Seismic window Pitfalls revisited On my bookshelf is a small 20 page booklet with the title“Pitfalls Revisited”. It was published in 1982 and is a follow up to the original 1971 SEG Monograph “Pitfalls in Seismic Interpretation”. How relevant are the topics discussed in these books nearly 50 years ago? It seems the problems in seismic interpretation have not really changed over the intervening years because most of the 16 pitfalls described are still applicable today. Pitfall 1 – Assuming the marine subsurface needs no low velocity correction. In fact the shallow section has numerous channels, reefs and faults which can lower signal – noise and introduce lateral velocity variations that distort the reflections at the target horizon. Today we can model velocity variations with full waveform inversion, and pre-stack depth migration aims to locate reflectors accurately in depth, but these are relatively recent additions to the seismic processing sequence. Pitfall 2 – Assuming a sonic log gives true velocity and a VSP is a continuous recording. Well ties still require some editing, stretching, squeezing and bulk shifting of the sonic log to match the seismic. Today we can record sonic while drilling, and a wireline sonic log is often not included in the evaluation programme. While this may be good enough for P wave velocity, we often need S wave velocities to solve rock physics problems. In practice an estimate, rather than a true S wave velocity, is produced by sonic while drilling and the tool has no way of recording variations caused by azimuthal anisotropy. Pitfall 3 is about Fresnell zones which are now well understood, but still largely ignored by many interpreters. Pitfall 4 – Neglecting the use of the diffraction overlay. Not even I am old enough to remember whatever that is, but I guess it is no longer needed now migration is standard. Pitfall 5 – Flattening to restore geological history. Ideally this should be done using depth migrated sections but it is regularly applied to time sections. Even on today’s workstations, flattening across a fault is not handled well and the area beneath a fault plane is distorted and not very useful. Pitfall 6 – Over-migrating. In the past it was expensive to migrate data and 2D strike lines contained side swipe reflectors which could not be migrated. With the advent of 3D seismic it seems this pitfall is no longer an issue. Pitfall 7 – Undocumented changes in acquisition and recording parameters. This will always be a pitfall. Only last month I had a problem with an unrecorded change in shot delay. Pitfall 8 – Using only a variable area display on cheap paper to interpret. Luckily this is a thing of the past and interpreters can almost instantly adjust scales and colours to suit their requirements. Pitfall 9 – not including a data processor in the team is still common. Pitfall 10 – Overlooking the need to coordinate stratigraphic and structural mapping. Today’s interpreters should have a sound knowledge of structural geology and sequence stratigraphy as well as geophysics. Some exposure to reservoir engineering also helps. Many of us do, but not everyone tries to put geology into their interpretation. This is still an issue, especially as many of the experienced interpreters are heading for retirement. Pitfall 11 – Not using or misusing seismic velocities. There should be no excuse for ignoring seismic velocities today. The trick, however, is to figure out which velocity to use (e.g. average velocity, interval velocity, migration velocity to name a few). A modern 3D seismic survey will often be delivered with about a dozen different velocity files including some anisotropy measures like epsilon and delta. Which one should be used for depth conversion? Velocity can also be used as an attribute to determine geological characteristics such as channel fill or the lithology of a shallow prograding sequence. Pitfall 12 – Using only one scale ratio – this is another problem of the paper section era, though it is still difficult to display long, regional lines on a workstation screen. Pitfall 13 – Ignoring pitfalls. Nothing has changed on this one! Pitfall 14 – Paying lip service to coordinated geological and geophysical evaluation. A team approach is required, even more so than 50 years ago as prospects get more subtle. Sure, some geopeople call themselves geoscientists (usually these are geologists trying to progress themselves), but nobody knows everything. Pitfall 15 – Job switching for temporary advantage is not a problem these days. In the last four years there has been no word from the plethora of head hunters that invaded our industry from overseas when the oil price was over $100/barrel. Pitfall 16 – Emphasis on drilling expenditure compared to interpretation costs. Ill- conceived wells will still be drilled because spending on pre-drill interpretations has been slashed as companies down size or try to maximise the number of projects assigned to geophysicists and geologists. Good prospects still need good ideas, and people with enough time to think of them. This quick analysis indicates about half the pitfalls identified 40 to 50 years ago in interpretation are still valid today, especially where velocities and the makeup team members are concerned. Velocities can be dealt with easily, but building and keeping good exploration teams together requires a high priority corporate commitment, which is often lacking. References Tucker, P. M. 1982. Pitfalls Revisited, SEG Geophysical Monograph Series 3. Tucker, P. M., and H. J. Yorston. 1973. Pitfalls in Seismic Interpretation. SEG Geophysical Monograph Series 2. Michael Micenko Associate Editor for Petroleum micenko@bigpond.com 40 PREVIEW Seismic window JUNE 2019