REFLEXION OF ELASTIC WAVES 147 
care must be taken that the shot is sufficiently close to the first seis- 
mometer to give the true velocity of the surface zone. Corrections 
made in this manner should not leave a residual error in the computed 
Viola datum in central Oklahoma of more than 15 feet. 
A typical example of the successful application of this method in 
Oklahoma is furnished by results obtained at Lucien, in Noblee 
County, reproduced in Figure 8; field-work was carried out by the 
Shell Petroleum Corporation. The, surface evidence for this structure 
is very meagre and it can justly be claimed as a reflexion seismic dis- 
covery. Drilling carried out subsequent to completion of the seismic 

gnot ee 
SURFACE N 
GROUND WATER TABLE Se Se 
Fic. 7.—Showing method of correcting for surface zones. 
survey has amply confirmed the general accuracy of the predictions. 
In deriving the contouring depicted in Figure 8 a group of reflexions 
from the Mississippi and Viola limestones were followed; over the 
center of the structure the upper of these limestones, the Mississippi, 
is missing and the chance of miscorrelating reflexions is therefore very 
great. It seems, however, that in this instance the pitfalls were 
avoided and the results achieved by the method can justly be de- 
scribed as brilliant. 
MAPPING BY ‘ DIP” 
In many localities we are confronted with the problem of mapping 
structure in sediments which do not contain limestones or other reflect- 
ing horizons continuous over appreciable areas. Such a cundition as 
is represented in Figure 9 can obviously not be investigated by a 
method involving correlation over more than a very limited distance. 
Experience has shown that in many such localities we do obtain re- 
807 
