Journal of Natural Disaster Science, Volume 38，Number 1，2017，pp85-104

Observations using an unmanned aerial vehicle in an area in danger of
volcanic eruptions at Kuchinoerabu-jima Volcano, southern Kyushu, Japan
Takao Ohminato*, Takayuki Kaneko*, Takao Koyama*, Atsushi Watanabe*, Wataru Kanda**,Takeshi Tameguri***, Ryunosuke Kazahaya***** Earthquake Research Institute, University of Tokyo** Tokyo Institute of Technology*** Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University**** National Institute of Advanced Industrial Science and Technology（Received: Sep. 28, 2016

Accepted: Feb. 22, 2017）

ABSTRACT
Kuchinoerabu-jima is a volcanic island in southern Kyushu, Japan. On August 3, 2014, a moderate summit
eruption occurred, destroying all the observation stations near the summit. By using an unmanned aerial
vehicle (UAV), a helicopter in our case, we installed four stations in the summit area in April 2015. We also
conducted multi-parameter observations including an aero-magnetic survey, visual and infrared observation,and gas measurements and sampling. A summit eruption occurred again on May 29, 2015. It was far larger
than the previous one and the entire island was evacuated. The seismometers installed in April were all
destroyed but they detected changes in seismic activity a few days before the eruption.
In September 2015, we installed five seismometers again. We also conducted multi-parameter
observations as in April. A comparison of the two observations in April and September shows a clear decline
in volcanic activity. Proximal data are sensitive to the volcanic activity, but difficult to acquire. UAV
observation clearly compensates for the lack of data near the summit of the volcano. Together with otherinformation, the UAV-acquired information contributed to reducing the alert level by the local government(Yakushima Town), and thus contributed to the evacuees being able to return in December 2015.
Keywords: unmanned helicopter, risk-free near summit observation, multi-parameter observations,
Kuchinoerabu-jima Volcano
1. INTRODUCTION
Kuchinoerabu-jima is an andesitic stratovolcano located 100 km south of the main island of Kyushu, Japan(Fig.1). An active vent is located atop Shindake, which is 700 m high. Fishing and sightseeing are the mainindustries on the island. All the residential areas are near the coastline (Fig.1). Major eruptions in the last
200 years have occurred at 30 to 90 year intervals (Geshi and Kobayashi, 2007). Some of these eruptions
have caused the loss of human life. The Disaster Prevention Research Institute, Kyoto University has beenmonitoring volcanic earthquakes since 1991. Other national agencies also operate observation stations inthe summit area, the hillside, and the foot of Shindake.
Since 1999, seismic activity had increased and upheaval and expansion of the edifice had been
detected by the Global Navigation Satellite System (GNSS) monitoring and leveling (Iguchi et al., 2007;
Triastuty et al., 2009). Anomalous increases in temperature and gas emissions have been observed as well
(Iguchi and Nakamichi, 2015; Iguchi et al., 2016). On August 3, 2014, a moderate eruption occurred at theShindake crater, and destroyed all the seismic, geodetic, and electro-magnetic stations around the summit
crater. After the eruption, Japan Meteorological Agency (JMA) raised the volcano alert level to Level 3 and85

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issued a notification restricting people from the area of 2 km from the summit crater.

Fig.1 Location of Kuchinoerabu-jima (inset). Red circles are seismic stations. Pink shade is thesummit crater of Shindake. Green squares and triangle are the position of the base stations in the April
2015 and September 2015 operations, respectively. Residential areas are shown in yellow.
In order to recover the lost summit stations, we conducted observations using an unmanned aerial
vehicle (UAV) from April 14 to 18, 2015. The observations include the installation of four seismic sensors
around the active summit crater, visual and infrared imaging, an aero-magnetic survey, and volcanic gas
measurements and sampling.
Forty-one days after the UAV operation, a moderate phreatomagmatic eruption occurred on May
29. The eruption column reached 9,000 m high and the pyroclastic flow reached some residential areas.
JMA raised the alert level from Level 3 to Level 5, the maximum warning level issued by JMA. The
restricted area was expanded to 3 km. An evacuation order by the local government (Yakushima Town) was
issued and all 140 people were evacuated from the island. All the summit stations installed in April werelost during this eruption.
In September, 2015, we conducted multi-parameter helicopter observations again. In this operation,
we installed five seismic stations and conducted the same multi-parameter observations by using UAV. A
comparison of the two observations in April and September showed a clear decline in volcanic activity.
On October 21, based on the various observation data including our data acquired by the helicopter
operation, JMA decided to reduce the restricted area from 3 km to 2 km, although the western portion was
kept at 2.5 km due to the danger of pyroclastic flow from a sudden eruption. The evacuation order was lifted
on December 25 and the evacuees returned to the island. The chronology of the Kuchinoerabu-jima eruption
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is summarized in Fig.2.

