![]() That's why seismic data loading and well ties are so *much* fun across multiple vintges of 2D land data where you have rugged topography, especially if processed by different contractors. I guess the issue is that its tends to be the most junior and inexperienced individual who gets to write out the SEG-Y archives. That creates a whole "if I remove a -6ms static shift which way does the data go, up or down?" problem I've learned the hard way to *always* check carefully before and after, because no one does it quite the same.įound plenty of SEGY datasets archived that had the shifts backwards or not applied, despite the meta-data claiming it was. Some software keeps all that stuff in a database and so it's never really applied to the data, just used as a reference point for calculations.Įven when you export the data how it is stored in the headers varies, with (I kid you not) different sign conventions being used by different software. There's a real issue here in terms of how the software works and manages the datum, and how it was exported and so on. Hope that is clear - its a complex topic. ![]() You don't want to use seismic velocities on a floating datum and then apply those uncorrected to data that has been corrected to the final SRD, and so on. The key thing is to know how the velocity models work - again that depends on the software too. Some needs the data relative to the surface and you can correct to a fixed or floating datum internally, sometimes that's and option, and sometimes it isn't. Now - when it comes to PreSTM and PreSDM it kind of depends on the software. SO - the fixed datum and SRD are both usually flat. Some processing and interpretation software always has an SRD equal to Mean Sea Level - in that case you will have seismic data displayed in "negative time" above MSL. Both are ways to join data.datum(values, key) rather for static datavizualisation not needing updates, binds data directly into an element to inject. But in common use of the term, it is not just any. It is true that a benchmark is a datum in and of itself. Some folks use the word benchmark interchangeably with the word datum. This is usually called the SRD - seismic reference datum. As a term it has many uses, but in design and construction, a datum refers to a vertical measurement taken in relation to a universal elevation, such as sea level. Usually you'd process at the floating datum, but view and QC the data with a correction to a final, fixed, flat datum. We use a smoother version of the topography - a "floating datum" and correct relative to that. If you have rugged topography, the corrections needed might be bigger than that. How small is small? Well it depends, but usually less than (say) 12-16ms larger than that and you start to distort raypaths. That a "vertical ray path" approximation, which is okay for small shifts. These corrections are made as static shifts to the shots and receivers. You usually pick this data to be above the highest topography essentially its like filling in the topography with a (magic) rock of constant seismic velocity. ![]() When you process that data, to remove the effects of the topography, you would apply elevation corrections to a fixed, flat datum. Normally speaking seismic data on land is recorded relative to the surface.
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