77

Bulletin of Glaciological Research ,- (,**0) 11ῌ2.
ῌJapanese Society of Snow and Ice

Study of the internal structure of the Kuranosuke snow patch in central Japan using
ground penetrating radar survey
Hideo SAKAI+, Yasuhiro URA+, Takayuki NAKANO+, Hajime IIDA, and Katsunori MUROI,+ Department of Earth Sciences, Toyama University, Gofuku -+3*, Toyama 3-*ῌ2///, Japan, Tateyama Caldera Sabo Museum, Tateyama-machi, Toyama 3-*ῌ+.*/, Japan(Received October /, ,**/; Revised manuscript accepted November +., ,**/)

Abstract
We studied the internal structure of the Kuranosuke snow patch in the Northern Japanese Alpsusing ground penetrating radar (GPR) survey. The Kuranosuke snow patch consists of a perennial
snow zone and an older ice zone. A GPR survey in Sep. ,**. elucidated the unconformity between theperennial snow zone and the older ice zone at a depth of / to 1 m. The thickness of the snow patch isover ,* m and ﬁve gravel layers were identiﬁed in the older ice zone. The gravel layers in the older ice
zone seem to decline at di#erent angles from the unconformity, which suggests they were formed
under di#erent conditions from the present. The Kuranosuke snow patch seems slightly shallower
than ,* years ago, which may be mainly caused by melting of the perennial snow zone. GPR studies
may be useful for ascertaining the formation and change of the Kuranosuke snow patch, and therelated paleoenvironment.

+.

Introduction

There are many perennial snow patches found in
the mountain ranges of central Japan. The Kuranosuke snow patch (-0῍-/῍N, +-1῍-1῍E, ,,2** m a.s.l., Fig.
+) located in the Northern Japanese Alps is one ofthem, where snow accumulation is more than +* m inwinter, with nearly the same amount of snowmelt insummer. It is in a cirque formed during the last ice age
and moulin-type vertical holes appear once every few
years after the ﬁrn has melted. Its internal structure
was studied by utilizing these vertical holes (Yoshida
et al., +32-; Iida et al., +33*), indicating that the snow
patch consists of a perennial snow zone and an older
ice zone. The thickness of the snow patch was over ,*m. The +.C dating of plants collected from the lower
ice zone in the vertical holes ranged between +*** to+1** BP (Nakamura, +33*). This indicates that the Kuranosuke snow patch is the oldest fossil ice body inJapan (dating back to the Kofun era).
Former studies using the vertical holes gave important ﬁndings, however the holes only permit limited examination of the general structure of the Kuranosuke snow patch. Ground penetrating radar (GPR)
survey is a useful investigation method in glaciology
and in the study of shallow snow/ice bodies throughtheir dielectric property. At the Kuranosuke snow
patch, GPR studies have been done on September +.,+32- by Yamamoto et al. (+320) and Yamamoto andYoshida (+321) and on September -*, +331 by Sakai etal. (+333a, +333b). Their surveys were conducted along
several lines at the snow patch with +** and +.* MHzantennas. Several important features of the snow
patch such as its basal structure, and the existence ofgravel layers, were ascertained.
In this study, we will show the results of a GPR
survey conducted at the Kuranosuke snow patch on
September ,2 and ,3, ,**.. The survey was done in aseason similar to the former GPR studies. Therefore,
comparison of the studies at three times (+32-, +331
and ,**.) can reveal the changes in depth and structure of the snow patch. The ,**. survey was especially concentrated around the deepest part of thesnow patch. By using a closely-spaced GPR survey,
and analyzing horizontal distribution maps of thereﬂection amplitude, we investigated the internal structure of the Kuranosuke snow patch in detail andincreased our understandings of its variation over
time.

,.

