diff --git a/docs/offset_investigation.md b/docs/offset_investigation.md index d69f0a1..3b44a3e 100644 --- a/docs/offset_investigation.md +++ b/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. |