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Author: Mattias Oscarsson-Nagel, Walter Söllner, Øystein Trætten, B. Armstrong, D. Nams, P. Yeatman
EAGE - 3 June 2019
There is a desire by the marine geophysical industry for a seismic source with a low environmental footprint.
Received sound pressure level (SPL) and sound exposure level (SEL) can restrict how seismic surveys can be conducted in sensitive areas.
To address this desire, we have developed a broadband non-impulsive source based on the concept of modular transducer elements. The source has a controlled output that can emit arbitrary signals, enables flexible source geometries, and can produce ultra-low frequency content that facilitates robust full waveform inversion.
Sea trials and testing indicate that the developed source will meet the challenging output demands and exhibit the necessary robustness to be a viable seismic source for the future. Large scale testing is planned in 2019.
Author: Matthew Pyett
EAGE - 3 June 2019
Located in the easternmost part of the Mediterranean Sea, offshore Lebanon remains a frontier, untested basin surrounded by many discoveries and proven working petroleum systems. Large gas fields have been discovered in clastic reservoirs within the southern portion of the Levantine Basin, these sediments are derived from the Nile Delta cone to the south, proving the existence of good quality pre-salt clastic reservoirs. These Nile Delta derived sediments are predicted to extend into the northern sector of the Levantine Basin, thus depositing offshore Lebanon. The 10,000 sqkm 3D dataset helps to reduce the geological uncertainty and helps identify the presence of channels and fan systems across the Levantine Basin. Biogenic gas is now proven in many clastic and carbonate discoveries in close proximity to this region. In addition, the narrow margin and deeper sections of the basin are believed to have the potential for additional thermogenic oil prone source rocks charging both deeper basin clastics and rifted Mesozoic carbonates units along the Levant Margin. With the opening of the 2nd Licensing Round imminent and the first wells offshore Lebanon scheduled for 2019 the industries attention remains focused offshore Lebanon and the general Eastern Mediterranean province.
Author: Jaime Ramos-Martinez, Alejandro Valenciano, Xiaoyang Jiang, Nizar Chemingui
EAGE - 3 June 2019
Full Waveform Inversion (FWI) success depends on producing seamless short- and long-wavelength model updates while avoiding cycle skipping. In its traditional implementation, FWI risks converging to an inaccurate result if the data lacks sufficient low frequencies or the starting model is far from the true one. Additionally, the model
updates may display a reflectivity imprint before the long-wavelength features are fully recovered. A solution to these fundamental challenges combines the quadratic form of the Wasserstein distance (W2-norm) for measuring the data misfit with a robust implementation of a velocity gradient. The W2-norm reduces the risk of cycle skipping
whereas the velocity gradient effectively eliminates the reflectivity imprint and emphasizes the long-wavelength model updates. We illustrate the performance of the new solution on a field survey acquired offshore Brazil. There, we demonstrate how FWI successfully updates the earth model and resolves a high-velocity carbonate layer that was missing from the starting model.
[PDF] Improved HSEQ and Survey Efficiency Demonstrated with a New Remotely Operated Streamer Cleaning Tool
Author: Trygve Skadberg, Rune Tønnessen, James Wright
EAGE - 3 June 2019
Barnacle growth on seismic streamers creates noise on the recorded signals. Keeping a low noise level in barnacle areas is challenging to the seismic crews. Frequent workboat trips are required for streamer cleaning. Workboat operations are weather dependent and are considered undesirable from an HSE perspective. In 2007 a purely mechanical and autonomous Streamer Cleaner Unit (SCU) was introduced. The SCU is launched and recovered from the workboat. In 2016 Tønnessen and Skadberg (2016) presented a Remotely Operated Streamer Tool (ROST) capable of launching and recovering SCUs to/from the streamer without the use of a workboat, and which is less dependent on weather. The ROST is operated from a Support vessel. Experiences from four surveys are presented. Operation in high sea states is demonstrated on a survey offshore Namibia and operation in extreme currents demonstrated on another survey east of South Africa. A noise removal method is presented that allows operation of the system while online. The method was first applied on a survey offshore South Africa and later on two surveys offshore Angola. It is demonstrated that work boat hours can be reduced by 70-80%, and a 14 streamer spread can be cleaned twice a week while acquiring seismic.
[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 2019
Barnacle 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.
