docs(offset): CONFIRMED — BE12599 is a geophone-assembly fault, not the recorder

The first mechanism this investigation has established physically rather than
inferred. Brian pulled BE12599 and bench-tested it; a geophone swap settles it.

Same recorder, twenty minutes apart: Vert overswing 2026.0 with its own
geophone, 3.4 with BE9888's known-good one. Frequency 2.2 -> 7.4 Hz. All three
channels Passed, no offsets, no self-triggering, and a clean damped impulse with
broadband content. The recorder is functional and merely out of calibration.

A bench control -- trigger forced by slapping the microphone, geophone untouched
-- shows Vert producing 0.19 in/s of bipolar wander with no mechanical input at
all. The fault is continuous, not episodic. All three channels are affected:
Tran and Long sit on a small DC while their coils still pass the swing test,
Vert has lost damping entirely. That is leakage on several contacts plus one
gone open, not three elements failing in sequence.

The consequence that reaches past this unit: an autozero adjusts the RECORDER's
zero reference and cannot fix a geophone. If most offsets are geophone-assembly
faults, a re-zero succeeds only on the minority where the recorder really is at
fault -- which is exactly the ~10% rate recorded in section 5 and unexplained
since August. Procedure change: swap the geophone first.

Corrects 8e's title, which called it a connector rather than a geophone: the
swap narrows the fault to the assembly, and element / cable / connector remain
open within it. And reweights the cause -- the geophone is buried, not in the
enclosure with the nest, and the unit sat ~6 ft from I-80 WB, so road salt now
outranks the mice.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog

docs/offset_investigation.md

@@ -934,6 +934,120 @@ which looks like a different failure mode wearing the same signature.
 
 ---
 
+### 8f. BE12599 — CONFIRMED on the bench: the geophone assembly, not the recorder (2026-09-23)
+
+**The first mechanism in this investigation established physically rather than
+inferred.** Brian pulled BE12599 from I-80 Abut 1 West on 2026-09-21 and bench-
+tested it. A geophone swap settles it.
+
+#### The swap test
+
+Same recorder, twenty minutes apart, sensor self-check:
+
+| geophone fitted | Tran | **Vert** | Long |
+|---|---|---|---|
+| BE12599's own (2026-09-22 21:10) | 4.2 | **2026.0** | 4.8 |
+| BE9888's known-good (2026-09-22 22:48) | 3.6 | **3.4** | 3.6 |
+
+Vert frequency likewise **2.2 Hz -> 7.4 Hz**. Fleet reference: overswing 3.3–5.1
+(median 3.8, max 5.1 across 42,979 passing records), frequency 7.4–7.8 Hz.
+
+With the good geophone the unit shows **no offsets, no self-triggering, all three
+channels Passed**, and records a clean damped impulse with broadband 5–100 Hz
+content. **BE12599 the recorder is functional** — it is only out of calibration
+(due 2025-08-12).
+
+**The fault is in the geophone assembly: element, cable, or its connector.**
+The swap does not separate those three, because the cable is permanently glanded
+into the geophone.
+
+#### The bench evidence, with ground motion removed
+
+17 events recorded on a workbench with nothing touching the sensor.
+**The control** — `N599LRY0.4B0`, trigger forced by slapping the *microphone*,
+geophone untouched:
+
+| channel | peak in/s | frac < 0 | mean/peak |
+|---|---|---|---|
+| Tran | 0.070 | **0.00** | 0.87 |
+| **Vert** | **0.190** | **0.35** | 0.13 |
+| Long | 0.070 | **0.00** | 0.84 |
+
+Vert produces 0.19 in/s of bipolar wander with **no mechanical input at all**.
+The fault is *continuous*, not episodic — every field event was simply the
+moment it crossed the trigger.
+
+**All three channels are affected**, consistently across all 17 events:
+Tran and Long parked at a small positive DC (`frac<0 = 0.00`, mp 0.64–0.95);
+Vert wandering at 0.89–1.78 Hz, peaks 0.14–0.54. Yet the swing test calls Tran
+and Long **Passed** (4.2 / 4.8). So their *coils* are healthy and merely sitting
+on an offset, while Vert's circuit has lost damping entirely.
+
+That reads as **leakage on several contacts plus one gone open** — which is what
+progressive corrosion at a shared connection does. Three independent geophone
+elements do not fail in sequence.
+
+**It is still progressing.** Tran's DC offset was ~0.011 in/s on 2026-09-14 and
+~0.064 on 2026-09-22 — 6x in eight days. Vert's overswing went 7.5 -> 2026.0 over
+the same window.
+
+#### ⚠ This explains the autozero's 10% success rate
+
+The journal has framed this fault around the **recorder** throughout: Instantel's
+autozero, the X1/X8 gains, the 2027–2069 window, `SUB 0x0E` as the best lead.
+All of that adjusts the recorder's zero reference.
+
+**An autozero cannot fix a geophone.** If most offsets are geophone-assembly
+faults, a re-zero would succeed only on the minority where the recorder really is
+at fault — which is exactly the ~10% Brian's father reported and which has sat
+unexplained in §5 since 2026-08-28.
+
+**Procedure change: swap the geophone first.** It is a five-minute field test
+costing nothing. On this unit it would have answered the question in August.
+
+⚠ Scoped to one unit. The other four confirmed units are the obvious check.
+
+#### Correction to §8e
+
+§8e is titled "a connector, not a geophone" and concluded the element was fine
+because the fault hopped channels. The channel-hopping observation stands and the
+swap test supports the general shape, but **that title overstated what was known**:
+the swap narrows the fault to the *assembly*, and element / cable / connector are
+still open within it. §8e's reasoning was sound; its confidence was not.
+
+#### Cause — road salt now outranks the mice
+
+The enclosure held a mouse nest on the battery ([[§8e]]), and that framed the
+2026-09-22 analysis. But **the geophone is buried in the ground, not in the
+enclosure** — they are two independent contamination paths, and the fault is in
+the buried half.
+
+The unit sat **~6 feet from I-80 westbound**. Winter road salt spray and highway
+runoff put a buried geophone in chloride-rich water for months a year. Chloride
+drives exactly this failure: corrosion at terminations, leakage across
+insulation, eventually an open circuit.
+
+The mice remain a real finding for the *enclosure* and the recorder end. They are
+probably not the cause of this fault.
+
+**Testable consequence:** road salt is a property of the **site**, not the unit.
+Units deployed near salted highways should carry higher geophone-assembly failure
+rates than units on quiet sites. Nothing in the archive records proximity to a
+road, so this needs the deployment records rather than the event data.
+
+#### Open — to be settled by opening the geophone
+
+1. **Meter at the geophone plug** before disassembly: three coil pairs, and each
+   to shell. Prediction: **Vert open or very high** (that is the 2026 overswing);
+   Tran and Long matched and normal; **insulation to shell finite on all three**,
+   Vert worst (that is the leakage putting DC on healthy coils).
+2. **Then open it.** Look for a water line, white/green corrosion at the coil
+   terminations, the state of the damping shunt, and the cable gland from the
+   inside. Measure the coils again **at the internal terminals** — if Vert reads
+   fine there, the fault is in the internal wiring or the gland, not the element.
+
+---
+
 ### ⚠ Serial prefixes — four of these units are BlastMates, not MiniMates
 
