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[PDF] Towed Streamer EM - Survey Design and Acquisition
TechNote - 4 July 2017
Towed Streamer EM delivers high density 3D CSEM data with exceptional acquisition
efficiency. Daily acquisition rates in the order of 140 sq. km are frequently achieved with the current record at over 200 sq. km in a single day. PGS acquires 2D GeoStreamer® data simultaneously with EM data, or 3D EM data over existing or planned 3D seismic. -
[PDF] Towed Streamer EM - Feasibility Studies
TechNote - 4 July 2017
PGS conducts EM feasibility studies in order to confirm that the Towed Streamer EM system will be sensitive to changes in target resistivity. A typical feasibility study involves forward modeling for a range of water depths, target depths, target sizes and resistivity scenarios. A feasibility study is not a pre-requisite to EM acquisition; rather it is a useful first step when looking at new areas or play types.
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[PDF] Towed Streamer EM - Processing and Inversion
TechNote - 4 July 2017
Towed Streamer EM data is monitored on board the vessel in real time as it is acquired. Once a sail line is complete offline QC is conducted, including a 1D QC inversion of every shot acquired. The EM field deliverable is denoised, navigation merged data, and this is delivered to both the customer and PGS’ EM imaging team for 2.5D and 3D anisotropic inversion which is performed onshore.
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[PDF] Rough Sea Surface Reflection Coefficient Estimation and Its Implication On Hydrophone-Only Pre-Stack Deghosting
12 June 2017
Deghosting a hydrophone-only measurement requires accurate description of seismic wavefield scattering at the sea surface. In recent years, attempts have been made to deghost hydrophone-only data using a frequency and angle dependent sea surface reflection coefficient. Nevertheless, rough sea surface reflection coefficients have both specular and non-specular contributions. In pre-stack hydrophone-only deghosting, the non-specular contributions are often assumed to be negligible. In this paper, we characterize the sea surface scattering using numerical modelling and quantify the contributions of both specular and non-specular reflections from two different realistic sea states. The results show that the magnitude of the specular reflection coefficient decreases with frequency and increases with angle. Moreover, the rougher the sea surface, the higher the magnitude of the non-specular contributions.
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[PDF] Modelling of Time-varying Rough Sea Surface Ghosts and Source Deghosting by Integral Inversion
12 June 2017
A major impediment to the full understanding of the data acquired in marine seismic is the restricting assumption of a flat and stationary sea surface used in certain pre-processing tools. A first step towards removing this assumption is to accurately account for the sea state (time varying free surface) in the deghosting process. On the receiver side, this is handled properly by using dual-sensor streamer. In this work, we present an integral approach to model the source side ghost effects from time-varying rough sea surfaces and show that the interaction with time-varying sea surfaces affect the subsurface reflections and may have a significant impact on seismic repeatability. We then continue with a theoretical derivation where we develop deghosting operator based on an integral inversion of the modeling operator. This formulation for source deghosting can account for the time-variation of rough sea surfaces.
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[PDF] High Fidelity Velocity Model Building, Imaging and Reflectivity Inversion – a Case Study Over the Viking Graben Area, Norwegian North Sea
12 June 2017
A case study was carried out over the Viking Graben area with special focus on the Volund field, where a high fidelity velocity model was built to solve the complex velocity variations in the overburden resulting from a widespread presence of channels and numerous small-scale sand injectites. Significant improvements to the image quality were achieved with the use of the detailed velocity model and by incorporating a variable quality field to compensate for visco-elastic effects. Alternative imaging algorithms and reflectivity inversion techniques were explored to overcome limitations of the imaging system. This further improved the spatial resolution, enhanced the signal and helped the interpretation of the remobilized sand system over the Volund field.
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[PDF] Very Sparse Seabed Seismic Acquisition for 3D/4D Reservoir Imaging with High-order Multiples. Application to Jubarte PRM
12 June 2017
A receiver decimation study has been performed for assessing the potential of using very sparse receiver grids in conjunction with high-order multiple imaging techniques for 4D reservoir monitoring. Reducing the density of receivers in a PRM (permanent reservoir monitoring) system will reduce the capital expenditure. For OBN (ocean bottom node) acquisition, cost saving can be achieve from more effective node deployment. The imaging process makes use of the multiple sea-surface reflections to significantly increase the fold, hence compensating for the reduced number of sensors. We will show that with the above imaging technology it is possible to use sparser seabed recording geometry without compromising the 3D or 4D image quality. Using the Jubarte PRM
dataset acquired by Petrobras, the decimation test consists of taking out receiver included into circular patches within a diameter of 600 m. The number of sensors is then reduced by a factor two. The results demonstrate that it is possible to break some established limit in term of seabed sensor sparseness without compromising the resolution of 3D/4D imaging. -
[PDF] A Robust FWI Method for Model Updating in High Contrast Bodies
12 June 2017
We present a robust method for Full Waveform Inversion (FWI), enabling the recovery of long-wavelength features of a velocity model. By using both transmitted and reflected waves the dynamically weighted FWI gradient enables high-resolution model building deeper than those achieved by diving waves alone. This reduces the dependency on long offset data acquisition.
The FWI approach uses a sophisticated regularization scheme to stabilize the inversion space. This methodology, which forms an extra constraint on the objective function, overcomes some of the limitations of the inversion in the presence of high contrast bodies. The implementation uses the split Bregman method, making it efficient and accurate.
We demonstrate the benefits of using the new gradient and regularization scheme by presenting the results on an intra-volcanic reservoir velocity model build from the Faroes-Shetland Basin. -
[PDF] A robust FWI gradient for high-resolution velocity model building
Author: Jaime Ramos-Martínez, Sean Crawley, Zuihong Zou, Alejandro Valenciano, Elena Klochikhina, Nizar Chemingui
SEG - 1 October 2016We 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.
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[PDF] Quantification of the Resolution in the Inversion results from Towed Streamer EM Data
Author: Joel Skogman, Johan Mattsson,
SEG - 1 October 2016The resolution of resistivity models from 3D inversion of Towed Streamer EM data is analyzed using the resolution matrix. In particular, a new resolution length derived from the resolution matrix is introduced. This measure is obtained by dividing the volume of an inversion cell with the corresponding diagonal element in the resolution matrix. Hence, the measure is independent on the actual cell discretization of the inversion domain. Resolution lengths are calculated both for a hypothetical case with noise free data and for resistivity sections obtained with regularized 3D inversion. The resolution of the ideal case with no noise constitutes the upper limit of the best possible resolving power of the Towed Streamer EM data. This limit is then compared against the results where smoothing regularization has been applied in the 3D inversion. A comparison is illustrated for inversion results of Towed Steamer EM data from a survey line in the Barents Sea.