Chapter 11. The Borehole Hanger
11.1 Description
The Affinity Borehole Hanger is a welded steel frame, which is designed to fit into the top of a borehole casing, in place of a trap plate. It is made to securely carry an Affinity unit flush with the top of the borehole casing.
The Borehole Hanger top plate serves in place of a trap plate, giving a secure surface against which to install a clamp on the installation strain wire.
The hanger frame is designed so that the Affinity unit may be removed for service without disturbing the seismometer installation.
11.2 Installation
To install the borehole hanger, follow the procedure below:
Install the sensor system in the borehole. Fit the last cable clamp between the signal cable and the strain wire such that it will be approximately 1 m below the top of the borehole casing at the final depth, with approximately 1.5 m of signal cable loose above the clamp.
If surplus cable must be accommodated, it may be formed into a loop and secured below the hanger frame to the strain wire, or to the cross-bar on the hanger frame, such that the weight of the cable is not applied to the connector on the Affinity.
Remove the trap plate from the top of the hanger frame.
Feed the free end of the signal cable up through the hanger frame. It should pass the bar at the bottom of the hanger on the side towards the straight edge of the hole in the lower plate.
Feed the hanger into the borehole, passing the strain wire through the slot in the top plate so that the top plate rests on the borehole casing.
Fit a clamp to the strain wire just above the hanger top plate and lower the strain wire so that the weight of the cables is carried by the top plate.
Connect the signal cable to the Affinity digitiser and lower it into the hanger frame. Engage the two pins on the frame lower plate with the peg holes in the end bulkhead of the Affinity. This may be facilitated by connecting one or two of the system cables to the top of the Affinity and using their connectors for purchase.
Caution: Do not suspend the Affinity by the cables but grip the connector bodies.
Fit the trap plate, engaging its pins in a similar manner.