docs/offset_investigation.md

@@ -1,5 +1,26 @@
 # The "offset" fault — investigation journal
 
+> ## ⚠ CORRECTED 2026-08-28 (same day) — the v1 detector was wrong
+>
+> Brian pushed back on the finding that offsets "come and go": in the field,
+> once a unit develops one it stays broken until the geophone is replaced.
+> He was right, and the challenge exposed **two real flaws** in the v1 detector:
+>
+> 1. **It scored only the axis with the largest peak.** A real event on one axis
+>    hid a persistent pedestal on another. BE12599 on 2026-08-21 read "clean"
+>    solely because Long had a 1.065 in/s event — Tran was sitting at
+>    **+0.4732 in/s** at that moment and was never examined.
+> 2. **It used the MEAN**, which a real transient perturbs. The **median** is the
+>    resting baseline — most samples sit at it, so a blast does not move it.
+>    Same event, Long channel: mean **+0.0783** vs median **-0.0050**.
+>
+> Both flaws manufactured false recoveries. The corrected detector
+> (`scratch/offset_scan2.py`, per-channel median) shows the pedestal is
+> **persistent**, exactly as the field experience says. See §2b and §3b.
+>
+> Sections below that were written against v1 are marked; v1 numbers are kept
+> for the reasoning trail, not as current fact.
+
 A running record of the **offset** hardware fault on Instantel Series III
 seismographs: a geophone channel whose trace sits displaced from zero rather
 than centred on it.
@@ -85,6 +106,60 @@ see offsets large enough to *dominate* the trace. A mild offset on a real blast
 is invisible. Instantel's A/D-mode check (§5) is the only thing that sees the
 mild end. Our base rate is therefore a **gross-offset** rate.
 
+### 2b. Detector v2 — per-channel median (CURRENT)
+
+`scratch/offset_scan2.py`. Supersedes the above.
+
+```
+for each series-3 waveform binary:
+    for each geo channel independently:
+        pedestal = median(samples)          # resting baseline, robust to blasts
+        flag the CHANNEL when |pedestal| >= 0.025 in/s   (5 A/D counts)
+a unit has a real fault when a channel is flagged on >=3 CONSECUTIVE events
+```
+
+Why median: a DC pedestal shifts every sample, so it moves the median. A real
+event moves only a minority of samples, so it does not. This removes the need
+for the `m/p` ratio guard entirely — that guard existed only to compensate for
+using the mean.
+
+Why per-channel: the fault is on one geophone axis. Scoring only the dominant
+axis means any event with motion elsewhere hides it.
+
+Why "3 consecutive": the 0.025 in/s floor is only ~2x a healthy channel's
+resting median (observed 0.010-0.015), so isolated flags are noise. Persistence
+is the discriminator — and it is what the field experience predicts.
+
+---
+
+## 3b. Archive results, corrected (v2)
+
+| | v1 (dominant axis, mean) | **v2 (per-channel median)** |
+|---|---|---|
+| units with any flagged event | 6 of 45 | 19 of 45 |
+| **units with a sustained pedestal (>=3 consecutive)** | — | **8 of 45 (18%)** |
+| runs of >=3 consecutive | — | 29 |
+| runs of 1-2 events (noise) | — | 69 |
+
+Units with a sustained pedestal: **BE9558, BE10895, BE11007, BE11529, BE12599,
+BE13117, BE18003, BE18438**. BE10895 and BE18003 were invisible to v1.
+
+**The affected channel is most often Vert**, which v1 got wrong — it named
+whichever axis had the largest peak. BE13117 and BE18438 are both Vert faults.
+
+Longest / clearest runs:
+
+| unit | ch | span | events | median in/s |
+|---|---|---|---|---|
+| BE13117 | Vert | 2023-05-03 → 05-04 | 194 | 0.035 → **1.915** |
+| BE18438 | Vert | 2026-02-25 → 02-26 | 75 | 0.180 → 0.370 |
+| BE9558 | Vert | 2020-02-11 (6 h) | 33 | 0.065 → 0.090 |
+| BE12599 | Tran | 2026-08-14 → 08-23 | 8 | 0.030 → **0.565** |
+| BE18003 | Vert | 2021-03-17 → 06-11 | 3 | 0.040 → 0.060 |
+
+BE12599 began **2026-08-14**, not 08-17 as v1 reported, and was still faulting
+at the last event in the archive.
+
 ---
 
 ## 3. Archive results (2026-08-28)
@@ -253,6 +328,16 @@ swing test measures geophone frequency response and damping — it never examine
 DC zero. **A grossly offset unit passes its own self-check.** This is why the
 fault goes unnoticed until somebody looks at waveforms.
 
