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1 January 0001
[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: Maz Farouki, Andy Bromley, David Cavalin
APGCE - 29 October 2019
The East Java Basin is a prolific hydrocarbon province in Indonesia in which exploration plays have typically targeted the pinnacle reefs of the Oligocene-Miocene Kujung carbonates. Robust imaging of the deeper section however has historically been challenged due in part to the limited bandwidth of conventional seismic data. A further challenge arises from the imaging artifacts introduced at the overlying Wonocolo carbonate platform, where the slow velocities of deep channel incisions are in strong contrast with the fast carbonate velocity. Prestack depth migration can be used to address this issue, but requires a velocity model with sufficiently high spatial and temporal resolution to precisely capture such velocity variations. To this aim, Full Waveform Inversion is used in the velocity model building. Whereas legacy data in the basin has struggled to image deeper than the Kujung level, seismic acquisition and imaging methods are now providing data with greater bandwidth and deeper penetration of signal. In Madura, the deeper Eocene Ngimbang formation as well as basement are now much better resolved, opening up the potential play in the Ngimbang clastics.
[PDF] Application of Full Waveform Inversion to resolve an eroded shallow carbonate platform, North Madura, East Java, Indonesia
Author: David Cavalin, Nurrul Ismail, Tom Paten, Kola Agbebi, Dave Lim
IPA - 2 September 2019
Proven plays in North Madura have been identified in the Miocene carbonate and syn-rift Eocene clastic systems. 3D broadband seismic data was acquired in order to obtain higher resolution and deeper imaging of potential prospects and leads within these systems.
Besides improving resolution, penetration, imaging and seismic attributes, broadband data has another major advantage; it allows the low frequencies of the recorded data to drive a more complex velocity model update technique: Full Waveform Inversion (FWI).
Standard traveltime reflection tomography techniques provide long to mid wavelength velocity updates but generally fail in updating shallow water environments while giving limited resolution in the rest of the velocity model. However, a more accurate velocity model is needed to correct rapid vertical and lateral velocity heterogeneities. Small-scale velocity anomalies in this survey include gas bearing river channels, whereas eroded shallow carbonate platforms present additional challenges related to structural distortions observed on the time domain outputs. Such velocity anomalies must be resolved prior to imaging the deeper section.
FWI operates by minimizing residuals calculated between recorded shot records and modeled shots, within a certain frequency band. An iterative approach was used to update the velocity model starting with low frequencies available from the broadband seismic data. Using the lowest possible frequency data, containing coherent signal, minimizes the risk of cycle skipping thus allowing the FWI update to start from a benign velocity model. The successive passes of FWI introduced details into the velocity model conforming to the geological challenges identified at the beginning of the project.
Combining both broadband data and FWI velocity model building (VMB) is key in correcting for structural distortion and amplitude dimming particularly associated with shallow velocity anomalies. This methodology allowed us to confidently position in depth the potential plays and leads affected by velocity anomalies in the shallower section.
Author: Jens Fredrik Wisløff, Daniel Barker, Stian Hegna, Alex Goertz, Florent Pesnel, Dorian Richelmi
SEG - 26 October 2014
The current implementations of marine source modeling theory have been calibrated and adjusted against measured signatures with a goal of high modeling accuracy within a limited frequency band. As multicomponent streamers and source deghosting allows for utilizing a significantly broader range of frequencies in seismic imaging, adjustments to the modeling are necessary in order to achieve a better match between measured and modelled signatures over the expanded frequency band. This includes significant changes to the calibration process such as considering deghosted measurements and avoiding the historically rooted standard DFS V filtering. The modeling results after applying the improved calibration show a very good match with measured array signatures over a wide frequency range.
Author: Chaoguang Zhou, Zijian Liu, Dan Whitmore, Samuel Brown
SEG - 26 October 2014
Anisotropic depth model building using surface seismic data alone is non-unique and one of the major reasons is that there is ambiguity among the anisotropy parameters.Additional well data can help reduce such ambiguity and thus yield a more accurate anisotropic model for depth imaging. In this paper, we present a tomographic model building approach that uses well data together with surface seismic data. It consists of four major steps: preparing data, estimating the local anisotropic parameters at the well locations, extrapolating the local parameters to generate a volumetric anisotropic model for further tomographic update, and finally tomographic updating with well control.
