SEISMIC WEATHERED OR AERATEDSURFACELAYER 61 
Since the recognition of this stratum, various methods have been 
applied to correct for the abnormally slow travel time through it 
(particularly in reflection work), but little or no attempt has been 
made to explain its nature, or to determine the cause of its important 
characteristic. 
Data taken in various places, over a wide area, from Kansas and 
Oklahoma to the gulf coasts of Mississippi, Louisiana, and Texas, 
have indicated that this layer may vary in thickness from practically 
zero-to more than 1so0 feet, though the general average is approxi- 
mately 50 feet. The velocity of sound in this ‘‘weathered” material, 
though variable, is always comparatively low and a general average 
might be placed between 2,000 and 2,500 feet per second; while the 
average velocity of sound in the same materials immediately below is 
found to be generally about 5,600 feet per second. 
As this top layer is at the surface of the ground and exposed, as 
its low velocity characteristic exists almost universally regardless of 
composition, and as this low velocity characteristic could result from 
a broken, disintegrated or loosely bedded condition, the term “‘weath- 
ered layer” is perhaps natural. This term has been in use for some time 
for the sake of convenience and for lack of a better one, though it has 
been granted that this layer does not, in many cases, conform to the 
geologic term ‘‘weathered.” In the Gulf Coast region, for example, 
the geophysical weathered layer is exceptionally uniform and thin, 
though the geologic term ‘‘weathered”’ could be applied to the sedi- 
ments at considerable depths. 
The suggestion that the ‘“‘weathered layer’? might be associated 
with the regional ground-water table has often been made, but though 
in some areas the thickness of this material seems to be determined 
by the depth of the water table, this has not been found to be true in 
a sufficient number of cases to permit a general conclusion. 
The extremely low velocities (approaching, or even less than, the 
velocity of sound in air) that have been obtained in some areas, led 
to the idea of air inclusion in the surface materials as the cause.’ 
With this idea in mind it was predicted that short (shallow) re- 
fraction profiles would show lower and lower velocities with shallower 
depth penetrations, approaching if not actually equaling the velocity 
1 The discussion here refers to air mixture in a free state—not in solution. As 
V= : air or gases may be considered together, as opposed to fluids or solids, be- 
cause of the order of magnitude of difference in E and P for these two classes of ma- 
terials. H=elasticity: P=density. 
391 
