Reformatting & geometry QC

Part 2 is the chain of operations between “data exists on a hard drive” and “velocity analysis can start.” None of it is glamorous. All of it is essential. The first step is the un-sexiest of all: reformatting — converting the vendor-delivered SEG-Y into the processing system’s internal format — and its conjoined twin geometry QC, where you verify that every trace is what its header says it is.

1. Reformatting in ten bullet points

• Read SEG-Y files from the vendor (often multi-terabyte).
• Parse the 3200-byte textual header — document any peculiarities.
• Parse the 400-byte binary header — extract sample rate, trace count, format code (IBM floats, IEEE, 16-bit int, etc.).
• For each trace, parse its 240-byte trace header plus N samples.
• Apply the coordinate scalar from bytes 71–72: if +100 or +1000, multiply; if −100 or −1000, divide.
• Handle byte-order correctly — SEG-Y spec says big-endian, but many legacy writers emit little-endian.
• Re-pack into the internal format (PROMAX, Petrel, OpenCPS, Madagascar, SeisSpace, etc.).
• Check trace count against the vendor report — a mismatch means traces were lost, duplicated, or the format spec was wrong.
• Sanity-check sample rate, sample count, and record length — every downstream operator uses these.
• Output a first geometry QC report — fold map, shot map, receiver map, histograms of trace header values.

2. Six fold maps to recognize at a glance

The fold map is the single most informative diagnostic. If you can only display one picture after reformatting, make it a fold map and compare it to the survey-design planned fold. The widget below shows the same synthetic survey under six different header-loading scenarios — study the signatures until you can name them from across the room.

3. What to look for

• Smooth trapezoidal plateau — geometry loaded cleanly. Max fold matches survey design. Edge ramps equal half the spread length. Proceed.
• Everything piled near origin — coordinate scalar applied in the wrong direction. Multiply or divide coordinates by the scalar value (100 or 1000) and re-load.
• Fold map shifted from survey plan — CMP field in header is stale. Re-compute CMP from shot/receiver X-Y in the loader, discard stored CMP.
• Random-looking speckle — byte-order wrong. Flip endian interpretation of the coordinate integers.
• Stripe or hole in fold — traces missing for a subset of shots/receivers. Check against the vendor report; re-request the missing SEG-Y file.
• Unusual offset histogram — shot/receiver fields might be swapped. Display a |x_r − x_s| histogram; negative or implausibly large offsets are a red flag.

4. The ten-minute QC checklist

Run these in order on every new dataset. Stop at the first failure and fix before continuing.

• Fold map matches survey-design planned fold.
• Shot map shows every planned shot line and spacing.
• Receiver map shows every planned receiver line and spacing.
• Offset histogram peaks within the planned offset range; no negative or huge values.
• Azimuth rose matches the survey design.
• Sample rate matches the vendor report (e.g. 2 ms).
• Trace length matches the vendor report (e.g. 6 s).
• Total trace count matches the vendor report ± 0.1 %.
• First-break time range is plausible for the target depths.
• A sample shot gather shows primaries at expected t₀ values.

Only after all ten pass do you proceed to any further processing. Every seismic project that went sideways in production started with a QC item that was silently skipped.

5. Why the investment pays

Every minute spent here is a day saved later. A scalar bug that goes undiagnosed for two weeks creates two weeks of processing products on the wrong geometry — velocity picks, statics, migrations — all of which need to be redone. A missing receiver line noticed in geometry QC takes a phone call; noticed at interpretation, it takes a reshoot.

Where this goes next

Section §2.2 starts the actual waveform processing. Trace editing removes bad samples and dead traces; amplitude recovery compensates for the known physical decay of wave amplitudes with travel time. Both are simple operations that every later step depends on.

References

• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge UP.
• Yilmaz, Ö. (2001). Seismic Data Analysis (2 vols.). SEG.
• Claerbout, J. F. (1976). Fundamentals of Geophysical Data Processing. McGraw-Hill.