Guralp Systems CMG-3TB - Manual (Page 43)

Operator's guide
The best option is often to attach the sensor housing to an earth line
brought out to the surface and attached to a metal stake driven into the
ground nearby.
The sensor's load bearing cable is suitable for this purpose, provided it
is secured to the sensor's lifting loop with a metallic clamp as shown
below. This provides an additional firm contact between the sonde and
the load-bearing cable. Installations with downhole digitizers will need
similar arrangements at the top and bottom of the digitizer module, or
a separate cable for this purpose.
November 2006 43