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.

6. The sort domains, and what each is for

Raw data arrives in shot order, but a processor re-sorts it constantly, because each problem is easiest to see, and each correction easiest to apply, in a particular domain. The same traces, re-ordered, expose different things, and knowing which domain to display is half of QC.

• Shot (common-source) gather. The recording domain. First breaks, refraction statics, source-generated noise (ground roll, air blast), and shot-to-shot amplitude balancing all live here, because they are organized by source. Linear noise really is linear in this domain, which is why FK and radial filters are designed on a shot gather.
• Common-receiver gather. All traces sharing one receiver. This is where a single bad geophone gives itself away: a dead, reversed, or ringing channel is a stripe here and nearly invisible anywhere else. Receiver statics and receiver-consistent amplitude terms are solved in this domain.
• CMP (common-midpoint) gather. The workhorse. Velocity analysis, NMO, residual statics, stack, and AVO all happen here, because every trace in the gather images the same subsurface point at a different offset. Almost everything from Part 3 onward assumes you are in this domain.
• Common-offset gather, or section. One offset, every midpoint. A single common-offset section is a rough, zero-effort image of the structure, handy for a quick structural look, for migration QC, and for catching offset-dependent problems such as a statics bust that only shows at far offset. DMO and some migration QC operate here.
• Offset order, inside a CMP. Less a separate domain than the sort within the CMP gather: by absolute offset for a tidy, monotonic moveout display, or by signed offset so a velocity error reads as a symmetric smile or frown (§3.3).

The working rule: when something looks wrong on the stack, ask which domain would organize the problem into a straight line or a single stripe, then display it there. A receiver fault is invisible on a CMP gather and obvious on a common-receiver gather; a velocity error is the other way around.

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.