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- Volume 34, Issue 11, 2016
First Break - Volume 34, Issue 11, 2016
Volume 34, Issue 11, 2016
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The transformation of seabed seismic
Authors Tim Bunting and John MosesSeabed seismic surveys have been part of the hydrocarbon exploration industry for many decades. Initial implementations deployed cables which were populated with hydrophones and generally used for shallow water and transition zone projects. The cables were directly connected to a recording vessel to power the in-sea hardware, manage the spread and record the sensor measurements. Barr et al. in the 1980s implemented a technique, first postulated in 1954 by Haggerty and Backus, which uses a dual component measurement (pressure and vertical particle velocity) to eliminate the receiver side ghost, extending the reach of the technique into deeper waters. The potential of the seabed technique was further extended to make a measurement of the shear reflectivity through the addition of horizontal particle velocity components (four-component recording or 4-C). In the late 1990s the industry started experimenting with buried seafloor cables as part of permanent seismic installations to assist in production monitoring of the reservoir. As well as ocean bottom cable (OBC) systems, it is possible to make a seabed seismic measurement with an array of ocean bottom nodes (OBN). An ocean bottom node is an autonomous recording device with a self-contained recording system, clock and battery. As there is no connection with the surface, there is no limitation on length of the receiver line, no downtime due to telemetry/power line failures and no lost time associated with moving of the recording vessel. Ocean bottom nodes have been in use for many decades, but their use had primarily been limited to long-offset refraction surveys for studies of earth tectonics. The majority of the early node technologies were developed in academia and were neither industrially engineered nor designed with a focus on operational efficiency. After early development by Statoil in the 1990s, the first seismic reflection ocean bottom node survey was acquired in 2004 over Pemex’s Cantarell Field in the Gulf of Mexico. Ocean bottom nodes, when used for hydrocarbon exploration, were initially deployed by remotely operated vehicles (ROVs), but more recently ropes or wires have also been employed.
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Exploiting the control of phase in marine vibrators
Authors Robert M. Laws, David F. Halliday, Ali Özbek and Jon-Fredrik HopperstadMarine seismic vibrators are considered to be more environmentally friendly than air guns and this has prompted a resurgence of interest in their use. However, these devices have novel features that are potentially of great benefit to seismic imaging and that have not yet been exploited - in particular, we can control the phase. With marine vibrators, the emitted waveform can be chosen freely, provided that it lies within the envelope of what the device can emit. Typically, the waveform is a swept-frequency sinusoid. In this paper, we show how the sweep phase can be used to suppress the residual shot noise (RSN) and to separate simultaneous sources of high multiplicity. This work is also described by Laws and Halliday (2013) and is closely related to that of Laws (2012). The control of phase is a benefit of marine vibrators that can be exploited to great effect. First, we look at the RSN situation. The conventional wisdom is that roughly a 10s shot-time interval is required in marine seismic data acquisition so that the RSN is acceptably small relative to the signal. Because both signal and RSN originate from the seismic source, the ratio of signal/RSN is not improved by using a larger source (pace Landrø, 2008). We show a simple case of RSN attenuation using alternating sine and cosine sweeps. That is to say, the sequence of source phases goes, for successive shots, [0° 90° 0° 90° 0° 90° 0° 90°... and so on]. We show schematically how this ‘phase sequencing’ leads to a dramatic attenuation of the RSN. It does so by moving the RSN into parts of the frequency-wavenumber spectrum where there is no signal. We then use frequency-wavenumber filtering to remove the RSN. This can be done with simple filtering up to the limit imposed by spatial aliasing and beyond that limit by using wavefield reconstruction methods, such as generalized matching pursuit (Özbek et al., 2010; Vassallo et al., 2010). Reconstruction methods decompose the seismic wavefield into a set of basis functions that allow the signal and RSN to be identified and separated. Then, we look at the simultaneous source situation. The RSN removal problem can be considered as a special case of the more general simultaneous source separation problem. As an example, in the case of two simultaneous sources, one source can be swept with consistent phase, while the other source is swept with alternating phase, i.e., the sequence of relative phases is [0° 180° 0° 180° 0° 180° 0° 180°... and so on]. The simultaneous source separation is demonstrated using the same type of reconstruction method as described above for the RSN. See also Moore et al. (2008) and Ji et al. (2012). The use of phase sequencing opens up possibilities for simultaneous source separation using large numbers of sources. By having complete control over the phases of all the sources it is possible, in certain domains, to make simultaneous source data look similar to aliased data from a single source. This opens the door to using many established wavefield interpolation and dealiasing-techniques to perform high-multiplicity simultaneous source separation.