Fig.2 Chronology of the Kuchinoerabu-jima eruption. The red shaded areas on the three maps on theright show the restricted areas for each period.
Various types of unmanned volcano observation tools have been developed. One famous andpioneering development was the Dante project conducted by Carnegie Mellon University and others(Krotkov et al., 1994). In the project, walking robots targeting robotic exploration of lunar and planetary
terrains, were developed and tested at Mt. Erebus Volcano in Antarctica in 1993 and Mt. Spurr Volcano in
Alaska in 1994. In the latter test, the robot operated for five days and successfully retrieved a gas sample.
After the Dante project, wheel or caterpillar based unmanned ground vehicles targeting volcano
observations have been developed in many countries including Japan, Italy and others, but all of them arestill at the research and development stage.
Airborne approaches using UAVs are more promising. Various types of UAV approaches havebeen used in volcano observations recently. In particular, "drones," battery-powered multicopters, are oneof the most useful tools in this field. Drones have already been utilized for taking pictures or volcanic gas
measurements at active volcanoes (e.g., Mori et al., 2016). However, battery powered drones are inadequate
for the purpose of multi-parameter observations at Kuchinoerabu-jima. We used a gasoline powered
helicopter, instead.
In this paper, we describe the details of the helicopter observations at Kuchinoerabu-jima Volcano.
We also discuss the importance of the unmanned observation tools that make it possible to perform datacollection without humans having to enter dangerous disaster areas such as areas damaged by volcanic
eruptions.
2. RISK-FREE OBSERVATION TOOL
As a volcano observation tool, we adopted a commercial UAV, unmanned autonomous helicopter, model
RMAX-G1 (Photo 1), developed and manufactured by Yamaha-Motor Co., Ltd. The UAV system consistsof the main body of a helicopter (Photo 1(a)) and a base station (Photo 1 (b, c)) (Suzuki, 2005; Kaneko etal., 2011). The main body is 3.6 m long and weighs 84 kg including a full tank of fuel and the onboard

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camera system. The payload and the maximum flight time are approximately 10 kg and 90 minutes,
respectively, depending on the amount of fuel. The helicopter can fly autonomously along a previously
programmed path within meter accuracy using real-time kinematics differential GPS. The radius of action
for autonomous aviation is approximately 5 km from the base station. The base station, consisting of a setof antennas, GPS equipment and mobile PCs (Photo 1 (b, c)), is easy to move or set up by human hand.
During the flight operation, the helicopter must be visible from the base station so that wirelesscommunication between the helicopter and the base station using 0.8 GHz and 2.3 GHz bands is notinterrupted. The flight course is prepared beforehand and the helicopter flies along the track specified by
the course data transmitted from the base station. It is also possible to change the flight path by transmitting
new path information during the flight, or change the flight mode from automatic mode to manual modeanytime. The capabilities and specifications of the helicopter are summarized in Table 1.

Photo1 (a) Main body of the unmanned autonomous helicopter RMAX-G1. (b) Inside the car used forthe base station. (c) A car used for the base station. Wireless antennas are also seen. Photos were taken
at Kirishima in 2011.

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Table1 Capability and Specifications of the UAV.
The most important advantage of UAV observations is the capability of risk-free operation indangerous circumstances such as the summit areas of active volcanoes. UAV observations have two othermajor advantages over observations by manned helicopters. One is that unmanned helicopters can fly atmuch lower altitudes than manned helicopters. Due to the civil aeronautics act in Japan, the lowest flight
altitude allowed for manned helicopters is 150 m, except for takeoff and landing, while there is no lower
altitude limitation for unmanned helicopters because they are light weight (less than 100 kg). This low
altitude capability is important for various types of surveys, such as aero-magnetic surveys, visual andinfrared imagery, or installing sensors on the ground. Another advantage over manned helicopters is that theflight path is far more accurate than that of manned helicopters. This advantage makes it possible to conduct
a measurement at exactly the same location again and again. Repeatability is important for comparing results
taken at the same location with a certain time interval.
By using the UAV, we conducted various types of geophysical and geochemical surveys, including
a seismic survey, aero-magnetic survey, visual and infrared imagery, and volcanic gas sampling at
Kuchinoerabu-jima Volcano in April and September, 2015. Base stations at which the helicopter takes-offor landed were set 2.9 km and 4 km from the summit in April and September, respectively (Fig.1). In thefollowing sections, we explain these observations in detail.

