Geology Reference
In-Depth Information
13
SEISMIC REFRACTION
Refraction surveys are widely used to study the water table and, for engi-
neering purposes, the poorly consolidated layers near the ground surface,
and also to determine corrections for the near-surface low-velocity layer
(LVL) in deep reflection work. Travel times are usually only a few tens
of milliseconds and there is little separation between arrivals of different
types of wave or of waves that have travelled by different paths. Usually
only the first-arrivals, which are always of a P-wave, can be picked with any
confidence.
13.1 Refraction Surveys
Ideally the interfaces studied in small refraction surveys should be shallow,
roughly planar and dip at less than 10 . Provided that velocity increases
with depth at each interface, the first-arrivals at the surface will come from
successively deeper interfaces as distance from the shot-point increases.
Survey results are displayed as plots of distance (on the horizontal axis)
against arrival time (vertical axis). The gradient of any line on such a plot is
the reciprocal of a velocity, i.e. steep slopes correspond to slow velocities.
13.1.1 The principal refractors
P-wave velocities for common rocks were shown schematically in Fig-
ure 11.2. In shallow refraction work it is often sufficient to consider the
ground in terms of dry overburden, wet overburden and weathered and fresh
bedrock. It is very difficult to deal with more than three interfaces.
The P-wave velocity of dry overburden is sometimes as low as 350 m s 1 ,
the velocity of sound in air, and is seldom more than 800 m s 1 .Thereis
usually a slow increase with depth, which is almost impossible to measure,
followed by an abrupt increase to 1500-1800 m s 1 at the water table.
Fresh bedrock generally has a P-wave velocity of more than 2500 m s 1
but is likely to be overlain by a transitional weathered layer where the
velocity, which may be initially less than 2000 m s 1 , increases with depth
and the accompanying reduction in weathering.
 
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