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Large-scale seismically guided anisotropic inversion of towed-streamer EM data in the Barents Sea
This paper presents a fast and efficient large-scale anisotropic inversion technique for towed streamer electromagnetic (EM) data, which incorporates seismic constraints. The inversion algorithm is based on the 3D contraction integral equation method and utilizes a reweighted regularized conjugate gradient technique to minimize the objective functional (Zhdanov et al. 2014a, b). We have also introduced the concept of a moving sensitivity domain for seismically guided EM inversion, originally developed for airborne EM surveys (Zhdanov and Cox, 2013), which makes it possible to invert the entire large-scale towed streamer EM survey data while keeping the accuracy of the computation of the EM fields. The developed algorithm and software can take into account the constraints based on seismic and well-log data, and provide the inversion guided by these constraints. To demonstrate the practical effectiveness of this approach for large-scale inversion of marine EM data, as well as integration with seismic data, we apply the method to the inversion of about 2000 line-km of towed streamer EM data. The data form part of a larger survey in the Norwegian Barents Sea (McKay et al., 2016). We show that the technique produces a single resistivity model that is consistent with both the measured EM data and the main seismically defined structures. Thus, the resistivity model is ready to be interpreted and used in further quantitative interpretation studies.
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Signal apparition applied to towed marine simultaneous sources – a case study on synthesized real data from the Viking Graben
Encoding sources using random dithers for simultaneous source acquisition offers significant scope for enhanced towed marine operations. However, so far it has proven to be challenging to reduce the decoding residual noise from these techniques to a level that is widely accepted by the industry. The emerging technology of signal apparition offers a new approach to overcome this challenge. In contrast to established thinking, the signal apparition concept capitalizes on periodic rather than random shot-to-shot modulation functions to facilitate shot separation. The approach allows us to efficiently populate the available data space in, for instance, the frequency wavenumber domain with energy from different simultaneous sources. The introduction of periodic modulation functions in seismic acquisition produces an effect where parts of the energy of one or more sources are shifted to different empty parts within the frequency wavenumber (f-k) domain. This so-called apparated energy can then be used to perfectly predict at low frequencies the remaining part of the signal in the regions of the f-k domain where the wavefields from the different sources overlap and to deterministically separate the sources. On a synthesized real data set from the Viking Graben in the North Sea, the suitability of this concept to marine seismic simultaneous sources is demonstrated. An operational scenario for a single vessel is designed where the source arrays are excited simultaneously. Having two closely spaced simultaneous sources is a challenging case for shot separation but, if successful, such a cost-effective single-vessel configuration offers highly significant productivity gains. The wavefield decoding results analysed pre-stack and post-stack are very encouraging and indicate that the signal apparition concept indeed has the potential to deliver the high quality source decoding required for many seismic applications.
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Transforming ocean bottom seismic technology into an exploration tool
More LessIt has been one of the enduring features of the oil and gas E&P industry that ocean bottom seismic (OBS) survey technology has yet to fulfil its potential. Ten years ago a similar observation was made (Berg, 2007). It was suggested then that towed-streamer surveys dominated the offshore survey market because the technology was wellestablished and inexpensive compared to ocean bottom techniques. The message was that an ocean bottom node (OBN) seismic acquisition could be ‘well worth the additional expense’. This was perhaps an unfortunate turn of phrase reinforcing an industry perception that seabed seismic was indeed a costly operation. In the intervening period, according to our best estimate, OBS surveys have in fact been slowly eroding the previous dollar value market share held by towed streamers, rising from around 4% in 2006 to approximately 15% in 2015 (see Table 1). It is also clear that the application of ocean bottom node systems rather than ocean bottom cable (OBC) is becoming the technology of choice for oil companies. More than 50% of OBS surveys now use some form of seabed node receivers (see Table 2). These have been developed over the years by a number of suppliers. Those OBC surveys that are being carried out are largely dependent on legacy systems with no similar record of recent innovation. We contend in this article that the technical case for node-based seabed seismic has been made in theory (See Ronen et al., 2009) and, in practice, judging by the increasing number of OBN surveys worldwide. It is also the case that in the current oil price crisis that oil companies for the foreseeable future are likely to concentrate their investment dollars on optimizing output from existing reservoirs in order to replenish reserves. This implies an increased role for OBS if the technology is offered at the right price point. We also believe that in a number of future exploration scenarios, e.g., complex geological settings in known oil provinces, OBN can be a more than viable alternative to towed-streamer solutions.
