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Third EAGE Marine Acquisition Workshop
- Conference date: August 22-24, 2022
- Location: Oslo, Norway
- Published: 22 August 2022
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Improved Low Frequency Output from Seismic Air Guns by Firing at Shallow Depths?
Authors M. Landrø and L. AmundsenSummaryBased on a field test in the Trondheimsfjord performed some years ago, we study the effect of changing the source depth on the low frequency content in the source spectrum. We find that extremely shallow source depths will increase the low frequency signal generated by air guns. Further, we conclude that there is a qualitative difference between enhancing the low frequency content by repeated bubble oscillations and by creating a broadband single peak signal. A single peak gives a relatively flat spectrum also for low frequencies, while a bubble tuned signature will cause notches and higher amplitudes for some frequencies. For processing and analysis purposes it is better to use a single peak compared to repeated bubbles that are hard to fully exploit at the processing step. A short source signal is preferred prior to a long bubbly source signal for several causes: non-repeatable effects for the bubble oscillations, need to estimate a relatively precise source signature for designature purposes. It our view a cheap and efficient way to avoid bubbles and increase both high and low frequencies from single air guns is to fire the air gun close to the surface, which is of interest for high-resolution and ultra-high resolution surveys for wind farm projects, marine mineral exploration and CO2 monitoring.
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Virtual Receiver Interferometry for Offshore Drill-Bit Seismic
Authors A. Goertz, E. Vange Bergfjord and B. AtkinsonSummaryWe present a novel interferometric method to reconstruct zero-offset VSP data from drill-bit vibrations that have been passively recorded with a seismic ocean-bottom array placed in the vicinity of a drilling rig. The method has been successfully applied during drilling of an exploration well for a 4 km deep target in the North Sea. At this borehole, significant depth uncertainty of the drilling target below a high-velocity formation required a check-shot velocity update and seismic look-ahead during drilling for optimal placement of the lowermost casing shoe. Drill-bit seismic can produce this information in real time without any impact on the drilling operation but is often hampered by lower signal-to-noise at zero offset due to the radiation pattern of PDC bits. We show that we can overcome such issues with the novel interferometric method, by reconstructing zero-offset data from a wide-aperture array. The resulting dataset is much denser than traditional VSP data. With about a tenfold increase in trace levels along the borehole, it is comparable to DAS VSP data. In addition, we obtain a high-fold 3D VSP image of the vicinity around the wellbore that can be used for reservoir characterization during the appraisal phase.
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A Very Low Frequency Marine Vibrator
Authors R. Tenghamn and A. MattssonSummaryThere has been an increasing interest in the industry for powerful very low frequency (VLF) seismic sources. The lowest frequency range is important for deriving the elastic properties of the subsurface by seismic full wave field inversion (FWI). Accordingly, there has been a need for powerful low frequency marine sound sources operating in the frequency band of 1 to 5 Hz. This paper describes a new low frequency source that is easy to deploy and operate from a small vessel. The new very low frequency source is built on a novel source concept using resonance to increase acoustic efficiency and at the same time decrease complexity of the source. The resonance frequency of the source can easily be adjusted to a certain frequency or adaptive in case of running sweeps in the frequency band of 1 to 5 Hz. This less complex source can be deployed at a water depth of 5 m or less and will give a SPL of more than 190 dB rel. 1 micro-Pa in the frequency range from 2 to 5 Hz.
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Development of the MVJIP Marine Vibrator: What’s Next?
Authors R. Alfaro, S. Secker, A. Cozzens, N. Henderson, M. Jenkerson, V. Nechayuk and G. JohnsonSummaryThe Marine Vibrator Joint Industry Project (MVJIP), setup by TotalEnergies, ExxonMobil and Shell to develop marine seismic vibrators, have been working for almost 10 years on the Integrated Projector Node (IPN) developed by General Dynamics Applied Physical Sciences. The IPN, an electromagnetic marine vibrator, has reached various milestones over the course of its development, with its geophysical characterisation test being shared most recently at EAGE 2018. Since then, the MVJIP have performed several tests including: 1) endurance tests where the IPN was run continuously over a period of months to force any premature failures that could happen in the field and take subsequent corrective action, 2) a tow test using two dummy IPNs (replicas to size, scale and weight) to allow the development on an appropriate launch and recovery system and 3) qualification testing of two IPNs to learn how well they perform in unison. Each test has been specifically designed to allow the green lighting of a pilot test to take place in late 2022 where for the first time real seismic data will be collected using two of the MVJIP IPNs. This pilot will benchmark the IPNs vs airguns alongside addressing other processing opportunities (e.g., Doppler shift correction).
