Assigning geological ages to horizon surfaces

A method of processing stratigraphic data (e.g. obtained from labeling seismic data) comprising a plurality of horizon surfaces S1-S10 within a geological volume includes the steps of: extending a plurality of spaced sampling traces (identified by surface coordinates (xn,yn)) through the volume to traverse the horizon surfaces; and assigning the horizon surfaces respective relative geological ages. On each sampling trace, the relative geological age of each horizon surface traversed by the sampling trace in the direction from geologically younger to geologically older horizon surfaces is increased in relation to the relative geological age of its preceding horizon surface, under the condition that each horizon surface takes the same relative geological age across all the sampling traces by which it is traversed. A sequence order of the unique identifiers may be determined, and represented as a directed acyclic graph DAG. The horizons and their ages may be represented in a chronostratigraphic chart. The processed data may be used to control the trajectory of a well drilling operation. Also disclosed is a method of processing seismic data in first and second representations, one of which is a chronostratigraphic representation.