Geology Reference
In-Depth Information
Figure 11.6
'Post-office relay' impact switch on the back of a sledgehammer
handle.
shot instant. This technique can be used on the rare occasions when charges
are fired using lit fuses.
Hammer surveys usually rely on making rather than breaking circuits.
One method is to connect the hammer head to one side of the trigger circuit
and the plate (assuming it is metal, not rubber) to the other. Although this
sounds simple and foolproof, in practice the repeated shocks suffered by
the various connections are too severe for long-term reliability. In any case,
the plates themselves have rather short lives, after which new connections
have to be made. It is more practical to mount a relay on the back of the
hammer handle, just behind the head, that closes momentarily when the ham-
mer hits the plate (Figure 11.6). It will close late, or not at all, if the hammer is
used the wrong way round. Solid-state switches sold by some seismograph
manufacturers give more repeatable results but are expensive and rather
easily damaged.
The cable linking the trigger switch on a hammer to the recorder is always
vulnerable, tending to snake across the plate just before impact. If it is cut,
the culprit is traditionally required to both repair the damage and ease the
thirst of all the witnesses.
Where the source is a heavy weight dropped from a considerable height,
a relay switch can be attached to its top surface but may not trigger if the
