Jökull - 31.12.2001, Page 59

Seismic soundings on Skeiðarársandur
SKS1 SKS5 SKS4 SKS2 SKS3
FR
HS
SV SKS6 SKS7
50
100-
150
200
250 -
300
Figure 4. Stratigraphic columns for the profiles showing the layering obtained from the seismic soundings. The
velocities in each layer are shown in km s-1. The weathered layer detected in SKS2, SKS4 and SKS5 is 1-4 m
thick. Its thickness marks the depth to the groundwater table in these profiles. The nature of the high-velocity
layer in SKS3 at 40 m depth is unclear; it may be an altered sedimentary layer but the velocity is such that
it does not rule out an intersedimentary lava flow. - Lagskipting endurkastssniða á Skeiðarársandi. Tölurnar
eru bylgjuhraðar í lögunum. Þykktin á yfrborðslaginu í sniðum SKS2, SKS4 og SKS5 (1-4 m) er dýpi niður á
grunnvatnsborð. Háhraðalagið á 40 m dýpi í SKS3 gœti verið ummyndað setlag en ekki er hœgt að útiloka að
þar sé hraunlag grafið í setlögunum.
of the results from the shots at each end. This applies
to majority of profiles and in most cases the difference
in calculated interval velocity is less than 0.15 km s_1.
However, the interval velocities for the deeper reflec-
tions must be regarded as very approximate since they
are sensitive to dip of layers.
RESULTS
SEISMIC REFLECTIONS
Reflections were detected at all locations (Figures 3
and 4). The number of reflections ranged from one
(FR) to four (SKS6). The lowermost reflection is con-
sidered to be from the igneous bedrock. This was con-
firmed at only one location (SKSl) where refracted
waves with a velocity of 5.3 km s_1 from the low-
ermost reflector were recorded. This refractor was
detected in one direction only. It may be from an
upward dipping boundary and the velocity therefore
too high. The expected bedrock velocity in this re-
gion is 4.0-4.5 km s_1, that of Layer 1 of Pálma-
son (1971). However, since no velocity information
exists for the assumed bedrock in most places, the
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