302 
PACIFIC SCIENCE, VoL XIX, July 1965 
tions upon the depth to the plug and the seis- 
mic velocities in the plug and the overlying 
material can be determined (Fig. 6). 
In this analysis we note that for the critical 
angle (i c ) 
and 
• • Vo 
sin i c = — 
V P 
h 
tan i c = — 
H 
where H is the thickness of the layer and h the 
half distance from the point of origin to emer- 
gence. 
Or if h is observed 
then 
If the plug is of only limited lateral extent, 
then the diffractions off the trailing edge of the 
plug will start with the end of the segment of 
refracted arrivals and cross over the first arrivals 
to lead into the early portion of the early arrival 
segment. If, however, the plug is of sufficient 
lateral extent, then the refraction segment rep- 
resenting the energy traveling through the plug 
will overtake the first arrivals and may even be 
composed of the first arrivals. The first case is 
illustrated in Figure 1, used to predict the ex- 
pected results; the latter is seen in Figure 5, 
the interpretation made here. The strong energy 
in the first motion in the "diffraction” range 
supports this interpretation. 
Based on the slope of the first arrivals at the 
downward flexure of the travel-time curve, a 
lower bound of 6.98 km/sec (say 7 km/sec) is 
obtained for the velocity in the plug. Inspection 
of the data in Figure 5 on which this is based 
indicates the relatively poor quality of the esti- 
mate. 
An estimate of the depth of the dome at the 
point where the critically refracted ray enters 
can be obtained by the construction of the 
aplanatic line in the vertical plane containing 
both the shotpoint and the detector at the 
crossover station, 7.27 km. This has been done 
and the maximum depth is indicated as being 
1630 m. This point corresponds to the point of 
the aplanatic curve which would be intersected 
by a ray leaving the origin at 42°, hence it is 
taken to be the depth. The reliability of this 
depth, then, depends on the validity of the 
assumption that the top of the plug is hori- 
Distance, (kilometers) 
i 
1 
Fig. 5. Interpretation of the travel-time data. 