Author: Nizar Chemingui, Alejandro Valenciano, Tony Martin
EAGE/PESGB Workshop on Velocities - 4 April 2019
Conventional velocity model building (VMB) in complex regimes, such as intra and subsalt data, requires time consuming manual intervention. It is a process that can produce unreliable models, leading to an increase in uncertainty for subsalt lead evaluation. We demonstrate an application of Full Waveform Inversion (FWI) to refine legacy velocity models generated by conventional VMB. We present our solution on a simultaneous long offsets (SLO) dataset from the Gulf of Mexico, acquired with dual-sensor streamers, which provided low frequency rich data. The SLO configuration recorded data with 16 km of offset, enabling both refractions and reflections to update the deeper parts of the velocity model. We employ an FWI velocity gradient that eliminates the migration isochrones. This provides support for the intra and subsalt model updates by removing the reflectivity imprint from the updated models. The FWI application successfully refined the geometry of the salt bodies including the base salt and the intrasalt enclosures. RTM images show a marked uplift, particularly for both the salt flanks and subsalt reflectors.
Author: Didier Lecerf
EAGE - 3 June 2019
Successful time-lapse studies require special care when it comes to the removal of undesirable artefacts caused by the differences in acquisition geometries. By attempting to repeat the source and receiver geometries between surveys as precisely as possible, any subsequent 4D noise is minimized. However, in some cases it is not possible
to repeat the survey geometries between vintages. This is the case when a towed streamer survey is compared with an OBS acquisition. The image domain approach for correcting illumination differences between 4D datasets builds on wave equation reflectivity inversion using Point Spread Functions (PSFs). In this two-step least-squares
imaging method, the reflectivity is recovered by explicitly computing multi-dimensional PSFs using wave-equation modeling and de-convolving these PSFs with the final migrated image.
We define a 4D formulation which is not dependent on geological and/or reservoir production constraints by introducing the concept of cross-survey PSFs (XPSFs). As shown using synthetic data examples, the joint reflectivity inversion process delivers superior results when compared to separate inversions as it ensures a more robust recovery of the 4D effects. The presented new methodology using cross-survey Point Spread Functions (XPSFs) ensures consistency of the wavefields for recovering the 4D signal.
[PDF] Innovative Inversion Schemes for Model Building and Reflectivity Estimation: A Deep Water West African Case Study
Author: Tony Martin, Marion Barbaray, Gareth Venfield, Vikash Chavda
EAGE - 3 June 2019
Late Cretaceous channel systems create structural uncertainty and impact amplitude fidelity of both Late and Early Cretaceous plays in deep water Côte d’Ivoire seismic data. We present a case study using a combination of Full Waveform Inversion (FWI) and image domain Least-squares migration (LSM) to resolve the impact of complex Late Cretaceous channel systems on deeper targets. A full wavefield FWI approach using a velocity kernel that eliminates the reflectivity imprint, created an accurate velocity model. This removed the structural uncertainty when used in the imaging step. A reflectivity estimate was then determined using LSM. The final dataset had improved amplitude fidelity by compensating for the loss of bandwidth and illumination associated with the Late Cretaceous channels.
[PDF] Deep-seated focused fluid migration as indicator for hydrocarbon leads in the East Shetland Platform, North Sea Province
Author: Jens Karstens, Philipp Müller, Christian Berndt, Stefano Patruno
Geological Society London Special Publications SP494 - 1 July 2019
Hydrocarbon exploration in the North Sea Basin has revealed a multitude of focused fluid conduits, which manifest in seismic data as pipe or chimney structures that in some instances are connected to underlying hydrocarbon reservoirs. 3D seismic data from the eastern margin of the East Shetland Platform (ESP) reveal the presence of more than 450 focused fluid conduits. Most of these initiate at the Base Tertiary Unconformity (BTU) and crosscut the overlying sediments. The focused fluid conduits correlate with intra-platform basin structures beneath the BTU and with permeable sediments lobes, channels and deltaic units in the overlying Paleocene to Eocene successions, which include known hydrocarbon reservoirs (e.g. Bressay, Bentley, Skipper or Piper). Clusters of pipes associated with other channels and deltaic units may indicate the presence of additional prospects at the eastern margin of the ESP. Our study highlights the potential of using seismically imaged focused fluid system analyses in hydrocarbon exploration in platform areas on both sides of the Viking Graben and other frontier areas as they reveal the presence of working hydrocarbon charge pathways.
Author: Volker Dirks, Senira Kattah
Hart E&P - 1 November 2016
WEI leads to improved amplitude fidelity and image resolution in the Jequitinhonha Basin offshore Brazil.