b'Seismic window Seismic windowMichael Micenko Associate Editor for Petroleummicenko@bigpond.comFlat reflectionsThis is the 50th anniversary of the ASEG, and if my calculations are correct it is also my 50th contribution to Seismic window. With this auspicious occasion in mind, I thought Id give a brief history of theFigure 1.Seismic line across an oil field .The full column. Seismic window was first writtenstack (top) data has an indication of a flat reflectorFigure 2.Seismic line across an oil field. The full by Rob Kirk in the 1990s. Rob had across-cutting a number of dipping horizons.stack (top) data has a less obvious indication of wealth of international experience at theApplying an optical stack (centre) enhances thea flat reflector. Applying an optical stack (centre) forefront of seismic stratigraphy, and heflat reflector slightly, but the best enhancementenhances the flat reflector slightly, but the best shared some of his observations in theis seen when a 20 x 20 averaging filter is appliedenhancement is seen when a 20 x 20 averaging early issues of Preview. These were tutorial(bottom). filter is applied (bottom).like and consisted mostly of annotated seismic displays. When Rob decided hemiddle panels of Figures 1 and 2 showapplying a horizontal averaging filter. could no longer continue writing therethat the flat reflector is enhanced byThis filter has removed or smeared was a short period where guest writersthis technique while dipping events areout most of the dipping events and contributed to fill the gap. Eventuallydiminished. significantly enhanced the flat events. I was approached, and I wrote theThe oil-water reflection is now quite occasional short article about interestingOptical stacking is not available toobvious.things I had observed. These short noteseveryone because it is not built into were intermittent, but now Im told theall software packages, but there isIt is possible to go even further by readers expect something in every issue.something just as good that can beapplying a vertical averaging filter. So, 20 years later, I have hit the 50 mark. applied using generally available filters.Figure 3 is a time slice and shows a flat he lower panels of Figures 1 and 2 showspot, but because of velocity variations Now on to seismic flat spots. that flat spots can be enhanced bythe reflection is not truly flat and it dips Flat spots can be reflections from the base of a hydrocarbon column, so they are of interest to interpreters. Sometimes they are hidden amongst other reflections, or are not flat in two-way time because of velocity variations, but we can apply some appropriate filters to enhance the flat events.Figure 1 shows a fairly obvious flat reflection in an offshore oil field, while Figure 2 has a hidden event closer to the edge of the oil field. A common practice is to apply an optical stack to the data. An optical stack applies a transparency to a number of adjacentFigure 3.Seismic time slice at the approximate TWT of the oil-water contact shows an area (the yellow traces (3040 say) and sums them. Thepolygon around the blob in the centre of the picture) dominated by a flat event but the field extends further.31 PREVIEW FEBRUARY 2020'