REFLEXION OF ELASTIC WAVES I51 
Since a relatively steep dip results in only a very small change in 
the time interval being measured, it is necessary that the correction 
for surface effects be either negligible or very carefully determined. 
To realise the importance of this I quote an actual example from the 
Gulf Coast of Texas. In measuring dips on a surface at about 6,000 
feet in depth and using a seismometer spread of 600 feet, a dip of 5 
degrees will produce a variation from the normal running time of 0.01 
second. It is obvious that a similar error in determining the difference 
between corrections for surface layers at the extremities of the seis- 
mometer spread will produce an error of like amount in the computed 
dip. It is not the purpose of this paper to discuss possible errors in 
Shot Pomt Seismometer Spread 
". (aaa a rea aaa 

6= Vt 
a’=Vt 
where t is running time to first seismometer 
and t'- - -  * last 
V ts average velocihy to depth d 
Fic. 11.—Graphical method of computing dip. 
great detail, but a warning that many insidious possibilities of error 
exist must be given. The problem is strictly a three-dimensional one, 
and yet we usually derive our interpretation assuming only two di- 
mensions. Propagation paths are curved, and yet our limited knowl- 
edge usually constrains us to assume straight line propagation. 
Nevertheless, this method also has achieved brilliant results. Figure 
12 shows a profile obtained by this method across a deeply buried 
salt dome in southern Louisiana. This salt dome gives no surface indi- 
cation of its presence, and yet by this method we can define the prob- 
able location of the crest of the associated uplift in the sediments to 
within limits of error of 1,500 feet. 
POSSIBLE APPLICATIONS 
The ideal conditions under which to apply reflexion shooting 
exist when a limestone or series of limestones are present in contact 
with shale or sand at depths of from 2,000 feet to 7,000 feet or even 
more; this condition guarantees the reflexion of energy. The best sur- 
811 
