1088 Subsurface Geologic Methods 



to many operators, particularly in areas such as the Edwards Plateau 

 of west Texas. The creation of an elastic-wave train within the earth by 

 the explosion of dynamite has been described by Morris.^^ 



The instant of the explosion is transmitted to the recording unit by 

 either radio or wire and is placed on the record. The distance between 

 the receiving points and the shot may be a number of miles in the re- 

 fraction method or may be only several hundred feet in the reflection 

 method. At each receiving point one or more geophones or detectors 

 convert the mechanical energy of the seismic waves into electrical energy. 

 These electrical impulses are carried to the recording unit by wire and 

 fed into specially constructed amplifiers, which not only act to increase 

 the intensity of the impulse but also act as discriminators. Filters are 

 included in the amplifiers, which allow the operator to emphasize the 

 desired frequencies and discriminate against all others. As reflections 

 ordinarily fall into the frequency band from 30 cycles per second to 

 70 cycles and as, "ground roll" is generally below 20 cycles per sec- 

 ond, the disturbance due to ground unrest from either the surface low- 

 frequency seismic waves or the higher-frequency wind "noise" may be 

 reduced. The size of a seismograph unit is indicated by the number 

 of channels or traces that may be recorded at any given time. In 

 common use today are instruments of 24 and even 48 channels. A given 

 channel may receive energy from one or several geophones, depending 

 upon the area. The output of the amplifiers is fed to recording moving- 

 coil-type galvanometers and thence recorded on rapidly moving photo- 

 graphic paper. Time lines are established on the paper at one one- 

 hundredth-second intervals so that any event may be timed with an 

 accuracy of one one-thousandth of a second. 



Many companies prefer "mixed" records, in which a certain portion 

 of the energy from each channel is fed to the following channel. This is 

 accomplished within the recording unit by the use of a mixer tube in 

 the amplifiers or by a resistance network, in which a portion of the 

 output is fed to the next channel. Results similar to those obtained from 

 multiple geophones are obtained by this method. Mixing and multiple 

 geophones have the advantage of lowering the disturbance level on the 

 record due to ground roll and the like and of strengthening the reflections 

 recorded. In faulted areas and in regions of appreciable dip, erroneous 

 interpretations may result from the use of mixed records, owing to the 

 effect of the mixing on the "step-out times" or "dip times." Most com- 

 panies today will shoot mixed records, but correlations are not attempted 

 on reflecting horizons that do not appear on the unmixed or pure record. 

 Frequently reflections that do not actually exist may be "created" by the 

 use of mixing. 



■*' Morris. George, Some Considerations of the Mechanism of the Generation of Seismic Waves by 

 Explosives: Geophysics, vol. 15, no. 1, p. 61-69, Jan. 1950. 