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3. OBSERVATION RESULTS
3.1 Seismic observations
We designed seismometers or seismic observation modules suitable exclusively for installation by helicopter.
Details of the seismic observations at Kuchinoerabu-jima are documented by Ohminato et al., 2016. Here,
we summarize our seismic observations.
Photo 2 shows the seismic observation modules used in the observations. A module is powered by
6 W solar cells with 10 Ah nickel hydride batteries. The mainframe is an aluminum low-height tripod,
approximately 1 m-wide and 40 cm-high. The AD board is the same as that used in the data logger HKS9700 manufactured by Keisokugiken Co. Seismic signals are digitized at 100 Hz with 24-bit resolution.
Electronics are put in a water-proof box. We used vertical-component moving coil velocity sensors with
4.5-Hz corner frequency. The seismometer weighs only 5 kg so as to fit within the helicopter's payload. Datatransmission is via a commercial cellular phone network. Data transmission consumes most of the power.
In order to reduce the power consumption, data transmission is conducted every 10 minutes, except at thetime of module deployment.

Photo 2 Seismic sensor used in Kuchinoerabu-jima UAV observations. In April, 2015, four sensors
were installed. In September 2015, five sensors were installed.

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As explained below, modules are not necessarily deployed horizontally, and we decided to use thevertical component only. We needed a special way to attach the sensors to the module legs so that the sensor
orientation was nearly vertical. We used three vertical sensors for one seismic module and a special sensor
arrangement was adopted in order for at least one of the three vertical sensors to be approximately vertical.
Each sensor was attached to each leg with an inclination angle of 5° from vertical. Three sensors werearranged so that they were rotationally symmetrical around the vertical axis of the module at equal intervalsof 120°. Although the module was deployed with a certain inclination, at least one of the three sensors was
closer to the vertical direction due to the sensor arrangement. Details of how we tested the sensor
arrangement are explained in Ohminato et al., (2016).
Figure 3 shows how we installed seismometers from the UAV. A winch was attached at the bottomof the helicopter. At the end of the wire from the winch, a wire detachment device was attached. A shortwire with a ring at the tip was attached to the seismometer. The detachment device had a remotely controlled
pin. By hooking the ring at the tip of the seismometer wire to the pin, the seismometer was suspended fromthe detachment device. The helicopter flew from the base station to the target position suspending aseismometer (Photo 3). When the helicopter arrived above the target position, the seismometer was slowly
lowered by winch until the sensor module touched the ground. The wire becoming slack when the sensor
module touched the ground surface indicated that the module had landed with the onboard camera. During
the sensor installation, the UAV continued to hover above the target position. If the sensor inclination
became significantly large, by referring to the real-time image of the onboard camera, we attempted themodule landing again and again until the module was approximately horizontal. In addition to referring tothe onboard camera image, we monitored the seismic signals transmitted from the module in order to check
whether the sensor inclination was sufficiently small. Once the sensor installation was confirmed by the
onboard camera image and by the transmitted seismic signals, the wire that was suspending the seismometer
was detached from the wire detachment device by remotely retracting the pin. The wire of the winch waswound up and the UAV returned to the base station.

Fig.3 How to install seismic sensors by using the UAV.

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Photo 3 The UAV just after take-off, suspending a seismic observation module.
We installed four seismic modules on April 17. For event detection, we used the ratio between the
short time average (STA) and the long time average (LTA) of the seismic amplitude. When the STA/LTA
ratio exceeded 5 at more than three stations, the signal was regarded as a seismic event. Before being
destroyed by the May 29 eruption, the four-station seismic network detected unusual seismic activity at thesummit area. Figure 4 shows the daily number of earthquakes in May 2015. The eruption on May 29
occurred at 9:59 local time. The number of earthquakes started increasing on May 26, three days before theFig.4 Daily number of earthquakes in May 2015 detected by UAV-installed seismic network at thesummit area of Shindake.

