# 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.
>
> **Then Brian proposed a better detector still** — measure the floor during
> the *pre-trigger* window, and require it to hold across pre/middle/end.
> That is now the detector of record (§2c). Final answer: **5 of 45 units
> (11%)**, stable across a 2x threshold range.
>
> 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.

This is a *journal*, not a spec. Findings are dated, dead ends are kept with
the reason they died, and every number says where it came from. When something
here is superseded, strike it and say why rather than deleting it — the point
is that a future session can tell what was actually established from what was
merely believed at the time.

Companion material:
- `scratch/offset_scan.py` — the detector
- `scratch/verify_against_ascii.py` — decoder verification harness
- `docs/instantel_protocol_reference.md` — wire protocol, incl. the
  unimplemented `SUB 0x0E` this investigation now wants

---

## TL;DR (current state, 2026-08-28)

- **It is real device data, not a decode bug.** Settled early and confirmed
  against Blastware's own ASCII exports.
- **Base rate: 5–6 of 45 units (11–13%)** across the full DL2 archive,
  2018–2026. This *confirms* the earlier 2-of-21 (9.5%) estimate from the much
  smaller Terra-View DB — survivorship bias from deleted events had **not**
  concealed a wave of cases.
- **The fault is bimodal, not a drift continuum.** A unit is either clean or
  grossly off. Loosening the amplitude threshold 11× adds no new units.
- **The unit's own sensor check cannot see it.** 102 offset events, zero
  sensor-check failures. Do not try to use it as a screen.
- **Cause is still unsettled.** Instantel's autozero fixes the minority of
  cases; the rest are hardware. We cannot yet tell which is which remotely.
- **Best open lead:** `SUB 0x0E` (channel sensor data, 8 channels × 10 bytes,
  unimplemented) may carry the very numbers Instantel says to check against
  **2027–2069**. Untested.

---

## 1. What the fault looks like

A healthy geophone trace is centred on zero. An offset channel is parked away
from zero, so the channel **mean approaches its own peak**. In Blastware the
signature is "parallel lines above or below the zero line" (Instantel's own
wording).

Consequences observed in the field:
- The unit can **self-trigger on its own offset** when the displacement exceeds
  the geo trigger level, producing streams of junk events with no ground
  motion. Instantel has a separate FAQ for this symptom (13-0-22, *"Unit
  triggers continuously without activity"*).
- Recorded PPV for that channel is meaningless while the fault persists.

---

## 2. The detector

Implemented in `scratch/offset_scan.py`. Operates on raw BW binaries only — no
DB, no sidecars.

```
for each series-3 waveform binary:
    decode -> per-channel ADC counts
    dominant axis = channel with the largest |peak|
    flag when   |mean| / peak  >  0.70
          and   |mean|         >= 0.90 x the unit's geo trigger level
episodes = per-serial runs of flagged events, split on a >12 h gap
```

Why each term:

| term | purpose |
|---|---|
| `\|mean\|/peak > 0.7` | the discriminator. A DC-parked trace has mean ≈ peak. |
| `\|mean\| >= 0.9 × trigger` | amplitude floor — suppresses quiet traces where mean and peak are both tiny and the ratio is meaningless. |
| dominant axis only | the fault is per-channel; scoring all three dilutes it. |
| 12 h episode gap | separates deployments/visits rather than counting events. |

Trigger level comes from a paired `_ASCII.TXT` when one exists, else the
per-serial median learned from that unit's ASCII files, else 0.2 in/s.

**Known limitation.** Event traces contain real ground motion, so this can only
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.

