Brian's method, and better than v2's whole-record median: the pre-trigger
window is definitionally quiet (the buffer captured before the trigger fired),
whereas a median is merely robust to the event. Requiring the floor to hold
across pre-trigger / middle / end rejects transients that a median cannot.
per channel: pre/mid/end medians, spread = max - min
offset when |pre| >= floor AND spread <= 0.02 in/s
real fault >= 3 consecutive flagged events on that channel
The empirical noise floor justifies the threshold and validates the decoder:
across 19,244 non-flagged channel-events the pre-trigger floor is 62.7% exactly
0.000, 94.5% within +/-1 quantisation unit, median +0.0000, mean -0.0008. There
is no systematic zero-point bias — an independent confirmation of the
32000-count geo scale.
The result is threshold-insensitive across a 2x range (0.020 to 0.040 in/s),
which is what separates a real signal from a tuned one:
FINAL: 5 of 45 units (11%) — BE9558, BE11529, BE12599, BE13117, BE18438
BE11007 and BE10895 drop out; the spread test identifies them as transients
rather than pedestals. v1's 11% headline was right by luck — it included
BE11007 and named the wrong channel on most units.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog
Brian challenged the v1 finding that offsets "come and go", against field
experience that a unit which develops one stays broken until the geophone is
replaced. He was right; v1 had two flaws, both of which manufactured false
recoveries:
1. It scored only the axis with the largest peak, so a real event on one axis
hid a persistent pedestal on another. BE12599 on 2026-08-21 read "clean"
because Long had a 1.065 in/s event, while Tran sat at +0.4732 in/s and was
never examined.
2. It used the mean, which a real transient perturbs. The median is the resting
baseline and a blast does not move it. Same event, Long channel:
mean +0.0783 vs median -0.0050.
offset_scan2.py flags a CHANNEL when |median| >= 0.025 in/s (5 A/D counts,
Instantel's own criterion) and treats >=3 consecutive flagged events as the
real signal. No m/p ratio guard is needed — that existed only to compensate for
the mean.
Corrected results:
units with any flagged event 6 -> 19 of 45
units with a sustained pedestal 8 of 45 (18%)
runs >=3 consecutive 29; 1-2 event runs (noise) 69
Also corrected: the affected channel is most often Vert, not Tran (v1 named
whichever axis had the largest peak, so it was frequently wrong). BE10895 and
BE18003 were invisible to v1. BE12599's fault began 2026-08-14, not 08-17.
The decode itself was never in question and is confirmed against Blastware's
own ASCII export: on K558LJN3.BK0W, BW shows Tran parked at +0.265..+0.375 in/s
for the entire record while Vert and Long sit at ~0.005 — Instantel's "parallel
lines above or below the zero line".
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog
Adds docs/offset_investigation.md — a dated journal of the "offset" hardware
fault, in the style of the codec status docs: findings with provenance, dead
ends kept with the reason they died, and per-unit case files.
Contents:
- base rate 5-6 of 45 units (11-13%) over the DL2 archive, 2018-2026,
confirming rather than overturning the earlier 2-of-21 estimate
- the detector, with the rationale for each term and its known blind spot
(event traces carry real motion, so only trace-dominating offsets show)
- the bimodality result: relaxing the amplitude floor 11x adds no new units
- Instantel's own procedure and thresholds from their FAQs 13-0-21 / 12-0-10:
A/D-mode ">5 counts", the autozero key sequence, and the 2027-2069 X1/X8
acceptance window that explains the ~10% field success rate of a re-zero
- four ruled-out hypotheses, each with the evidence that killed it:
condensation, clipping, the sensor check as a predictor (102 offset events,
zero failures — a grossly offset unit passes its own self-check), and the
calibration-timing correlation (confounded, one unit per time bucket)
- open questions, chiefly whether SUB 0x0E carries the autozero numbers
Cross-referenced from CLAUDE.md and Appendix E of the protocol reference
(whose CRLF line endings are preserved).
Separately: CLAUDE.md had 793 NUL bytes appended after its last line. They
predate this work (present at least as far back as e42956a / v0.21.0) and made
grep treat the file as binary, silently skipping it. Stripped.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog
serversdownandClaude Opus 5