504 SEISMIC METHODS [Chap. 9 



which contains as many galvanometer elements as there are channels. 

 Communication with the shot point is generally maintained by wire and 

 the same wire is used for the transmission of the shot instant. An ap- 

 paratus that is adapted to extreme requirements of portability and does 

 not necessitate the use of a truck or tent is illustrated in Fig. 9-114. 



C. Refraction Methods 



1. General. The travel of seismic waves in the earth is controlled by 

 the same laws as is the propagation of light rays. Seismic waves are re- 

 fracted and reflected on any interface at which there is a change in veloc- 

 ity. Therefore, a deviation from normal ttavel time is observed when 

 media of different velocities occur below. When the variation of travel 

 time with distance has been determined, depths and nature of the refracting 

 beds may be deduced from the travel-time curves. If the velocity within a 

 given layer is constant, the seismic rays may be considered straight. The 

 theory of wave propagation is based on Snell's law of refraction and the 

 principle of Fermat which states that seismic energy follows that path 

 which enables it to travel from the shot point to the receiving point in a 

 minimum of time. 



In refraction shooting, a charge is placed at one location and a number 

 of seismographs are set up in a straight line, preferably at equal intervals. 

 Profiles may be laid out in the direction of the strike, at right angles thereto, 

 or in both directions. For dipping formations, profiles are shot both up 

 slope and down slope. Depth calculations are based on a time-impulse 

 analysis of the seismogram. Impulses are located which correspond to 

 the arrival of different types of waves ; their arrival time is measured from 

 the instant of the shot. In practice, only impulses of longitudinal waves 

 are used. The first impulses or "breaks" are due to the deepest high-speed 

 bed within range. If later impulses (due to shallower beds of lower 

 velocity) are noticed, they may also be timed. However, depth calcula- 

 tions may be based on first impulses only. Travel times are plotted 

 against the distances of the various receiving stations from the shot point. 

 Thus, the travel-time curve is obtained. It consists of one Une if only 

 first impulses have been plotted, but it may have several branches if later 

 impulses have been utilized. If the travel time curve is straight and has 

 essentially the same slope for all distances, no higher-speed beds have been 

 reached. When breaks (changes of angles) occur, they may be due to a 

 variety of conditions. Several simple types are discussed below. 



2. Refraction equipment. Seismic refraction equipment is similar to 

 and in many cases identical in general design with fan-shooting equip- 

 ment, described on the preceding page. However, in refraction shooting 

 there is greater opportunity to use multielement equipment with a com- 



