Figure 4.14 shows the RE20h10v of data recorded with a 50 MHz setup of the pulseEKKO
4.4. Application of
RE
to Higher Frequency Data
Figure 4.14: RE20h10v of a 50 MHz profile between S15 and S12 (calculated using a fixed
number of 100 bins). The brine layer reflections starts about 600 m along the profile at an approximate depth of 45 m. The deeper reflection visible earlier on probably represents the bottom of the ice shelf. The vertical exaggeration is roughly 23:1.
intrusion extending several kilometres into the ice shelf (Kovacs et al., 1982b). This layer contains salt water which reflects and absorbs most of the radar energy and therefore prevents any deeper reflectors to be detected. The data shown in Fig. 4.14 corresponds to a 4.8 km
long part of the transect across the ice shelf between the stakes S15 and S12, moving south (see Fig. 2.15). Only the dewow and the absolute value calculations were performed prior to calculating the RE20h10v, since these two steps are easy to apply even in the field while
outlier removal is not. Accordingly, the numerous black boxes are related to outliers caused by the same fibre-optic cable problems as in the 25 MHz glacier profile (e.g. Fig. 4.4). The three dark vertical lines are caused by more erroneous data through interference from radio transmissions (the 50 MHz antennae were unshielded).
The reflection at approximately 120 m and slowly decreasing in depth along the first 700
m in Fig. 4.14 is probably the bottom reflection from the ice shelf / sea water interface. This reflection is suddenly cut off and a different one appears at about 40 m depth, which is most likely associated with the brine layer. It also slowly decreases in depth along the profile as the ice shelf becomes thinner in this direction (Kovacs et al., 1982b). The vertical scale in Fig. 4.14 assumes an average wave velocity of 0.2 m
ns (corresponding to an average
snow density of 591 mkg3). This is a reasonable estimate down to the brine layer, but too large when deeper reflections are concerned, i.e. the actual thickness of the ice shelf is probably overestimated in Fig. 4.14.
Due to the strong reflection and some absorption of the radar wave by the brine layer, the bottom of the ice shelf is not visible in those parts of the profile where the brine layer is present, except between 600 and 700 m where both reflections can be observed. This region marks the maximum extent of the brine intrusion into the ice shelf and therefore the layer is probably very thin at this point, reducing the amount of reflection and absorption, and allowing some of the radar energy to penetrate through to the bottom of the ice shelf (Kovacs et al., 1982a). The apparent horizon at around 80 m depth that runs parallel to the brine layer is not an actual reflection, but a secondary image (sometimes referred to as a ‘ghost’ or ‘multiple’) of the brine layer reflection. This is a common artefact when strong reflectors are present and is caused by a part of the returned radar signal being reflected back down by the snow-air interface and therefore travelling between the surface and the brine layer twice. Figure 4.15a is a close-up of the top 40 m of Fig. 4.14. The brine layer shows up as a continuous band of low RE20h10v between two bands of higher values (and another thinner
band of low RE20h10v below these). The brighter bands correspond to the increasing and
decreasing amplitude gradients of the reflection, while the lower values in between relate to the centre of the amplitude peak. This is similar to the pattern of the RE for the 5th and
6th peak in Fig. 4.1b. While the horizontal continuity of the pattern clearly delineates this
reflection from its surroundings, the distinction between highRE20h10v values related to this
particular reflection and similarly high RE20h10v values above, is difficult. Comparison with
the RE20h10v calculated in the same manner for a 500 MHz recording along the same line
(Fig. 4.15b) illustrates the coarseness of the resolution of the 50 MHz system. Clearly, the 500 MHz system has a considerably better resolution than the 50 MHz setup, resulting in a much narrower band of high RE20h10v corresponding to the brine layer reflection with a
higher contrast relative to the RE20h10v of the surroundings. Furthermore, several internal
layers between the surface and the brine infiltration can be identified in Fig. 4.15b, e.g. at about 11 m depth and at 18 m depth, none of which are visible in Fig. 4.15a.
All internal layers move closer to the surface from left to right. While this trend is similar to the one seen in the brine layer, it is not as steep. A closer look at Fig. 4.15b shows that the brine layer ‘cuts through’ some of the internal layers, which underlines the fact that the brine infiltration happened after the formation of the rest of the internal structure (see also Fig. 4.16a and b). However, most of the other internal layers are relatively blurred and the contrast to the background is low. Similar to Fig. 4.14, the high signal variability close to the surface (top quarter of the profile) causes relatively high RE20h10v values everywhere,
disguising any distinct individual reflectors.
The overall raised RE20h10v in Fig. 4.15a is probably due to high variability in the GPR
signal caused by numerous internal layers in the upper part of the snow pack, all of which partially reflect the radar wave. These reflections are weaker than the one from the brine layer, as is illustrated in Fig. 4.16a which shows the underlying radargram for Fig. 4.15a. Several continuous reflections from internal layers can be made out between the surface and the brine layer but the reflection amplitudes are smaller. The ubiquity of internal layers is further illustrated by the 500 MHz GPR radargram of the same profile in Fig. 4.16b. The higher resolution of the 500 MHz system allows the identification of an even larger number of layers, some of which are also highlighted by the RE20h10v in Fig. 4.15b. The resolution
4.4. Application of
RE
to Higher Frequency Data
Figure 4.15: RE20h10v of (a) 50 MHz and (b) 500 MHz GPR data collected along the same
line between the stakes S15 and S12 on the McMurdo Ice Shelf (see Fig. 2.15b) in 2009. In both cases only the dewow filter is applied before calculating theRE20h10v from the absolute
values of the data using a fixed number of 100 bins. The vertical exaggeration is roughly 47:1.
Figure 4.16: Radargrams underlying the RE20h10v profiles in Fig. 4.15. In addition to the
dewow filter a linear gain was applied to both (a) the 50 MHz and (b) the 500 MHz data. Furthermore, ten-fold stacking was performed on the 500 MHz data. The vertical scale is expanded in (b) to improve the visibility of weak internal reflections. The resulting vertical exaggeration in (b) is approximately 82:1.