Figure 8. Wide angle reflection and refrac-
tion profile performed at about 400 MHz
near the OCAF. Arrow 1 points to the
directly coupled air wave, followed by the
directly coupled ground wave (arrow 2). All
other events become parallel with the ground
wave. The slope of the ground wave corre-
sponds to n = 1.28, equivalent to an approxi-
mate snow density of 0.33 g/cm3. Bright
spots and noise bands are artifacts of the
automatic gain control used to amplify the
weak air wave.
buried building after it was abandoned in the
ated above it since the meltwater occupies less vol-
mid-1970's.*
ume than the dense snow it replaces. At the time of
the survey, the surface of the pool was 70.1 m (230
The migration dielectric constant of 1.93 is an
integrated value. The fact that ε increases with
ft) below the snow surface and its diameter was
thought to be approximately 9 m (30 ft). The air
depth is demonstrated by the results of a WARR
bulb began at about 60 m depth and so there was
profile taken near the site (Fig. 8). The WARR
approximately 10 m of air space above the water
used two Model 3102 units, one as a transmitter
surface.
and the other as receiver; only the transmitter
Three sewage sumps located nearby are labeled
was moved. At the expense of noise amplifica-
utilidor vent, sump 1 and sump 2 on Figure 5. The
tion, automatic gain control was applied to this
objective here was to determine if the radar could
record to make visible the first event, the weak
detect these sumps and, if possible, measure their
direct air wave coupling. The timedistance slope
lateral extent, known to be about 21 m. We also
of the direct air wave coupling gives a velocity of
hoped to obtain an effective dielectric constant of
31.0 cm/ns, an error of about 3% over the actual
deeper firn by using the diffractions from the well,
value of 30 cm/ns. The ground wave slope gives
whose depth was known.
a value of n = 1.28. This value of n corresponds to
a near-surface density of about 0.35 g/cm3 and,
A 100-MHz time section profile along the line
PQ (Fig. 5) across the well is shown in Figure 9.
therefore, represents only the upper layers of
The 100-MHz transducer was used because the
snow. All subsequent subsurface reflections be-
sampling rate was sufficient to reproduce the sig-
come parallel with the ground wave as they re-
nals at the necessary time range. The slight surface
fract into the faster near-surface media; thus, re-
elevation difference of about 2 m from the north to
fractions spend more time near the surface.
the south end of the profile was inconsequential in
view of the large time range used to reach the well.
New water well and nearby sewage sumps
The wide hyperbola peak at 762-ns time delay is
The new water well is located just south of the
the response to the water in the well, which is a
main station (Fig. 5). It was being formed during
prominent target with large radar cross section
our visit. First, a shaft was melted with hot water
and a positive dielectric anomaly (higher dielec-
down through permeable snow to a depth of
tric constant than that of the snow). This is consis-
about 60 m where water began to pond. This wa-
tent with the phase polarity of the response; a re-
ter was brought up, heated and returned to en-
sponse to the hemispherical air bulb, which has a
large the pool. As the pool grew, a cavity was cre-
low radar cross section, would have the opposite
phase because it is a negative dielectric anomaly.
* Personal communication with A. Hogan, CRREL, 1995.
9
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