Models with enough detail and with correct rock properties are crucial for reliable results. Interpreted horizons from seismic data create the structure of the models. P-wave velocities are derived from seismic data and calibrated to well logs. Density and shear velocity are obtained from well logs or approximated. Raytracing requires that time models be converted to depth. Inverse raytracing can be used to create a more accurate model than a vertical shift. In illumination studies, a constant rock property contrast on the reflecting horizon is created that isolates the effects of the overlying structure and velocity variations.
Raytracing in complex geology, such as subsalt illumination studies, requires offset raytracing that simulates actual shooting geometry. 3-D raytracing is usually required in complex areas. 2-D models can be valuable in understanding effects of variations in individual structure or velocity features (refer to 2-D results above). Rays are sorted CRP gathers to simulate a 3-D PreSDM process. Results include division into offset bans to assist in AVO analysis and calculation of reflection angles.