Isabelle Lecomte

Resolution and illumination analyses in PSDM: A ray-based approach

Prestack depth migration (PSDM) should be the ultimate goal of seismic processing, producing angle-dependant depth images of the subsurface reflectivity. But the expected quality of PSDM images is constrained by many factors. Understanding all of these factors is necessary to improve depth imaging of geologic structures. In all PSDM approaches, e.g., Kirchhoff or wave-equation, migration always includes compensating for wave propagation in the overburden (back propagation, downward continuation, etc.), before focusing back the reflected/diffracted energy at each considered location in depth (imaging). Ideally, we would like to retrieve the reflectivity of the ground as detailed as possible to invert for the elastic parameters. But the waves perceive the reflectivity through “thick glasses,” seeing blurred structures, and not necessarily all of them, depending on the illumination. Only a filtered version of the true reflectivity is therefore retrieved. Being able to estimate these filters, the so-called re...

Simulated Prestack Local Imaging: a robust and efficient interpretation tool to control illumination, resolution, and time-lapse properties of reservoirs.

Summary The “Simulated Prestack Local Imaging” (SimPLI 1 ) concept is a new method helping interpreters of migrated seismic data to control and constrain their interpretation of local structures such as oil/gas reservoirs. There is no need in this approach to first generate synthetic data, and then migrate to calculate the migrated seismic response of reservoirs models. Simulated prestack migrated sections are quickly obtained as functions of the survey, emitted pulse, wave-modes, and local reservoir structure. An interpreter can therefore check interactively various reservoir scenarios in terms of illumination (survey-planning, reservoir characterization), resolution, and time-lapse evolution related to rock parameter changes during the production. Input can be interpreted horizons with attributes, property grids from reservoir models and inversion results, or hypothetic models. The results are prestack migrated sections, in depth or time. The SimPLI method is a convolution technique, which goes far beyond the classic 1D convolution as being able to predict 2D/3D effects of illumination, resolution and prestack acquisition, but without requiring experts in modeling and migration.

Improved applicability of ray tracing in seismic acquisition, imaging, and interpretation

Ray-based seismic modeling methods can be applied at various stages of the exploration and production process. The standard ray method has several advantages, e.g., computational efficiency and the possibility of simulating propagation of elementary waves. As a high-frequency approximation, the method also has a number of limitations, particularly with respect to a lack of amplitude reliability in the presence of rapid changes of the model functions representing elastic parameters and interfaces. Given the objective of improving the applicability of the standard ray method, we present a strategy that does not require specific extension to finite frequencies. Instead, we define each ray-based process as an element of a system that, as a composite process, is able to obtain better results than the ray-based process applied alone. Other elements of the composite process can be finitedifference modeling or numerical solutions for surface and volume integrals, which are basic constituents of Kirchhoff modeling and imaging. We also include among the process elements recently developed techniques for simulating the migration amplitude on a target reflector and in a local volume, e.g., a reservoir zone. The model is decomposed according to its complexity into volume elements, surface elements, or a combination. The composite process consists of a specified interaction between process elements and model elements, which fits well with the philosophy of modern software design. Model elements that will be exposed to ray-tracing algorithms may need appropriate preparation, e.g., smoothing and resampling. We demonstrate specifically, in a tutorial example, that simulating the seismic response from a reflector by ray-based composite processes can yield better results than applying standard ray tracing alone.

Review of Ray Theory Applications in Modelling and Imaging of Seismic Data

Throughout the last twenty years, 3D seismic ray modelling has developed from a research tool to a more operational tool that has gained growing interest in the petroleum industry. Various areas of application have been established and new ones are under development. Many of these applications require a modelling system with flexible, robust and efficient modelling algorithms in the core. The present paper reviews the basic elements of such a system, based on the ‘open model’ concept and the ‘wavefront construction’ technique. In the latter, Červenýs dynamic ray tracing is an intrinsic part. The modelling system can be used for generating ray attributes and synthetic seismograms for realistic 3D surveys with tens of thousands of shots and receivers. Moreover, some other types of application areas are illustrated: Production of Greens functions for prestack depth migration and hybrid modelling (combined ray and finite-difference modelling), attribute mapping and illumination analysis, both for survey planning and interpretation. Finally, the concepts of ‘isochron rays’ and ‘velocity rays’ related to seismic isochrons have been introduced recently, with very interesting future applications.

Have a look at the resolution of prestack depth migration for any model, survey and wavefields.