### 2c. Detector v3 — PRE-TRIGGER floor + constant-floor test (CURRENT)

`scratch/offset_scan3.py`. Brian's method, and better than v2 for a reason
worth naming: **the pre-trigger window is definitionally quiet** — it is the
buffer captured before the trigger fired — whereas a whole-record median is
merely *robust* to the event. `pretrig_samples` comes from the STRT record.

```
per channel:
    pre    = median of the first pretrig_samples samples
    mid    = median of the middle third
    end    = median of the final third
    spread = max(pre,mid,end) - min(pre,mid,end)

    offset when  |pre| >= floor  AND  spread <= 0.02 in/s
real fault when a channel is flagged on >=3 CONSECUTIVE events
```

A DC offset is a **constant floor** — present before the trigger, during, and
after. The spread test rejects transients (settling, handling, a long event
tail) that move one segment relative to the others, which is what v2's
whole-record median could not do.

**The empirical noise floor justifies the threshold.** Across 19,244
non-flagged channel-events the pre-trigger floor distributes as:

| floor | share |
|---|---|
| −1 unit (−0.005) | 18.4% |
| **0.000** | **62.7%** |
| +1 unit (+0.005) | 13.4% |

**94.5% within ±1 quantisation unit; median exactly +0.0000, mean −0.0008.**
So there is **no systematic zero-point bias in the decoder** — an independent
confirmation of the 32000-count scale. A healthy channel really does read
0.000, and "any constant floor that is not 0.000" is the right signal, with
±1 unit of slack for quantisation.

**The result is threshold-insensitive**, which is what distinguishes a real
signal from a tuned one:

| floor | units flagged | sustained units |
|---|---|---|
| 2 units (0.010) | 34 | 15 ← into the noise |
| 3 units (0.015) | 26 | 8 |
| **4 units (0.020)** | 17 | **5** |
| **5 units (0.025)** — Instantel's | 12 | **5** |
| **8 units (0.040)** | 8 | **5** |

### FINAL RESULT: 5 of 45 units (11%)

**BE9558, BE11529, BE12599, BE13117, BE18438.**

Unchanged across a 2x threshold range. BE11007 and BE10895 drop out — the
spread test identifies them as transients, not pedestals.

The 11% headline happens to match v1's, but the reasoning and the unit list
differ: v1 included BE11007 and named the wrong *channel* on most units.

---

## 3. Archive results (2026-08-28)

Source: DL2 event export, 6,577 **unique** series-3 waveforms, 45 units.
See [`dl2-archive`](#8-data-and-tooling) for the `Sent/` mirror trap.

**283 suspect events, 15 episodes, 6 of 45 units (13.3%).**
Excluding BE11007 (§4, likely not an offset at all): **5 of 45 = 11.1%**.

### Threshold sensitivity — the bimodality result

Re-scoring the same corpus at a range of amplitude floors, with two
ratio cut-offs (1 A/D count = 0.005 in/s, see §5):

| \|offset\| floor | m/p > 0.7 | m/p > 0.9 |
|---|---|---|
| 5 cts (0.025 in/s) — *Instantel's own* | 333 ev / 6 units | 279 ev / **5 units** |
| 10 cts (0.050) | 294 / 6 | 274 / 5 |
| 20 cts (0.100) | 250 / 5 | 244 / 4 |
| 40 cts (0.200) | 209 / 5 | 203 / 4 |
| 80 cts (0.400) | 152 / 4 | 148 / 2 |
| 160 cts (0.800) | 144 / 2 | 141 / 1 |

Relaxing the floor by 11× (0.27 → 0.025 in/s) adds ~14% more events and **no
new units**. There is no population of mild offsets hiding below our threshold
*in event data*. Either a unit is clean or it is grossly off.

---

## 4. Per-unit case files

Ordered by severity. `m/p` medians are on the offending channel.

### BE13117 — one violent day, never again
`145 / 454 events (32%)`, **1 episode**, 2023-05-04, 6.8 h.
Offset climbed **0.393 → 1.875 in/s within the episode**. `m/p` median
**0.996** — the trace is almost pure DC. No recurrence in the rest of its 454
events. No ASCII files in the archive, so no calibration history.

