Jökull - 01.12.1989, Page 8

Table II. Seismic velocities as calculated by the
X2t2 method. V^s is the root-mean-square velocity
for the ice-water reflection and V2 is the interval
velocity of layer 2. T0 is the reflection time at zero
offset.
Taflall. Bylgjuhraði reiknaður samkvæmt X2t2 að-
ferð. Vrms er veginn meðalhraði fyrir alla íshelluna
en V2 er hraði í ísnum neðan 30 m dýpis.
Shotpoint v v rms m/s T0 ms v2 m/s
4 3690 143.3 3800
5 3670 138.0 3780
12 3820 136.2 3940
13 3530 133.3 3630
26 3360 135.6 3450
27 3490 134.1 3590
28 3560 133.5 3660
29 3440 127.5 3540
39 3440 139.3 3530
40 3670 137.0 3780
mean 3570±100 3670±110
The mean of the two velocity values are given with
95% confidence intervals.
and lateral changes in velocity are very small, if
present at all.
For the purposes of velocity analysis it was
decided to regard the ice shelf as consisting of two
layers, above and below 30 m below the surface.
The thickness of the upper layer is equal to the
depth of the shotholes and corresponds roughly to
the layer in which the compaction of snow and its
transition to ice takes place.
Out of 40 spht spread shotpoints in the Grímsvötn
survey, 12 were either at the edges of the ice shelf or
on the slopes surrounding it. Out of the 28 on the
ice shelf proper, 10 were chosen for X2t2 analysis.
The criteria were that there should be little or no
dip, that the ice water reflection was free from irre-
gularities and that the reflection times could be read
with good accuracy (+1 ms). Shotpoints 4-13 are
situated on line 1, 26-29 on line 2 and 39 and 40 on
Seismic velocity <m/s)
Fig. 4. A velocity profile of the ice shelf. The solid
line shows the velocity profile of the uppermost
40 m as calculated by the W-H method (Table I).
The broken line shows the interval velocity of layer
2 as calculated by the X2t2-method.
Mynd 4. Bylgjuhraði í íshellunni semfall afdýpi.
line 3 (Fig. 5). A least squares routine was used to
calculate the RMS-velocities for each shotpoint.
The mean RMS velocity for the 10 shotpoints for
the reflection from the ice-water interface is
3570 m/s. It is perhaps of interest to note that there
are indications of higher velocities in line 1 (bearing
east-west) than lines 2 and 3 (bearing north-south).
The mean RMS velocity for shotpoints 4-13 is
3680 m/s, but for shotpoints 26-40 the mean is
3490 m/s. A more detailed study of the data is
needed to see whether this phenomena is real or not.
However, for the purposes of the present survey it is
not important and the mean value of 3570 m/s is
used for the data processing.
6 JÖKULL, No. 39, 1989