Guralp Systems CMG-3TB - Manual (Page 42)

CMG-3TB
An earth strap can be used to ensure a good connection.
If the lower borehole is filled with salt water, the instrument will be
adequately grounded without any further action. Fresh water is an
inferior conductor.
In a dry or sand-filled borehole, or one with a non-conducting casing,
you will need to ensure the sonde is grounded by some other means.
42 Issue C