reflections that the radar wavelet could not resolve.
forcement within the bunker roof. Close inspec-
Noise added to the signal from high-gain amplifi-
tion of this horizon reveals that it contains many
cation prevents us from seeing deeper than 7 m
closely spaced, small diffractions from individual
into the ground.
components of the steel reinforcement mesh. A
The 400-MHz profile (Fig. 9b and 10b) shows
series of hyperbolas apparent between 0 and 10 m
details in the upper 1.5 m of the subsurface. There is
distance define a ramp-shaped mound adjacent
a second discontinuous horizon that can be tracked
to the western utilidor. Its closeness to the utilidor
from about 1 m depth at the east end of the transect
suggests that it defines the surface of an earthen
to about 40 m distance, where it is truncated near
berm built during site construction.
the ground surface. This horizon reaches a maxi-
Transect 3 recorded at 400 MHz extends 50 m
mum depth of about 1.3 m between 18 and 30 m
across the bunker, being approximately 10 m north
along the line. Small diffractions at 2, 23, and 38 m
of transect 2 (Fig. 5). The features recorded by this
originate about 20 cm above the discontinuous
profile are similar to those of transect 2. The west
horizon. These diffractions probably originate from
utilidor is clearly evident between 3 and 4 m dis-
buried pipes or cables that were placed on a hard
tance, as are the walls of the bunker. The small,
packed surface prior to backfilling.
closely spaced diffractions evident from 7 to 43 m
indicate the steel reinforcement mesh (Fig. 13 and
14a). A tighter spacing of these diffractions and
Transects 2, 3, and 5
Transects 2, 3, and 5 delineate the bunker and
an apparent drop in the ceiling between the dis-
the adjoining utilidors. Transect 2 extends 60 m
tances of 13 and 35 m suggest either a second
from the overgrowth west of the bunker to the top
reinforcement mesh or a double ceiling over an
of the concrete slab of the Transmitter Annex at
inner bunker (Fig. 14b). The breaks in small dif-
the south end of the bunker (Fig. 5). The 100-MHz
fractions between 1113 m and 3538 m (Fig. 13)
profile is distinguished by the limited penetration
appear to be caused by edge effects and signal
where the antennas were directly over the bun-
resonance at the outer walls of the inner bunker
ker, and by strong diffractions at its edges (Fig.
(Fig. 14b). A steel pipe, probably a vent pipe, was
11a and 12a). There is a near-horizontal reflector
recorded at a distance mark of 23 m as a sharp
at a depth of 5 m on both sides of the bunker (Fig.
resonance that extends through the entire record
11a). The depth of this reflector corresponds to the
(Fig. 13).
strong reflector in transect 1 (Fig. 9a and 10a), sug-
Transect 5 was located at the north end of the
gesting that it is probably the same excavation
bunker and transmitter annex, extending towards
surface. The reflection bands at a depth of 50 to 70
the woods to the east (Fig. 5). The 100- and 400-
cm above the bunker (Fig. 11b) appear to reflect
MHz profiles record responses from the ceiling of
its upper surface (Fig. 12b).
the outer bunker, with its reinforcement mesh, and
The 400-MHz profile shows distinct diffractions
edge effects at the outer bunker wall and utilidors
that originate from the utilidor structures on
(Fig. 15 and 16a,b). Several horizons are apparent
either side of the bunker (Fig. 12b). These diffrac-
on the profiles east of the bunker. The horizons
tions appear as broad arches in the 100-MHz pro-
adjacent to the bunker are aligned subhorizontally,
file, where the wavelength is about 1.3 m long (Fig.
with occasional hyperbolas originating at reflector
11a). Two distinct hyperbola sets can be distin-
horizons. These horizons on-lap a westerly dipping
guished in the 400-MHz data with the smaller (30-
horizon that truncates gently dipping reflectors to
cm) wavelength (Fig. 11b). The higher resolution
the east. We interpret the horizons closest to the
allows us to distinguish between the outer and
bunker as layered fill. The westerly dipping hori-
inner walls of each utilidor. The top of the utilidor
zon appears to be an excavation surface that trun-
appears to be at about 1 m depth; a lower diffrac-
cates natural depositional horizons to the east. The
tion at 1.5 m depth and 54 m distance along the
hyperbolas in the fill probably represent buried
transect may locate the utilidor floor (Fig. 12b and
objects, such as pipes and conduits, or boulders
12c). Utilidors seen during the investigation were
introduced during backfill (Fig. 15b and 16b,c).
constructed of concrete walls, with railroad ties
Transect 6 is located north of the bunker and
covered with soil for a ceiling and an earthen floor.
extended 120 m (Fig. 5). The 100-MHz profile in
The distinct horizon at 70 cm depth on the 400-
Figure 17 shows a prominent reflector that extends
MHz profile (Fig. 11b) represents the top surface
across the transect between 4 and 7 m depth. This
of the bunker. A second, slightly deeper horizon
reflector truncates several smaller, less well-
of multiple reflections represents the steel rein-
defined subhorizontal reflectors, likely to repre-
11
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