Geology Reference
In-Depth Information
S
G
V 1
i 3
sin
i
=
V 1
/V
c
2
i 3
i c
i c
Q
P
sin i /sin
i
=
V
/V
i c
3
3
c
1
2
3
i c
i c
V 2
sin
i
=
V
/V
3
3
c
2
3
3
sin
i
=
V 1
/V
3
3
V 3
Figure 13.1 Critical refraction at two interfaces. The relationships between
the angles and the velocities all follow from Snell's Law.
13.1.3 Lengths of refraction spreads
A line of geophones laid out for a refraction survey is known as a spread ,
the term array being reserved for geophones feeding a single recording
channel. Arrays, which are common in reflection work, are almost unknown
in refraction surveys, where the sharpest possible arrivals are needed.
Sufficient information on the direct wave and reasonable coverage of the
refractor is obtained if the length of the spread is about three times the
crossover distance. A simple but often inaccurate rule of thumb states that
the spread length should be eight times the expected refractor depth.
13.1.4 Shot positioning
In most refraction surveys, short shots are fired very close to the ends of
the spread. Interpretation is simplified if these shots are actually at the end-
geophone positions so that travel times between shot-points are recorded
directly. If this system is used, the geophone normally at the short-shot
location should be moved half-way towards the next in line before the shot
is actually fired (and must be replaced afterwards). Damage to the geophone
is avoided and some extra information is obtained on the direct wave.
Long shots are placed sufficiently far from the spread for all the first-
arrivals to have come via the deepest refractor, and short-shot data may
therefore be needed to determine the minimum acceptable long-shot offsets.
Distances to long shots need be measured accurately only if continuous
 
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