### BE18438 — recurring, months apart
`87 / 293 (30%)`, **2 episodes**: 2025-11-15 (1.2 h, n=12, 0.279 → 0.369) and
2026-02-25 (**28.8 h**, n=75, 0.183 → 0.366). `m/p` median 0.967.
Clean across all 196 events preceding its 2025-08-12 calibration.

### BE9558 — six years apart
`38 / 196 (19%)`, **4 episodes**: 2020-02-11 (6.3 h, n=33, but only
0.068 → 0.086 — very mild), then 2026-04-14, 2026-04-29, 2026-05-04
(0.28–0.45). `m/p` median 0.919. Calibrated 2026-06-26; 0/7 events flagged
after, but n=7 is far too small to call it fixed.

### BE12599 — the live case ⚠
`6 / 77 (8%)`, **6 single-event episodes, one per day at exactly 05:00**,
2026-08-17 → 2026-08-23. Offset rose 0.383 → 0.565 then fell back to 0.345.
`m/p` ≈ 0.965, geo trigger 0.3 in/s — **the offset exceeds the trigger level,
so the unit is triggering on its own fault**. Last calibrated 2025-08-12.

This is the most recent and the most useful: a currently-faulting unit is the
natural experiment for the re-zero-vs-repair question (§7).

### BE11529 — marginal
`4 / 99 (4%)`, 1 episode 2025-07-08, 0.4 h, offsets only 0.051 → 0.058 in/s.
`m/p` median 0.959, so DC-dominated, but the magnitude is near the noise of
this method. Treat as unconfirmed.

### BE11007 — probably NOT an offset
`3 / 70 (4%)`, 1 episode 2022-01-17, offsets 5.500 → 6.904 in/s — by far the
largest. But `m/p` is only **0.719–0.738** against ≥0.9 for every other unit,
and the peaks are 7.6–9.4 in/s on a 10 in/s range. That reads as a **large
real blast with asymmetric ground motion**, not a parked trace. Excluded from
the headline base rate.

---

## 5. Instantel's own procedure and thresholds

From two Instantel technical-support FAQs supplied 2026-08-28
(answers **13-0-21** *"How to determine offsets"* and **12-0-10** *"Removing
offsets on an Instantel Series III monitor"*; created 2008/2007, last updated
2009-03-06).

### Identifying (13-0-21)

1. Create or use an event with the **manual minimum trigger** set for the
   connected geophone and microphone — i.e. an event that recorded no real data.
2. Save it and open in Blastware.
3. An offset shows as **parallel lines above or below the zero line**.
4. Put the unit in **A/D mode** — on Series III, press and hold `OPTION`, then
   press `START MONITOR`.
5. **Display counts higher than 5**, with no vibration or overpressure present,
   indicate an offset.

### Removing — the autozero (12-0-10)

1. Be in a **quiet area with low vibration**.
2. Power on the Blastmate III / Minimate Plus.
3. Connect the geophone and microphone — **LINEAR mic only**.
   ⚠ *Do not connect an "A" weight microphone, regardless of what the monitor
   displays.*
4. Press `Test`.
5. Wait for the **Sensor Check** results to appear.
6. Press `OPTION` and `START MONITOR` **simultaneously**.
7. `Performing Autozero` appears; press `Enter`.
8. Confirm the sensors are properly connected; press `Enter`.
9. Wait for the autozero to complete.
10. Press `Enter` twice → Main Menu, *Ready To Monitor*, offset corrected.

### The go/no-go number — 2027 to 2069

> When you perform an Autozero on any Series III unit, the lists of numbers in
> the **X1 and X8 gains should all be between 2027 and 2069**. If not, repeat
> the Autozero. **If the numbers are extremely out of the specified range, then
> the unit should be sent in for repair.**
>
> If this process does not remove the offset problem, return the unit **and
> sensors** to Instantel for repair.

This is the documented explanation for the field experience (Brian's dad,
2026-08-28) that **a re-zero works maybe 10% of the time** — the autozero only
recovers units whose zero reference is still near-correct.

