Report of the Round Table Session
Hedgeland, D.1*, Jenkerson, M.2*, Abma, R.3, Correa, C.4, Hayes, S.5, Lambert, I.6, Norton J.7, and Robinson, N.8
1 BP, UK
2 Exxon-Mobil, USA
3 BP, USA
4 Repsol, Spain
5 Genesis Oil and Gas Consultants Ltd, UK
6 CGG, France
7 Teledyne-Bolt, USA
8 Gardline Environmental Limited, UK
This report can be referenced as:
Hedgeland, D., Jenkerson, M., Abma, R., Correa, C., Hayes, S., Lambert, I., Norton J., and Robinson, N. (2015). Report of the Seismic Survey Session, Oceanoise2015, Vilanova i la Geltrú, Barcelona, Spain, 10-15 May. (Editors Michel André & Peter Sigray). Retrieved from http://oceanoise2015.com
Seismic surveys are an important tool for oil and gas exploration and production activities offshore and are also widely used for imaging geology for research purposes. Airgun seismic source arrays currently provide the most efficient, robust and safe sound source that is commercially available for conducting seismic surveys.
As well as seismic surveys and E&P activities, many other industries and stakeholders also use the maritime space for activities such as commercial shipping, fishing, renewable energy development, military, recreation and construction; all of which either use sound for various activities or introduce sound as a result of activities.
The aim of the session was to cover three main topic areas providing the audience with an overview of; typical geophysical survey methods and sound sources, inputs to processes for assessing potential implications of sound to marine life and finally development efforts related to seismic source technologies.
Questions from the audience further explored some of the technical and physical aspects of survey methods and sources modelling, as well as drivers, resources and timeframe challenges for implementing new methods and technologies.
Technical and physical aspects of survey methods and source modelling
A new methodology was presented for measuring sound output from seismic sources using the same hydrophone streamers (typically towed at 10m depth for conventional data acquisition) that are used to collect seismic data to produce an image of the sub-surface geology.
The seismic hydrophone streamers are calibrated to manufacturing specifications and typically regularly tested via the functionality of the on-board recording system. Seismic streamers used for 2D/3D seismic surveys are typically between 6 to 8km or longer. Each streamer is made up of sections that can be changed as required if physically damaged or there are failing hydrophone channels. Streamer sections with any repeatedly failing hydrophone channels would typically be replaced during deployment/recovery operations as part of the nominal equipment maintenance schedule or if needed could be changed in-situ once deployed from the seismic vessel if weather conditions allow and rigorous safety conditions are fulfilled.
The shallow towing depth and nominal positioning, behind a seismic source array (endfire aspect relative to the array) is not able to provide a measure of the sound output from the side (or abeam/broadside aspect relative to the array). Other data collection systems and methods were available and in use for characterising sound sources, however such systems would typically incur additional cost and added operational complexity compared with using equipment that was already in use during seismic surveys. Whilst equipment used in other seismic survey methods may also offer opportunities for characterising sound sources, such as towed Broadband seismic where receivers are towed at deeper depths or Ocean Bottom Seismic (OBS) where receivers are located on the seabed. The case study was a valuable first step towards understanding the challenges of using existing/conventional seismic equipment to collect meaningful sound data.
Some geophysical service providers have developed marine mammal passive acoustic monitoring systems using hydrophones in the seismic streamers. However the use of individual hydrophones grouped together to form ‘channels’ with improved directivity in the vertical direction versus horizontal may not be optimal for passive acoustic detection of marine mammals. At least one of these systems used individual sensor modules that could be positioned at various locations along the length of the seismic streamers. Also seismic streamer technologies were available that used single hydrophone sensors that could then be added together to provide an equivalent response to having grouped sensors.
A Joint Industry Programme (JIP) is underway to develop and test a number of prototype Marine Vibrator transducer units that could be used as alternative seismic source to airguns.
A nominal benchmark technical specification for source output has been set to guide system developers. This includes a source level specification for an array of transducers in the frequency domain for the (downward) vertical direction, which is a commonly used directivity measure for airgun source arrays.
