Seismic | OCEANOISE2023

Report of the Seismic Session

Hedgeland, D.¹*, Jenkerson, M.²*, Roy, D.³, Hager, E.⁴, Southall, B.⁵, Nechayuk, V.⁶, Racca, R.⁷, Stenton, C.⁸

¹BP, UK
²MJ.Pegasus, LLC
³APS, USA
⁴Shearwater, UK
⁵Southall Environmental Associates, USA
⁶ExxonMobil, USA
⁷JASCO Applied Sciences, Canada
⁸Ocean Science consultants, UK

* Session Chairs and Corresponding Authors; E-mail: David.Hedgeland@uk.bp.com – MJ.pegasus@outlook.com

This report can be referenced as: Hedgeland, D., Jenkerson, M., Roy, D., Hager, E., Southall, B., Nechayuk, V., Racca, R., and Stenton, C. (2023). Report of the Seismic Session, OCEANOISE2023, Vilanova i la Geltrú, Barcelona, Spain, 22-26 May. Retrieved from https://2023.oceanoise.com

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Introduction

Offshore geophysical surveys continue to be an important tool for imaging the seabed and subsurface geology, which are used to enable planning, siting and construction of offshore hydrocarbon, renewable energy and other industry infrastructure developments.

Airgun seismic source arrays currently continue to provide the most efficient, robust and safe sound sources that are commercially available for conducting seismic surveys to image both deep and shallow geologic structures below the seabed. Whereas higher frequency active and other impulsive sources, such as sparkers are most commonly used for higher resolution seabed and shallow depth subsurface imaging for site characterisation studies and to assist in the planning and placement of infrastructure on the seabed, such as foundations for offshore wind turbines.

The session provided the audience with an update on the continued development activities for marine ‘deep’ imaging seismic survey sources, including both marine vibrators, modifications to compressed air sources and operational planning mitigation measures. In addition, the session provided an update on research activities designed to improve our understanding of the potential implications of sound from these and other seabed imaging survey sources to marine life.

Questions from the audience further explored some of the technical aspects of the studies being presented

Revisit of Oceanoise2017 Seismic session

The Oceanoise2017 session provided an overview of typical geophysical survey methods and sound sources and efforts that were underway to develop alternative seismic source technologies.

The session Co-chairs promised and were pleased to return at the next Oceanoise opportunity in 2023 to provide an update on progress of the various source and survey technology development efforts.

Update on development activities of alternative seismic sources

The development status for two separate prototype Marine Vibrator (MV) transducer technologies were presented. Both technologies (Applied Physical Sciences (APS) and Shearwater) are at a prototype testing stage and could be used as alternative seismic sources to airgun arrays. Both activities demonstrated the multi-year history of industry engagement in developing source technologies with reduced sound emission characteristics.

Development of the first device started in 2013. As part of a Joint Industry Project (MVJIP), a number of prototype technologies were assessed against nominal source output technical specifications to achieve geophysical objectives that were set to guide system developers. Following extensive model validation and testing, the device progressed to open-water prototype testing in 2022 with real-time QC and data processing being implemented via dedicated applications. Efforts are ongoing to reduce both the size and weight of in-sea equipment, which will help simplify deployment, recovery, operations, and maintenance of the system in preparation for deployment of the device as part of a behavioural response study (BRS).

The second marine vibrator technology is being developed ‘from idea to operation’ by an international geophysical contractor company with the goal of reducing potential environmental impact, by reducing peak output as well as less total energy emitted. A commercial reality for any new source technology to succeed in the market is a need to be flexible enough `to suit different survey objectives, safe and reliable to be able to operate in a real-life environment. Data from early prototype testing has demonstrated desirable signal characteristics enabling the product to progress from an engineering idea to its first offshore field trial.

