122 EES ROSATRE AND SOS ele 
COMPUTATION OF SUBSURFACE DIP FROM REFLECTIONS 
Figure 1 illustrates how dips are measured by reflections. By 
plotting the image of the shot point for the recording geophones, the 
problem is resolved into one of triangulation upon this image point by 
means of sound-wave paths. The distance between the recording 
geophones serves as a base line, and the other two sides of the triangle 
are the path lengths of the recorded reflections. Since only the travel 
times for these reflections are determined directly, the triangle is 
completed by multiplying these travel times by the appropriate veloc- 
ity with which the sound waves have traveled. This computation is 
] 
he Shoot ing 
Recordin 
an Down Slope 
sp { shooting 
Recording Along Level Shootin { 
y Recording PUP SIope SPI 
SP= Shot Point R.P «Recording Point SP1=Shot Poin} Image 
FIGURE I. 

Fic. 1.—Geometrical solution of dip determination. 
modified by certain corrections necessitated by a thin zone of low- 
velocity material almost invariably present at the earth’s surface. 
The methods of correcting for this zone, known as the “aerated layer,” 
are familiar to seismologists, but are too involved to describe here. 
The velocity of the sound waves below the aerated layer can be 
measured in any of several ways. One method is to lower a recorder 
down a well, and record the travel time from an explosion at the sur- 
face. A second method is the analysis of refraction data from a profile 
of appropriate length and recorder density. Thirdly, the appropriate 
velocity may also be determined empirically by an analysis of the re- 
flection times themselves, and due to the general lack of available 
refraction profiles or well recordings, this is the most widely used 
method in the Gulf Coast. 
ACCURACY AND CONTROL 
Dip reflection surveys for the purpose of local preliminary recon- 
naissance consist essentially of unclosed traverses, in general made up 
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