### Scale derivation (inference, well-supported — not proven)

2048 is 12-bit midscale. Our codec's geo full scale is 32000 internal counts =
10 in/s, with 1 decoder unit = 16 counts = exactly 0.005 in/s
(`geo-full-scale-is-32000-counts`). ±2000 A/D counts about 2048 therefore maps
to ±10 in/s at **0.005 in/s per A/D count**. That makes:

- Instantel's ">5 counts" threshold ≈ **0.025 in/s**
- the 2027–2069 window = **±21 counts = ±0.105 in/s** of tolerated zero error

Consistent and mutually corroborating, but we have not confirmed the A/D-count
scale directly from a device reading.

---

## 6. Ruled out — keep these dead

### Condensation / humidity — DEAD (2026-08-25)
Proposed, then killed by its own controls: BE18438 stayed flat across a 10-hour
overnight gap, and only 2 of 21 units showed the fault while 19 sat in the same
weather. The apparent "diurnal cycle" was an artifact of binning by hour-of-day
across two days. See `waveform-dc-offset-is-real-device-data`.

### Clipping as a false-positive source — RULED OUT (2026-08-28)
A rail-hitting trace would fake an offset (mean → peak). It isn't happening:
median suspect peak is only **10% of full scale**, p90 is 18.6%. Only BE11007's
3 events exceed 50% FS, and none reach 98%.

### The sensor check as a predictor — DOES NOT WORK (2026-08-28)
Tested on 102 offset events across 4 units:

| unit | state | n | failed | median ratio | median freq |
|---|---|---|---|---|---|
| BE11529 | offset | 4 | **0** | 3.90 | 7.6 |
| BE11529 | clean | 14 | 0 | 3.80 | 7.5 |
| BE12599 | offset | 6 | **0** | 4.00 | 7.4 |
| BE12599 | clean | 13 | 0 | 4.00 | 7.6 |
| BE18438 | offset | 87 | **0** | 3.70 | 7.6 |
| BE18438 | clean | 25 | 0 | 3.80 | 7.5 |
| BE9558 | offset | 5 | **0** | 3.90 | 7.8 |
| BE9558 | clean | 44 | 0 | 3.80 | 7.5 |

Zero failures on either side and indistinguishable ratios/frequencies. The
swing test measures geophone frequency response and damping — it never examines
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:

| unit | suspect before latest cal | after |
|---|---|---|
| BE18438 | 0 / 196 | 87 / 97 |
| BE12599 | 0 / 62 | 6 / 15 |
| BE11529 | 0 / 82 | 4 / 17 |
| BE9558 | 38 / 189 | 0 / 7 |

But bucketing suspects by months-since-calibration gives **one unit per bucket**:
`0–3mo={BE11529}`, `3–6 & 6–9mo={BE18438}`, `9–12mo={BE9558}`,
`12–15mo={BE12599}`. The apparent "51% failure rate at 6–9 months" is entirely
BE18438's single February 2026 episode. Five units with roughly one episode
each cannot support a population trend. **Do not re-derive this.**

Also note: all affected units are calibrated on a **~12–13 month cadence**, so
"sent to Instantel" is the routine annual schedule, not evidence of a
fault-driven return.

---

## 7. Open questions

### Q1 — Is it a latched bad zero or analog degradation?
The question that decides everything. A latched zero is correctable (possibly
over the wire); degradation means a repair. Instantel's 2027–2069 rule implies
*both* populations exist, with the split roughly 10/90 in the field.

