SEISMIC METHODS 809 



of a seismometer record may be from 10^ to 1 up to 10^ to 1, Consequently, 

 some variable control of the amplification or response of the recording 

 equipment is required in order that the entire wave train be legible. Three 

 methods of overcoming this difficulty are in use. 



A simple method from the point of view of recording equipment is to 

 shoot several shots, setting the amplifier at dilTerent levels of gain to 

 obtain legible amplitude at successively different reflection times. An 

 equally simple method is to use different amounts of explosive with the 

 same level of gain. Neither of these techniques, however, is practical for 

 commercial work, due to high explosive cost and loss of time. 



A second method utilizes manual gain control. This may be accom- 

 plished by means of a gang control which simultaneously actuates a poten- 

 tiometer in each amplifier. Usually one galvanometer trace will record 

 the gain versus time on the record. A skilled operator can obtain good 

 results with the manual type of control, by varying the gain in accordance 

 with prior experience in the area, usually so as to maintain a relatively 

 constant average amplitude of excursion on the galvanometer traces. For 

 this purpose, the recording camera must be equipped with a split or double 

 beam to allow visual observation as the recording is made. 



Another method utilizes a cam-operated gain control, which is varied 

 with time.f The time control may be a separate timing mechanism driving 

 the cam. An alternate arrangement mechanically couples the cam with the 

 paper feed mechanism in the camera. A somewhat more complicated ar- 

 rangement utilizes an electronic trigger tube which starts the cam mechan- 

 ism when the shot is fired or when the first burst of seismic energy is 

 received. The shape of the cam is governed by the reflection characteristics 

 of the area and the overall gain characteristics of the amplifier, filter, and 

 galvanometer. 



Another and more prevalent procedure utilizes automatic volume con- 

 trol or expander systems^ which are essentially the same as those employed 

 for radio and television reception. § The rectified output of one of the early 

 amplification stages is applied to a variable Mu tube or to the grid, or as a 

 cathode follower to one of the succeeding tubes, so that the amplification 

 is increased as the incoming signal decreases in overall gain. 



The automatic control is designed to be responsive to a group of wave 

 trains or the envelope of the waves and not to each individual peak or 



t H. R. Prescott, "Method and Apparatus for Making Geological Explorations," U. S. Patent 

 2,158,198, issued May 16, 1939. 



t R. G. Piety, "Automatic Volume Control for Seismograph Amplifiers," U. S. Patent 2,430,246, 

 Nov. 4, 1947. 



H. Hoover, Jr., "Seismic Amplifying System." U. S. Patent 2.430.983, Nov. 18, 1947. 

 J. O. Parr, Jr., "Seismic Surveying," U. S. Patent 2,424,705, July 29, 1947. 



E. J. Shimek and G. M. Groenendyke, "Gain Controlling System for Seismographs," U. S. 

 Patent 2,420.571, May 13, 1947. 



R. Maillet, "Geophysical Prospecting," U. S. Patent 2,420,672, May 20, 1947. 

 § Reuben Lee, "Amplifier Circuits," Industrial Electronics Reference Book, John Wiley and 

 Sons, New York, 1948. 



Radiotron Designers Handbook, R. C. A., 1941, Parts 1 and 2. 