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Acquisition of long-offset data offshore Gabon shows how synchronized source technology adds flexibility to tailored acquisition solutions
Authors Thomas Mensch, Krzysztof Cichy, Risto Siliqi, Jo Firth and Benoit JupinetThe current climate in the oil exploration industry has engendered a strong push towards efficiency in acquisition. One technique that offers this is synchronized sources, or SyncSource, where sources are activated before the recording of data from the previous shot has been completed. As this may result in significant overlap of seismic data between successive shot records, it means that the data must be de-blended to recover the individual contribution from each source. However, this makes it possible to acquire data with higher trace density, smaller bins or longer records than can be achieved using conventional acquisition techniques. The simultaneous shooting technique has been commonly used in land acquisition for some years and its potential for ocean bottom and towed-streamer acquisition has been well documented (e.g. Moore et al., 2012; Davies et al., 2013; Poole et al., 2014). As long as the de-blending can be performed successfully, without compromising the data quality, there are many advantages to synchronized source acquisition in terms of quality and efficiency, based on the fact that the sources can be activated more frequently. This offers options for new acquisition geometries that had previously been either impossible or prohibitively expensive. It becomes possible to maximize efficiency either by increasing vessel speed without increasing shotpoint interval or reducing record length, or by reducing crossline bin spacing by adding a source rather than by reducing cable separation. Superior resolution can be achieved with a finer shot grid and a higher trace density to deliver higher-fold images with better signal-to-noise ratio and smaller bin size. Better illumination at depth can be achieved by increasing the recording time with the optimal source-receiver offset and azimuth, through the deployment of additional source vessels, without compromising the shot density or acquisition time. Recent advances in acquisition equipment and technology have combined with advances in de-blending and cross-talk attenuation algorithms in processing to allow blended acquisition offshore to become a realistic option. We have successfully conducted a full-scale commercial survey of 2500 km2 using this technology in order to acquire ultra-long offset data offshore Gabon. In Gabon’s South Basin, the geology is very complex with both pre- and post-salt plays, while compressional zones and salt basins cause challenges for illumination and imaging. This area is in a continental passive margin and exhibits the classic characteristics of a gravity-driven collapse system (Figure 1). Upslope, there is a broad region of extension, with normal faulting, rollover anticlines, thin salt with pillows and carbonate rafts. Downslope, there is a compressional domain with thrust faulting, folds, tilted diapirs and complex extruded salt structures. In between there is a transitional zone of upright diapirs and local welds (Xiao et al., 2016).
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The use of seismic attributes to enhance imaging of salt structures in the Barents Sea
Authors Luis Alberto Rojo, Alejandro Escalona and Lothar SchulteSeismic imaging and interpretation of salt bodies and surrounding minibasins have been always a challenge in areas with salt tectonics. Complex steeply dipping flanks along salt structures and adjacent stratigraphy remain problematic areas owing to the complex ray paths of the seismic waves travelling through the salt. This fact avoids a clear image of the salt-sediment interface, increasing the uncertainty of the interpretation of salt structures and surrounding minibasins. In this study, a 3D seismic cube located in the Nordkapp Basin (western Barents Sea), has been used to develop suitable interpretation attribute workflows which will allow the interpreter to: (1) improve interpretation and mapping techniques of salt bodies, salt-related structural elements, and adjacent minibasin stratigraphy; and (2) obtain a better understanding of salt kinematics, trapping geometry, and reservoir distribution adjacent to salt structures. The mapping and characterization of salt structures have been carried out using the attribute dip-illumination and variance. In contrast, chaos and anttrack highlight the presence of radial faults and have been used to better understand diapir kinematics. The surrounding minibasins have been interpreted using frequency filters and cosine of instantaneous phase, which have been essential to identify the different types of composite halokinetic sequences and the presence of sedimentary wedges which might act as potential reservoirs.
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Main components of full-waveform inversion for reservoir characterization
Authors Ehsan Zabihi Naeini, Tariq Alkhalifah, Ilya Tsvankin, Nishant Kamath and Jiubing ChengBuilding a 3D reservoir model, which has become a key part of reservoir management, is a challenging task. Classic seismic inversion techniques, both deterministic and stochastic, have attempted to reduce the uncertainty in reservoir modelling. However, these inversion methods are typically based just on amplitude and make a number of critical simplifying assumptions, such as that migrated data are accurate and can be modelled by 1D convolution (using reflection coefficients computed from the Zoeppritz equation or its linear approximations). Migration algorithms themselves may suffer from inadequate amplitude and multiple-scattering treatments. Full-waveform inversion (FWI), specifically designed for the direct estimation of the reservoir parameters, is proposed here as an alternative method for seismic reservoir characterization. This is clearly an ambitious goal, and it is not our intention to claim we have already achieved it. Instead, we intend to introduce the main components of such reservoir-oriented inversion and discuss a strategy for elastic, anisotropic FWI constrained by rockphysics models and facies types. Among the many challenges, we focus mostly on understanding the physics and describing some elements of an efficient forward-modelling engine. In particular, we show how analysing the radiation patterns helps to optimise the parameterisation and could reduce the null space.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)