Whilst measurement data for marine vibrator sources was not yet available to enable a direct comparison of cumulative sound levels between the anticipated longer duration, lower peak sound level signal of a marine vibrator with a short pulse signal of a conventional seismic source. Theoretical assumptions and experience based on the use of vibrator source technology for land seismic surveys suggests that energy outside the frequency range of geophysical interest (up to 100-150Hz) can be significantly reduced. Therefore overall cumulative sound levels outside the geophysical frequency range of interest are anticipated to be lower for marine vibrator sources compared to conventional seismic sources. Whereas cumulative sound levels are anticipated to be comparable within the geophysical frequency range of interest.
It was suggested that surveys using Ocean Bottom Seismic survey (OBS) methods might offer an opportunity to use lower source sound levels as hydrophone receiver systems are located on the seabed rather than towed close to the sea surface behind a moving vessel. This is considered unlikely to be the case as the OBS method only removed the need for the upward water column pathway for reflected energy as the receivers were placed on the seabed. Source sound levels similar to those used during towed streamer surveys were required so that sufficient energy was reflected back from the sub-surface geology.
Underwater sound modelling is commonly used during studies to assess the likelihood of potential marine life disturbance due to sound from seismic surveys. Efforts are ongoing to measure sound from seismic sources in order to provide input to such studies to help ensure as far as possible that the various characteristics of sound from seismic sources is captured as accurately as possible during the initial stages of any model based assessment process.
The output from sound sources are commonly referenced in the vertical downward direction whereas the sound field is commonly modelled in the horizontal direction without the effects of a surface ghost reflection. An industry supported 3D Source Characterisation Study has been conducted with data recorded across all azimuths and take off angles from vertical.
Drivers, resource and timeframe challenges
In addition to development efforts related to marine vibrator seismic sources, there are also a number of ongoing efforts related to the use of existing or modified airgun technology. The E-Source, which via a re-design of existing airgun sources, provides a reduction in sound energy outside the frequency range of geophysical interest. The other; Popcorn is a source method under development that utilised modified activation times of individual source elements within a source array (rather than the conventional use with all source elements being activated simultaneously) and applying data processing capabilities in order to reconstruct the data to produce an image of at least an equivalent quality to using a conventional source method. The Popcorn method offered a number of potential benefits, such as using existing source technology, improved data collection efficiency and reduced peak sound level output.
The approximate cost for replacing a typical seismic source array made up of 30-40 source elements with a new type such as the E-Source would be in the order of 1-2 millions US dollars. However it may be possible to reduce the cost burden by replacing a sub-set of source elements rather than a complete array.
There was some discussion related to whether Popcorn offered an improvement in data quality from a geophysical perspective, which would likely be a significant driver for it to being adopted in addition to any potential environmental aspect such as reducing peak output sound level. Results from theoretical modelling and early measurement data suggested that data quality could be at least equal to that achieved from a conventional source array. Tested to date had been limited to a stationary 3-4 source element configuration. It was noted that historically source controllers had been designed to activate source elements simultaneously and it was unclear whether currently available source controller systems and the physical activation mechanisms of existing source technology would be able to accommodate the method applied to a full scale moving scenario.
A range of survey methods are currently available, which are used not only for E&P activities but also for a range of oceanographic and geologic research. Whilst airgun seismic source arrays currently provide the most efficient, robust and safe sound source that is commercially available for conducting seismic surveys. Efforts are underway within the E&P industry to understand the characteristics of sound from seismic surveys and reduce output of sound energy associated with existing seismic sources that is outside the geophysical frequency range of interest.
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.
Source technology development takes time and resources with timelines to commercial availability being dependent upon achieving confidence and balance between meeting geophysical data quality objectives, operational safety and reliability requirements, stewardship of environmental sensitivities and regulatory compliance
Considerations for the future:
- What is the status of the various source technology developments?
- How can we more effectively estimate potential ‘impacts’ of our sources under differing operational conditions?
- What is the optimum source design from an environmental perspective that can still fulfil the geophysical and operational objectives?