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eruption, and peaked two days before the eruption. The eruption on May 29 occurred during a decline in thenumber of events. Just before the eruption, no significant change in seismic activity was observed. Figure 5
shows temporal changes in hypocenter locations before the May 29 eruption. Although the number ofearthquakes showed a significant change before the eruption (Fig.4), the hypocenter distribution change
before the eruption is not clear. The apparent gradual expansion in epicenter distribution was mainly due toan increase in the number of events that have unclear onset and thus have larger location errors.

Fig.5 Change in the hypocenter location from May 1 to May 29, 2015. Dotted lines show the locationof the center of the summit crater. Top: north-south position. Middle: east-west position. Bottom:
vertical position.

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T.OHMINATO, T KANEKO, T KOYAMA, A WATANABE, W KANDA, T TAMEGURI,KAZAHAYAThe characteristic temporal change in seismic activity before the eruption (Fig.4) was detected onlyby the seismic network installed by the UAV. Nearly 100 events, most of which were smaller than M0.0,
were localized from May 26 to May 29 by our summit network. On the other hand, only two earthquakes
were located near the summit area at the depth of around 1 km below sea level (Fig.26 of Japan
Meteorological Agency (2015)). This JMA report means that far stations that survived the eruption in
August 2014 were insufficiently sensitive to detect shallow seismic activity near the summit. The opposite
was the case with events that occurred far from the summit area. On the western side of Shindake, an intense
seismic activity followed the May 23 earthquake. This activity was well detected by far stations (Fig.26 ofJapan Meteorological Agency (2015)), while only a few events larger than M1.2 in this activity weredetected by the summit stations. These two observational facts indicate that the seismic network in thesummit area was sensitive to the seismic activity near the summit area and was less sensitive to the events
far from the summit, and vice versa for far stations. The sensitivity difference comes from the difference inthe source-receiver distance. The characteristic change in seismic activity shown in Fig.4, which might bea precursor to the eruption, was a phenomenon that could only be detected by near source stations. Thischange was not detected clearly by the distant stations operated at the foot of the edifice of the volcano.
Since all the stations installed in April were destroyed by the May 29 eruption, we re-installed fiveseismic stations on September 11. The recovered seismic network revealed that the summit seismic activity
before the May eruption and that after the station re-installation in September were significantly different.
We classified earthquakes into three types, volcano-tectonic (VT), low-frequency (LF), andmonochromatic following the event type definition of McNutt (1996). According to McNutt (1996), VT, LF
and monochromatic events have the following characteristics. VT earthquakes have a source mechanism
similar to the ordinal tectonic earthquakes and are believed to reflect the change in the stress field in the
volcanos. LF events are caused by various fluid activities in the volcanic edifice, such as magma movement,
hydro-thermal activity, or volcanic gas movement. Monochromatic events are often observed when magma
ascends close to the ground surface.
Figure 6 shows the numbers of various types of volcanic earthquakes, VT, LF, or monochromatic,from April 2015 to September 2016. Sharp increases just before the May 2015 eruption were seen in all
types of events (Fig.4). The activity after the second installation in September 2015 shows several
differences compared to the activity before the May 2015 eruption. The total number of events after
September 2015 was as low as that before the increase in late May, 2015 and a similar low activity level
lasted for about a year. April 2015 pre-eruption activity was dominated by LP events, while the seismic
surge in September 2015 mainly came from VT activity. VT activity shows two short-lived increases in
September 2015 and May to June in 2016. The timing of the LF event increase was different from that of
VT. Monochromatic events activity was low except for in the period before the May 2015 eruption.
3.2. Aeromagnetic survey
Aeromagnetic surveys of an unmanned helicopter have been conducted at various volcanoes in recent years.
Examples are those conducted at Izu-Oshima (Kaneko et al., 2011), Tarumae Volcano, Hokkaido(Hashimoto et al., 2012), and Kirishima Volcano (Koyama et al., 2013). In all of these surveys, a portable
Cesium optical-pumping magnetometer G-858 (MagMapper) from Geometrics, Inc., USA was used.
Technical details on an aeromagnetic survey by UAV are well documented by Kaneko et al., (2011).
We conducted two aeromagnetic surveys, one in April 2015 and one in September 2015, using thesame instruments and techniques as for the previous observations mentioned above. In Fig.7, the top andbottom panels show the flight path and magnetic field intensity along the flight path in April and September,respectively. Flight altitudes were 100 to 150 m above the ground surface along the topography. Thus, they
are draped surveys. Path intervals in the horizontal direction were approximately 100 m. The total flightpath lengths were 80 km and 60 km in the April and September operations, respectively. In the April
operation, two base stations were used to cover both eastern and western sides of the edifice of Shindake.
In the September operation, due to the danger of sudden eruptions, it was impossible to use the two basestations used in the April operation. Instead, we had to use only one base station near the top of Banyagamine hill located 4 km west of Shindake. This is why the flight path coverage in September was smaller in94

