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[PDF] Source Deghosting and Demultiple for Calm and Rough Weather Conditions
Author: Elsa Cecconello, Endrias Asgedom, Walter Söllner
EAGE - 3 June 2019Source deghosting and demultiple algorithms have been extensively studied but mostly in the case of a flat seasurface. In this work, we consider time-varying sea-surfaces in different types of weather conditions and derive an inversion approach for removal of sea-surface effects. Starting from Rayleigh’s reciprocity theorem, we model seismic data including time-dependent wavefields scattered at the sea-surface, and highlight the temporal variation of these wavefields through simple synthetic examples, comparing two different weather conditions (calm and rough). We also reveal a limitation of source deghosting in the context of time-dependent wavefields: source deghosting affects the sea-surface multiples and can compromise the success of demultiple processing, which is applied in a later step. Results shows that this limitation is also important under calm weather conditions. To overcome this limitation, we propose simultaneously source deghosting and demultiple, therefore, removing all seasurface effects in one-step. Synthetic data examples are shown using the Sigsbee2B geological model.
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[PDF] Full Waveform Inversion of Simultaneous Long-Offset Data
Author: Nizar Chemingui, Alejandro Valenciano
EAGE - 3 June 2019The standard workflow for velocity model building (VMB) in complex regimes is an interpretive process that requires time-consuming manual intervention, and remains an error-prone process that can produce suboptimal results.
Here we discuss an application of Full Waveform Inversion (FWI) to automate the refinement of legacy velocity models generated by conventional workflows. We demonstrate our solution on a full-azimuth (FAZ) survey acquired in the Gulf of Mexico using dual-sensor streamers and blended sources in the form of simultaneous long-offsets (SLO). The dual-sensor acquisition provides low-frequency data while the SLO configuration enables the recording of long offsets in excess of 16 km. The long offsets and low frequencies were key to using both refractions and reflections to update the deeper parts of the velocity model. Our solution directly inverts the simultaneous data as
acquired in the field. We also employed an FWI velocity gradient that eliminates the migration isochrones, removing the reflectivity imprint from the model updates. The FWI application to the field survey successfully refined the geometry of the salt bodies including the base salt and the intra-salt enclosures. It also improved the RTM image
particularly the salt flanks and the subsalt reflectors. -
[PDF] A Robust Iterative Deblending Method for Simultaneous Source Acquisition
Author: Lian Duan, Maiza Bekara, Edwin Hodges, Paolo Terenghi
EAGE - 3 June 2019This paper presents a new method for simultaneous source deblending developed in the context of spatial coherency preservation. The deblending is formulated as an inverse problem which is solved in local overlapping windows extracted from the blended data. To constrain the solution, the unknown sources are assumed to be quasipredictive in the spatial domains. During the signal estimation, this will promote tolerance on impulsive blending noise and provide flexibility and robustness in coherency preservation depending upon the signal to noise ratio at a given frequency or within a given window. The proposed method is generic and can be applied to all configurations of simultaneous source acquisition and can deal with an arbitrary number of sources. Tests on numerically blended real towed streamer marine data show good deblending results with an excellent compromise between signal preservation and cross-talk contamination across a wide frequency range, particularly towards the low end. The proposed method is robust and flexible and can be applied in the early stages of a typical marine seismic processing
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[PDF] Improved HSEQ and survey efficiency demonstrated with a new remotely operated streamer cleaning tool
Author: Andrew Long, Rune Tønnessen, Trygve Skadberg, James Wright
AEGC - 2 September 2019Barnacle growth on seismic streamers creates noise on the recorded signals. Keeping a low noise level in barnacle areas is challenging to seismic crews: Frequent workboat trips are required for streamer cleaning, and workboat operations are weather-dependent and considered undesirable from an HSE perspective. In 2007 a purely mechanical and autonomous streamer cleaning unit (SCU) was introduced that is launched and recovered from the workboat. In 2016 we subsequently developed a
remotely operated streamer tool (ROST) that is capable of launching and recovering SCUs to/from the streamer without the use of a workboat, and which is therefore less
affected by weather. The ROST is operated from a support vessel that is independent of the seismic vessel. We present experiences from four surveys that were
subject to different operating conditions. Operation in high sea states is demonstrated on a survey offshore Namibia, and operation in extreme currents is demonstrated on another survey east of South Africa. A noise removal method is also presented that allows operation of the ROST while online during seismic recording. The method was first applied on the offshore South African survey, and later on two surveys in offshore Angola. It is demonstrated that work boat exposure hours can be reduced by 70-80%, and a 14 streamer spread can be cleaned twice a week while acquiring seismic in all types of operating conditions. -
[PDF] Methodology Utilizing Continuous Source and Receiver Wavefields - Signal to Noise Ratio Considerations
Author: Stian Hegna, Tilman Klüver, Jostein Lima
EAGE - 3 June 2019The peak sound pressure levels (SPL) and the sound exposure levels (SEL) are significantly reduced with the seismic methodology based on the emission and recording of continuous source and receiver wavefields. The peak sound pressure levels are reduced because the emitted energy is spread out in time approaching the properties of white noise, in contrast to conventional marine sources where the emitted wavefield is approaching the properties of a spike. The energy levels emitted by marine seismic sources and its impact on the signal to noise ratio has been discussed in a number of papers. The ability to achieve comparable penetration in depth with the continuous wavefield method despite of significantly reduced peak SPL and SEL levels compared to the conventional marine seismic method is related to several aspects such as attenuation of shot generated noise, improved spatial sampling of source positions, and improved ability to attenuate noise with long continuous records. In this paper we will discuss the phase spectrum of the wavefield emitted by a source, and its influence on the signal to noise ratio after deconvolving the emitted wavefield from the received wavefield.
