SEISMIC WEATHEREDOR AERATEDSURFACELAYER 63 
As can be seen from the curve, a velocity within the weathered 
layer is indicated (slope of curve near origin) as low as 550 feet per 
second, while the velocity of sound in air alone is approximately 1,100 
feet per second. Assuming the low velocity to be due to the inclusion 
of air as postulated, we would naturally expect the velocity to be 










[oP il | 
Nigra Re ris a Be 
Scaled alia Ws os alee Sa 
el 
cv ESE 
amen Ey 
fe a BE 
elem evnttete etsy e te 
Tithe i 
a ll dU CU 
254 il 
Bn a ales lal 
as a 
2s i 
= aes et ea a 
Urner = Seue 
SK a | 
(SCG. Sou & 
Eu a SPB 
CEP ERaseece 
PROPORTION AIR TO EARTH BY VOLUME 
Fic. 1 
lowest nearest the surface, where the proportion of air should be 
greatest. Referring this velocity (550 feet per second) to the curve 
(Fig. 1), the corresponding air concentration is 0.oor part by volume, 
which seems probable in the near-surface materials. A computation 
of the depth corresponding with the ‘‘break-point”’ of the time-distance 
curve indicates 714 feet as the thickness of the “‘aerated layer.” 
393 
