Slashes on Seismic Records

The mechanism involved in the generation of large-amplitude pulses caused by electrical bursts, which appear from time to time on seismic recordings, is analyzed in terms of a VLF transmitter/ receiver system. This method affords a systematic approach to the generation, propagation, and reception of static bursts and the way they are transformed into low-frequency pulses by the seismic recording instruments. To begin with, the basic building blocks of the transmitter/receiver system are defined in terms of the seismic recording system. Subsequently, each block is analyzed in some detail to determine the condition necessary for the reception of static bursts, in particular those generated by lightning, from great distances. Experimental results are included to test several of the hypotheses formulated in the course of the analysis. The main cause responsible for the high sensitivity of the recording instruments to static bursts is traced to the low common-mode rejection of the input transformer in the VLF band. Most of the electromagnetic energy radiated by lightning discharges is concentrated within this band.