Jökull - 01.01.2005, Side 158

Þröstur Þorsteinsson et al
Distance (m)
D
ep
th
(m
)
Bed
Surface
N S
0 500 1000 1500 2000 2500 3000
0
100
200
300
400
500
Figure 5. Radar-profile along MD10 to MD02 at the western margin of the study area. The depth ranges from
about 100 m to 250 m. A strong echo, most likely from an ash layer, is visible near the surface in the southern
part of the profile. – Ísjár-snið milli MD10 til MD12, á vesturhlið mælisvæðisins. Dýpið er breytilegt, frá um
100 m til 250 m. Sterkt endurkast, líklegast frá öskulagi, sést nálægt yfirborði í syðrihluta þversniðsins.
RESULTS
Radar
Figure 6 shows the surface topography calculated
from the pressure record attained during the radar
measurements and the elevation of the GPS-points.
The origin of the local coordinate is at the UTM co-
ordinate of 5.91x105 m easting and 7.068x106 m nor-
thing, in the 28N zone (Figure 1b).
The ice thickness (Figure 7) was calculated from
the two-way travel time, assuming a radio-wave speed
of 169 m µs−1. The ice thickness decreases from SE
to NW, from some 300 m in the SE part of the survey
region to about 112 m in the NW part.
Figure 8 shows the bed topography inferred from
surface elevation and radar data (GPR-data). The bed
relief within the study area is about 185 m.
Location of measurement marks in April
Data processing revealed extremely small errors in the
data, < 2 cm in the horizontal direction and 2.5–3.5 cm
in the vertical direction (given 1 cm horizontal set-up
error and 2 cm vertical set-up error). Redundant oc-
cupations of 50% of the network and extremely high
quality data (even with the Trimble 4000’s) contribute
to the excellent data quality. Little or no data edit-
ing was necessary to attain statistically valid solu-
tions. Most of the short baselines solved well with sin-
gle frequency solutions. Survey conditions pn April
20 were extraordinarily quiet (the best GPS data S.
O’Neel the UNAVCO GPS specialist had ever seen).
Continuous GPS survey April 22–May 7, 2003
Continuous GPS measurements were carried out at
MD00, MD11, and MD12 (Figure 2). The MD00 sta-
158 JÖKULL No. 55, 2005