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area on the eastern side of Shindake than that in April.

Fig.6 Daily number of earthquakes from April, 2015 to September 2016. From top to bottom, total
number of events, VT events, LF events, and monochromatic events.

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Fig.7 Flight paths of aero-magnetic survey in April (top) and September (bottom). Colors indicate themagnetic field intensity along the path.
By using the magnetic total intensity data, the rock magnetization intensity around the summit
area of Shindake was estimated following the three steps explained in Koyama et al., (2013). The first stepis extraction of the magnetic rock contribution from the observed magnetic field by removing the global
geomagnetic reference field and the locally obtained reference geomagnetic field from the observed
magnetic field. The second step is to estimate the averaged magnetization intensity of the Shindake volcanic
edifice. The irregular topography of the edifice is expressed as the sum of elementary magnetization prisms
10 m by 10 m horizontally and extending vertically from the ground surface to 650 m below sea level. Eachprism is uniformly magnetized in the identical orientation of the geomagnetic main field. For the surface

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topography, 10 m mesh DEM data published by the Geospatial Information Authority of Japan is used.
Averaged magnetization intensity is calculated by minimizing the difference between the observed magnetic
field and the magnetic field composed of contributions of all uniformly magnetized elementary prisms. The
averaged magnetization intensity obtained in this step is 1.8 A/m. The third step is to estimate the horizontal
distribution of the magnetization intensity assuming that each prism is magnetized uniformly in the verticaldirection but horizontal magnetization differs from prism to prism. Horizontal distribution of magnetization
is determined so as to minimize the data misfit and model roughness.
Next, we compared the total magnetic fields obtained in April and September. The results of thecomparison of two aeromagnetic surveys in April and September, 2015 are shown in Fig.8. The left panel
shows the observed change in the magnetic field between the April and September operations. The rightpanel shows the computed change in the magnetic field that best explains the observed changes.

