An aeromagnetic survey of Shinmoe-dake volcano, Kirishima, Japan, after the 2011 eruption using an unmanned autonomous helicopter
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 9 November 2012
Accepted: 9 March 2013
Published: 8 July 2013
In January 2011, magmatic eruptions at Shinmoe-dake, Kirishima, Japan, started for the first time in 300 years. After three sub-plinian explosive events, lava accumulation started at the summit crater. The accumulation lasted for about three months, and the final volume of the accumulated lava reached 1.5 × 107 m3. We conducted aeromagnetic surveys using an unmanned autonomous helicopter over the Shinmoe-dake summit crater in late May 2011, and late October to early November, 2011, in order to investigate the magnetization of this area and changes in the magnetic field associated with the 2011 eruption. The averaged magnetization intensity around Shinmoe-dake is 1.5 A/m. A demagnetized area elongated in the north to northwest direction from Shinmoe-dake has been detected. We also detected a clear change in the total magnetic field between the two aeromagnetic observations characterized by positive and negative changes in the south and north, respectively, of the Shinmoe-dake edifice. These changes are well reproduced by a model in which 20–70% of lava accumulated in the summit crater was cooled down below the Curie temperature and has been magnetized.
In January 2011, the Shinmoe-dake volcano, Kirishima, Japan, began its first magmatic eruption in 300 years. During the intense eruptive activity including three sub-plinian explosive events, lava accumulation occurred at the summit crater. During the lava accumulation for three months, the main phase lasted for about a month. The lava feeding rate was high in the first three days. It was estimated to be 8 × 106 m3/day, and the final volume of the accumulated lava reached 1.5 × 107 m3 by the end of April (Nakada et al., 2013).
In Japan, when an eruption starts at a volcano, there are restrictions on approaching the active vent. For example, at the Sakura-jima volcano, one of the most active volcanoes in Japan, no one is allowed to approach closer than two kilometers from the summit craters. The restriction is imposed not only for ground-based approaches, but also for manned aircraft. This restriction causes data gaps for almost all types of geophysical measurements, including aeromagnetic surveys (e.g., Okubo et al., 2009). Observations near the active summit area is also strictly restricted for Shinmoe-dake.
Before the 2011 eruption, the Earthquake Research Institute, University of Tokyo, had operated five magnetometers around the summit area of Shinmoe-dake. Approaching within three kilometers of the summit of Shinmoe-dake has been prohibited since the beginning of the eruption in January 2011. By the time of the largest eruptions in late January and early February, 2011, four of five magnetometers were lost, and it was impossible to fix or to replace them because of the restriction. Thus, we needed to construct a substitute geomagnetic observation system in order to monitor the volcanic activity of Shinmoe-dake.
We needed an observation method in which aeromagnetic surveys could be conducted in the summit area without risk to human lives. For this purpose, we used an aeromag-netic survey system based on an unmanned autonomous helicopter. By using this system, we were able to conduct aeromagnetic surveys above the summit crater in late May 2011, and late October through early November, 2011, in order to investigate the magnetization of Shinmoe-dake, and the surrounding area, and changes associated with the 2011 eruption.
An aeromagnetic survey targeting the entire Kyushu area was conducted in 1981 (Okubo et al., 1985), in which the flight path spacing was 3 km and thus the resolution around Shinmoe-dake was insufficient for our purposes. In that sense, our measurement is the first attempt at a highresolution aeromagnetic survey focusing on Shinmoe-dake. In this paper, we introduce the aeromagnetic survey system using an unmanned autonomous helicopter, following a brief description of the 2011 eruption of the Shinmoe-dake volcano, Kirishima, in Kyushu, Japan. After describing the analysis method and its results, we interpret the obtained magnetization distribution and the change in magnetic field intensity.
2. Unmanned Aerial Vehicle (UAV) for Aeromag-netic Surveys at Active Volcanoes
2.1 Unmanned autonomous helicopter used for volcano observations
The advantages of an aeromagnetic survey using an unmanned helicopter is not limited to being risk-free for human lives. There are two other major advantages: one is that an unmanned helicopter can fly at a much lower altitude. Due to the civil aeronautics act in Japan, the lowest flight altitude allowed for manned helicopters is 150 m, while there is no altitude limitation for our unmanned helicopter because it weighs less than 100 kg. Since the magnetic field rapidly attenuates as one over the cube of the source-receiver distance, being able to fly at low altitudes is a big advantage for obtaining data with a high signal-to-noise ratio. Another advantage is that the flight path is far more accurate. By conducting aeromagnetic surveys several times using exactly the same path, small changes in the magnetic field, and, thus, small changes in the temperature inside volcanoes, can be detected without being hidden by the noise due to inaccurate flight paths.
