Guralp Systems CMG-3TB - Manual (Page 7)

Operator's guide
A digital borehole installation can be provided with RS232, RS422 or
fibre-optic links to the surface, depending on the depth of the
borehole.
When a downhole digitizer is present, it takes the place of the strain
relief unit in the borehole. The surface unit also takes a slightly
different form, with a serial connector allowing you to attach a modem
or other communications link. In this type of installation, instead of
using the surface unit to pass control signals to the sensor, all
functions can be accessed remotely via the digitizer.
If you prefer to install a stand-alone digitizer at the surface, it should
be connected to the 19-pin
RECORDER socket of the breakout box.
1.3 The hole lock system
The hole lock clamp unit in a 3TB instrument provides a stable
platform for the sensor modules mounted above and below it. It is
designed to maintain a positive pressure on the borehole casing over a
prolonged period of time without attention, and to fix the sonde in
place whilst avoiding transmitting any stresses.
Güralp Systems hole locks are constructed to order from accurate
measurements of your borehole at the depth you wish to install the
instrument. Either single-jaw or three-jaw hole lock units can be
manufactured.
In installations with sand backfill, or where the instrument rests on the
bottom of the borehole, a hole lock may be unnecessary.
The single-jaw hole lock
The single jaw hole lock is the standard option for triaxial borehole
instruments. It consists of an active clamp arm and a number of skids
or studs on the sonde body. The arm is attached to a compression
spring, which forces it to swing out from the sonde and wedge the
body against the borehole wall. A serrated steel jaw at the end of the
arm provides maximum grip against the borehole casing. The skids or
studs and the locking arm together form a multi-point clamp, which
aligns the sonde body parallel to the axis of the borehole and holds it
firmly in place so that it cannot twist or slip under the influence of
ground vibrations.
There are several configurations of skids and studs which can provide
a suitable clamp. Either
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the locking jaw pushes two steel skids against the side of the
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