Journal of Natural Disaster Science, Volume 36, Number 2, 2015, pp25-33

Detection of buried snowpack in landslide sediments using ground penetrating radar

Yoshinori Izumi*, Hideo Sakai** and Isao Kamiishi**** Graduate School of Science and Engineering for Education, University of Toyama** Graduate School of Science and Engineering for Research, University of Toyama***Snow and Ice Research Center, National Research Institute for Earth Science and Disaster
Prevention

ABSTRACT
When landslides occur in snow-covered areas, the presence of snow may cause a secondary
hazard. In the Tatsunokuchi area of Tsunan-machi, Niigata Prefecture, Japan, a large landslide was
induced by the seismic motion resulting from the North Nagano Prefecture earthquake (magnitude of
6.7) on March 12, 2011, which caused a snow avalanche at the same time.
In this study, a ground penetrating radar (GPR) survey was applied to find areas of buriedsnow and ice underground in the landslide area. We first conducted experimental field surveys ofartificially buried snow and ice to verify the effectiveness of the GPR survey in terms of investigating
ice and snow within collapsed land. The results of the experiments successfully detected strong radarreflections from areas of ice and snow. When the ice and snow began to melt, the survey data showedboth multiple reflections and the attenuation of reflections by meltwater.
Then, we investigated the presence of buried snow in the landslide area of Tatsunokuchi using
GPR surveys. A GPR survey conducted in August 2011, five months after the landslide event, showeda clear response indicating the presence of a snowpack underlying the sediment. A response related toan underground snowpack was also observed in November 2012, about one year after the first GPR
study. An excavation was conducted and the underground snowpack was ascertained. The snowpack
was found to be compressed into an ice-like state that may be appropriate for long-term storage
underground. The survey results of the buried snowpack in the landslide area at Tatsunokuchi wereconsistent with the results of the experimental surveys. Snowmelt water may cause the ground toloosen, which can increase the risk of secondary hazards such as subsidence and sediment discharge
by heavy rains.
The results of our study indicate that GPR surveys are useful for examining buried snowpacks inlandslide areas, as well identifying the potential areas where secondary hazards can be caused by themelting of snow and ice within the landslide area.

Keywords: Ground Penetrating Radar, Landslide, Snowpack, North Nagano Prefecture Earthquake

1. Introduction
There have been many earthquakes and related
sediment hazards such as slope failure in Japan. Landslides
that occur during heavy winter snowfall may lead toaccumulation of the snow within the collapsed mass. The
snow beneath the landslide area may then cause secondary
hazards because of the meltwater causing the ground toloosen and subside. In landslide areas involving snow, it isimportant to investigate the condition of the snowpack toY.Izumi, H.Sakai, I. Kamiishi

avoid secondary hazards.
In Tsunan-machi, Niigata Prefecture, a large landslide
was induced by the seismic motion of the North Nagano
Prefecture earthquake, a 6.7 magnitude earthquake that
occurred on March 12, 2011. Following the earthquake,a mudslide, consisting of mud and snow, was induced,
causing large damage around the lower reaches of the slope.
In August 2011, snow was observed in the damaged area,
which was considered to have remained buried undergroundsince the hazard.
A GPR survey identified its utility by detecting thechange in snow density, the snow depth and the number andthickness of the internal ice layers (Sakai et al., 1999).This
method has been widely used in the field of glaciology to
study frozen river lagoons, seasonally frozen ground and soforth (Moldoveanu et al., 2004, Nakano et al., 2008).
In the previous GPR researches, there has been little
study on underground snow.
In this study, we conducted GPR surveys ofareas where ice and snow were buried undergroundexperimentally, and verified the effectiveness of this nondestructive survey method. After that, we investigated thelandslide hazard area by GPR survey in August 2011 and in
November 2012.
2. Outline of GPR survey
A GPR survey is a method of locating undergroundst r uct u res by detecting the physical response (e.g.
ref lection, penetration, and attenuation) of radio waves(Fig. 1). The depth range and the resolution depend on
the frequency of the GPR antenna. When the frequency
is higher, the resolution improves, but the depth range
decreases. In our GPR surveys, we used the Noggin plus,
the pulse EKKO 1000 and the pulse EKKO Pro (Sensors& Software Inc., Canada). Several types of antennas withdifferent center frequencies were used.
We analyzed the survey data using GPR profiles thatplace the travel time of radio waves and the amplitude ofreflection waves in chronological order. The vertical axisof the profile shows the travel time of the radio waves andthe estimated depth, while the horizontal axis shows the
distance along the survey line. In the profile, the gradesof shading represent the amplitude difference of reflected
waves. The peaks of waves are colored red, the troughs areblue and the even waves are white.
A topographical correction to the GPR profile wasnecessary because the study area was a slope. We measured
the elevation of the survey line by D-GPS and corrected theeffect of the surface slope on the GPR profile. Furthermore,
we used timeslice mapping to create horizontal distribution
maps of the ref lection amplitude from all GPR profiles(Conyers and Goodman, 1997). We created timeslice maps
at several depths using different colors to represent thestrength or weakness of the reflected waves.