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New Marine Seismic Acquisition Equipment
Authors T. Elboth, S. Rentsch-Smith, L. Combee and K. IranpourSummaryRecent application of Full-Waveform-Inversion (FWI) have convincingly shown the value of acquiring both high resolution near offset data and long offset low frequency data. In this presentation we show some ongoing acquisition hardware developments that aim to support the acquisition of such data. New and versatile ocean bottom nodes with a long battery life, a novel low frequency source and a high-fidelity marine vibrator system represents the next generation of acquisition systems. In these systems we no longer go bigger and larger, but rather more nimble and smarter. The challenge for the industry will be to take full advantage of these new developments in the years to come.
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Low-Volume Apparition Sources: Source Trials from the Bass Strait
Authors S. Redfearn, L. Casasanta, S. Grion and J. CockerSummaryTowed-streamer blended acquisition using either overlapping or simultaneous sources is designed to deliver increased shot sampling and fold, enhanced resolution, and to optimize operational efficiency. During Q4 2021, several test lines of streamer data were acquired in the Australian Bass Strait to assess the impact of novel source configurations on data quality. The processing results on these 2D test lines show a comparable data quality and bandwidth for low-volume apparition sources compared to a conventional, high-volume source. Here the extra shot points from the apparition data have been used to increase the stack fold, however in a 3D application the apparition source configuration offers the opportunity the make use of the additional sources for better resolution in the crossline direction.
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Investigation into Methods for Interpolating Missing Frequencies in Seismic Data
Authors S. Tuppen, T. Elboth, K. Iranpour and D. Van ManenSummaryFor the last 50 years, airgun-based seismic sources have dominated the market due to their reliable and repeatable nature. However, the industry is looking at alternative source designs, for example, marine vibrators. Such marine vibrator sources offer a high-fidelity control of the emitted waveform and can be built or optimized for use in a particular frequency band. During acquisition, a marine vibrator survey may consist of several marine vibrators whereby each vibrator emits non-overlapping frequency bands with frequency gaps in between. In this case, the missing frequency bands must be interpolated. In this paper, we show that a convolutional neural network and a sparse solver approach both show promising estimates of the missing frequency bands of each vibrator source.
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Enhancing Low-Frequencies in Airgun Arrays
Authors S. Rentsch, E. Hager, L. Wing and S. HollandSummaryRecently, towed marine seismic acquisitions are intently focusing on enhancing low frequencies on the source and receiver side. In this abstract we document our journey to understand ways to enhance the low-frequency output of airgun arrays for towed marine seismic acquisitions. We investigate the physical effect of (partial) frequency locking of the pressure field from adjacent airguns or clusters, which can give a larger effective bubble volume, lower the oscillation frequency, and thus increase the low-frequency content emitted over a traditional design in which frequency locking does not occur. We show how we conducted a series of field experiments and demonstrate that we achieved (partial) frequency locking with a 70% increase in effective bubble volume for a 2-cluster scenario and a 95% increase for a 3-cluster scenario, respectively. Furthermore, we verify spectra from estimated far field signatures with the help of PRM system measurements and show a significant low-frequency uplift below 5 Hz for a frequency-locked airgun array compared to a classical array.
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Challenges and Solutions in Shallow and Deep Water Ocean Bottom Seismic
Authors C. Bagaini and D. BoieroSummaryOcean-bottom seismic (OBS) has been gaining popularity in the seismic exploration community in the last decade because of the benefits that the multi-azimuth and broadband OBS data can bring to the final image of the subsurface. Modern OBS acquisition systems can acquire data in water depths that range from a minimum (e.g., 10 m), which is essentially controlled by the vessel’s draft, to a maximum (e.g., 3000 m in acquisition systems based on autonomous nodes), which is constrained by the maximum operational depth of the acquisition system.
This huge range of water depths can be simplistically divided into three zones: shallow, medium, and deep waters. We outline the challenges and solutions for each of them, with special attention to shallow and deep waters, and propose some criteria for this classification.
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Optimization of Operational Costs when Acquiring Hybrid Surveys with Free-Fall, Self Pop-Up Nodes
Authors G. Mellier, F. Josse, C. L’Her, N. Tellier and P. PelleauSummaryRecent advances in marine seismic processing are increasingly supporting the use of sparse OBN in conjunction with low-frequency sources in order to provide ever-lower frequencies, longer offset and wider azimuth datasets well suited to FWI purposes. However, conventional OBN systems show operational limitations when employed for sparse nodal operations. On one hand, node-on-a-rope (NOAR) operations are limited to relatively shallow water depths and are not optimal for sparse deployment as they require long cable sections between each OBN and with deployment only at a slow vessel speeds (∼ 2 knots). On the other hand, OBN deployment by ROV (Remotely Operated Vehicles) is well adapted to operations in deep waters, but is a very costly approach, all the more so when nodes are deployed over a wide area. Free-fall seismometers are less known by the seismic industry, albeit they have been in use for decades for scientific purposes. This paper will provide some indications on how this type of OBN system can be efficiently operated to fit with today’s highly productive seismic operations.
