Thomas Hertweck

Data stacking beyond CMP

Stacking has been used in seismic data processing for a long time. In fact, stacked sections (or volumes in 3D) are standard deliverables in the industry, and the concepts of common-midpoint (CMP) gathers and normal moveout (NMO) correction are mentioned in almost every textbook on seismic processing. Although the general trend is toward prestack imaging (either in time or in depth), the construction of stacked sections remains an important step within the seismic processing flow, since they are almost always the first available interpretable images of the subsurface.

Aperture effects in 2.5D Kirchhoff migration: A geometrical explanation

Seismic images obtained by Kirchhoff time or depth migration are always accompanied by some artifacts known as migration noise, migration boundary effects, or diffraction smiles, which may severely affect the quality of the migration result. Most of these undesirable effects are caused by a limited aperture if the algorithms make no special disposition to avoid them. Strong amplitude variation along reflection events may cause similar artifacts. All of these effects can be explained mathematically by means of the method of stationary phase. However, such a purely theoretical explication is not always easily understood by applied geophysicists. A geometrical interpretation of the terms of the stationary-phase approximation in relation to the diffraction and reflection traveltime curves in the time domain can help to develop a more intuitive understanding of the migration artifacts. A simple numerical experiment for poststack (zero-offset) data indicates the problem and helps to demonstrate the effects and the methods to avoid them.

Traveltime-Based True-Amplitude Migration

True-amplitude Kirchhoff migration (TAKM) is an important tool in seismic-reflection imaging. In addition to a structural image, it leads to reflectivity maps of the subsurface. TAKM is carried out in terms of a weighted diffraction stack where the weight functions are computed with dynamic ray tracing (DRT) in addition to the diffraction traveltimes. DRT, however, is time-consuming and imposes restrictions on the velocity models, which are not always acceptable. An alternative approach to TAKM is proposed in which the weight functions are directly determined from the diffraction traveltimes. Because other methods exist for the generation of traveltimes, this approach is not limited by the requirements for DRT. Applications to a complex synthetic model and real data demonstrate that the image quality and accuracy of the reconstructed amplitudes are equivalent to those obtained from TAKM with DRT-generated weight functions.

A seismic reflection imaging workflow based on the Common-Reflection-Surface (CRS) stack: theoretical background and case study

In recent years, many case studies have demonstrated that the Common-Reflection-Surface (CRS) stack produces reliable stack sections with an excellent signal-to-noise ratio. In addition, an entire set of physically interpretable stacking parameters, so-called kinematic wavefield or CRS attributes, is determined. These attributes can be applied in further processing in such a way that a complete and consistent seismic reflection imaging workflow can be established which leads from the preprocessed multicoverage data in the time domain to migrated sections in the depth domain. The basic steps of this CRS-stack-based seismic reflection imaging workflow are the CRS stack itself, the determination of a smooth macrovelocity model by means of CRS attributes, and limited-aperture preand poststack Kirchhoff-type depth migration where the aperture is possibly optimized by means of the determined attributes. Our workflow approach has been applied to a recently acquired seismic dataset and revealed superior results compared to standard processing based on NMO/DMO/stack with a subsequent time migration and depth conversion.

De-noising seismic data in the time-frequency domain

Marine seismic data are quite often affected by noise. For successful imaging, it is crucial that as much coherent and incoherent noise as possible is removed in an early stage of processing. An effective method to handle a broad range of noise problems is a time-frequency de-noising (TFDN) algorithm. In this paper, we present background information on the physics of weather and flow noise generation on seismic streamers, some details regarding the implementation of a TFDN method, and three examples where it has been successfully applied to seismic data.

True-amplitude Kirchhoff Migration From Topography

In areas with topographic variations, acquisition and processing of seismic data is a challenging task for geophysicists. Although there exist methods to adjust the measured data to a flat datum, it is sometimes necessary or favored to migrate the data directly from topography in order to get high-quality migrated images. Kirchhoff migration is a suitable tool to handle such irregular geometries in an efficient and amplitude-preserving way. However, several important aspects must be considered: Migration weights must refer to the actual topographic measurement surface and its local dip and need to honor the local acquisition geometry. In addition, a careful estimation of the velocity model and, thus, the traveltime tables is necessary. Then, Kirchhoff true-amplitude prestack migration does not only produce kinematically correct images but also enables further studies that rely on the dynamic information of the migration output. A prominent example for such investigations is the analysis of amplitude variation with offset or angle in the prestack migrated images.

Automatic Fracture Detection and Velocity/Anisotropic Parameter Analysis for HTI Media from Marine NAZ 3D Super-gathers

Media exhibiting horizontal transverse isotropy are usually associated with an oriented stress field or open fractures. Seismic reflection traveltimes beneath such media vary with the source-receiver azimuth. Earlier, we proposed a method that searches simultaneously for three characteristic HTI parameters to achieve an optimum moveout correction of the narrow-azimuth NAZ gathers where multi-azimuth processing techniques are not ideal. The dominant fracturing orientation was assumed known. Here, we extend the method to detect the fracture direction and to run the automatic analysis on 3D super-gathers. Application to the same marine multi-source/streamer NAZ data demonstrates the effectiveness of the extended method.

On the Aperture Effects In 2.5-D Kirchhoff Migration

Seismic images obtained by Kirchhoff time or depth migration are always accompanied by some artifacts known as “migration noise”, “migration boundary effects”, or “diffraction smiles”, which may severely affect the quality of the migration result. Most of these undesirable effects are caused by a limited aperture if the algorithms make no special disposition to avoid them. Likewise, strong amplitude variation along reflection events may also cause similar artifacts. All these effects can be explained mathematically by means of the Method of Stationary Phase. However, such a purely theoretical explication is not always easy to understand for applied geophysicists. By relating the terms of the stationary-phase approximation to simple geometrical situations, a more physical interpretation of the migration artifacts can be obtained. A simple numerical experiment for poststack (zero-offset) data indicates the problem and helps to develop an intuitive understanding of the effects and the methods to avoid them.

Cascaded Seismic Migration And Demigration

Summary We present a tool capable of performing 4D seismic monitoring investigations based on the so-called Unified Approach (Hubral et al. (1996),Tygel et al. (1996)). We show how inversion and forward modeling problems can be solved using the same algorithm: amplitude-preserving Kirchhoff-type depth migration enables to investigate reservoir properties, whereas the approach can also be used for forward modeling purposes (Santos et al., 1998). With this method, selected parts of the wavefield and/or selected target zones can be modeled in a fast, efficient and amplitude-preserving way which proves to be especially advantageous for poststack scenarios. Thus, time-dependent variations of subsurface properties can be investigated. The sensitivity to inconsistencies in selected target zones well below the surface (as is the case in e.g. subsalt imaging) can be tested by a repeated modeling and inversion procedure. In this paper, we present a practical implementation of the theory which enables us to perform prestack- and poststack migration and demigration within the same algorithm for arbitrary sourceand receiver configurations. We address practical problems occuring when implementing the method such as reducing necessary data volumes to a feasible minimum. The assemblage of summation operators requires the knowledge of an a-priori macro-velocity model. In order to reduce storage of wavefield attributes from this macro-velocity model, we present criteria, how sparse these informations can be sampled in the Green’s Function Table without losing resolution.