2.2 Cesium magnetometer
A portable Cesium optical-pumping magnetometer G-858 (MagMapper) from Geometrics, Inc., USA, is used in this system. This instrument consists of a sensor and a logging console, which are connected with a 4.5-m-long cable. For weight saving, the power is supplied from the main body of the helicopter via a DC/DC converter. To avoid magnetic influence from the main body of the helicopter itself, the sensor needed to be placed at least three meters from the helicopter in order to obtain a noise level of less than 10 nT (Kaneko et al., 2011). We adopted a bird-type installation, where a 4.5-m-long rope is suspended from the helicopter with a sensor attached at the end. The sensor was placed in a plastic cone in order to insulate it from possible impact at the time of landing (Kaneko et al., 2011).
2.3 Previous use of unmanned helicopters in aeromag-netic surveys
As a first application to aeromagnetic surveys of an unmanned helicopter with a cesium magnetometer, we carried out high resolution aeromagnetic measurements at Izu-Oshima, Japan, which revealed a detailed shallow magmatic structure inside the summit caldera (Kaneko et al., 2010, 2011). This system was also used for surveys of the northern part of Sakurajima in 2010 and 2011. The latest application of this system was at the Tarumae volcano, Hokkaido, Japan (Hashimoto et al., 2012).
3. Aeromagnetic Measurement at the Shinmoe-dake Volcano
3.1 Outline of the Shinmoe-dake volcano
Shinmoe-dake is one of more than 20 Quaternary eruptive craters, which constitute the Kirishima volcano in southern Kyushu, Japan. Shinmoe-dake is 18,000 years old and its edifice is andesitic-dacitic. In 1716, a large amount of pumice and lithic fragments were thrown from Shinmoe-dake and formed a thick layer at the eastern foot. In 1959, a pheriatic explosion left a 500-m-long fissure, passing the summit crater and extending to its western flank. Shinmoe-dake had a crater lake. According to electromagnetic surveys, hydrothermal water flow travels in a southwesterly direction from the summit crater of Shinmoe-dake (Kagiyama et al., 1996).
In January 2011, magmatic eruptions at Shinmoe-dake, Kirishima, Japan, began. It was the first magmatic eruption in 300 years. During the intense eruptive activity, including three sub-plinian explosive events, lava accumulation started at the summit crater. The lava accumulation lasted for about three months. The major phase of lava accumulation was the first month. The lava feeding rate was especially high in the first three days. It is estimated to have been 8 × 106 m3/day, and the final volume of the accumulated lava reached 1.5 × 107 m3 (Nakada et al., 2013).
3.2 Aeromagnetic measurements in May 2011
3.3 Aeromagnetic measurement in October and November, 2011
On October 31, and November 8, 2011, we again conducted an aeromagnetic survey over Shinmoe-dake. The flight conditions were almost identical to those of the May aeromagnetic survey. We again measured the geomagnetic total intensity at 10 Hz, using the same magnetometer. Although the flight path was not exactly the same as for the May survey due to the difference in weather conditions, the area covered during the October-November survey is the same as in May (Fig. 3, bottom). We slightly modified the flight path so that the areas covered by coarse intervals due to bad weather conditions in May had denser flight intervals.
4. Data Processing
By using the magnetic total intensity data obtained by the autonomous unmanned helicopter surveys, the rock magnetization intensity around the Shinmoe-dake edifice was estimated following the three steps explained below.
4.1 Step 1: Extraction of the magnetic field originating from the magnetized rock
4.2 Step 2: Estimation of the averaged magnetization intensity around Shinmoe-dake
By using the anomalous magnetic intensity dF R , the rock magnetization underground can be estimated. As a second step, the averaged magnetization over the whole survey area is estimated under the approximation that the orientation of the rock magnetization is parallel to the direction of the main field, and also that the magnetization is uniform vertically from the ground surface down to 6 km below sea level (BSL), which is slightly shallower than the Curie point depth (Okubo et al., 1985). For the surface topography, we use the 10-m-mesh DEM data published by the Geographical Survey Institute Japan. The whole area is divided into elemental magnetization prisms extending in the vertical direction. Each prism has a horizontal size of 10 m by 10 m and extends from the ground surface to 6000 m BSL. We then calculate the contribution of each prism to the total magnetic intensity using the analytical expression below. By summing up the contributions of all prisms, we are able to calculate the total magnetic anomaly. Finally, the averaged magnetization is calculated using the least squares method.
4.3 Step 3: Estimate of the horizontal distribution of the magnetization intensity
In general, calculation of an inverse matrix is a difficult method for solving an inverse problem numerically. To obtain just a single solution m, an iterative method, such as, say, a conjugate gradient (CG) method, is definitely faster and thus more useful. That is why conventional studies have commonly used the CG method (e.g., Nakatsuka, 1995; Ueda, 2007). In this study, however, we use a direct method, the singular value decomposition (SVD). The reason why we chose SVD rather than CG is not only that the size of the problem was feasible for our computational environment, but also that the diagonal elements of the resolution matrix could be easily obtained by the SVD method (e.g., Menke, 1989).
