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who is far enough from the shotpoint to be safe but close enough to see what is happening. This can be achieved if, after the shot hole has been charged, the detonator is first connected to one end of an expendable cable 20 or 30 m long. Only when the shot point is clear should the other end of this cable be connected to the cable from the firing unit. Firing can then be prevented at any time by pulling the two cables apart. Unless ‘sweaty’ gelignite is being used (and the sight of oily nitro-glycerine oozing out of the packets should be sufficient warning to even the least experienced), modern explosives are reasonably insensitive to both heat and shock. Detonators are the commonest causes of accidents. Although their explosive power is small, they have caused loss of fingers and even hands. If fired on their own as low energy sources, they should always be placed in well-tamped holes, since damage or serious injury can be caused by fragments of the metal casing. 3.5 Time breaks In any seismic survey, the time at which the seismic wave is initiated must be known. In some instruments this appears on the record as a break in one of the traces (the shot break or time break). On most modern instruments it actually defines the start of the record. Time-break pulses may be produced in many different ways. A geophone may be placed close to the source, although this is very hard on the geophone. Explosive sources are usually fired electrically, and the cessation of current flow in the detonator circuit can provide the required signal. Alternatively, a wire can be looped around the main explosive charge, to be broken at the 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 hammer hits the plate (Figure 1.4).
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