20 METHODS OF GEOPHYSICAL EXPLORATION [Chap. 2 



properties with depth may be measured by noting how certain physical 

 quantities change in horizontal direction. Seismic methods are therefore 

 well adapted to depth determination of horizontal formation boundaries. 

 In seismic exploration a charge of dynamite is fired at or near the surface 

 and the elastic impulses are picked up by vibration detectors, likewise at 

 the surface. The time which elapses between generation and reception of 

 the elastic impulses ("travel time") is measured by recording also the in- 

 stant of the explosion and time marks (usually at 1/100 sec. interval). 



The simplest form of seismic exploration is the "fan shooting" method, 

 which has for its objective the determination of the nature of the media 

 occurring between the shot point and a number of detectors set up in a 

 circle around it. A second important seismic method is the "refraction 

 method," in which travel times of first arrivals are observed along a 

 profile. The variation of this travel time with distance or the "travel 

 time curve" makes it possible to determine true velocities and depths of 

 the refracting formations. A third, and now the most important, method 

 of seismic prospecting is the "reflection method," in which the time re- 

 quired for an elastic impulse to travel to and from a reflecting bed is 

 measured. From the travel times it is possible to make a direct calcula- 

 tion of the depths of the reflecting sufaces but not an evaluation of the 

 elastic wave speeds within the reflecting formations. 



Seismic equipment. Seismic equipment falls into two groups, that used 

 at the shot point and that used at the receiving points. For the genera- 

 tion of elastic impulses, dynamite is employed, although weights dropped 

 from scaffolds or towers and unbalanced flywheel machines have been 

 applied to generate nonperiodic and periodic impulses (see Chapter 12). 

 The dynamite is set off by special electric blasting caps, and the break in 

 the firing circuit is transmitted by wire or radio to the recording truck. 

 Seismic shot holes are drilled by special rotaries, spudders, or centrifugal 

 pumps. In reflection work a special detector is set up at the shot hole 

 for transmitting the time elapsed between the firing of the shot and the 

 arrival of the wave at the surface. 



The equipment at the receiving points consists of as many vibration 

 detectors as there are receiving points (6 or 12), connected to as many 

 amplifiers and a recording camera in a specially designed truck. The 

 function of this equipment is to detect, amplify, and record the ground 

 vibrations on rapidly moving photographic paper (see Figs. 2-9, 2-10, and 

 2-11). The detectors, also referred to as "geophones," phones, or pickups, 

 record the vertical component of the vibration and are constructed like 

 microphones. "Inductive" detectors are built like moving coil dynamic 

 microphones, "reluctance" detectors like phonograph pickups and "capaci- 

 tive" detectors like condenser microphones and "piezo-electric" detectors 