Fig. 1. Diagram of GPR survey

3. Experiment with buried snow and ice
To verify the effectiveness of GPR surveys as amethod of detecting ice and snow in collapsed land, we
conducted an experimental field survey of artificiallyburied snow and ice.
The experiment was conducted twice, in the summer
and winter. Ice was buried during the summer experiment,
while snow was buried during the winter experiment. The
velocity of the radar waves was estimated from the depth of
the buried samples.
3.1. GPR experiment with buried ice
A GPR su r vey of the exper iment al bu r ied ice
wa s c o n d u c t e d a t Ho d a k a O b s e r va t o r y of Kyot oUniversity in September 2011. We set up a survey areaof 2m×2m and buried a block of ice with dimensions of
0.5m(W)×0.4m(D)×0.2m(H) in the center of the survey area
(Fig. 2). The top surface of the ice was a few centimeters
below ground and the soil in that region was categorized as
humus. The survey lines were set at intervals of 0.25m anda 500MHz antenna (Noggin plus) was used for the survey.
We conducted a GPR survey of the area describedabove for the following three cases:(1) No ice(2) Buried ice(3) Buried ice containing water that represented the state ofwater on the surface of ice

Journal of Natural Disaster Science, Volume 36, Number 2, 2015, pp25-33

Fig. 2. GPR survey of the buried ice and the survey lines.

Figure 3 shows the GPR profile for all three cases,
with the survey line crossing the middle of the area ofburied ice and water. In the GPR profile for case (1), there
was no significant anomaly. In the GPR profile of case(2), a strong reflection of radar waves appeared at the icearea, marked as the black dashed area in the figure. In the
GPR profile of case (3), multiple reflections were observed
because of the reflection of ice in addition to that of thewater.

Fig. 3. GPR profiles: (1) No ice, (2) Buried ice, (3) Buried ice containing water

We constructed timeslice maps from the GPR profiles
at depths of 0.12-0.3m (4-10ns), which were calculated by
the thickness of the ice and underground depth, for thethree cases (Fig. 4). An area of strong reflection appears at

the buried ice area in case (2). The timeslice map of case(3) shows that the strong reflection caused by the buried ice
was attenuated by the presence of water.

Fig. 4. Timeslice maps at a depth of 0.12-0.3m (4-10ns): (1) No ice, (2) Buried ice, (3) Buried ice containing water.

Y.Izumi, H.Sakai, I. Kamiishi

3.2. GPR experiment with buried snow
A GPR survey of the buried snow was conducted
at a field site, with the humus soil mixed with red soil,in Toyama city in March 2012. We set up a survey areaof 1m×1m and buried a block of compressed snow withdimensions of 0.2m(W)×0.2m(D)×0.15m(H) in the centerof the survey area (Fig. 5). The top surface of the snow
block was a few centimeters below ground. The snow hada density of 592.6kg/m3, temperature of 0℃ and watercontent of 3.1%. The survey lines were set at intervals of

0.1m and a 1200MHz antenna (pulse EKKO 1000) wasused.
We conducted the GPR survey on the area describedabove for the following three cases:(1) No snow(2) Immediately after snow was buried(3) 15 minutes after snow was buried

Fig. 5. GPR survey experiment of the buried snow and the survey lines.

Figure 6 shows the GPR profile for the three cases
where the survey line crosses the middle of the area ofburied snow. In the GPR profile of case (1), there was no
significant anomaly. In the GPR profile of case (2), a strong

ref lection of radar waves appeared at the area of snow,marked as the black dashed area in the figure. In the GPR
profile of case (3), multiple reflections from snow appeared,in addition to reflections from snowmelt water.