 Corrected 2026-09-06, after Brian queried "BA10895?" against a report that
@@ -985,3 +1099,7 @@ decoder lost the geo threshold along with the serial. See commit `9ceff65`.
 | 2026-09-06 | The spread gate is **blind to onsets by construction** — it rejects a moving floor, which is what an onset is. It rejected the very record in which the ramp is visible. |
 | 2026-09-07 | **The non-motion test** (Brian: "it doesn't cross zero"). `\|mean\|/peak` is bimodal with a 1.09% dead zone and returns exactly the 5 confirmed units from physics, not a threshold. Independent corroboration of the unit list. **BE11007 settled as NOT an offset** — a one-sided 9 in/s blast. |
 | 2026-09-07 | **BE12599 is a connector fault, not a geophone fault.** Unipolar spikes with a 26→118 ms RC tail progressing to a flat pedestal, and the fault MOVES between Long and Tran while the sensor self-check passes on every event. An element cannot hop channels; a connector can. Inspect before reseating. |
+| 2026-09-23 | **CONFIRMED: BE12599's fault is the geophone assembly, not the recorder.** Geophone swap on the same unit: Vert overswing **2026.0 -> 3.4**, frequency 2.2 -> 7.4 Hz, all three channels Passed, no self-triggering. First physically established mechanism in this investigation. |
+| 2026-09-23 | **This explains the autozero's ~10% success rate** (§5): an autozero adjusts the *recorder's* zero reference and cannot fix a geophone. **Procedure change: swap the geophone first.** |
+| 2026-09-23 | Bench control event (mic-slap trigger, geophone untouched) shows Vert wandering **0.19 in/s with no mechanical input** — the fault is continuous, not episodic. All three channels affected; Tran/Long parked on DC while their coils pass the swing test. |
+| 2026-09-23 | **Cause: road salt now outranks the mice.** The geophone is buried, not in the enclosure — two independent contamination paths, and the fault is in the buried half. The unit sat ~6 ft from I-80 WB. Salt is a *site* property, so this predicts higher failure rates near salted highways. |