78 beers. GEOPHYSICAL PROBLEMS [Ch. 4 



More than half the money spent for geophysics is devoted to the seis- 

 mograph, an instrument which measures the travel time of sound 

 waves into the earth and return. The principle is as simple as the 

 echo returning from a sharp discontinuity: a brick wall, the side of 

 a cliff, or the edge of a forest. The principle was first developed in 

 echo depth sounding after the sinking of the steamship Titanic by an 

 iceberg in 1913. Since then the echo principle has been developed 

 in many fields, finally resulting in the evolution of radar. 



In its application to oil finding, the seismograph employs miniature 

 earthquakes set off by small explosions of dynamite just below the 

 surface of the ground. Sound waves traveling through the earth are 

 in part returned to the surface by sudden changes in sound velocity. 

 The sequence of layered rocks commonly found in oil provinces makes 

 an ideal setting for the return of a series of echoes. Each reflection is 

 recorded on a moving film which can later be analyzed to identify 

 the origin of a long series of echoes. The reflection seismograph 

 makes echo determinations over a network of points spaced from a few 

 hundred feet to one mile apart on the surface of the ground. At each 

 of these points there results a sub-sea-level datum value of each of 

 the reflection horizons which appear on the prospect under survey. 

 The numerical datum values, under the guidance of a competent ge- 

 ologist, may be contoured to show the attitude at depth of one or 

 more reflecting horizons. If a horizon is related to the occurrence of 

 oil and gas in the form of a confining trap, this fact will be inferred 

 by the geologist from the contours. He will then recommend the 

 drilling of a well upon a favorable site to investigate the conditions 

 of permeability and saturation in the objective horizon. 



By established refinements in the reflection technique it is possible 

 to achieve a quantity of subsurface data which are equivalent to 

 drilling a well at each reflection point. An accurate structural picture 

 can be obtained as well as valuable stratigraphic data. If lateral 

 changes in sedimentation occur, they may often be inferred from the 

 character of reflection records received along the transition zone. 

 Full use of the resolving power of the reflection seismograph in this 

 application is rarely employed, but if fine detail is required it is 

 available at additional cost. There are a number of controls over 

 the resolving power which enable one to delineate fine structure 

 within the grosser features if the expense is warranted. 



Among the refinements which the reflection seismograph may em- 

 ploy with advantage is the accurate determination of the velocity of 

 sound through the sequence of beds. For this purpose a special seismic 

 detector is lowered into a well drilled through the rock formations. 