Fig.8 Comparison between observed (left) and theoretical (right) values of the changes in geomagnetic
intensity during the six-month period from April to September, 2015. Observed changes are well
reproduced by the theoretical calculations.
The best model is obtained by inverting the differential magnetic field between the April
observation and the September observation. Assuming that the subsurface magnetization anomaly
corresponding to the differential magnetic field is distributed at a very shallow depth, we express theanomaly by elementary magnetization prisms of 10 m×10 m×10 m, or 10 m cubes. There is no assumption
in the horizontal distribution of magnetic anomalies. The differential magnetization intensity with respect
to the averaged magnetization intensity of 1.8 A/m of each cube is inverted. It is assumed that eachelementary cube acquires differential magnetic intensity when its temperature is below the Curie
temperature. The result of the inversion shows that the magnetization volume that is obtained by the volume
integration of the magnetization intensity of all elementary cubes is 5×105 m3.
The erupted mass during the May 29, 2015 eruption is estimated as 6.6×105 – 1.1×106 ton(Meteorological Research Institute, 2015), which corresponds to the volume range of 2.6 - 4.4×105 m3.
Since not all the mass contained in the volcanic plume falls near the summit area, the volume of volcanic
deposits just around the summit crater would be smaller than this volume. If we assume that the volume ofcooled deposits around the summit crater is 4×105 m3, which would be the larger side of the estimation
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range, we would need an additional magnetized volume of 105 m3 to explain the estimated magnetization
volume. A demagnetized volume of 105 m3 or more would distribute at the very shallow part of the Shindakesummit area.
The result of the aeromagnetic survey indicates that cooling in the shallow part of the volcanic
edifice progressed in six months.
3.3. Visual and infrared imagery
Visual and infrared images were obtained during the two UAV observations in April and September 2015.
We used infrared camera FLIR/SC 620. Infrared images are affected by various factors. For example, when
the ground is masked by drifting volcanic gas, the measured temperature is affected by gas concentration,
gas temperature, gas composition etc. and thus would be different from the actual ground temperature. The
topography of the volcanic edifice is rugged and thus a point seen from one direction in the air is notnecessarily seen from other directions. Thus, the temperature indicated in one image is not necessarily thesame as that in another image including the same point but taken from another direction. The discussions
below on temperature changes are based on such limited temperature information.
Four distinct "hot spots" were identified in the April infrared images. These locations are shownby red open circles in the map at the top of Fig.9. Spot A is a congregation of intense fumarolic vents on thewestern slope of Shindake. Spot B is also a congregation of vents and is located at the bottom of a 100-m
wide concave topography on the western slope of Shindake. Spot B is closer to the summit crater than Spot
A. Spot C corresponds to a congregation of active fumaroles on the western wall inside the summit crater.
Spot D is at the southern bottom of the summit crater but its details are not clear because Spot D was covered
with dense fumarolic gas during the measurements. The measured temperature at Spot D can beunderestimated due to the coverage of dense fumarolic gases.
Regarding Spots A and B, a comparison of visual images clearly indicates that the surface
fumarolic activities in September significantly declined compared with the activities in April. Spots C andD showed a similar decline in fumarolic activity. Although fumarolic gas was ubiquitous in the summit
crater during the two observations, the intensity of gas emissions from fumaroles in Spots C and D seemed
to be weaker in September than in April. From these comparisons of visual images, we can say that thesurface fumarolic activity significantly declined during the five-month interval between the two
observations.
On the other hand, the maximum temperature changes at the four spots are not straightforward. As
shown in Fig.9, changes in the maximum temperature at the four spots were different from one another. The
temperature at Spot A was almost unchanged at 247˚C in April and 241˚C in September. Spot B showed aclear temperature drop from 379˚C to 71˚C. Spot C also showed a clear temperature decline from 314˚C to
189˚C, but this change may have been affected by the fumarolic gas that covers the spot. Spot D showed aslight rise from 250˚C to 285˚C, but this rise may be due to fumarolic gas in the crater.
The heat source of the hot spots would be high temperature volcanic gas. The observed gas
temperature near the surface was lower than the temperature of the magma from which gases were released.
There are several factors that cause gas temperature to decline. Gas temperature decreases as a result of heatloss during the gas ascent and the decompression of water vapor, which occupies the majority of volcanic
gas content.
The original temperature of volcanic gas released from magma would be similar in April and
September. In contrast to this, the visual observations showed that the amounts of gas in April and September
were significantly different. Complicated temperature changes observed at four spots would originate fromthe difference in gas ascending conditions from the gas source to the surface through the volcanic edifice.
Changes in magma head depth were also part of the conditions. Deep investigation of these conditions isbeyond the scope of this paper. Here, we only summarize the conditions that each spot has to satisfy.
At Spot A, it seems that the gas temperature decline occurred by a mechanism that is not strongly
affected by the change in the gas flow rate. On the other hand, at Spots B and C, the gas temperature decline
mechanism was strongly affected by the change in the gas flow rate.

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Fig.9 Changes in fumarolic activity at four hot spots at the summit area. Top: Locations of four hot
spots A, B, C, and D (red open circles). Left: visual images of hot spots taken in April 18, 2015.
Highest temperature Right: visual images of hot spots taken in September 11, 2015.

