Chap. 2J METHODS OF GEOPHYSICAL EXPLORATION 21 



like crystal microphones. At present the inductive and reluctance types 

 predominate. The amplifiers have usually three to four stages, are either 

 straight transformer or resistance-impedance coupled, and include auto- 

 matic volume control and amplitude expanding circuits to offset the de- 

 crease of (reflection) amplitude with depth. Recording units are coil 

 galvanometers, bifilar oscillographs, or unifilar string galvanometers. 



Fan shooting. In this method receivers are grouped at equal distances 

 along the circumference of a circle, at the center of which the shot is fired. 

 This gives the arrangement the appearance of a fan. An area is covered 

 with a series of overlapping fans for the purpose of determining the char- 



Heiland Research Corp. 



Fig. 2-9. Representative seismic i-ecording channel. From left to right: detector; 

 three-stage self-contained amplifier; six-element camera with six electromagnetically 

 damped galvanometers and timing mechanism. 



acter of a medium intervening between shot point and receiving points. 

 A salt dome or other high speed medium will appear as a reduction of the 

 normal travel time for the particular distance and area, or as a time 

 "lead." By plotting these leads for each fan line a salt dome, anticline, or 

 the like can be outlined rapidly. Usually, fan-shooting indications are 

 detailed by other geophysical methods. In mining, the method has been 

 applied to the location of gold placer channels which appear as time lags 

 instead of leads. 



Refraction methods. In refraction shooting, travel times (of first im- 

 pulses) are determined and plotted as functions of the distance of recep- 

 tors arranged in a profile. If the medium betAveen source and reception 



