O. C. LESTER, JR., AND S. W. WILCOX 
The two traces of this record, representing two seismograph positions 
on a refraction profile with a chained difference in distance of 500’, 
from the shot point, were recorded simultaneously with the same oscillo- 
graph. The longer trace, which can be identified by the air waves 
(C, C’), definitely shows a shorter arrival time of the intial energy 
(B, B’). 
This so-called negative velocity, while definitely shown to exist, is 
considered to be of academic, rather than practical interest, and that 
portion of the curves will not be discussed in the following: 
A UNIVERSAL DIP CHART 
The curve (Fig. 1) obtained from equation A is, of course, applic- 
able to only one pair of beds of velosity Vi and V2. This necessitates 
the construction of a new curve for each pair of velocities under con- 
sideration. If, however, the equation A be writtes as: 
Vv i 

V2 cos < + Bsin « 
A family of curves may be drawn, using B as a parametér, which 
will be applicable to all velocity ratios. Such a family of curves is 
shown in Fig. 4 (Minus the negative velocity portion). A tangent curve 
is given (at left) for convenience in converting tangents to actual dip. 
These curves may be used for the following purposes: 
(See Figure 4) 
1. Given apparent velocity up dip—to find the angle of dip <. 
V: and V2, known. 
2. Given apparent velocity down dip—to find the angle of dip . 
Vi and V2, known. 
3. Given apparent velocities both up and down dip (from reversed 
profiles)—to determine true velocity V:, and angle ™. 
For case numbers: 
V: 

1. a) Determine B = cot sin™* 
4 
vi 
b) Determine K = —— 
V: 
c) Select up dip curve (parameter) corresponding to above value of 
B. (Interpolating between curves if necessary). Project above 
