Guralp Systems CMG-3TB - Manual (Page 41)

Operator's guide
which is connected to a local ground plate. All earth lines for
equipment in the installation, such as the casings of the transformers
and of the sensor electronics, as well as the signal ground line from the
sensor, should be connected to this plate.
The best local earth point in many installations is the borehole itself.
For this to work, the borehole must have a conductive casing and be
situated close (<30 m) to the surface installation. In such an
installation you need only connect a cable (green wire in the
photograph below) from the local ground plate to the borehole casing.
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