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T.OHMINATO, T KANEKO, T KOYAMA, A WATANABE, W KANDA, T TAMEGURI,KAZAHAYAAs for Spot D, considering its location at the bottom of the summit crater, it seems natural tosuppose that this spot would have been connected to the gas source more directly than other spots, andtherefore its temperature should be higher than that of other spots. However, in practice, the temperature atSpot D was not exceptionally high compared to that of other spots, and the direction of temperature change
was different from that of other spots. Spot D was always covered by fumarolic gas and thus the measured
temperatures would have been affected by the gas. If we could have measured the actual temperature at SpotD, we suppose it would have been much higher than that of other spots. We also suppose that the actual
temperature at Spot D would have dropped significantly between April and September.
The visual images showed a clear decline in surface fumarolic activities. Complicated temperaturechanges could be interpreted as the result of the complicated gas-path structure and the decline in volcanic
gas emissions.
3.4. Multi-gas measurements and sampling
We also conducted volcanic gas measurements and sampling in April and September in 2015. The volcanic
gas composition is controlled by magma degassing processes and conditions. It is important to measure andsample gases before they are contaminated by the surrounding atmosphere. Thus, we need to conduct gas
measurements and sampling as close to the source as possible. An unmanned helicopter makes it possible
to fly into the volcanic plume and to measure and sample dense volcanic gas near the source.
Photo 4 shows how the gas measurement and sampling instruments were installed to the UAV during
the operation. A multi-gas instrument (Shinohara, 2005; Aiuppa et al., 2005; Shinohara et al., 2011, 2015)
was attached under the helicopter body. The instrument included a CO2-H2O analyzer, SO2 and H2Selectrochemical sensors, and an H2 semi-conductor sensor for the flying measurements. Alkaline-filter packtechniques (Shinohara and Witter, 2005) were also included. S, Cl and F were collected by two filter packs
and analyzed in the laboratory. A gas intake pipe was set so that the intake point was positioned above thegas exhaust pipe of the helicopter. Owing to the helicopter's downwash, this pipe positioning makes it
possible to avoid contamination from the helicopter's exhaust-gas.

Photo4 The UAV with multi-gas sampler at the bottom of the body.

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From the gas data, changes in the gas composition and the reduction in apparent equilibrium
temperature between the two observations were obtained. The apparent equilibrium temperature (AET)
corresponds to the reaction SO2 + 3H2 = H2S + 2H2O. The obtained AETs were higher than the surface
temperatures obtained by the infrared measurements mentioned in Section 3.3. High estimates of AET
indicated that the measured gases were derived from a high-temperature source, and were cooled during thegas ascent to the surface. AET in September was lower than AET in April but it was still high, suggestingthat a high-temperature gas source still existed somewhere in the volcanic edifice. In future, we will perform
detailed analyses of the gas data and interpret the cause of the high AETs and their changes.
4. DISCUSSION
4.1. UAVs for near summit observations
In volcanic observations, it is important to conduct observations near the summit area because, in
general, geophysical and geochemical signals decay rapidly with distance from the source. It is also
important to conduct observations as soon as possible after volcanic eruptions because volcanic ash deposits
are easily washed away by precipitation, and also because various observation parameters often decay
rapidly over time. A lack of information on volcanic activity makes it difficult for those who are responsiblefor deciding on an evacuation plan. In many cases, however, observation in the proximity of the summit
area is not easy. The main reason is that, approaching the summit area before, during, and after an eruption
is dangerous because of the risk of sudden further eruptions.
In order to overcome the problem of observation in close proximity to the summit area, various
types of unmanned volcano observation tools have been developed. As explained in the introduction of thispaper, ground-based approaches are still at a research and development stage but airborne approaches areadvancing to the stage of practical use.
Drones have already been used in various situations in volcano observations. In particular, among
the several types of drones, battery-powered drones are one of the most popular airborne approaches andare being adopted for observations at volcanoes worldwide. Battery-powered drones have the big advantage
of being easy to operate. However, they are not perfect due to their limited payload and flight time. Thereis a trade-off between payload and flight-time. For example, the seismic sensor installation at Kuchinoerabujima requires 10 kg of payload and 40 to 50 minutes flight time. A UAV for this purpose must be able tohover during the sensor deployment. Real-time image transmission at long distance is also essential. No
commercially available battery-powered drone can satisfy all these conditions. Only a gasoline-engine basedhelicopter, such as the one used in our research, satisfies the requirements.
Of course, a gasoline-engine helicopter has several disadvantages as well. Gasoline-engine
helicopters are expensive to operate and trained experts are required. Thus, it is difficult to put this type of
UAV into operation in active volcanic areas immediately after an eruption. In the case of Kuchinoerabujima, two operations in April and September both required three to four months of preparation. Because ofthis long preparation period, we were able to measure volcanic gases only twice during the six-month period,
which is far less frequent than by SO2 monitoring using another method (Mori et al., 2017).
We need to use various types of UAVs appropriately according to various situations. For example,
battery-powered drones can be used soon after the eruption for visual observation and for small payload or
short flight-time observations, including gas-sampling. Gasoline-engine helicopters are adequate for large
payload operations such as installing sensors, sampling rocks, or long distance and long-flight time
operations, such as aero-magnetic surveys.
4.2. Lessons learned from UAV operations at Kuchinoerabu-jima
In the case of Kuchinoerabu-jima, our helicopter approach was the only way to reconstruct the seismic
network and to conduct multi-parameter observations near the summit. The proximal data obtained by the
UAV, such as the seismic, magnetic, visual, and chemical data, cannot be obtained by other approaches and101

