GEOLOGICAL APPLICATION OF SEISMOGRAPHY 1317 
SHOT PECORPOER 

SHOT RECOROER 
Yj 
lic. 3.—Shortest time paths (upper) in sediments and (lower) with salt dome 
present. 
the high-speed salt. For a dome a mile in diameter, which is a fair av- 
erage, the saving in time is approximately 0.6 second; obviously, the pres- 
ence of the dome is easily detected. 
As the method was applied to progressively deeper domes, the time 
anomalies to be expected became correspondingly less. It became 
necessary to increase the accuracy of timing to at least 0.01 second. 
Whereas the distance between shot and recorder points was previously 
(letermined by timing the air wave travelling from shot to recorder, it 
now became necessary carefully to survey this distance, especially as, 
for the deeper domes, it was necessary to place the recorder farther awav 
from the shot point. Though a mile or two had been sufficient for the 
very shallow domes, it was found necessary to use 6-8 miles for the deep 
domes. This. of course, meant larger charges of dynamite, charges as 
large as 1,000 pounds being used for a single shot. The large charge re- 
quired and the increased accuracy necessary in determining the distance 
between shot and recorder made the work increasingly expensive; also, 
the smaller time anomalies exhibited by the deeper domes made the 
results less reliable. However, in a period of 7 years many domes have 
been discovered by the seismograph. 
REFRACTION METHOD; STRUCTURE DETERMINATION 
In the study of the comparatively simple problem of searching for 
salt domes, the attempt was cautiously made to extend the range of 
usefulness of the seismograph method. Having found a dome, it was 
desirable to define it in as much detail as possible. It was possible to 
contour the top of the dome and determine its top edge. 
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