+### "Offsets are transient / come and go on their own" — RETRACTED 2026-08-28
+v1 reported episodes lasting hours that ended spontaneously. **This was an
+artifact of the v1 detector** (see the banner at the top). With the per-channel
+median, the pedestal persists. Every clear case reads clean again only after a
+multi-day-to-multi-month gap consistent with service: BE13117 6 days, BE18438
+24 days, BE9558 63 days **with a confirmed Instantel calibration inside the
+gap**. BE12599 never reads clean — it is still faulting at the end of the
+archive. This matches the operational experience: once a unit develops an
+offset it stays broken until the geophone is replaced.
+
 ### "Offsets develop N months after calibration" — CONFOUNDED, NOT A FINDING
 Tempting, and it looked strong:
 
@@ -361,3 +446,4 @@ doubled two reported figures before it was caught.
 | 2026-08-28 | Calibration-timing correlation attempted and **rejected as confounded**. |
 | 2026-08-28 | Instantel FAQs supplied: autozero procedure, the **2027–2069** window, the **>5 counts** threshold. Explains the ~10% re-zero success rate. |
 | 2026-08-28 | Bimodality established; sensor check proven **blind** to offsets; `SUB 0x0E` identified as the best open lead. |
+| 2026-08-28 | **v1 detector retracted.** Brian challenged the "come and go" finding against field experience. Two flaws found: dominant-axis-only scoring and mean-instead-of-median. Corrected detector shows persistent pedestals on **8 of 45 units**, and the gaps are service windows. |

scratch/offset_scan2.py

@@ -0,0 +1,108 @@
+#!/usr/bin/env python3
+"""Offset detector v2 — per-channel MEDIAN pedestal.
+
+Supersedes the dominant-axis / mean detector in offset_scan.py, which had two
+flaws that 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 2026-08-21 read "clean"
+     because Long had a 1.065 in/s event, while Tran sat at +0.47 in/s.
+  2. It used the MEAN, which a real transient perturbs.  The median is the
+     resting baseline: most samples sit at it, so a blast does not move it.
+     Same event, Long: mean +0.0783 vs median -0.0050.
+
+Flags a CHANNEL when |median| >= --floor in/s (default 0.025 = 5 A/D counts,
+Instantel's own criterion; 1 A/D count = 0.005 in/s).
+
+Emits one row per (event, channel) so persistence can be tracked per channel.
+"""
+from __future__ import annotations
+import argparse, csv, re, statistics, sys
+from concurrent.futures import ProcessPoolExecutor, as_completed
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+from minimateplus.event_file_io import read_blastware_file
+
+GEO = ("Tran", "Vert", "Long")
+K = 10.0 / 32000.0                      # ADC counts -> in/s at the 10 in/s range
+_WAVE_RE = re.compile(r"\.[A-Za-z0-9]{2}0[Ww]$")
+_STEM_RE = re.compile(r"^([B-Z])(\d{3})")
+
+
+def serial_from_name(n):
+    m = _STEM_RE.match(n)
+    return f"BE{(ord(m.group(1))-ord('B'))*1000+int(m.group(2))}" if m else "?"
+
+
+def scan_one(ps):
+    p = Path(ps)
+    try:
+        ev = read_blastware_file(p)
+        s = ev.raw_samples or {}
+        if not all(s.get(c) for c in GEO):
+            return None
+        ts = ev.timestamp
+        stamp = (f"{ts.year:04d}-{ts.month:02d}-{ts.day:02d}T"
+                 f"{ts.hour:02d}:{ts.minute:02d}:{ts.second:02d}") if ts else ""
+        out = []
+        for ch in GEO:
+            a = s[ch]
+            out.append({
+                "serial": serial_from_name(p.name), "timestamp": stamp,
+                "filename": p.name, "channel": ch,
+                "median_ips": round(statistics.median(a) * K, 4),
+                "mean_ips":   round(statistics.fmean(a) * K, 4),
+                "peak_ips":   round(max(abs(v) for v in a) * K, 4),
+            })
+        return out
+    except Exception:
+        return None
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--dir", required=True)
+    ap.add_argument("--jobs", type=int, default=4)
+    ap.add_argument("--floor", type=float, default=0.025)
+    ap.add_argument("--out", required=True)
+    a = ap.parse_args()
+
+    seen, files = set(), []
+    for q in sorted(Path(a.dir).rglob("*")):
+        if q.is_file() and _WAVE_RE.search(q.name) and q.name not in seen:
+            seen.add(q.name); files.append(str(q))
+    print(f"unique waveform binaries: {len(files)}", flush=True)
+
+    rows = []
+    with ProcessPoolExecutor(max_workers=a.jobs) as ex:
+        for n, f in enumerate(as_completed([ex.submit(scan_one, p) for p in files]), 1):
+            r = f.result()
+            if r: rows.extend(r)
+            if n % 2000 == 0: print(f"  {n}/{len(files)}", flush=True)
+
+    for r in rows:
+        r["offset"] = int(abs(r["median_ips"]) >= a.floor)
+
+    cols = ["serial","timestamp","filename","channel","median_ips","mean_ips","peak_ips","offset"]
+    with open(a.out, "w", newline="") as fh:
+        w = csv.DictWriter(fh, fieldnames=cols); w.writeheader(); w.writerows(rows)
+
+    from collections import defaultdict
+    ev_flagged = {(r["serial"], r["filename"]) for r in rows if r["offset"]}
+    ev_all     = {(r["serial"], r["filename"]) for r in rows}
+    per = defaultdict(set)
+    for r in rows:
+        if r["offset"]: per[r["serial"]].add(r["filename"])
+    tot = defaultdict(set)
+    for r in rows: tot[r["serial"]].add(r["filename"])
+    print(f"\nfloor = {a.floor} in/s ({a.floor/0.005:.0f} A/D counts)")
+    print(f"events with >=1 offset channel: {len(ev_flagged)} of {len(ev_all)}")
+    print(f"units affected: {len(per)} of {len(tot)}")
+    for s in sorted(per, key=lambda s: -len(per[s])):
+        print(f"  {s:9} {len(per[s]):4} / {len(tot[s]):4} events")
+    print(f"\nwrote {a.out}")
+
+
+if __name__ == "__main__":
+    main()