Factors that Affect Seismic Amplitudes and Processing for AVO Analysis
Seismic processing is usually conducted to achieve objectives such as structural imaging or stratigraphic resolution, rather than amplitude-variation-with-offset (AVO) analysis. Although the ultimate goal of processing surface seismic data is to obtain a migrated stack section, interpretation objectives have changed over the years. In the 1960s and early 1970s, structural interpretation and reservoir delineation required well-balanced amplitudes and clear, sharp interfaces. Thus, final migrated sections usually had an automatic gain control (AGC) applied. By the middle 1970s, stratigraphic interpretation had caught up, and along with that advance came the awareness that amplitude information on migrated sections is important. At that time, seismic processors began to retain relative amplitudes during processing. That information also helped with detection of bright spots (anomalous reflectors corresponding to gas-charged sands) and flat spots (anomalous subhorizontal reflectors associated with fluid contacts, usually gas on water) on relative-amplitude-processed (RAP) sections. Ostrander's introduction of AVO analysis in 1984 led to the confirmation of bright spots and other anomalous reflections on RAP sections (Ostrander, 1984). Since then, geophysicists have persisted in trying to extract more information from seismic amplitudes, during both prestack and poststack processing. Along with AVO analysis, a plethora of seismic attributes has been studied and is known to add value to the interpretation carried out on such data (Chopra and Marfurt, 2007). The crucial point in amplitude analysis has been the realization that the reliability of the results depends on the nature of the data acquisition and the quality of the processing that generated the amplitudes.