The use of horizontally larger prisms with several depth intervals may be more realistic. However, there is a large ambiguity in inverting the potential field to obtain the magnetic anomaly distribution. The ambiguity is especially large in the vertical direction. We note that the horizontal anomaly pattern does not change significantly as a function of prism height, only the bulk magnetization value of each prism does. We therefore decided to focus on elucidating the horizontal distribution of the magnetic anomaly, by using prisms that were small in the horizontal directions.
5. Results and Discussion
5.1 Averaged magnetization intensity and the magma supply path
As explained in Subsection 4.2, the estimated averaged magnetization intensity of Shinmoe-dake is about 1.5 A/m. The magnetization intensity reflects the content of magnetic minerals in the rock. In general, mafic rocks show a higher intensity, and silicic rocks show a lower intensity. The average magnetization of the dacitic Unzen volcano in Kyushu, Japan, is 2.9–3.1 A/m (Nakatsuka, 1994; Okubo et al., 2005). In Izu-Oshima, a typical basaltic volcano located 100 km south of Tokyo, Japan, the average magnetization is 10.4–12.1 A/m (Ueda, 2007). The expected magnetization intensity at Shinmoe-dake would be between these two values (3–12 A/m), since (1) Shinmoe-dake is mainly covered with andesitic rock (Imura and Kobayashi, 2001), and (2) the SiO2 variation of the ash emitted in the 2011 eruption is 57–58 wt% (Suzuki et al., 2013b). This idea is also supported by laboratory measurements, showing that the magnetization values of andesite rock from Shinmoe-dake sampled before the 2011 eruption were between 2.3 and 10.0 A/m (Utada et al., 2000). Thus, the estimated value of 1.5 A/m is lower than expected based on rock properties alone.
Shinmoe-dake is predominantly built of hot andesitic lava, which cools in situ and acquires a magnetization parallel to the Earth’s field. As mentioned above, this seems to contradict the rather low magnetization. What processes can reduce the magnetization of Shinmoe-dake?
The basaltic Stromboli volcano in Italy has an average magnetization intensity of 2.2 A/m, which is lower than expected for basaltic rocks. It is commonly believed that the magnetic properties of the edifice of Stromboli are affected by past and ongoing volcanic activity (Okuma et al., 2009). Similarly, at Shinmoe-dake, volcanic activity is ongoing and its edifice is covered with relatively new volcanic ash. This may partially explain the low magnetic intensity there.
Could the explanation lie with reverse-magnetized lava flows embedded in normally magnetized ones? Shinmoe-dake was formed as little as 20,000 years ago, thus, there is no reverse-magnetized material there. Another possibility is a locally shallow Curie point depth due to a shallow heat source. Okubo et al. (1985) reported that the Curie point depth in the Kirishima region is 7–8 km below sea level. Although this depth is shallower than that of the surrounding area, it is highly unlikely that the average depth in the entire area is as shallow as 2 or 3 km. Such a shallow depth may be possible only in a small area.
A more plausible explanation of low magnetization would be post-eruption alteration. For example, low magnetization observed at Hawaii is partially explained by rock alteration (Hildenbrand et al., 1993). The resistivity structure beneath Shinmoe-dake is characterized by a shallow low-resistive layer of 2–10 Ω m with thickness of 1 km. This low resistivity layer is thought to be an aquifer (Kagiyama et al., 1996). Hydrothermal alteration would occur in such an aquifer. A reduced magnetization layer produced by the alteration of magnetic minerals (such as magnetite) to nonmagnetic minerals (such as hematite) may explain the observed low magnetization at Shinmoe-dake.
As shown in Fig. 4(a), the horizontal distribution of magnetization demonstrates that a weakly-magnetized zone extends from the edifice of Shinmoe-dake in both the north and the northwest directions. The location of the main magma reservoir that supplied the erupted magma to Shinmoe-dake is estimated from GPS observations 6 km northwest from the summit of Shinmoe-dake at a depth of 9 km (e.g. Imakiire and Oowaki 2011; Nakao et al. 2012). The GPS observations also suggest that there is another shallow magma reservoir at a depth of 3–5 km beneath the Shinmoe-dake summit (Imakiire and Oowaki 2011). The geometrical configuration of the two magma reservoirs suggests that there is a magma path that connects these two reservoirs. Since a weakly-magnetized area extends northwest from Shinmoe-dake along the inferred magma path, it is highly likely that this weakly-magnetized area is the result of heating from the magma transported from the main deep magma reservoir to the shallow magma reservoir beneath Shinmoe-dake.
