Thorbjørn Rekdal

Imaging complex geologic structure with single-arrival Kirchhoff prestack depth migration

We compare various forms of single‐arrival Kirchhoff prestack depth migration to a full‐waveform, finite‐difference migration image, using synthetic seismic data generated from the structurally complex 2-D Marmousi velocity model. First‐arrival‐traveltime Kirchhoff migration produces severe artifacts and image contamination in regions of the depth model where significant reflection energy propagates as late or multiple arrivals in the total reflection wavefield. Kirchhoff migrations using maximum‐energy‐arrival traveltime trajectories significantly improve the image in the complex zone of the Marmousi model, but are not as coherent as the finite‐difference migration image. By carefully incorporating continuous phase estimates with the associated maximum‐energy arrival traveltimes, we obtain single‐arrival Kirchhoff images that are similar in quality to the finite‐difference migration image. Furthermore, maximum‐energy Greens function traveltime and phase values calculated within the seismic frequency ban...

Improving the data quality in marine streamer seismic by increased cross‐line sampling

The efficiency for marine streamer acquisition has been increased significantly during the last decade, extending in particular the towing width. Recent developments have focused on increasing the sampling of the wavefield to improve the resolution and image quality. This occurred mainly in the inline direction, e.g. by single sensor systems or single source configurations. The cross-line sampling with a much sparser sampling compared to the inline direction, has remained more or less the same. If we want to improve the data quality even further, do we have to go back to less streamers and more narrow tow width? Our analysis from synthetic modeling tests and real data examples shows that denser sampling of sources and receivers, in particular in the cross-line direction, can improve the resolution for towed streamer seismic. A larger cross-line offset improves the focusing of diffractors. In connection with denser sampling by overlap shooting, the acquisition and processing footprint is reduced. Hence, higher trace density and wider cross-line spread can improve the resolution and the image quality in difficult areas.

Determining infill specifications based on geophysical criteria

During marine seismic acquisition, areas of deficient coverage can arise. If these deficient areas are sufficiently large to be considered outside the specifications, additional infill lines must be acquired. It is desirable that this infill acquisition be targeted such that the additional acquisition requirements are minimised whilst maintaining adequate data quality. This study provides a method for predicting the effects of coverage holes on the migrated image on a case-by-case basis. Synthetic modelling followed by pre-stack time migration is used to quantify these effects for the particular model and targets of interest. These results may then be used to assess infill requirements based on a permitted level of data degradation. Such modelling can be performed at the pre-survey planning stage, thus allowing infill considerations to be taken into account during the survey design process.

Paraxial Ray Tracing In the Marmousi Model

First arrival traveltime methods have proven to fail to image deeper structure, when most of the reflected energy is transported by later arrivals. The family of ray tracing methods is one of the few alternatives, which not only computes multiple arrivals, but also computes amplitude and phase-shifts. Even in rough structures ray methods can be used to compute useful Greens functions, if the model is properly smoothed. In this paper, we discuss ways to obtain a good image with Greens functions computed with the paraxial ray tracing method in the 2D Marmousi model. We demonstrate that smoothing is preferred not only to make the model more “ray valid”, but to make the high frequency solution more like a band-limited solution. Smoothing over a length proportional to the local wavelength of the centerfrequency gave a better image than if we just apply a constant sized smoothing operator. We also found that using the two most energetic arrivals, instead of just the most energetic, improved the image. We applied a merging technique to reduce two nearby arrivals to one, which reduces the high frequency noise in the image significantly for similar cost as using just the most energetic arrival.

A modified form of diffraction tomography to image boundary structures

Wavefield extrapolation downward from the surface, as applied in migration and associated inversion methods, is a common procedure to image subsurface reflectors. These methods require adequate (i.e., extensive and unaliased) sampling of the surface wavefield. Seismic tomography on the other hand, relates parameters of the upward propagated wavefield to the diffracting image, and sampling requirements are less severe; it is usually the only option to image deep structures from sparse data. The ordinary form of ray tomography, however, imposes a severe smoothness constraint on the boundary; in particular the “tops” and “valleys” of a relatively rough structure are not well‐resolved. We have implemented a generalized form of tomography, which uses both the ray term and the diffraction term linearized in the boundary perturbation. We introduce a generalized reflection coefficient that can be linearized in terms of the (unknown) boundary gradient, and we demonstrate the adequacy of this approximation with the...