Fig. 6. GPR profiles: (1) No snow buried, (2) Immediately after snow was buried and(3) 15 minutes after snow was buried.

Journal of Natural Disaster Science, Volume 36, Number 2, 2015, pp25-33

We constructed timeslice maps from the GPR profiles
over a depth range of 0.06-0.18m (2-5ns), which were
calculated by the thickness of ice and underground depth,
for the three cases (Fig. 7). An area of strong reflection

appears at the buried snow area in the timeslice map ofcase (2). The timeslice map of case (3) shows that thereflection caused by the snow area was attenuated due tothe snowmelt water.

Fig. 7. Timeslice maps (2-5ns: 0.06-0.18m): (1) No buried snow, (2) Immediately after snow was buried and(3) 15 minutes after snow was buried.

4. Survey of snowpack in the sediment4.1. Outline of survey
During the North Nagano Prefecture earthquake
on March 12, 2011, the seismic motion induced a large
landslide over an area of 130m×130m on the northeast slopeof Eboshigatayama of the Tatsunokuchi area in Tsunan-

machi. The landslide moved downslope about 760m over awidth of 50-100m, covering Route 353 with mud consistingof a mixture of snow and soil (Kamiishi et al., 2012;
Hoyanagi et al., 2012). Figure 8 shows the condition of the
field site in August, five months after the landslide event. A
snow block was observed at the site, which indicated thatthe landslide sediment may contain a snowpack.

Fig. 8. Map of the Tatsunokuchi area and photograph of the landslide area. The arrows of the figure on the left show
the survey lines. The arrow’s length is the distance of the lines. The arrow’s direction is the surveyed course.
◎ is the excavation point.

Y.Izumi, H.Sakai, I. Kamiishi

4.2. Result(1) GPR survey in August 2011
A GPR su r vey of the collapsed sedi ment was
conducted about 700m from the landslide area. We set a

survey line length of 36.5m and used two types of antennas
with center frequencies of 250MHz (Noggin plus) and100MHz (pulse EKKO Pro) because the depth of the buriedsnow was unclear. A photograph of the survey in August
2011 and elevation of the survey line are shown in Fig. 9.

Fig. 9. Photograph of survey in August 2011 and elevation of the survey line.

T he aver age velocit y of t he r a d a r waves wa s
calculated to be 0.06m/ns using a common mid-point(CMP) survey (Annan and Cosway, 1992), and the velocity
was used to draw the GPR profile. Figures 10 (a) and 10(b) show the GPR survey profiles using the 250MHz and
100MHz antennas, respectively. In both the profiles, strong
reflections of radar waves were identified at a distance of

14-24m from the starting point of the survey. The profile
from the 100MHz antenna showed a clearer reflection thanthat from the 250MHz antenna. As the snowpack in the
ground showed as strong reflections from our experiment,
the reflection of the radar in these profiles was possibly due
to the snowpack area. The area marked by dashed lines inFig. 10 is the estimated snowpack area under the sediment.

Fig. 10. GPR survey profiles in 2011: (a) 250 MHz antenna, (b) 100 MHz antenna

Journal of Natural Disaster Science, Volume 36, Number 2, 2015, pp25-33(2) GPR survey in November 2012
In October 2012, about a year after the sur vey
conducted i n Aug u st 2011, workers (f rom Niigat a
P refect u re) who were restor i ng t he la ndslide a rea

discovered that water at a temperature of 0℃ was flowing
away from a drainage hole. The sediment near the drainage
hole was excavated (Fig. 11), and a 1-m thick snowpack was
discovered at a depth of 4.2m below ground. The snowpack
had similar hardness to ice with the compression.

Fig. 11. The excavation site showing a snowpack in the landslide sediment.

We conducted a GPR survey in the area where thesnowpack was found. A survey line length of 40m was set
and antennas with a center frequency of 250MHz (Nogginplus) and 100MHz (pulse EKKO Pro) were used. Figure
12 shows a photograph of the survey in November 2012and elevation of the survey line. The survey results wereanalyzed using a radar velocity of 0.05m/ns, estimatedfrom a CMP survey. During the survey, the temperature of
the water flowing from the drainage hole was about 5℃,
suggesting that the snowpack had been melting gradually.

