SEISMOLOGICAL DISCOVERY OF VERMILION BAY 55 
an arc indicating normal arrival times. If minus time differences are 
plotted out from the shot point, an approximation is made to the 
outline of the wave front at the recording points. In the case of the 
Vermilion Bay data, the resulting pattern of time differences un- 
mistakably indicated the presence of a previously unknown salt 
dome. 
The smooth curve used as the “‘normal” in Figure 1 is calculated 
from the travel-time relationship 
ee 2b aT 
xX = — sin h— 
a 2 
for a linear increase of velocity with depth. The constants 6 (surface 
velocity) and a (increase in velocity per foot of depth) were deter- 
mined by trial, and found to be 
b = 5,633 feet per second 
@ = 0.5483 foot per second. 
The penetration, Z, for sucha curve is 
Coen b 
L= eee 
4 a? a 
Short refraction data.—A short negative profile was shot north 
from SP 1, to serve as a “normal” in mapping the top of the dome. 
This is shown in Figure 3. 
An interesting feature of this profile is that the extrapolated 
travel-time curve indicates a negative time for zero distances. This 
is to be expected for explosion-generated data, and indicates the ab- 
normally high velocity of sound in the immediate neighborhood of 
the explosion. However, in comparison with the greater number of 
such short refraction profiles, these data are exceptional, since the 
usual data indicate a plus time at zero distance, due to the general 
existence of a low-speed layer at the surface of the ground (weathered 
or aerated zone). In the case of the Vermilion Bay data, there is ap- 
parently no aerated zone,! so that there is opportunity to observe the 
effect of the abnormally high sound velocities in the immediate 
vicinity of the explosion.’ 
1 This is not necessarily an obvious conclusion, since in some cases of water- 
covered marsh, definite geophysical evidence of an aerated zone has been found on the 
shorter travel-time curves. As Paul Weaver has pointed out in “(Geophysics of the 
Soil,” this is to be expected in the case of thick layers of soil being formed from rotting 
vegetation. 
2 A. B. Wood, Textbook on Sound, p. 266. 
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