently generated. This volumetric change is one of the
`advantages of the flextensional shell design of the Geokinetics
`marine vibrator.
`Another advantage that the Geokinetics marine vibrator
`has is that it is an all-electric system. Many previous vibrator
`designs used hydraulics to drive them. The hydraulic servo-
`valves that create the oscillating motion of a vibrator have
`limitations with respect to the speed at which they can react
`which limits the complexity of the waveforms they can gener-
`ate. The all-electric drive system of the Geokinetics marine
`vibrator does not have this limitation and also permits a more
`sophisticated control system able to control harmonics up
`to several kilohertz. The control of harmonics was identified
`in the JIP Environmental Assessment as an important aspect
`of any future marine vibrator design. In addition, hydraulic
`systems always have the potential to leak which can create an
`environmental hazard.
`To improve the reliability of the Geokinetics marine
`vibrator, the original drive system was redesigned. The
`original proto-type vibrators used electromagnetic voice coils,
`much like what is found in home audio system speakers, to
`drive the vibrators. While this type of system is capable of
`generating significant displacement, it does so with relatively
`low force. The new drive system uses Terfenol-D which is
`a highly magnetostrictive metal. Magnetostrictive materials
`respond to magnetic fields by changing their shape and dimen-
`sions. For example, when a rod of Terfenol-D is placed in a
`magnetic field, it grows longer and narrower, and it does this
`with significant force. The drawback is that the displacement
`is relatively small. The Geokinetics marine vibrator overcomes
`this by employing a two-stage, highly efficient motion ampli-
`fier design, increasing the displacement achieved by the
`flextensional shell while maintaining a significant level of
`force. The reliability of this drive system lies in the fact that it
`is essentially a solid-state drive element with no moving parts.
`
`The future of marine Vibroseis
`Although marine Vibroseis has not yet been adopted by the
`seismic industry as an alternative to airguns for marine seismic
`exploration, given the current environmental and regulatory
`landscape, their acceptance appears inevitable. The industry
`is pushing to develop this technology and several marine
`vibrator designs are currently being contemplated. However,
`several challenges must be met before marine vibrators will see
`widespread usage around the world.
`
`The impact of marine vibrators on the environment is still
`uncertain. Oil industry and environmental groups all agree
`that marine Vibroseis should have a significantly smaller
`impact than airguns. The Environmental Assessment of
`Marine Vibroseis sponsored by the Joint Industry Programme,
`E&P Sound and Marine Life concludes that marine vibrators
`‘should in most respects have less environmental impact that
`surveys using airgun arrays’. The study goes on to say that
`marine vibrators will have reduced impacts in all habitats and
`environments, and for behavioral, auditory and physiological
`effects.
`A challenge still facing marine Vibroseis is to demonstrate
`that it is capable of acquiring seismic data with the same
`fidelity and efficiency as airguns. Seismic data acquired with
`airguns provides a wealth of information about the subsurface
`which is required for the safe and efficient exploration for,
`and extraction of, hydrocarbon reserves. The industry is not
`likely to accept inferior seismic data which cannot provide the
`same levels of confidence for their exploration programmes. In
`addition, the efficiency in both time and cost must at least be
`comparable to what can be achieved with airguns unless there
`are overwhelming environmental considerations.
`When these challenges have been met, the desire and will-
`ingness of companies to use marine Vibroseis will increase. As
`in any market where demand is increasing, development and
`innovation will correspondingly increase further improving
`the ability of marine Vibroseis to compete with airguns as a
`viable marine seismic source. Just as the industry has learnt
`how to make land Vibroseis competitive with explosives,
`marine Vibroseis will become competitive with airguns. But
`also, just as there are examples in land seismic acquisition
`where explosives are preferable to Vibroseis for economical,
`logistical or environmental reasons, there will be situations in
`marine seismic acquisition where airguns will still provide the
`best choice. Marine Vibroseis may not replace airguns as the
`preferred seismic source for marine seismic acquisition, but
`it will most certainly become a significant component in the
`array of options available to the seismic industry in its quest
`to provide the highest quality, most cost effective and most
`environmentally responsible options for their clients.
`
`References
`LGL and MAI. [2011] Environmental Assessment of Marine Vibroseis.
`LGL Rep. TA4604-1: JIP contract 22 07-12. Rep. from LGL Ltd.,
`environ. Res. Assoc. King City, Ont., Canada and marine Acoustics
`Inc., Arlington, VA U.S.A., for Joint Industry Programme, E&P
`Sound and Marine Life, Intern. Assoc of Oil & Gas Producers,
`London, UK.
`Schlundt, C.E., Finneran, J.J., Carder, D.A., Ridgway, S.H. [2000]
`Temporary shift in masked hearing thresholds of bottlenose dol-
`phins, Tursiops truncatus, and white whales, Delphinapterus leucas,
`after exposure to intense tones. The Journal of the Acoustical Society
`of America 107(6), 3496-3508.
`
`72
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`special topic
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`www.firstbreak.org © 2013 EAGE
`
`first break volume 31, November 2013
`
`PGS Exhibit 2005
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