Critical reflection illuminations analysis

A method for evaluating the illumination of a subsurface geological formation of interest that is located beneath a velocity boundary or interface with a material having high velocity seismic propagating properties above it and a subjacent material having relatively slower velocity seismic propagating properties;

• the method comprising:
• a) creating a theorized target horizon representing the subsurface zone of interest;
• b) creating a theorized velocity contrast horizon representing the subsurface velocity boundary;
• c) creating a velocity model representing the speeds at which seismic energy is expected to travel from the theorized target horizon up to and through the theorized velocity contrast horizon;
• d) selecting a plurality of locations along the theorized target horizon for critical reflection analysis;
• e) calculating the propagation of seismic energy from each of the plurality of locations along at least one pair of respective trajectories satisfying Snell's law for arriving and departing wave paths to determine incidence angles for each path of the pair of trajectories at the theorized velocity contrast horizon;
• f) comparing the incidence angles for each trajectory of the pair of trajectories to one or more critical angles, wherein the velocity contrast refracts the seismic energy to progress along the velocity contrast horizon such that the seismic energy intersecting the velocity contrast horizon at the critical angle is unlikely to progress to the earth's surface in a lateral range or at a strength to be meaningful for seismic prospecting;
• g) tabulating the number of pairs of trajectories at each selected location where both trajectories are pre-critical, or less than the critical angle, in comparison to the number of pairs of trajectories where either or both of the trajectories of the pair are critical or post-critical, or equal to or greater than at least one critical angle wherein a pair is deemed pre-critical if each trajectory of the pair is pre-critical and wherein a pair is deem post-critical if either trajectory or both trajectories are post-critical;
• and
• h) creating a display, critical reflection illumination map, that identifies areas of the target horizon for which selected locations have more pairs of pre-critical trajectories and therefore fewer pairs of post-critical trajectories and also identifies areas of the target horizon for which selected locations have fewer pairs of pre-critical trajectories and therefore more pairs of post-critical trajectories, wherein the areas that have more pairs of pre-critical trajectories are likely to yield useful seismic data and areas that have fewer pairs of pre-critical trajectories are less likely to yield useful seismic data when recorded above the velocity contrast.