NORSAR-3D

Ray-based seismic modeling methods can be applied at various stages of the exploration and production process. The standard ray method has several advantages, include computational efficiency and the possibility of simulating propagation of elementary waves. As a high-frequency approximation, the method also has some 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 tracing method, we introduce Kirchhoff modelling to our software portfolio. It is based on the well-known Kirchhoff integral solution for calculating the seismic wavefield between a shot and receiver in the given Background Model, for a given target. The Kirchhoff integral provides a robust method for implementing seismic modelling and prestack depth migration. Thus, with this approach we are able to convert a velocity model given in depth domain to seismic data in time domain and to migrate the shot data to a depth image. Kirchhoff modelling keeps the flexibility of ray tracing (selection of a part of the wavefield, efficiency) while adding benefits known from FD modelling (diffractions). Kirchhoff modelling is slower than raytracing, but more efficient and flexible than FD modelling.

Kirchhoff Prestack Data Workflow

Forward modelling of seismic data is the use of geological models of the earth to simulate seismic field experiments. Models can be one, two, or three dimensional and consist of depth horizons and associated elastic properties, such as P wave velocities, S wave velocities, and densities. An application of forward modelling is for verification of structural and stratigraphic interpretations. For example, synthetic seismic sections derived from forward modelling can be compared to stacked sections to verify the original interpretation. If needed, the original interpretation or model is altered, and the process repeated until a desired correlation between the actual data and modelled results is observed.

In this contribution, we present Kirchhoff prestack data workflow, which is a powerful, robust, and flexible method to generate synthetic traces. This workflow allows to generate synthetic seismograms on either the complete set of horizons or a sub-selected set of horizons.

Generate the data you need!

The new Kirchhoff modelling software allows to select which part of the wave field should be included, thus it is easier to identify specific events in you real data by correlating with the synthetic seismic traces.

Kirchhoff Target Migration Workflow

Seismic data processing is aimed to image the earth's subsurface, that is, obtaining a picture of subsurface structure from the seismic waves recorded at the earth's surface.

Of the many processes applied to seismic data, seismic migration is the one most directly associated with the notion of imaging. Until the migration step, seismic data are merely recorded traces of waves that have been reflected/diffracted from anomalies in the subsurface. In its simplest form, seismic migration is the process that converts information as a function of recording time to features in subsurface depth. It moves dipping events to their correct positions, collapses diffractions, increases spatial resolution and is probably the most important of all processing stages.

In order to provide the complete picture to the user, we have integrated a Kirchhoff target migration workflow into the existing modelling environment. Here, the generated synthetic data will be directly transferred into the corresponding picture of the sub-surface for a given input set up.

Find out how sensitive the migrated result is to velocity deviations.

Are the targets well illuminated? Are there shadow zones? How large migration aperture is required? How long streamers are required?

Illumination maps show how well a target horizon is illuminated by a survey. The basic map is the hit map, which shows for a survey how many rays from shots to receivers reflect on the different parts of the horizon. Shadow zones clearly stand out on hit maps. There are more maps; they show different parameters computed along the rays, including: Travel time, reflection angle, required migration aperture, and amplitude. The maps are divided into grid cells, and for each parameter the maps show how minimum, maximum, and average value of the parameters vary across the horizon. A map can be restricted to a certain interval of another parameter. For example, separate maps can be made for different offset intervals, to show which offsets are important in a survey. Most commonly, the illumination information is put down on the target, but similar maps can also be made with the information posted in the shot or receiver locations. These maps show which shot and receiver positions contribute most to the illumination, and the maps may reveal that shots or receivers may be skipped in some areas.

Comparison of the nominal fold, here shown on the surface, and the expected fold at the target, computed as a hit map.

Illumination map showing maximum horizontal difference between common reflection point (CRP) and common midpoint (CMP). This is the required migration aperture if all reflections are to be depth migrated.

Which survey azimuths and offsets are required to illuminate a point on the target? Does it change along the target? Which reflection angles and azimuths does a survey cover in a target point? How does the angular coverage change along the target? Where do rays propagate down to the target?

See this Illumination Ray Concept presentation for more information.

Incorporate anisotropy into your model

Anisotropy within rock units results in a variation in seismic velocity that depends on the direction in which the ray is propagating. Introducing anisotropy into seismic modelling is essential to accurately reproduce the transmission of seismic energy through the earth.

The anisotropy option within NORSAR-3D upgrades the software package to enable TTI anisotropy to be included in layered models. This option enables a more realistic earth model to be constructed, resulting in e.g. an improvement in traveltimes for PSDM and tomography applications.

We have been very pleased with the imaging using the VTI anisotropy... The data on which we tested the imaging with the anisotropy option showed significant enhancement of structural detail over the migration using the isotropic travel times, which in turn was far superior to the previous prestack time migration.

-- Dr. Andrew V. Barrett, Kelman Technologies

An example of a salt structure with several adjacent transversely isotropic shale layers where the axis of symmetry is normal to the bedding (the two top pictures). Depth migrated images are shown (from left to right) using an isotropic model, a VTI model and the TTI model. It can be seen that a good subsurface image in the vicinity of the salt dome is obtained only when using the TTI model. The VTI model shows a misfit for the lower reflectors whereas the isotropic model images them poorly.