feed a single amplifier. In addition to recording information from the 24 to 

 60 channels which make up a modern reflection seismograph, the seismogram 

 photographs timing lines, the initiation of the shot and the arrival of direct 

 energy at the top of the shot hole, events A, D, and C of Figure 26-1. 



The reflection seismic amplifier is complicated by an automatic gain control 

 to hold weak and strong seismic pulses to a similar excursion level on the seis- 

 mogram, a set of filters to emphasize the frequency range of the desired signals 

 and a crossover network for mixing signals among adjacent channels. The 

 conventional reflection seismograph has a frequency pass band of 20 to 90 

 cycles per second. Wide range seismographs adapted for magnetic tape record- 

 ing and later selective play-back may have a 5 to 500 cycles per second response. 

 Such systems are suitable for refraction, conventional reflection and shallow or 

 high-frequency reflection seismic prospecting. The versatility of data display 

 permitted by magnetic recording will be discussed later. 



Although chiefly conducted on land, seismic operations are now extensively 

 performed in the exploration of water-covered areas. Boats replace trucks, 

 seismometers or pressure transducers are trailed, and shots are set off in the 

 water. 



In land operations, elastic waves, generated near the ground surface, 

 usually by a buried explosive charge but sometimes by falling weights or charges 

 above the ground surface, are broadcast, or in special cases partially beamed, 

 from the vicinity of the source throughout the adjacent region of the earth's 

 crust. Much of the energy from a buried explosive charge or shot is dissipated 

 in fracturing and compacting the material immediately surrounding the shot. 

 Of that energy which survives as an elastic wavelet or pulse, some reaches 

 vibration detectors distributed in the general vicinity. The relative placement 

 of these detectors with respect to the source and the relative arrival-times with 

 respect to the initiation of the shot by whatever path this elastic energy may 

 travel, comprise a set of data from which structural and lithologic information 

 may be gained. 



We measure, then, two variables — time and distance — in our effort to 

 determine the location, configuration, and attitude of our subsurface target. We 

 are sometimes guided by two others — relative amplitude and frequency. Be- 

 cause of the multiplicity of recording channels employed, we obtain a correspond- 

 ing number of simultaneous equations from which to determine the position and 

 attitude of the reflector segment. 



In common with some of the electrical prospecting methods, we create 

 the very force field whose reactions we determine, rather than measure those 

 uncontrolled fields inherent in the earth as encountered in magnetic or gravity 

 prospecting. There is a further and unique advantage in seismic prospecting: 

 we can examine and to some degree appraise the significance of the data as they 

 are obtained. 



558 