Outline of the GPR study

The GPR survey was done in the rectangular areaof /*ῌ/* m shown in Fig. + by using the Noggin GPR
system with a ,/* MHz antenna. In this area, a total of/+ survey lines (spacing between neighboring lines
being +m) from south to north were laid out. The

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Bulletin of Glaciological Research

Fig. +. Location and schematic map of the Kuranosuke snow patch. The red square in the map shows the survey area,
where the survey lines whose GPR data explained in section - are also indicated. The photographs show the survey
area and a snapshot of the survey.

length of each line was /* m. Referring to the study of
Iida et al. (+33*), the area was located over the deepest
part of the snow patch and the vertical holes.
GPR data along the /+ lines were examined inradar proﬁles such as Fig. ,. In the proﬁle, the relative
amplitude of radar reﬂection from the vertical zone isshown, where the abscissa is the horizontal distancealong the survey line and the ordinate shows theSakai et al.

travel time of the radar wave and the calculated depthon the left and right scales, respectively. In the calculation of depth, we used the velocity value of *.+0m nsῌ+, which is determined by ﬁtting a hyperbola todi#racting objects (Moldoveanu-Constantinescu andStewart, ,**.).
In the next section, representative GPR proﬁlesfrom the ﬁve lines shown in Fig. + are described. The
proﬁles are those of lines ++ and +. in the western area,that of line ,/ in the middle area and those of lines .*and ./ in the eastern area. Line +. runs in the westernarea with 3* m length beyond both edges of the rectangular area.
Further, all the GPR proﬁles from /+ lines werecombined into horizontal distribution maps of reﬂection amplitude (amplitude time-slice maps; Conyers
and Goodman, +331), which were examined at several
depths.

-.

Results and discussion

GPR proﬁle along line +.
The upper image in Fig. , shows the proﬁle alongline +., with lines and the inferred strata drawn around the distinct reﬂection areas.
We can identify a clear boundary C around / m indepth. It is consistent with the depth of the unconformity between the perennial snow zone and the older
ice zone found in the vertical holes near the survey
line (Iida et al., +33*). A layer with gravel around theunconformity may cause the distinct radar reﬂection.
In the perennial snow zone, layers A and B ofclear reﬂection are identiﬁed. Examination of the proﬁle by changing the gain of reﬂection amplitude proved that the two layers are a continuous layer. It may
be a dirt ice layer formed by concentrated (melted)
annual layers over several years.
As for the bottom of the ice zone and internallayers, the basal plane under the snow patch resembled a bowl (U-type) shape as a whole. The inclined
reﬂection over the distance of * to .* m is the boundary between bottom of the ice zone and the lower
zone. Regarding the geographical features, the lower
zone corresponds to a protalus rampart. The sloping
reﬂection at a distance of 1* to 2/ m in the northernside is the boundary between the ice zone and thecirque wall. The reﬂection is strong around the basal
plane at a distance from -/ to ./ m, which may be aconcentration of gravel which probably collapsed
from the cirque wall and protalus rampart. At the
middle of the line, the snow patch has a maximum
depth of about ,/ m. Proﬁles show ﬁve clear reﬂection
layers (D, E, F, G and H) between the unconformity
and the basal plane. These may be gravel layers in thefossil ice zone.

-.+.

-.,.

Other GPR proﬁles

79

The proﬁle of line ++ in Fig. , is another exampleof the results in the western area. We can identify thereﬂection inclined downward from south to north as
the boundary between the ice zone and the protalus
rampart. The cirque wall in the northern side is notreached in the survey line. At the right end of the
proﬁle, the snow patch reaches about ,- m in depth.
Several clear reﬂections in the fossil ice zone may
show layers of gravel.
The proﬁle of line ,/ shows the results in the
middle area. Di#ering from the proﬁles in the westernarea, the proﬁle shows a steeply inclined reﬂection at
the boundary with the protalus rampart.
Similar steeply inclined reﬂection at the boundary between the ice zone and the protalus rampart is
identiﬁed in the proﬁle of line .* in the eastern area,
although the incline turns into to a gradual slope fromthe next line, and the reﬂection in line ./ has an angle
similar to that in the western area (such as line ++).
In most of the survey lines apart from the easternarea, the unconformity is conﬁrmed at the depth of /to 1 m. The depth of the unconformity at line ./ isshallow compared with those in proﬁles from thewestern and middle areas. The proﬁles near the eastern end of the area have a shallower unconformity,
and in the proﬁle of line /*, it is about - m in depth.
We can identify a strong reﬂection (dark) region
at the unconformity in several lines, e.g. a region at a
distance of ,* to ,/ m in line ,/. The region may be aconcentration of gravel, possibly formed by fallingstones from the cirque wall, or transported debris
ﬂow. In other words, debris ﬂow or falling stones may
have accumulated while the unconformity layer was
an exposed surface.
In the proﬁles of lines ,/ and .*, sloping reﬂections in the ice zone were identiﬁed. Sloping reﬂections are also visible in several other proﬁles. They
may be inclined gravel layers, which have been formed under di#erent conditions (e.g. by glacier ﬂow)
from the present and/or the period when the unconformity was an exposed surface. Therefore it is important to ascertain the distribution and structure ofinclined reﬂections in the ice zone by GPR, as they
may be useful for investigating the sedimentary condition and the related paleoclimate.
Comparison with former GPR studies at the Kuranosuke snow patch
In Fig. -, the GPR proﬁle along line +. (the lower
image) was compared with the previous proﬁles studied in +32- (Yamamoto et al., +320) and in +331 (Sakai etal., +333a). All the survey lines have a NS strike andcover similar areas. Yamamoto et al. (+320) used a +.*MHz antenna and in the study of Sakai et al. (+333a), a+** MHz antenna was applied. In this study, a ,/* MHzantenna was used. Compared with the proﬁles fromformer studies, the proﬁle for ,**. has clearer reﬂe-.-.