Fig. 12. Photograph of survey in November 2012 and elevation of the survey line.

Y.Izumi, H.Sakai, I. Kamiishi

Figure 13 shows the representative GPR profiles
obtained from the 250MHz antenna (Fig. 13 (a)) and the
100MHz antenna (Fig. 13 (b)). In both the profiles, there
were multiple reflections at distances of 28-30m and 34mfrom the survey starting point. These reflections may becaused by puddles of rainwater on the ground surface. In
the profile shown in Fig. 13(b), another reflection anomaly

was identified at a distance of 22m from the survey starting
point, at a depth of 2.5-3m. As these reflection patterns
were thought not to be rocks and soil strata, and thesnowpack in the ground was identified as strong reflections
by the experiment, this was possibly a response from theremaining snowpack in the sediment.

Fig. 13. GPR profiles of the 2012 survey: (a) 250MHz antenna, (b) 100MHz antenna

5. Discussion
We applied the results of our experimental fieldsurveys of artificially buried ice and snow to interpret the
GPR survey data of the landslide area at Tatsunokuchi.
Strong ref lections were observed at three locations in
the 2011 survey (Fig. 14 (a)). All anomalies extended
vertically to depths of a few meters, similar to the multiple
ref lections caused by the meltwater of snow and ice inthe experimental survey. The results of the 2011 survey
therefore indicated that water contained in snowpack was
distributed inhomogeneously.
Multiple ref lections observed from puddles on the
ground were similar to the experimental results (Fig. 14(b)). The strong reflection at distances of 21-23m from thesurvey starting point, at a depth of 2.5-3.5m, was likely the
response of the remaining subsurface snow.

Fig. 14. GPR profiles at 100MHz. (a) 2011 results, (b) 2012 results. The black dashed lines show snow packs andmultiple reflections. The results were examined by the reflection’s pattern gained by the experiment, andare the enlarged area.

Journal of Natural Disaster Science, Volume 36, Number 2, 2015, pp25-33

The velocity of radar waves in the landslide sediment
was 0.05-0.06m/ns, which is slower than typical velocities
through the ground, indicating that the sediment was richin moisture (Society of Exploration Geophysicists, 1998).
The radar waves were sharply attenuated at a depth of
1.5m by increased moisture underground. However, thecompressed snowpack caused a strong anomaly, resulting
in the identification of a snowpack area.
The results showed that ent rained snow in thelandslide sediment could remain for more than one year,
indicating that there is a potential risk of a secondary
landslide due to meltwater from persistent snow in thesediment.
The average annual temperature in Tsunan-machi
is 10.5℃ and temperatures have occasionally reachedover 34℃ during 2011 and 2012, according to the Japan
Meteorological Agency. Snowpacks have remained in
the landslide sediment for more than a year despite thesetemperature ranges. Possible causes of the subsurface area
being less affected by temperatures above ground are theheat insulation effect by rich moisture of the sediment, andthe snow stable at a temperature of 0℃ being insulated by
air remaining in the compressed snow.
6. Conclusions
A landslide was triggered by the North Nagano
Prefecture earthquake in the Tatsunokuchi area of Tsunanmachi in Niigata Prefecture and the main mass entrained
the snow in the area. The results of GPR surveys in
the area indicated that the snowpacks can remain in
the landslide sediment for more than a year, which was
visually confirmed by the excavation. In the controlled field
experiments, where snow and ice were artificially buriedin the subsurface, strong ref lections observed from thelandslide sediment were likely caused by the response of asnowpack, and the vertical extension of the reflection was
the result of multiple reflections by meltwater.
The GPR surveys identified clear reflection patterns
caused by subsurface snow and ice and therefore allowed
the area of entrained snow in the landslide sediment to be
determined. Such snow could remain in the sediment for
more than one year, creating a potential risk for a secondary
hazard by the looseness of the ground (Ito, 2003). The GPR
survey is a highly effective method of investigating suchareas and monitoring changes. In future work, efficient
GPR methods in areas with debris should be developed
to conduct effective surveys of landslides. Furthermore,measurements of the electromagnetic properties of soil
mixed with snow would help estimate the amount of snowburied in the landslide sediment.

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