SEISMIC REFRACTION - Field Geophysics - page 250

W

Difference line zero

E

250

200

extrapolated
intercept time
calculated
intercept time

150

100

calculated
intercept
time

50

Geophone 12 in
offset position

Geophone 1 in
offset position

G11 G12

G1 G2 G3 G4 G5 G6 G7 G8 G9 G10

Geophone positions (20m intervals)

Figure 13.9 Complete time-distance plot for a four-shot refraction spread.
The long-shot difference times, indicated by diamonds, are referred to a zero
line arbitrarily placed at t = 280 ms. Note the difference at the G12 position
between the intercept times obtained by extrapolation of short-shot data
and by using the long-shot/short-shot difference times. Extrapolation of the
western short-shot refracted arrival line back to the G1 position would be
even more problematic and could lead to an even more erroneous estimate.

of axes (Figure 13.9). After plotting the data, interpretation proceeds in the
following stages (note that velocities are quoted only to the nearest 10 ms − 1 ;
even this overstates the attainable accuracy).

Stage 1 Base refractor intercept times
Measure LS(W)-SS(W) time differences: These are roughly constant and
close to 41 ms from G6 to G12, indicating that in this region the SS(W)
arrivals have come from the base refractor. Similarly, LS(E)-SS(E) time
differences are close to 59 ms from G1 to G4.

Intercept times:
LS(W) time at W end = 101 ms. Intercept time = 101 − 41 = 60 ms.
LS(E) time at E end = 208 ms. Intercept time = 208 − 59 = 149 ms.
Note the difference of 21 ms between the LS(E) estimate and the corre-
sponding extrapolated intercept time on Figure 13.9 of about 170 ms.