Oklahoma City structure, they successfully denned the flank of a known dome 

 near Dougherty, Oklahoma, on a properly migrated record section. 



At about the same time, L. Mintrop of Germany applied for a patent cover- 

 ing a refraction technique for locating the depth and type of subsurface forma- 

 tions. He brought his rather crude instruments to North America and in 1924, 

 working for the Gulf Oil Company, located the Orchard Dome of Bend County, 

 Texas, the first refraction discovery in the United States. In 1927, Amerada's 

 Geophysical Research Corporation under DeGolyer discovered the Maud Pool 

 in Oklahoma. This was the first success of the reflection method. 



These were the beginnings of a series of discoveries that established the 

 seismograph as an essential petroleum prospecting tool. Seismic activity in- 

 creased from 41 crew-months in 1930 (Lyons, 1955) to 11,000 crew-months in 

 1955 (Patrick, 1957), a gain of some 27,000 percent. Each year several hundred 

 million dollars are spent in seismic exploration. 



Because of its comparatively simple field operation and its remarkable 

 depth of penetration, the reflection method almost eclipsed the earlier refraction 

 method in the search for oil. Certain problem areas, however, produced no 

 reflections, and the seismologists resorted to refraction shooting. Today we 

 realize that each method has its place in exploration. Likewise, even the ardent 

 seismologist has come to realize the importance to the exploration picture of 

 the complete integration of all pertinent information regardless of its source. 



The common procedure employed in reflection seismic prospecting on 

 land is illustrated in Figure 26-1. It consists of electrically detonating an ex- 

 plosive charge and recording as a function of time the resulting seismic energy 

 as it arrives at a group of vibration detectors or seismometers disposed in a 

 particular array on the surface of the ground. 



Portions of the seismic energy thus generated reach the seismometers by 

 several different routes. One portion travels directly to the surface and then 

 spreads out over the ground as a surface wave. Another travels along a sub- 

 surface layer and is refracted to the surface. A third travels downward until 

 it reaches an abrupt change in lithology, then reflects back to the surface. 



Only energies traveling by refracted and reflected paths are identified on 

 the seismic record or seismogram shown in Figure 26-1. They are respectively 

 first arrival events E and reflected events F. The surface wave has been sup- 

 pressed in the recording process, thus leaving only background noise as ex- 

 traneous excursions. 



In the recording process, seismic energy reaching each seismometer is 

 transformed into electrical energy, which is amplified and fed to a mirror 

 galvanometer. Light reflected from this mirror traces on moving photosensitive 

 paper a line whose lateral excursions are a function of the ground motion. Each 

 combination of seismometer, amplifier, and galvanometer commonly is known as 

 a channel. A channel may contain a multiplicity of seismometers connected to 



556 



