Tadashi Nakatsuka

Aeromagnetic constraints on the subsurface structure of Usu Volcano, Hokkaido, Japan

Usu Volcano, Hokkaido, Japan consists mainly of dacitic volcanic rocks underlain by basaltic somma lava and Pliocene–Pleistocene andesitic volcanic rocks, and erupts every 20–30 years. The most recent eruption, in 2000, was the first since 1978. We conducted a helicopter-borne high-resolution aeromagnetic survey almost three months after the start of this eruption. We calculated magnetic anomalies on a smoothed observation surface using a reduction method, assuming equivalent anomalies below the actual observation surface. We conducted three-dimensional (3D) imaging of magnetic anomalies to constrain the subsurface structure. Our model indicates that there are magnetisation highs in the main edifice of Usu Volcano, which may reflect the subsurface distribution of the Usu somma lava. Meanwhile, magnetisation lows lie north-west of the Nishi-Yama Craters Area and on Higashi-Maruyama Cryptodome, where nearby Pliocene and Pleistocene volcanic rocks, respectively, are found. The reverse magnetisation observed at outcrops close to these sites could explain the magnetisation lows. Although the survey improved our understanding of the surface and subsurface distribution of volcanic rocks in the edifice and basement of Usu Volcano, some limitations remain. No information about the magmas intruded during the recent eruptions in 1977–1978 and 2000 was obtained by the survey, though some of these intrusions were revealed by other geophysical data. The small magnetic contrast between the intruded magmas and their host rocks is the most probable reason. Perhaps the intruded magmas (in particular, those of the most recent eruption) had not cooled enough to become strongly magnetised by the time the survey was conducted.

Aeromagnetic 3D subsurface imaging with effective source volume minimisation and its application to data from the Otoge cauldron, Shitara, Central Japan

Three-dimensional (3D) imaging techniques using the conjugate gradient solution are discussed for magnetic anomaly source reconstruction, especially for areas of rugged terrain, such as those of volcanoes. The analysis model configuration permits surface undulation and variable depth slicing. First, primitive source model data are put into the analysis to confirm the characteristic behaviour of the regularisation methods and parameter scaling. The compact regularisation method is then applied to synthetic geologic models to understand the resolving power of the method given the problem of the inherent weakness of a non-unique solution. Finally, the field data of a helicopter-borne magnetic survey of the Otoge cauldron are put into the 3D imaging analysis with the method. The analysis reveals a quite reasonable subsurface image of the magma reservoir and the Otoge cone sheets and Otoge stocks of post-cauldron activity, as inferred from geological studies.

Aeromagnetic constraints on the subsurface structure of the Unzen Graben, Kyushu, Japan

Aeromagnetic analyses have been conducted in and around the Unzen Volcano, Kyushu, Japan, in order to reveal the subsurface structure of the Unzen graben. First, we applied a magnetization intensity mapping method to analyze the aeromagnetic anomalies of the central part of the Shimabara peninsula. Magnetization highs and lows correspond to the Older Unzen (0.15–0.5 Ma) and the Younger Unzen (<0.15 Ma), respectively. However, the Mayu-Yama volcano is exceptionally high in the Younger Unzen. Moreover, it turns out that the Pre-Unzen (>0.5 Ma) or localized hydrothermally altered areas show magnetization lows. Next, magnetic models were constructed from aeromagnetic anomalies, drilling data and the result of magnetization intensity mapping. Finally, similar to the results of other geophysical data, it turns out that the Unzen graben has the features of a half-graben, with the northern fault (the Chijiwa fault) down in the western Unzen region and the southern fault (the Futsu and Fukae fault) down in the eastern Unzen region. Moreover, it clarified that the layers of low magnetization extend to the near-surface beneath Shimo-Dake, Kami-Dake, and the Unzen hot spring. These layers of low magnetization reflect the fractured or hydrothermally altered zones caused by the upflow of geothermal convection that exists in the central part in the graben.

Discussion: On ‘Magnetization structure of Aogashima Island using vector magnetic anomalies obtained by a helicopter-borne magnetometer’ (Isezaki, N., and J. Matsuo, 2009, Exploration Geophysics, 40, 17–26; Butsuri-Tansa, 62, 17–26; Mulli-Tamsa, 12, 17–26).

In ‘Magnetization structure of Aogashima Island using vector magnetic anomalies obtained by a helicopter-borne magnetometer’, Isezaki and Matsuo discussed a source of inaccuracy in the inversion of total intensity magnetic anomalies, and presented an example of a helicopter threecomponent magnetic survey and its inversion. In their paper, I consider there to be some problems and points to be discussed. Here I refer to the figures in their paper by the original figure numbers, and use the label D-1 for the new figure in this discussion. Isezaki and Matsuo described the algebra of the total intensity anomaly (TIA), and discussed the error introduced by the assumption that TIA is approximated by the projected total intensity anomaly (PTA). Their figures 3 and 4 are correct although the description of the horizontal axis of figure 4 could be clearer. It is obvious that the relative error eT/TIA ( = (TIA PTA)/ TIA) becomes infinitewhereTIA = 0.However, the situation is not as severe as their assertion implies, because such a large error occurs only in a quite limited region nearPTA = 0, but the result of analyses of magnetic anomalies is commonlymore dependent on the locations andvaluesof thehighand lowanomalypeaks,where the approximation above is fully valid. To illustrate this, I show a simple synthetic model. Figure D-1 shows an example of a magnetic anomaly caused by a point dipole source (equivalent to uniformly magnetized sphere) as observed on a horizontal plane. The source dipole, with amagneticmoment of 8 10Am (8 (A/m) 1 km) in the direction of 45 inclination and 7 declination, parallel to the ambientmagneticfield direction, is situated at the centre at a depth of 1000m below the observation plane. The left panel shows the PTA anomaly (~1400 nT p-p) at the contour interval of 50 nT, while the thin contours in the right panel are for the TIA anomaly. Itwill be almost impossible to discriminate between the two in the illustration, but there is an actual difference as shown by the thick broken contour lines (at a contour interval of 2 nT) on the right panel. The peak value of the difference eT is ~14 nT, although the same differencemight arise froma horizontal position inaccuracy of 6m. If we use this TIA data in an inversion analysis for PTA anomalies, how much error would be obtained in the result? The PTA and TIA data in the range of thick rectangle in Figure D-1 were put into inversion analyses for a single point dipole source (with six unknown parameters, three for position and three for magnetic moment) assuming input data are PTA anomalies. Table D-1 is the result for two source intensities, for the case shown in Figure D-1 and for the case of a smaller ( 0.3) magnetic moment. The result using PTA data recovered the

International Symposium on Airborne Geophysics

Airborne geophysics can be defined as the measurement of Earth properties from sensors in the sky. The airborne measurement platform is usually a traditional fixedwing airplane or helicopter, but could also include lighter-than-air craft, unmanned drones, or other specialty craft. The earliest history of airborne geophysics includes kite and hot-air balloon experiments. However, modern airborne geophysics dates from the mid-1940s when military submarine-hunting magnetometers were first used to map variations in the Earths magnetic field. The current gamut of airborne geophysical techniques spans a broad range, including potential fields (both gravity and magnetics), electromagnetics (EM), radiometrics, spectral imaging, and thermal imaging.