Chap. 9] SEISMIC METHODS 441 



and magnification, introduction of electrical amplification, increase in 

 photographic recording speed and restriction of the record to the vertical 

 component. However, the theory of wave propagation used in seismic 

 prospecting is again quite similar to the theory developed in earthquake 

 seismology. 



In the application of the seismic method to oil exploration, great suc- 

 cesses have been obtained in locating salt domes, anticlines and faults, 

 and in mapping the topography of basement rocks. Buried land surfaces 

 and limestone beds are usually good seismic key horizons. While in oil 

 exploration refraction methods dominated the field several years ago both 

 for reconnaissance and detail, the picture has now changed completely. 

 Ever increasing fields of application have been found for the reflection 

 method. It was soon discovered that not only limestone beds but hard 

 shales and other beds with seemingly small differences in elastic properties 

 would give reflections. On occasion, their lack of continuity gave rise to 

 serious difficulties, but these were overcome by the application of the dip • 

 shooting and the continuous-profiling methods. 



The field of civil engineering has also seen the application of seismic ex- 

 ploration in late years. Most foundation problems, such as determination 

 of depth to bedrock and investigation of tunnel and dam sites, may be 

 attacked by refraction methods. Another application of seismology in 

 civil engineering (engineering seismology) has as its objective the design 

 of earthquake-proof structures, the determination of dynamic response of 

 models of proposed structures, the investigation of damage done by traffic 

 and blasting vibrations, the determination of the elastic properties of 

 foundation sites, and the analysis of the frequency response of roadbeds, 

 bridges, dams, and buildings. A discussion of the fields of engineering 

 seismology and of acoustic methods is given in Chapter 12. 



II. PHYSICAL ROCK PROPERTIES IN SEISMIC EXPLORATION; 



SELECTED TOPICS ON THE THEORY OF ELASTIC 



DEFORMATIONS AND WAVE PROPAGATION 



A. General 



The elastic properties of rocks may be ascertained in the laboratory, and 

 from such data the velocity of the elastic waves in formations may be 

 determined. Direct velocity determinations may be made in the field 

 (1) by shooting on exposed formations; (2) by shooting in or near a well 

 at known depths (average velocity determinations in reflection work) ; and 

 (3) from the travel-time curve. A direct measurement of velocity on rock 

 samples in the laboratory has not been attempted. 



Elastic wave-producing forces are associated with two types of strains : 