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Bulletin of Glaciological Research

Fig. ,.

GPR proﬁles along lines +., ++, ,/, .* and ./. The darker parts represent stronger reﬂections.

ctions indicating the unconformity and the gravel
layers. This suggests that the ,/* MHz antenna is
more e#ective for clarifying the internal structure of
the snow patch up to several tens of meters in depth.
The three proﬁles show similar reﬂection patternin general, although di#erences such as the shape ofboundary between the ice zone and the lower zone arevisible. Locations of the survey lines were determinedby topographical surveys. The locations of the threelines in Fig. - are close each other, but not exactly theSakai et al.

81

Fig. -. Comparison of GPR proﬁles in the area of line +. from three surveys: (a) +32-(Yamamoto et al., +320), (b) +331
(Sakai et al., +333a) and (c) ,**. (the current study).

same. One reason for the di#erences among the three
proﬁles is considered to be the di#erence in location of
the survey lines.

Comparison between the proﬁles for +32- and ,**.
suggests a reduction in the thickness of the perennial
snow zone.

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Bulletin of Glaciological Research

Regarding the proﬁle for ,**. at line +., the proﬁles at the neighboring lines show a similar depth forthe perennial snow zone, suggesting that the unconformity is fairly ﬂat around the area of line +.. Therefore, the depth of the perennial snow zone in the
proﬁle in ,**. may not di#er greatly even acrossseveral survey lines.
The depth of the perennial snow zone may change depending on seasons. Concerning the season of
the surveys, the survey in +32- was conducted on
September +., the survey in +331 was on September -*and the survey in ,**. was on September ,2. Therefore, the di#erence in season cannot explain large
di#erences in the thickness of the perennial snow
zone in these proﬁles.
In other words, the GPR studies suggest that theperennial snow patch may have diminished in thickness during the past ,* years. Iida and his colleagues
are preparing to show that the surface area of thesnow patch also seems to be smaller than ,* years ago(Iida, H., personal communication).
Analysis of horizontal distribution maps of thereﬂection amplitude
All data from the GPR proﬁles for /+ lines in therectangular area were combined, and horizontal distribution maps of the reﬂection amplitude (amplitude
time-slice maps) were made.
The process of analysis is described by Conyers
and Goodman (+331) as follows: radar proﬁles whichhave been collected along parallel lines within an area
are sliced at a particular time interval. The relative
amplitudes of the reﬂected radar waves recorded between those times are then averaged and interpolated
prior to printing them in map form. The resulting
anomalies visible in a slice map therefore represent
the spatial distribution of reﬂection amplitudes between speciﬁc depths across the area.
In Fig. ., several maps at di#erent depths down toabout +0 m are shown. In these maps, the distributionof the relative intensity of reﬂection is shown by thecolor representation. The map at the depth *ῌ+.2/ mshows a distinct reﬂection as a whole, which is causedby the reﬂection from the surface snow.
Small circles drawn in each map show a strong
reﬂection region. The region corresponds to the vertical hole J which has been studied (Yoshida et al., +32-;
Iida et al., +33*) to ascertain the internal structure of
the snow patch.
The map at the depth /./. to 1.-2 m corresponds tothe unconformity layer. In this map, strong reﬂection
regions are widely visible. The strong reﬂection region such as in the left lower (southwestern) side in
the map suggests a concentration of gravel at theunconformity. This seems consistent with the proﬁlefor line ++ in Fig. ,, where a strong reﬂection (darkregion) is found at a similar position on the unconfor-...