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New Low Frequency Seismic Source—Tuning the Bandwidth for Subsurface Penetration and Reducing High Frequency Content
Authors J. Large, H. Chambon, M. Kryvohuz, H. Macintyre, S. Ronen, S. Laroche, N. Tellier and G. BaetenSummaryThe marine seismic industry has two key evolutions; reducing energy generated for marine mammals and increasing low frequency for complex imaging. The recorded data during a survey, up to 50 kHz, clearly show that the Tuned Pulse Source (TPS™) has a significant advantage. Following several years of development, optimization and field validation, a new generation of sources is now available. The TPS operates with larger volumes at lower pressure for reliable and safer operation. Compared to conventional pneumatic sources, the low frequency infrasound output of the TPS is increased. The data shows that : environmental impact of seismic sources is not characterized by the volume of the source, the environmental impact of one day of shooting airguns is therefore comparable to the impact of many days of shooting TPS.
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Recent Advances and Lessons Learned with Wide-Tow Multi-Sources in Marine Streamer Seismic
Authors C. Roalkvam, M. Widmaier, O. Orji and R. TønnessenSummaryMore than 10 marine seismic acquisition projects with novel wide-tow multi-source configurations have been acquired since the commercialization of the concept by PGS in 2019. The wide-tow source solutions were applied in advanced multi-azimuth (MAZ) projects in the North Sea, in high resolution surveys in the Barents Sea, in exploration surveys offshore Australia, and most recently offshore Malaysia. The wide-tow source projects achieved several industry records, new geophysical benchmarks were set, and the near-offset-rich data built the basis for new processing approaches. This presentation discusses recent experiences highlights and lessons learned with wide-tow multi-sources.
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Unbiased Hydrophone Position Errors can Lead to Biased Estimates of SEL
By R. LawsSummaryThe Sound Exposure Level (SEL) is a well-established metric for the impact of sound on marine life.
In an oft-used experimental configuration a sound source is operated, and a hydrophone measures the sound pressure waveform at some position in the water. This waveform is then processed to estimate the waveform at the position of the animal and the SEL is calculated from it.
A set of Monte-Carlo simulations of such an experiment was run in order to examine the effect upon the estimated SEL of positional perturbations of the hydrophone. The perturbations were drawn from a 3D normal distribution with zero mean and a standard deviation of 0.5 m. The data were processed using the nominal hydrophone position.
In the presence of positional perturbations, the estimated SEL suffers from error; that is only to be expected. What is less expected, however, is that there can be a significant bias in the estimated SEL even though the positional uncertainties have no bias.
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Towards Cost Efficient CCS Monitoring
Authors S. David, T. Holm-Trudeng, M. Rocke, F. Ten Kroode and M. ZajacSummaryCarbon Capture and Storage (CCS) will be a key instrument to reach the goal of net zero CO2 emissions by 2050. Monitoring will be required to ensure CO2 containment and to ascertain conformance with long term expectations about the CO2 behavior inside the storage complex. In the oil and gas industry, the value of time lapse seismic acquisition for field production monitoring has been recognized long ago. The lack of financial incentives in the CCS market is pushing the industry to find innovative and cost-effective ways to make seismic acquisition more affordable. Various geophysical technologies are being and will have to be developed to reduce costs. These solutions will be site dependent and a bespoke solution will have to be made for each location, depending on local geology, risk assessment and budget.
We present a cost-effective acquisition solution for CCS projects, both structural derisking and monitoring, that offers high resolution seismic data using short streamers: Extended High Resolution (XHR) acquisition. In future highly congested areas, streamer acquisition may however become infeasible and we will need to use ocean bottom based techniques for CCS monitoring. With this in mind, we sketch a path towards much more cost-effective OBN acquisition.
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OBN Clock Drift from an Onshore Processing Point of View
Authors G. Apeland and P. KristiansenSummaryOcean-bottom node (OBN) acquisition offers full-azimuth and long-offset seismic acquisition which is often needed for optimal imaging of the subsurface. A number of 3D and 4D case histories have shown the value of the OBN technology. On the other hand, there are challenges with OBN which we do not have with other surface seismic methods.
The autonomous nodes used in OBN acquisition cannot communicate with external sources during deployment to get exact world time for the samples they record, but instead must use an internal clock to time stamp the recorded samples. The source firing time is, on the other hand, recorded at world time. Over time the internal clock might drift relative to world time, and this is known as clock drift and has to be compensated for during processing of the OBN data. Hence an accurate and stable node clock is a key part of a successful OBN data acquisition. In this paper we discuss the need to standardize the information shared between the acquisition and processing companies to ensure that the clock drift compensation on OBN data can be handled accurately and rapidly.
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