714 



EXPLORATION GEOPHYSICS 



Hence, the approximation introduced by using time gradients in the direc- 

 tion of the spread to determine the component of dip in that direction is 

 subject to an error of the same degree as that by which the true dip 

 as a function of A7' for a given reflection time deviates from a linear 

 function. 



In these considerations it is assumed that the seismometer spread dis- 

 tance is sufficiently short so that equivalent rays may be used. When this 

 is not the case a correction should be considered. 



The Low Velocity Layer. — In practically all areas, a layer occurs 

 at the surface of the earth which is unconsolidated, and often heterogeneous 

 in character, and transmits waves at low velocity. The thickness of the 

 layer may vary from almost zero to several hundred feet; however, 

 the most common thickness is from about 25 feet to 100 feet. 



In literature on seismic prospecting, the term "weathered" is fre- 

 quently applied to this layer although this is a misnomer in that the layer 

 may not necessarily exhibit a weathering condition. Furthermore, the 

 term "weathered layer" as used by seismic investigators differs in meaning 

 from the well established term "zone of weathering" used by geologists.' 

 The terms, low velocity, aerated and unconsolidated layer, are also used. 



Of importance to seismic prospecting is the low velocity property of 

 this layer; most commonly between 1500 and 2500 ft./sec, in contrast to 

 velocities of about 6,000 ft./sec. and more in the deeper layers. The term 

 "low velocity layer," therefore, has found increasing favor and will be used 

 here.* The low velocity layer may consist of more than a single layer, and 

 it is customary to define this layer simply as one having a velocity less than 

 about 5,000 ft./sec. — or less than any other velocity when occasion arises. 



Special attention must always be directed to the low velocity layer in 

 seismic prospecting because the usual heterogeneous character of this 



layer would lead to inaccurate 



SEISMOMETER 



results if the anomalous ve- 

 locity in this layer were not 

 taken into account. The layer 

 plays but a small part in the 

 subsurface section despite its 

 relatively large effect on travel- 

 time data and must be taken 

 into account before the data 

 can be analyzed. 



Figure 437 illustrates a the- 

 oretical effect that may be ob- 



* To avoid excessive repetition of the terms velocity of the lozv velocity layer and 

 vertical travel-time in the lozv velocity layer, these terms v^rill be designated by Vu 

 and t„ respectively, and the symbols will be used. 



The subscript u may stand for unconsolidated or for upper layer. Consolidated 

 is here used in the sense of unaerated ; that is, a condition in which large interstices, 

 if any, between grains are filled by water or by cementation. 



Fig. 437. — Diagram illustrating the creation of mul- 

 tiple reflections (A) when the shot-point is placed near 

 the surface. 



