SEISMIC METHODS 893 



the surface A'. Using the points 0, 0', and O'', we may draw the path of 

 the rays which leave the origin and arrive at the seismometers ^ and Si, 

 which may be considered to represent the outside seismometers of a spht 

 spread. 



The reflection which appears to come from the imaginary image 0" is 

 actually reflected twice from the real reflecting horizon B and once from the 

 surface A. It travels the path OahcS, while a true reflection from bed B 

 will travel the path OdS. Such being the case, the multiple reflection 

 should have the following relationships to the single reflection from the 

 real reflecting horizon : ( 1 ) nearly double the reflection time ( for the seis- 

 mometer placed very close to the shot point, when the reflecting horizon is 

 substantially parallel to the surface), (2) approximately twice the correct 

 angle of dip or AT value, and (3) an average velocity equal to that in the 

 interval between the surface A and the reflecting horizon B, irrespective 

 of the higher velocity which usually exists at the depth which would 

 correspond to the reflection time from the image surface A' . 



Theoretically we should expect many cases of multiple reflections 

 between the surface and the shallow reflecting beds. Actually, however, 

 such reflections have seldom been observed in land operations. Lester 

 describes experimental work conducted in an attempt to obtain evidence 

 of energy reflected from the surface. "No sign of a surface reflection was 

 seen on any of the records."! Various theories have been advanced to 

 explain this fact, such as, that the surfaces used were not sufficiently 

 smooth to produce good reflections, or that the velocity boundary between 

 air and the surface of the earth is not the sharp boundary to be expected, 

 considering the relative densities of the air and the earth materials. "It 

 has been shown that velocities of less than that of sound in air are measur- 

 able in very-near-surface earth materials. Though such velocities increase 

 with depth at a rapid rate through the so-called seismic weathered layer, 

 it is not unusual to find that depths of 50 to more than 100 feet are pene- 

 trated before encountering unweathered ground velocities of approximately 

 5,000 feet per second. "$ 



It has been found, however, that the base of the low velocity or 

 "weathering" zone reflects a large percentage of the incident energy. 

 Velocities in the "weathering" zone often range from 1,000 to 2,000 feet 

 per second. At the base of this zone there is a discontinuity and the velocity 

 may increase abruptly to 5,000 or more feet per second. Using normal 

 values for density, we can expect a coefficient of reflection at this discon- 

 tinuity of from 35 to 75 per cent. Quite often multiple reflections occur 

 under these conditions, especially between the base of "weathering" and a 

 shallow basement or other reflecting horizon. 



In other words, the gradual change in velocity from the surface down 

 through the earth prevents the air-surface interface from acting as a reflect- 



t O. C. Lester, Jr., "Discussion of Multiple Reflections," Geophysics, XIII, No. 1, January, 

 1948. 



t O. C. Lester, ibid. 