In addition to marine vibrator sources, numerous operational and engineering options are either currently available or under development aimed to reduce the risk of potential impacts of sound from seismic surveys. These can be categorised as (1) efficient survey design to reduce either spatial coverage; durations of surveys to avoid sensitive areas and/or periods for a given area. (2) engineering modifications (eSource, BluePulse) to available devices to reduce the out of band energy generated by the devices; and/or (3) redesign of the array configuration or device to tilt the output of the system to lower frequencies (TPS, Harmony, Gemini).

Different techniques can reduce different components of the sound output characteristics (peak, bandwidth, SEL, duty cycle). Notwithstanding the commercial reality of any new technology, such methods offer the potential to reduce the environmental impact without compromising the quality of the seismic data.

Improving understanding of potential implications of sound from survey sources to marine life

The potential for marine vibrator sources to reduce the risk of auditory impacts on marine mammals has been demonstrated via modelling studies with reduced overall sound levels and limited frequency bandwidth resulting in predicted distances to hearing impairment threshold values being significantly reduced (Matthews et al 2021).

In preparation for a new study investigating the behavioral responses of large whales to marine vibroseis sources in 2024. Results from early-stage pilot studies implemented in 2022-23 were presented. The objective of which was to apply proven tools and learnings from implementing satellite tagging studies elsewhere to refine controlled exposure experiment (CEE) methods to obtain novel data on the behavioral responses of very low frequency baleen whales to marine vibrator signals in realistic operational conditions. Key field study elements include locating, tagging, and tracking blue and/or fin whales over both fine-scale short duration (hours to days) and coarser scale longer duration (days to weeks) data. Plans are underway for further field work to be conducted using a full-scale prototype MV device in the middle of 2024.

A case study showing the development an implementation of an effective mitigation strategy to reduce the risk of the impact of sound from seismic surveys to a population of Gray whales was presented (Racca et al 2023). Accurate mapping of sound emissions from the survey source arrays was used to estimate the received sound levels at the individual animal locations derived from visual monitoring. The geospatial quantification of received sound levels informed an analysis of the effects of the seismic surveys on whale distribution and behaviour.

Sub-Bottom Profiler (SBP) sources produce a highly directional sound signal at specific frequencies, pulse durations and rates to provide high resolution surface and shallow depth subsurface characteristics of the seafloor. In comparison to some other survey sound sources, sound levels, frequency distributions, and therefore potential impacts to marine species are poorly quantified and understood. A recent soon to be completed study has enabled the collection and analysis of sound level data from opportunistic measurements of two different SBP devices in an operational setting. Combining the measured data with propagation modelling investigated the effects of water-column depth, current speed, water temperature, sound-velocity profile, and seafloor substrate to derive propagation or transmission loss functions for each propagation scenario and produce simplified equations to estimate sound propagation and duration of exposure relative to available Temporary- and Permanent Threshold Shift (TTS and PTS) thresholds for different marine mammal groups (Southall et al 2019). Through discussion with the audience, it was confirmed that the modelling study considered sound levels at various orientations (off-axis) to the main direction of signal emission.

Summary

It is widely recognised that the timelines for technology development to attain commercial availability is dependent upon achieving confidence and balance between meeting geophysical data quality objectives, operational safety and reliability requirements, stewardship of environmental sensitivities and regulatory compliance. The various ongoing development efforts offer the possibility of greater flexibility and choice of source technologies and data acquisition methods for future survey activities relative to achieving the geophysical survey objectives and environmental sensitivities.

It was highlighted during other Oceanoise23 discussions that the development of behavioural impact threshold criteria (commonly used to assess potential impacts on marine species as part of impact assessment studies) are less developed for non or less impulsive sound sources. Following the many years of scientific focus on investigating potential impacts of impulsive sound on marine species, the discussion highlighted the need for further research and discussion regarding the potential impacts of non or less impulsive sound. Reducing scientific and regulatory uncertainty would help inform the development of new survey sound source technologies.

Whilst airgun seismic source arrays continue to provide the most efficient, robust and safe sound source that is commercially available for conducting seismic surveys. At least two development efforts to produce viable marine vibrator sound sources have progressed from prototype testing to trials in operational settings. The environmental advantages of these sources are also being evaluated.

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