Models of Amplitude as a function of Offset require compressional velocities, shear velocities, and bulk densities for a model.  Conventional methods of producing models for AVO models involve estimating these properties for homogenous layers of varying thicknesses.  Seismotech's method involves taking well logs, sonic and density logs, in conjunction with lithologic and other petrophysical information based on other logs (resistivity logs, gamma ray and spontaneous potential logs, porosity logs) to estimate shear wave velocities varying almost continuously using petrophysical models (as Biot-Gassmann theory) to estimate fluid effects on model velocities and density.  GeoGraphix's PRIZM and QLA2 are used for all log analysis tasks as estimating water saturation, average porosity and effective porosity.  Fluid substitution modeling and estimation of shear wave velocities are done in Rock Solid Images' PetroTools.
    An example of calculated fluid effects is shown below where gas has been substituted for water in a brine sand (Sw is changed from 100% to 30% in the sand).  The change in compressional sonic transit time is indicated in track 4  in red, and change in shear sonic transit time is indicated in blue.  In track 2 the bulk density for the gas case is indicated by the dashed brown curve, while the solid brown curve represents the bulk density in the brine case (before fluid substitution).  Porosity logs are shown in the fifth track for denisty porosity, neutron porosity, sonic porosity, effective wet porosity, and average porosity.

After compressional and shear velocities and bulk density is estimated for brine and gas models, GeoGraphix's GMAplus AVO modeling program is used to calculate synthetic gathers (as shown below for a gas case which is unrelated to the log example shown above) which can be compared with observed seismic data processed to maintain and enhance seismic amplitude information in order to test these models.

Hampson-Russell AVO program is also often used to calculate gradient and zero-offset (P-wave) stacks and angle or offset stacks for comparison with AVO models.  Hampson-Russell AVO is also capable of modeling fluid replacement effects although blocking of logs is necessary.