NORSAR-3D is a powerful tool for calculation of Green's functions, that is, tables of traveltimes and other parameters, for Kirchhoff type Pre-Stack Depth Migration. 

In the following we discuss some aspects of the use of NORSAR-3D in the PSDM context. A general knowledge of NORSAR-3D is assumed. 

The subsurface model
In NORSAR-3D the subsurface model can be a single velocity field, provided as a 3D (Seg-Y) cube, or a layered model with velocity discontinuities across the interfaces, or any combination of these. Notice that inside each block/volume in the model, the velocities must vary slowly. You can filter (smooth) velocity fields in NORSAR-3D to make the slowly varying field. The slowly-varying-velocities requirement also applies if the model consists of just one block (volume) with a single velocity field: The velocity may not change rapidly.

If there are large velocity contrasts in the model, we recommend that the contrast is represented by an interface, and that there is a velocity discontinuity across the interface. See the User's Guide for Version 5.0, section for details on how to split the velocity field properly.

In areas with salt bodies surrounded by sediment layers, with a quite continuous velocity increase in the sediments, the salt bodies are represented as separate blocks (volumes) with constant salt velocity, while the surrounding sediments is a single block with a continuous, smooth velocity field.

Also notice that you now have the VTI anisotropy option: You can provide the Thomsen Epsilon and Delta as constants, 2D grids or cubes individually for each block (volume) in the model. A model can be a mixture of anisotropic and isotropic blocks. 

The smoothness requirements discussed above also apply to densities and the Thomsen parameters.

Receivers and shots in the modelling of PSDM traveltime tables
In the Common Shot Survey Generator you can make a 3-dimensional grid of receivers. In PSDM we use the depth points as this 3D 'receiver' grid. In practice it must be desampled compared to the actual PSDM 3D depth point grid, using for example every other or third node along each direction. 

The shots in the modelling are the (regularized) shot/receiver positions on the surface.

Event attributes - what to store and how
In the Common Shot Wavefront Tracer in 'Functions>Set Event Attributes', select 'Prestack Depth Migration' to minimize the number of parameters stored for each event. You can tune the data storage further in the 'Edit Event Attribute Settings'. By default the PSDM option gives you the Static Event File Format and just one event per receiver. Increase this if you want multi-arrivals. The purpose of the sorting order ('Receiver Lines (slow-->fast)') is to fine-tune the event storage to how you will access the data later on in the PSDM process. It should not be necessary to change this parameter in initial tests.

Parameters for the Wavefront Tracing
Most default parameters for the wavefront tracing are in version 5.0 quite OK for the dimensions and level of details usually found in seismic exploration.

Set the Maximum Traveltime to match the max. traveltime recorded in the data. Notice that the max. traveltime here is for one-way propagation, from positions on the surface and down to depth points. 

If high transmission angles at interfaces are involved, increase the Maximum Incidence/Departing Angle.

Use 'No Shot Similarity'. The Shot Similarity may slow down the computations in PSDM applications. The reason is that there is a very large number of receivers, thus a large proportion of the computed wave field passes through receivers. It also takes a quite long time to collect info from adjacent 'full' shots to construct the initial wavefronts for the 'intermediate' shots.

Before you start a major modelling job, model just a few shots, and check the results in the interactive common-shot tracer and with the event file extractor.

For a large job, use the batch tracer and divide into smaller sub-jobs for parallel computing.

Inspect the results for selected shots and receivers in the Common Shot Wavefront Tracer and the Event Attribute Extractor. Use the NORSAR-3D Event File toolkit to access the NORSAR-3D results from your PSDM application.