836 EXPLORATION GEOPHYSICS 



Generally one trace nearest the shot-point is selected for the frequency 

 pattern studies. By comparison with the patterns of corresponding traces 

 at the other shot-points, the structure and certain physical properties of the 

 subsurface are revealed. 



Figure 517 is a schematic diagram of one form of instrumentation 

 employed in the frequency-pattern technique. The analysis is made in steps 

 of 5 cycles, and usually covers the range from 2 to 500 cps. The film or 

 magnetic tape is spliced into an endless loop and passed through the analyzer 

 in repetitive cycles. Approximately 1.5 seconds time is required for each 

 pass, and 100 passes are necessary to cover the desired frequency range. 

 The total time for producing the frequency pattern is about two minutes. 



A synchronous motor drives the reproducer through which the record 

 loop is cycled. The film drive is geared to a potentiometer which supplies 

 a D.C. bias sweep current to the cathode-ray tube, so that each sweep of the 

 cathode ray beam across the tube corresponds to one pass of the record loop. 



The output of the reproducer is passed through an amplifier and thence 

 into a band pass filter or heterodyne type wave analyzer. The output of the 

 filter is then amplified and actuates the vertical deflection coil of the 

 cathode-ray tube, thereby introducing vertical displacement as the cathode- 

 ray beam is moving horizontally across the tube. The beam is shifted 

 downward at each successive horizontal sweep, so that a series of parallel 

 lines is traced across the tube as the analysis proceeds. By superimposing 

 the patterns obtained at the various stations, the relative travel times or 

 AT values are indicated by the shift in the films at each station, at the best 

 correlation. 



ASSEMBLY OF EQUIPMENT FOR SEISMIC PROSPECTING 



A major portion of the geophysical equipment used in commercial 

 work is designed for mounting in trucks or other motor-propelled convey- 

 ances. Figure 518 illustrates 24-trace equipment designed to be placed in 

 a truck or boat. Each amplifier and filter channel is incorporated into a 

 separate panel unit. Figure 519 shows a similar type of truck equipment, 

 with 24 recording traces, utilizing three amplifier channels per unit. 



In contrast with the rather elaborate apparatus for truck mounting, 

 extremely portable and lighter-weight equipment has been designed for use 

 where trucks cannot operate. Simplification is the chief characteristic of 

 such equipment. For example, one or two filter settings are usually con- 

 sidered sufficient, and such items as multi-recording have not appeared in 

 portable versions. In most cases the equipment is divided into separate 

 packages which are interconnected by plug-in cables when in use. Figure 

 520 illustrates portable seismograph equipment. A similar 12-trace outfit, 

 set up at a field station, is illustrated in Figures 521 and 522. In general, 

 results obtained with portable seismograph equipment compare very favor- 

 ably with those achieved with the heavier, less portable equipment mounted 

 in trucks or boats. The main difference in the operations is that survey 