**BE12599 is the natural experiment** — faulting as of 2026-08-23. Read its
values, run the autozero, read them again.

### Q2 — Can we read the autozero numbers over the wire? (best lead)
Instantel says to check *"the lists of numbers in the **X1 and X8 gains**"* —
4 sensors × 2 gains = **8 channels**. The protocol reference already documents
an unimplemented command with exactly that shape:

```
SUB 0x0E  ->  RSP 0xF1     "channel sensor data"
  2-step read; channel selector in params[6:8] = 0x0000..0x0007
  data length 0x0A (10 bytes) per channel
```

Blastware's *Unit Channel Test* sequence:
`POLL×N → 0x15 → 0x01 → 0x08 → 0x01 → 0x0E×8 → 0x98×2 → 0x0E×8`
— note the **second `0x0E` pass carries live ADC readings**.

**Hypothesis (untested):** `0x0E` returns the numbers Instantel wants compared
against 2027–2069. If true, SFM could diagnose an offset remotely *and* predict
whether a re-zero will succeed — converting a 10%/90% shipping gamble into a
decision made before packing a box.

**How to test.** `bridges/ach_mitm.py` is a generic TCP proxy:

```bash
python bridges/ach_mitm.py --bw-host <MODEM_IP> --bw-port 9034 --listen-port 9999
```

Point Blastware at the proxy and run **Unit Channel Test**.
⚠ In this topology the output filenames are reversed — the tool labels the
*connecting* side "unit", so `raw_s3_*.bin` holds Blastware's bytes and
`raw_bw_*.bin` the unit's.

Capture priority: (1) BE12599 while faulting, (2) a known-good unit as control,
(3) before/after an autozero on the same unit. Eight 10-byte payloads with an
expected value near 2048 is a very constrained puzzle.

### Q3 — What is the mild-offset rate?
Unmeasurable from event files (§2). Only the A/D-mode check sees it. Would
need a fleet sweep in A/D mode, or Q2 to succeed.

### Q4 — Does an offset recur on the same unit after service?
BE9558 shows episodes in 2020 and 2026; BE18438 twice in four months. Suggestive
of recurrence, but service records aren't in the data — only calibration dates.

---

## 8. Data and tooling

| what | where |
|---|---|
| detector | `scratch/offset_scan.py` |
| current results | `/home/serversdown/dl2-archive/offset_archive.csv` |
| earlier candidate list (Terra-View DB, 274 events) | `scratch/offset_candidates.csv` |
| archive working copy | `/home/serversdown/dl2-archive/files/` |
| archive source | NAS `DeathStar` 10.0.0.2, `/volume1/Uploads/TMI/DL2-Event-backup-8-25-26/Event/autocall home/` |

⚠ **The DL2 export keeps a byte-identical `Sent/` mirror of its root.** 13,077
waveform paths are 6,577 distinct files. Always dedupe by basename — this
doubled two reported figures before it was caught.

---

## 9. Chronology

| date | event |
|---|---|
| 2026-08-25 | Reported as a *waveform decode bug* — traces with a DC offset. Investigation shows the offset is **real device data**; the decoder is correct. |
| 2026-08-25 | Brian relays his dad's description: a known hardware fault called an "offset"; usually sent to Instantel. |
| 2026-08-25 | First detection pass over the Terra-View DB: **2 of 21 units**, 274 events, 11 months. Flagged as vulnerable to survivorship bias — flooded events were routinely deleted. |
| 2026-08-25 | Condensation hypothesis proposed, then **killed by its own controls**. |
| 2026-08-25 | Parked pending the multi-year archive. |
| 2026-08-28 | DL2 archive pulled (33 GB, 546k files; 6.6 GB working set). |
| 2026-08-28 | Archive scan: **6 of 45 units**, 283 events, 15 episodes. Prior base rate **confirmed**, not overturned. |
| 2026-08-28 | Clipping ruled out; `m/p` established as the discriminator; BE11007 reclassified as probably a real blast. |
| 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. |
| 2026-08-28 | **Detector v3 (Brian's method):** pre-trigger floor + pre/mid/end consistency. Healthy channels proven to sit at 0.000 +/-1 unit (94.5%), confirming no decoder zero-point bias. Final: **5 of 45 units (11%)**, threshold-insensitive. |