SEISMIC METHODS 895 



beds 1 and 2 will travel the path OabcS'. From the record, the time for a 

 multiple reflection occurring between bed 1 and bed 2 will delineate the 

 image bed V, having a reflection time equal to that of bed 2, plus the 

 difference in the reflection times of the two actual reflecting beds. The 

 apparent path of travel being OadcSi this may be expressed mathematically 

 as follows : 



wherein Ti is the time for the reflection which came from the image bed 

 1', T2 is the reflecting time for bed 2, and 7\ is the reflecting time for bed 1, 

 the shallower bed. 



The apparent velocity of the multiple reflection will be influenced 

 more or less by the true velocity between beds 1 and 2, and will not exhibit 

 the higher velocities that would normally be expected if the wave traveled 

 to the deeper horizon occupied by the image bed 1'. If the distance between 

 beds 1 and 2 is an appreciable part of the total travel path, then the velocity 

 between beds 1 and 2 may influence the overall velocity to a measurable 

 amount. 



There are no easily applied rules for positively identifying multiple 

 reflections between the deeper lying beds. Generally, the subsurface condi- 

 tions are complicated, and do not bear the simple relationships which were 

 enumerated for the two previous cases. The condition is further compli- 

 cated in those areas where many reflections are obtained, because then it 

 is difficult to separate, by visual inspection, the true and the multiple 

 reflections. 



Often multiple reflections will be apparent when dip cross-sections are 

 plotted, and some discordant dips are obtained which usually have greater 

 dip than the true dip at the depth indicated. Another general test is to plot a 

 time-depth graph for the reflections. Usually a smooth curve may be 

 drawn through these points to show the relationship of velocity with depth. 

 This curve is similar to that obtained by refraction shooting. Since multiple 

 reflections do not follow the same time-depth relationships of true reflec- 

 tions, they will depart from the curve, and normally give values of time, 

 for a given indicated depth, greater than the curve. 



Multiple Reflections in Air Shooting. — In the Antarctic, T. C. Poul- 

 ter conducted a study of reverberation records between the air-snow 

 interface at the surface and the bottom of an ice flow. The height of the 

 ice-snow above sea level was accurately known. The velocity in this solidi- 

 fied snow was accurately measured and found to be 12,050 feet per second. 

 This mass of ice and snow was floating in two thousand feet of water, the 

 depth of which was determined by direct measurements and sonic depth 

 gear on the boats. This solidified but porous snow was flooded to sea level 

 with salt water and the velocity in the flooded portion was accurately deter- 

 mined at 4,980 feet per second. The water temperature was constant to 

 plus or minus one-half degree the year around. Under these favorable 