mity.
The maps for depth deeper than 1.-2 m show an
arc shaped pattern of reﬂection in the eastern area of
each map. This is the reﬂection from the boundarywith the end moraine (Fig. +). The radius of the arc issmaller in the deeper slice maps, which suggests thatthe snow patch is a bowl-shaped conﬁguration in thestudied area.

..

Conclusion

We conducted a GPR survey at the Kuranosuke
snow patch, which has a fossil ice zone in the lower
part, with its age going back to the Kofun era (c.a. +1**
BP). The survey was done in a rectangular area of /*ῌ
/* m located around the deep part of the snow patch
and its vertical holes. The /+ survey lines having a NS
strike with /* m in length were laid out in the area.
The GPR proﬁles show the unconformity between theperennial snow zone and the ice zone at the depth of /to 1 m. In the perennial snow zone, a clear reﬂection
layer is identiﬁed suggesting a dirt layer formed fromconcentrated annual layers over several years.
At the bottom of ice zone, a clear reﬂection shows
the boundary with the protalus rampart and the cirque wall. The ice zone has a maximum thickness ofabout ,* m and ﬁve reﬂection layers are identiﬁed
within the ice zone, suggesting gravel layers.
The GPR proﬁle for ,**. at the western side in thestudied area with 3* m in length was compared withthe proﬁles studied in +32- by Yamamoto et al. (+320)
and in +331 Sakai et al. (+333a). The proﬁle in ,**. wassurveyed with a ,/* MHz antenna (higher frequency
than those in the previous studies). Comparison
among three proﬁles shows that the proﬁle for ,**.
has a clearer reﬂection pattern, which suggests thatthe ,/* MHz antenna is more e#ective for clarifying
the internal structure of the Kuranosuke snow patch.
The maximum depth of the snow patch in thisstudy is shallow compared with the depth studied by
Yamamoto et al. (+320) and in particular, the depth ofthe perennial snow zone seems to be reduced. The
GPR study indicates that the perennial snow patch
may have diminished in size in recent years.
The proﬁles indicate that the unconformity isalmost parallel to the surface. The strong reﬂection
region such as in the southwestern side of the studied
area suggests a concentration of gravels at the unconformity. This may be formed by falling stones fromthe cirque wall, and/or transported debris ﬂow. During the accumulation of debris and falling stones, theunconformity may have been exposed as surface.
The maps at depth deeper than 1.-2 m show an arc
shaped pattern of reﬂection which may be the boundary with the end moraine. The radius of the arc issmaller in the deeper slice maps, suggesting that thesnow patch is the bowl-shape conﬁguration in theSakai et al.

83

Fig. .. Horizontal distribution maps of the reﬂection amplitude (amplitude time-slice maps) analyzed at several depths.
The distribution of the relative intensity of reﬂection is shown by the color representation. Strong reﬂections are
represented by red and weak reﬂections by dark blue. Small circles show a strong reﬂection region.

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Bulletin of Glaciological Researchstudied area.
Sloping reﬂections in the ice zone are identiﬁed inseveral proﬁles. They may be the inclined gravel layers, which were formed under di#erent conditions (e.g.
by glacier ﬂow) from the present and/or the period
when the unconformity was an exposed surface. The
distribution and structure of inclined reﬂections revealed by GPR may be useful for investigating thesedimentary environment and the related paleoclimate.
The Kuranosuke snow patch, which has the oldest fossil ice in Japan, is important for study of thepaleoclimate and paleoenvironment of the mountain
ranges in Japan. Detailed continuous investigations
are necessary and GPR may contribute to these studies.

Acknowledgments
We are grateful for constructive comments byProfessor Yutaka Ageta and Dr. Nozomu Naito on thispaper. This research was supported with funds fromthe “Discovering Toyama Prefecture’s Snow Packs
Project,” Toyama Prefecture.

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