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[PDF] Adaptive multiple subtraction by statistical curvelet matching
Author: Truong Nguyen, Richard Dyer
SEG - 1 October 2016In this paper, we present a novel algorithm that performs adaptive subtraction of a multiple model in the curvelet domain. The algorithm is based on the observation that the predicted model is an imperfect estimate of the actual multiples, containing two types of error: 1. a systematic error that manifests as an approximately constant phase and amplitude error within each subband and 2. a localized error that potentially varies from coefficient to coefficient. Adaptation of the model is automatically controlled by parameters provided by a statistical modelling step. Results show that the algorithm works well with different types of multiples and levels of noise.
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[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 2016Based 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.
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[PDF] A robust FWI gradient for high-resolution velocity model building
Author: Jaime Ramos-Martinez, 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] Petroleum Prospectivity in the Namibe and Southern Benguela Basins, Offshore Angola
1 June 2013
The underexplored offshore deepwater basins of Angola hold tremendous potential for hydrocarbons. These basins have marked similarities with the prolific hydrocarbon producing region of the Brazilian margin which abutted the Angola offshore area prior to the opening of the South Atlantic.
In 2011, PGS in association with Sonangol acquired a 12,700 km regional 2D grid using a dual-sensor towed streamer systems to provide broadband seismic imaging to address the hydrocarbon potential of the frontier Kwanza, Benguela and Namibe basins of Angola. This high quality seismic has been used to enhance the knowledge of potential petroleum systems of the Namibe and Southern Benguela basins.
Horst and graben structures that formed during rift phases are now visible in the depth-migrated seismic data and are overlain by 3-5 km of post-rift sediments. Thick salt, which is well known to the north, extends into the southern Benguela and northern Namibe basins. Post-rift channel and fan structures can be identified as high amplitude anomalies throughout the basin. These are observed to pinch out on the flanks of volcanic high structures and salt diapirs.
The improved imaging and resolution provided by these data significantly de-risk exploration in a frontier deepwater area where well costs are high.
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[PDF] Combining Velocity Model Building and Interpretation Techniques to Reduce Uncertainty in Pre-salt Imaging
1 June 2013
There is ever increasing interest in exploring pre-salt provinces across the South Atlantic Conjugate Margin The 2006 discovery of the Lula (Tupi) field in the Brazil Santos Basin and more recent discoveries such as the 2012 Azul Field in the Angolan Kwanza Basin in the Lower Cretaceous, have proven that there is a working hydrocarbon system across the South Atlantic Conjugate Margin.
The key to evaluating the prospectivity of these pre-salt provinces is generation of an accurate seismic image using pre-stack depth imaging methods. However, it is not possible to obtain a useable image below allochthonous salt using an approach entirely dependent upon algorithms to construct the PSDM velocity model.
The introduction of geological understanding through seismic interpretation, basin modelling and the (limited) available well data can aid PSDM velocity model accuracy and reduce uncertainty of the pre-salt image obtained (Merry, 2012). This paper reviews an approach taken to resolve the pre-salt image using data examples from Angola Blocks 24 and 25.
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[PDF] Simultaneous Long Offset (SLO) Towed Streamer Seismic Acquisition
1 June 2013
An innovative 3D towed streamer project in offshore Gabon used a dual-vessel continuous long offset streamer configuration to acquire 0-12 km offsets with ten dual-sensor streamers. Streamer control for the 6 km streamers was robust and avoided operational complications or logistical penalties. Simultaneous shooting maximized inline shot density for long record lengths, thus capturing unaliased deep target reflections from rugose base-salt and sub-salt horizons. Survey design benefited from prior 2D survey experiences with a variety of broadband source and streamer technologies, and the use of 2D streamers as long as 12 km. 3D illumination modelling further suggested that offsets as long as 16 km could be expected to yield useful base-salt and sub-salt reflections.
Wavefield separation processing yielded full receiver-side deghosting onboard, followed by an inversion-based separation of simultaneous shots onshore. The ultra-long 12 km offsets combined with strong amplitudes of deghosted low frequencies have yielded encouraging sub-salt and pre-salt imaging.
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[PDF] 3D High-Resolution Imaging Using Separated Wavefields
1 June 2013
Conventional wave equation migration extrapolates upcoming boundary data generated from primary reflections to image the subsurface. In marine seismic data processing, sea surface related multiples are a major challenge. Particularly, when the water bottom is shallow, short period multiples are difficult to deal with. Acquisition footprint of seismic imaging is another issue for shallow water bottom geology, where the primary reflections do not have the complete illumination coverage to the shallow targets, including the water bottom. In this paper, we review a method of separated wavefield imaging using sea surface related multiples. We present a data processing work flow of separated wavefield imaging including up and down-going wavefield separation using dual-sensor data, wave equation migration of wavefield extrapolation and imaging condition, and a post processing step of amplitude balancing. A shallow water example from Asia-Pacific is tested, where the separated wavefield imaging generates remarkable high resolution 3D images. The separated wavefield image has greater area of subsurface illumination, which mitigates the strong acquisition footprint. The method and results can be useful to reduce well drilling geohazards.
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[PDF] Global 3D acoustic FWI using sparse model parameterization
Author: Debanjan Datta, Mrinal K. Sen, Scott Morton, Faqi Liu
SEG - 1 September 2017Estimating a starting velocity model for Full Waveform Inversion can be challenging. It requires several passes of migration velocity analysis to obtain a model accurate enough to prevent cycle skipping. We present an alternative approach to estimate a 3D starting model using a global optimization method called Very Fast Simulated Annealing. To constrain the optimization problem with a large number of unknowns, we parameterize the 3D model with surfaces and velocities surrounding them and solve for the optimal parameters. The final estimated model from VFSA serves as a starting model for FWI. We demonstrate the effectiveness by comparing FWI results along a few 2D lines from the 3D model. The proposed method is largely automated and reduces numerous man hours required to build a starting model. We apply our proposed method to one toy model and one complex synthetic model.In both cases, we were able to obtain acceptable results. Use of VFSA with a sparse parameterization method makes our 3D global inversion a practical tool.
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[PDF] Application of a Full 3-D Designature Solution to a Wide Azimuth Survey
Author: Jack Kinkead, Jon Burren
SEG - 1 September 2017The current drive within the industry is towards a “Full Bandwidth” solution for seismic imaging coupled with acquisition solutions such as “Multi-Azimuth”, “Wide-Azimuth” and “Full-Azimuth”. Additionally, changes in source array geometry and multi-sensor receivers have also been employed. The proper processing of these types of data requires that special attention is given to how the wave field is produced, its propagation through the earth and finally how it is being captured. Of particular interest, and in order of their relative impact to the wave field, are:
- Surface ghost effects
- Source array effects
- Receiver array effects
- Earth effects
This paper will focus on the ghost and source array effects and the results achieved when they are handled properly.
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[PDF] Broadband Least-Squares Wave-Equation Migration
Author: Shaoping Lu, Xiang Li, Alejandro Valenciano, Nizar Chemingui, Cheng Cheng
SEG - 1 September 2017In general, Least-Squares Migration (LSM) can produce images corrected for overburden effects, variations in illumination, and incomplete acquisition geometry. The modeling engine of our LSM implementation consists of a visco-acoustic anisotropic one-way wave-equation extrapolator. It has the advantage of efficiently propagating high-frequency seismic data using high-resolution earth models (e.g. derived from Full Waveform Inversion). Applications to synthetic and field data examples (from the Gulf of Mexico and the North Sea) consistently delivered higher resolution images with better amplitude balance when compared to standard seismic migration