TGS has two methods to produce RTM angle gathers:

Method 1: 

Angles are obtained directly through calculation of the directions for the forward propagation waves from source and the backward propagation waves from receivers. The wave fields are separated into up- and down-going components before estimating the directions, and optical flow method is applied to improve the direction estimation accuracy compared to conventional Poynting vector method.  We also have the option to use structure dip obtained from stacked migration image to replace the wave propagation direction computation for the receiver side which is typically less robust in noisy area.

Method 2: 

TGS RTM angle gathers are obtained through mapping of shot-based partial imaging based on source-side only propagation directions, which can be calculated by Poynting vector method or ray tracing method. The advantage of this method is very efficient and accurate for smooth sediment area, and the disadvantage is lacking accuracy in some complex areas like subsalts, and it does not handle multi arrivals.