Fiber optic time break

Abstract

Geophysical methods that use explosive seismic sources need to produce an accurate time break signal at the time of the blast. This is generally achieved with a seismic detonator, a special variety of electrical detonator (or cap) that is designed to have minimal latency between the injection of electrical current into the detonator and initiation of the explosion, as well as having a slightly higher base charge and better water resistance. A time‐break signal is obtained by either electronically controlling or monitoring the blast current. Seismic detonators are guaranteed to have better than a millisecond latency if sufficient current is injected into the leads; the necessary current is usually 5 to 10 amps. A millisecond tolerance is acceptable for most seismic work but may not be sufficient for shallow studies or for crosswell tomography. However, in fairness to the seismic detonator, the actual performance is generally better: Burrows (1936) and, independently, Rolland and White (1937) reported a time lag of less than 0.3 ms with a deviation of 0.1 ms. These values have changed little since the 1940s. The MK 2 from C.I.L. Inc. (a division of I.C.I.) has an average lag of 0.3–0.4 ms (personal communication with I.C.I. explosives). Major improvements in electric detonator design have been in the areas of safety and durability. A much greater disadvantage for explosive sources are the strict regulations on the transport and storage of explosive devices (Tour, 1992). The cost of complying with these regulations may prohibit the use of explosives in small surveys or in remote areas. An example is the transport of explosives by aircraft: the only passengers allowed on board are those neccesary for completing the flight and for transporting the explosives. Chartering an aircraft to transport a small amount of explosive material is too costly for many geotechnical and mining geophysics surveys.