Picking the correct seismic line: A crucial factor when performing AVO analysis

Abstract

The observed variation in amplitude versus offset (AVO) in CDP gathers has been used by explorationists as a tool to help in the determination of fluid content and lithology. Picking the correct seismic line that properly images the zone of interest is one of the most important steps leading to a successful AVO analysis. Three essential aspects need to be considered when picking the seismic line: 1. the orientation of the line to the structure, 2. adequacy of acquisition parameters, and 3. seismic data quality. In this paper we will primarily examine the question of picking the line that correctly images the zone of interest. A three dimensional dome model and two marine examples will be used to demonstrate the potential problems that the interpreter and processing geophysicists will encounter when picking the seismic line to perform an AVO analysis on. Review of both the CDP stack and migrated sections can avoid picking a line that does not image the zone of interest. INTRODUCTION Most interpreters make their subsurface geological interpretations using time-migrated seismic sections. However when performing an AVO study we need to analyze pre-stack common-midpoint (CMP) gathers. Therefore it is necessary to accurately locate prospective events on unmigrated CMP stack sections. It is also important to use unmigrated stack sections when selecting lines for AVO analysis where the orientation of the available lines run oblique to the structural dip. In this case even if pre-stack migration has been performed on the data it is still necessary to use unmigrated stack sections to pick the correct line location. MODEL STUDY A three dimensional dome structure was built and two seismic lines were generated based on normal incidence ray tracing (Fig. 1). If the zone of interest is located at the top of the dome, line 2 (Fig. 1, 2) can be used for AVO study because normal incidence ray tracing shows that the line will properly image the crest of the dome. The event AA' (Fig. 1) sits on the flank of the dome. From the map view (Fig. 2) seismic line 3 goes through AA' and might initially be picked to perform an AVO analysis on. But Fig. 1 shows that the event AA' is actually imaged by line 1, which is down dip (or south) of line 3. Line 1 should be used in the AVO analysis of event AA' instead of line 3.