5.2 Lava cooling model
The most significant difference in the total magnetic field between the aeromagnetic measurement in May and that in October-November is a combination of positive and negative anomalies on the southern and northern slopes, respectively. The intensity of the magnetic anomaly is ±100 nT. A combination of positive and negative anomalies in the north and south of a volcanic edifice is often observed in active volcanoes, and is interpreted as magnetization or demagnetization associated with the ascent or withdrawal of magma. For example, the Aso volcano in Japan often shows this type of anomaly associated with volcanic activity (e.g., Tanaka, 1993). The interpretation of this type of anomaly is as follows. When magma approaches the ground surface, the temperature of the volcanic edifice increases by hot volcanic fluid being exsolved from the magma or by the heat of the magma itself. The heated edifice is demagnetized, and the magnetic field due to the demagnetization manifests as a positive anomaly in the north and a negative anomaly in the south. When magma descends, an opposite anomaly is observed.
The spatial pattern of the negative-north and a positive-south anomalies observed at Shinmoe-dake suggests magnetization, or cooling of the volcanic edifice. Below, we examine whether the observed magnetization can be explained by the cooling of the extruded lava in the summit crater of Shinmoe-dake.
Rainfall above the contour line of the bottom of the crater may have also contributed to lava cooling in the crater. The elevation of the bottom of the summit crater before the eruption was 1239 m above sea level, and the diameter of the contour line of 1250 m was approximately 1500 m. The amount of rainfall water in this wider area is 4 times larger than the rainfall in the summit crater. In order for the rainfall outside the crater to help cool the lava in the crater, the permeability distribution near the top of the volcanic edifice must result in water falling around the crater being directed into the area where the lava was pooling. If this condition is satisfied, then more than 70% of the heat necessary to cool the lava can be explained.
We calculate the theoretical residual magnetic field assuming that all of the 1.5 × 107 m3 of lava in the summit crater acquires a certain value of magnetization by cooling. The lava is divided into elemental magnetization prisms, and the theoretical magnetic field is given by a sum of contributions from all the prisms. The best-fit synthetic residual magnetic field is obtained when we assume that the magnetization change in the crater is about 1.5 A/m (Fig. 5, right).
It is surprising that the magnetization of the cooled lava in the crater matches the averaged magnetization in this area. As mentioned in Subsection 5.1, the expected magnetization intensity of lava extruded from Shinmoe-dake is higher than this value. One possible explanation for this reduced magnetization is that the summit area is covered with thick volcanic ash, which mainly cools in the air and acquires randomly directed magnetization, and thus causes reduced magnetization. The amount of rainfall may not be enough to fully magnetize the lava in the crater. In this case, it turns out that the assumption that the all the lava in the crater was fully magnetized is not correct. This would be another explanation for the low magnetization intensity of 1.5 A/m. If 20 to 70% of the lava is magnetized, then the expected magnetic intensity is 2 to 7.5 A/m, which is reasonable for the andesitic-dacitic magma of Shinmoe-dake.
We have conducted aeromagnetic surveys at Shinmoe-dake twice in 2011, using an unmanned autonomous helicopter. The average magnetization intensity around Shinmoe-dake is 1.5 A/m, suggesting that the volcanic edifice is covered with relatively new volcanic material, or has lost magnetic minerals by post-eruption alteration. The magnetization model shows a weakly-magnetized area extending to the north and northwest of Shinmoe-dake. This area may correspond to the magma supply path from the deep magma chamber to Shinmoe-dake. The differences in the total magnetic field observed between two observations show positive and negative magnetic anomalies in the south and north of the Shinmoe-dake edifice, respectively. The spatial pattern of the anomaly suggests lava cooling in the summit crater. The calculated magnetic anomaly, assuming completely cooled lava in the summit crater between the two aeromagnetic surveys, explains the observed anomaly. But the magnetic intensity that best explains the observation seems rather low. Lava cooling of 20–70% by rainfall in and around the summit area can explain the majority of the observed anomaly.
We thank T. Morishita, T. Kubono, J. Imai and other members of the Sky Operations division of YamahaMotor Co., Ltd for the operation of RMAX-G1. Y. Honda and K. Kajiwara at the Center for Environmental Remote Sensing, Chiba University, gave us valuable advice for helicopter operations. We are grateful to A. Takagi and M. Ukawa for their funding assistance. We also thank the Japan Meteorological Agency and Kirishima-city, Kagoshima prefecture, for their support in field operations. Constructive comments by Dr. Tony Hurst and one anonymous reviewer helped significantly to improve the quality of the paper. This research is supported by the Programs of Special Coordination Funds for Promoting Science and Technology, and a Director’s grant from the Earthquake Research Institute, University of Tokyo.
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