Operator report: attaching a different geophone to an affected unit
makes the offset go away. That rules out the unit's analog front-end
and any stored per-channel zero constant (a constant lives in the unit
and would survive a sensor swap).
The stored data agrees — MicL, a separate transducer on its own cable,
shows no offset during either episode (|mean|/peak 0.17 and 0.02) while
the geo channels on the same unit at the same moment are pinned.
Two distinct sensor-side patterns recorded:
BE18438 Vert 0.97, Tran 0.16, Long 0.18 -> one conductor pair
BE9558 Long 0.99, Tran 0.90, Vert 0.81 -> shared return / ground
Candidate mechanisms narrowed to three, since a geophone coil is passive
and cannot generate sustained DC: galvanic corrosion at a connector or
splice (matches the ~46 mV referred to the ADC input), a leakage path to
shield, or changed coil DC resistance interacting with the amplifier's
input bias current.
Also records the confound: swapping a sensor requires a monitoring
restart, and these units run Sensor Check "Before monitoring", so the
restart re-zeros too. The swap does not cleanly separate "new sensor"
from "the restart re-zeroed it". Controls and the single best
measurement (open-circuit DC across the suspect connector) documented.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog
serversdownandClaude Opus 5
4f8224a751