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[PDF] Continuous Wavefields Method – a New Marine Seismic Acquisition and Processing Approach
Author: Stian Hegna, Tilman Klüver, J. Lima
EAGE - 3 June 2019There are several challenges with the conventional 3D marine seismic method that are addressed with a novel seismic methodology based on the emission and recording of continuous source and receiver wavefields. The challenges that are discussed in this paper are coarse spatial sampling of source positions, environmental impact of marine seismic sources, and efficiency of the acquisition. The coarse spatial sampling of source positions is addressed by emitting continuous source wavefields. The environmental impact of marine seismic sources is reduced by spreading the emitted energy out in time. The efficiency of marine seismic acquisition can be improved with the continuous wavefields method because there are no vessel speed limitations caused by the method as such, and because source elements can be distributed crossline in multiple positions improving the coverage in each sail line and hence reducing the number of sail lines needed to cover a certain area.
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[PDF] Making the Transition from Discrete Shot Records to Continuous Wavefields – Real Data Application
Author: Tilman Klüver, Stian Hegna, Jostein Lima
EAGE - 4 June 2018In this paper, we present real data applications of the acquisition and processing method outlined in the companion paper (Hegna, S., Klüver, T., and Lima J., 2018, Making the transition from discrete shot records to continuous
wavefields – Methodology: Expanded abstract submitted to the EAGE annual meeting). We discuss the design of sources that emit continuous wavefields approaching the properties of white noise using air-guns. The design
minimizes correlation between sources and allows a six-source configuration with a vessel towing six strings of airguns, thereby increasing efficiency or cross-line sampling. The environmental benefit of spreading source energy in time will be demonstrated. We illustrate the processing method based on continuous source and receiver wavefields on seismic data acquired on top of data acquired previously based on discrete shot records. Both data sets will be compared at image level. -
[PDF] Intra-Chalk Porosity Variations in Norway-Danish Central Graben: Integrated Mapping Using Broadband Elastic Attributes
Author: Noémie Pernin, Tim Bird, Cyrille Reiser
EAGE - 4 June 2018Understanding the influence of porosity on elastic properties through rock physics analysis is essential in seismic reservoir characterisation and key in the context of the chalk play. It is known that acoustic impedance is strongly correlated with porosity in carbonates but also that the chalk properties can vary widely creating intra-chalk layers of varying reservoir quality. This integrated study shows how reliable broadband relative elastic attributes tie at the wells and are able to map lateral and vertical changes of porosity within the Chalk interval in the Norwegian and Danish North Sea Central Graben.
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[PDF] Shot to Shot Source Signature Variation Correction
Author: Hocine Tabti, Jens Fredrik Wisløff, Torben Høy, Martin Widmaier
EAGE - 4 June 2018In this paper we discuss the ability to correct for shot to shot source signature variations and show how this could impact current survey source specifications. Contractual survey specifications define acceptability limits as to which
changes can be tolerated based on source wavelet attribute changes. Parts of two acquisition sequences have been acquired with deliberate survey source out-of-spec configurations. The out-of-spec tests included both source dropouts and pressure drop in separate sequences. Shot by shot source signatures have been generated from near field measurements. These were used to correct the seismic data for source wavelet changes including those caused by the out-ofspec pressure and source dropouts. We have demonstrated in this work that changes in the source signature, including changes beyond out-of-spec limits, can be estimated at every shot point and can efficiently be corrected for in processing. Current source out-of-spec limits are possibly too rigid if source variations
can be estimated at each shot point and corrected for in processing. -
[PDF] 4D Broadband Towed-Streamer Assessment, West Africa Deep Water Case Study
Author: Didier Lecerf, David Raistrick, Bertrand Caselitz, M. Wingham, J. Bradley, B. Moseley
EAGE - 4 June 2018A 4D broadband assessment has been performed in a deep offshore West Africa environment using repeated dualsensor streamer acquisitions. Three sail lines have been re-acquired only a few weeks after the original acquisition with standard 4D acquisition consideration. The study has compared the data repeatability metric NRMS along the processing sequences using, on one side, a reconstructed band limited 4D data corresponding to a single hydrophone and on the other side an extended bandwidth 4D data using the up-going wavefield only. Because the datasets have been recorded using the same acquisition, the main differences come from the bandwidth discrepancy and the variable sea-state. The 4D “Up-going on Up-going” preserves, for all processing steps, around 1% NRMS benefit against the 4D hydrophone-only. Because the NRMS comparison is biased by the dominant frequency discrepancy, the 1% gain does not fully reflect the detectability advantage of the 4D broadband. The NRMS analysis on the low frequency part has demonstrated the clear improvement on 4D broadband and a
qualitative evaluation on the 4D differences has highlighted the negative effect of the sea-state on the hydrophoneonly 4D results.