750 



EXPLORATION GEOPHYSICS 



empirical velocity measurements made in deep bore holes. The D-scale 

 may be calculated from the conditions given by the velocity-depth function, 

 and then the R-scale may be obtained by subtracting the D-scale values 

 from the empirical measured time-depth curve. 



Another plotting instrument has been described by Daly,f for use in 

 areas where the velocity may be assumed to increase linearly with depth. 



REFRACTION METHOD 



In the refraction method of seismic prospecting, measurements are 

 made of the elapsed time between the shot and the arrival of the first wave 

 trains, i.e., the "first arrivals," and successive correlatable wave trains. 

 A series of seismometers is placed at measured distances from the shot- 

 point, and by plotting a time-distance graph for these first arrivals (the 

 refracted longitudinal, fastest-traveling waves) and the successive wave 

 trains which can be correlated, changes in velocity of the wave path are 

 shown by discontinuities or changes in slope of the graph. From the points 

 of inflection of the graph calculations may be made to determine the depth 

 to the boundaries separating the media of different elastic wave velocities. 

 This information is interpreted in terms of subsurface geology. 





1 J ! > { i 1' . I ' ' 



l(^»i^i»|^-Mv-^4<*»%v*^«**'^|'*y**V^^ 



P * R 



|.«j»ww'*yv--*'*y*«| \lw 





Fig. 456. — Seismogram showing the arrival of refracted waves P, reflected waves R, 

 and surface waves L. The shot was 1/8 pound of 60% dynamite and was exploded at a 

 depth of 3 feet. (Reproduced from B. Gutenberg, Beitrdge zur angewandten Geophysik, 

 Vol. 6, No. 2, 1936.) 



Subsurface Section Consisting of Two Horizontal Layers. — The 

 path of a refracted wave through a section consisting of two horizontal 

 layers has already been described. (Figures 411 and 412.) A seismogram 

 showing the arrival of refracted waves P, reflected waves R, and surface 

 waves L, is given in Figure 456. 



t J. W. Daly, "An Instrument for Plotting Reflection Data on the Assumption of Linear 

 Increase of Velocity," Geophysics, Vol. XIII, No. 2, April, 1948, pp. 153-162. 