Author: Samuel Brown, Alejandro Valenciano, Nizar Chemingui, Dan Whitmore, Paul Feldman, Bruno Virlouvet, Sverre Brandsberg-Dahl
SEG - 18 October 2015
We demonstrate a novel workflow for high fidelity full azimuth tilted transverse isotropy (TTI) velocity model building in an area of the Gulf of Mexico (GOM) with complex salt geometries. The workflow combines wavelet shift tomography for building a detailed overburden model, angle tomography driven by azimuth sectored reverse time migration (RTM) angle gathers for updating the sediment model in complex mini basins, underneath overhangs, and subsalt, and finally full waveform inversion (FWI) to resolve fine scale features in the overburden model. A full azimuth, long offset dual sensor acquisition provides the illumination needed in this notoriously difficult imaging area, as well the low frequencies and long offsets required for an optimal FWI solution.
Author: Alba Ordoñez, Walter Söllner, Tilman Klüver, Leiv Gelius
SEG - 18 October 2015
This paper discusses imaging using the wavefield separated into upgoing and downgoing components and including primaries and multiples. We image the reflectivity by solving Fredholm integral equations at every depth level of the image, after extrapolating the wavefields with a oneway wave equation propagator. The reflectivity, or reflected wavefield in the hypothetical experiment with point sources and receivers at the image level, is determined free of multiple scattering from the overburden. We also show how the reflectivity, obtained by inverting the matrix form of the Fredholm integral equations, can be extended to angle-dependent reflectivity at the image point.
[PDF] Imaging the total wavefields by reflectivity inversion using amplitude-normalized wavefield decomposition: field data example
Author: Alba Ordoñez, University of Oslo, Walter Söllner, Tilman Klüver
SEG - 1 October 2016
Based on an example acquired with dual-sensor towed-streamers and time and depth distributed sources, we image the total upand downgoing wavefields using primaries and multiples. The imaging framework is based on computing the subsurface impulse response (i.e., reflectivity). At every depth level, the latter can be obtained by inverting the matrix form of an integral equation defined in terms of the amplitude-normalized upgoing pressure and downgoing vertical velocity wavefields. This procedure gives the reflectivity matrix. The total upgoing wavefield used in the imaging scheme is composed of the scattered energy from primaries and multiples. The primary reflected wavefield is generated by a direct downgoing source wavefield, which is mostly passing the acquisition surface at offsets smaller than the nearest data channel. Hence, the most relevant part of the direct wavefield is not measured in the studied example. From the near-field pressure measurements, we predict the missing direct arrivals needed to image the primary reflected wavefield; and we then synthesize the total downgoing wavefield by adding the downgoing scattered energy. By downward extrapolating the total up- and downgoing wavefields, the information of the subsurface is extracted from the reflectivity matrix in the spatial and angular domains.
Author: Jaime Ramos-Martínez, Sean Crawley, Zuihong Zou, Alejandro Valenciano, Elena Klochikhina, Nizar Chemingui
SEG - 1 October 2016
We describe a robust method to produce long-wavelength updates in gradient-based Full Waveform Inversion (FWI). The gradient is computed by applying dynamic weights in the velocity sensitivity kernel derived from impedance and velocity parameterization of the classical objective function. The new kernel implementation effectively eliminates the migration isochrones produced by the specular reflections and emphasizes the low-wavenumber components in the gradient in heterogeneous media. The new gradient is able to provide velocity updates beyond penetration depth of diving waves. We use a synthetic example to illustrate how this dynamically weighted FWI gradient successfully recovers the background velocity from pre-critical reflections. We apply the new approach to 2D and 3D dual sensor data from deep-water Gulf of Mexico. Results show how the dynamically weighted FWI gradient can combine both transmitted and reflected energy in a global FWI scheme and provide high-resolution velocity models without migration imprint in the updates.