Summary Integral equation approaches to acoustic imaging within the Born or Kirchhoff approximation give compact algorithms for depth migration, using pre-calculated Green’s functions. They moreover give direct access to a fundamental parameter: the scattering wavenumber. This parameter controls the resolution of the depth migrated section. Using efficient methods to obtain asymptotic Green’s functions, resolution functions can now be calculated numerically in both 2D and 3D. Any location in any background model can be considered. All kind of surveys and subselections of shot/ receiver couples are possible. As multi-field imaging is available, new set of scattering wavenumbers may be considered. Resolution functions are important at a survey planning stage to define the optimal acquisition. They are also essential for the interpretation of depth migrated sections, i.e., to discriminate between true reflections and artificial events due to bandlimited signals and limited-aperture. Such resolution effects should be corrected prior to any measurements of reflectivity.

Shallow Landslides and Their Dynamics in Coastal and Deepwater Environments, Norway

In this manuscript, we present the first results of integrated slope stability studies to investigate smaller-scale mass movement processes in different physiographic settings of Norway. These include coastal areas (Sorfjord, Finneidfjord), and pristine open ocean settings in intermediate (Vesteralen) and deep waters (Lofoten) on the Norwegian margin. Triggers, pre-conditioning factors and sedimentary processes associated with these landslides are currently not well constrained.

Towards geophysical and geotechnical integration for quick-clay mapping in Norway

Quick clay is a known hazard in formerly-glaciated coastal areas in e.g., Norway, Sweden and Canada. In this paper, we review the physical properties of quick clays in order to find a suitable, integrated and multi-disciplinary approach to improve our possibilities to accurately identify the occurrence of quick clay and map its extent both vertically and laterally. As no single geophysical method yields optimal information, one should combine a variety of geophysical methods with geotechnical data (in situ measurements using Cone Penetration Testing (CPTU), Seismic CPTU (SCPTU) and Resistivity CPTU (RCPTU); laboratory tests) for an in-depth quick-clay assessment at a given site. In this respect, geophysical data are used to fill the gaps between geotechnical boreholes providing ground-truth. Such an integrated and multi-disciplinary approach brings us closer to 2D or pseudo-3D site characterization for quick clays and as such, an improved assessment of the potential hazard they pose. The integrated approach is applied in practice on two Norwegian quick-clay sites. The first site, Hvittingfoss, was remediated against potential landslides in 2008 whereas the second one, Rissa, was the scene of a major quick-clay landslide in 1978, quick clays being still present over a large area. The collected data and preliminary site characterizations illustrate the high diversity as well as the complexity and clearly emphasize the need for higher resolution, careful imaging and calibration of the data in order to accomplish the assessment of a quickclay hazard.

Ray-based seismic modeling of geologic models: Understanding and analyzing seismic images efficiently

AbstractOften, interpreters only have access to seismic sections and, at times, well data, when making an interpretation of structures and depositional features in the subsurface. The validity of the final interpretation is based on how well the seismic data are able to reproduce the actual geology, and seismic modeling can help constrain that. Ideally, modeling should create complete seismograms, which is often best achieved by finite-difference modeling with postprocessing to produce synthetic seismic sections for comparison purposes. Such extensive modeling is, however, not routinely affordable. A far more efficient option, using the simpler 1D convolution model with reflectivity logs extracted along verticals in velocity models, generates poor modeling results when lateral velocity variations are expected. A third and intermediate option is to use the various ray-based approaches available, which are efficient and flexible. However, standard ray methods, such as the normal-incidence point for unmigrat...

Efficient and flexible seismic modeling of reservoirs: A hybrid approach

Model-based analysis of seismic data, now recognized as a key to a better understanding of images of subsurface structure, began in the 1960s. But these models were inflexible, primarily due to hardware and software limitations, and generally were simple homogeneous isotropic horizontal layers—i.e., with only vertical velocity variations.

3D structure and formation of hydrothermal vent complexes at the Paleocene-Eocene transition, the Møre Basin, mid-Norwegian margin

AbstractThe mid-Norwegian margin is regarded as an example of a volcanic-rifted margin formed prior to and during the Paleogene breakup of the northeast Atlantic. The area is characterized by the presence of voluminous basaltic complexes such as extrusive lava and lava delta sequences, intrusive sills and dikes, and hydrothermal vent complexes. We have developed a detailed 3D seismic analysis of fluid- and gas-induced hydrothermal vent complexes in a 310  km2 area in the More Basin, offshore Norway. We find that formation of hydrothermal vent complexes is accommodated by deformation of the host rock when sills are emplaced. Fluids are generated by metamorphic reactions and pore-fluid expansion around sills and are focused around sill tips due to buoyancy. Hydrothermal vent complexes are associated with doming of the overlying strata, leading to the formation of draping mounds above the vent contemporary surface. The morphological characteristics of the upper part and the underlying feeder structure (condu...