Jökull - 31.12.2001, Side 58

Guðmundsson et al.
JkEGsG GEOMETRICS ES24É
FKOH1217-MT 14136 2MH(/1W
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Figure 3. An example of a 24 channel seismic section from Skeiðarársandur (Profile SKS2). The spacing be-
tween geophones was 5 m. Two reflectors appear on the section; at small offset they are partly obscured by
surface waves. b) X2T2 graph of the section shown to the left. - Dœmi um niðurstöður úr 24 rása endurkasts-
mælingu á Skeiðarársandi (snið SKS2). Bil milli nema var 5 m. Tvö endurköst komafram. Til hœgri er X2T2
línurit afsniði SKS2. RMS-hraðar fyrir lögin eru ákvarðaðir útfrá hallatölum á línuritinu.
INTERPRETATION
Interpretation of the seismic records involved the de-
termination of travel times of refracted and reflected
waves. Clear reflections were picked and RMS- and
interval velocities determined. The velocity of the
uppermost layer was determined from the first ar-
rivals and RMS-velocity of the first reflection (Fig-
ure 3a). Due to the limited length of the profiles, re-
fracted waves were usually not detected from deeper
layers. Velocities in the sediments were therefore de-
termined using the X2T2 method (Dix, 1955). This
method (Figure 3b) is based on the equation of the
time-distance hyperbola
t2 = to2 + ~2- (1)
Vrms
Here t is the travel time for a distance X between the
shotpoint and the geophone and fo is the travel time
for a vertical ray. Vrms is the root mean square ve-
locity of the layers overlying the nth reflector and is
defined from
Vrms =
J2uí2tí/J2'
.2=1
2=1
1/2
(2)
Here and r* are respectively the interval velocity
and travel time through layer number i. Finally, the
interval velocity of the layer above the nth reflector is
given by the Dix formula:
vn -
V2 i
vRMS,nLn
Vrm S ,n—\tn—l
tn. t
n— 1
1/2
(3)
where VRMs,n,tn and VRMs,n-i,tn-i are, respec-
tively, the root mean square velocities and travel times
from reflectors number n and n — 1.
When possible, reflections were picked on re-
versed shots. The interval velocities are then the rnean
56 JÖKULLNo. 51