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thus are all of high importance. Seismic data from the summit network were sent to the Disaster Prevention
Research Institute, Kyoto University in almost real time. Other information based on the UAV data waspromptly presented to the Coordinating Committee for Prediction of Volcanic Eruptions.
However, not all the data were considered to be decisive information. It only partially contributed
to the local government (Yakushima Town) decision-making process. One reason for this is the low
frequency of the data. As mentioned above, SO2 measurements by ferry or fishing boat were far more
frequent than the UAV measurements, which were conducted only twice in five months, and thus, the former
were regarded as more important.
Seismic data were almost continuous except for the 4-month data gap between the May eruption
and the September reinstallation, and there was no alternative data. However, as with other UAV data, theseismic data did not become decisive. Perhaps this was because the experts responsible for advising the
local government desired geodetic and gas information more than seismic activity information. The geodetic
information reflects the magma movement in the volcanic edifice. The gas information shows how close to the groundsurface the magma head is in the volcanic conduit. Compared with these types of information, an interpretation ofseismic information is far more difficult. And both geodetic and gas information were obtained far more frequently
than our UAV data.

Although the UAV data were not decisive in the case of Kuchinoerabu-jima, we still think thesedata were important. At the time of the second UAV observation in September 2015, the decline in volcanic
activity had already been detected by non-UAV observations, such as SO2 monitoring by ferry, geodetic
observation, visual survey by airplane from long distance, and seismic data obtained at stations far from thesummit area. But these non-UAV data were insufficient for decision makers to make the final decision. The
UAV data alone were not decisive but without them, decision making would have been delayed. For thisreason, we believe that the UAV data played an important role in convincing those who are responsible fordecision making about the decline in volcanic activity.
There are several ways to make the UAV approach more useful and more effective. One way would
be to develop GNSS modules that are installable by UAV. GNSS modules are at the development stage
today (Ohminato et al., 2012), and the future UAV operation would include GNSS observations. The othertask would be to reduce the preparation period before operations. This is far more difficult because it requires
more helicopters and operators. What we can do is to show the importance and effectiveness of UAV
observations, continuously and tenaciously, to those who are responsible for decision making during a
volcanic crisis.
5. CONCLUSIONS
The following is a summary of the UAV observations at Kuchinoerabu-jima Volcano and a perspective on
UAV observations at active volcanoes.
(1) The damaged seismic network at the summit area of Shindake by the August 2014 eruption was reestablished by the UAV operation without risking human life. The recovered seismic network detectedchanges in seismic activity at the summit area just before the May 2015 eruption. The seismic network reinstalled in September 2015 confirmed the recent low seismic activity at the summit area.
(2) Cooling of the volcanic edifice and a decline in volcanic activity were also confirmed by aero-magnetic
survey, and infrared and visual imagery.
(3) We believe that the information obtained by the UAV played an important role in convincing volcano
experts about the decline in volcanic activity, which had already been indicated by other information suchas gas monitoring and geodetic observations. In this sense, UAV data contributed to the return of evacuees
to Kuchinoerabu-jima.
(4) Various types of UAV are now used in volcano observations. Each of them has advantages anddisadvantages. It is desirable to use them properly and appropriately according to the situation of the target
volcano.

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ACKNOWLEDGEMENT
This research was supported by a grant for collaborative research from DPRI, Kyoto University (No. 27G6). It is also supported by JSPS KAKENHI Grant 26287103, MEXT KAKENHI Grant Number 15H05794
and by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its
Earthquake and Volcano Hazards Observation and Research Program.
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