Global Inversion of Grounded Electric Source Time-domain Electromagnetic Data Using Particle Swarm Optimization

Global optimization inversion of grounded wire time-domain electromagnetic (TDEM) data was implemented through application of the particle swarm optimization (PSO) algorithm. This probabilistic approach is an alternative to the widely used deterministic local-optimization approach. In the PSO algorithm, each particle that constitutes the swarm epitomizes a probable geophysical model comprised by subsurface resistivity values at several layers and layer thicknesses. The forward formulation of the TDEM problem for calculating the vertical component of the induced magnetic field is first expressed in the Laplace domain. Transformation of the magnetic field from the Laplace domain into the time domain is performed by applying the Gaver-Stehfest numerical method. The implementation of PSO inversion to the TDEM problem is straightforward. It only requires adjustment of a few inversion parameters such as inertia, acceleration coefficients and numbers of iteration and particles. The PSO inversion scheme was tested on synthetic noise-free data and noisy synthetic data as well as to field data recorded in a volcanic-geothermal area. The results suggest that the PSO inversion scheme can effectively solve the TDEM 1D stratified earth problem.

Seismic exploration at Fuji volcano with active sources : The outline of the experiment and the arrival time data

Fuji volcano (altitude 3,776m) is the largest basaltic stratovolcano in Japan. In late August and early September 2003, seismic exploration was conducted around Fuji volcano by the detonation of 500 kg charges of dynamite to investigate the seismic structure of that area. Seismographs with an eigenfrequency of 2 Hz were used for observation, positioned along a WSW-ENE line passing through the summit of the mountain. A total of 469 seismic stations were installed at intervals of 250-500 m. The data were stored in memory on-site using data loggers. The sampling interval was 4 ms. Charges were detonated at 5 points, one at each end of the observation line and 3 along its length. The first arrival times and the later-phase arrival times at each station for each detonation were recorded as data. P-wave velocities in the surface layer were estimated from the travel time curves near the explosion points, with results of 2.5 km/s obtained for the vicinity of Fuji volcano and 4.0 km5/s elsewhere

Precise remote-monitoring technique of water volume and temperature of a crater lake in Aso volcano, Japan : implications for a sensitive window of a volcanic hydrothermal system

A high-resolution Digital Surface Model and a commercial digital camera have enabled precise and continuous monitoring of the crater lake at Aso volcano. From July 2006 onwards, infrared (IR) thermometry has been used with this system, enabling more accurate measurements of lake volume and temperature based on simple and intensive observations than has been possible in any other previous studies. The heat discharge remained largely constant at approximately 220 MW, with the exception of an abrupt increase to 280 MW that coincided with a rapid decrease in the water level in August 2007. Simultaneously, an increase in temperature at a shallow depth was suggested by other observations. The crater lake was found to respond to even slight changes in volcanic fluid supply, which can be well quantified by our method. Thus, a crater lake can be monitored more precisely than subaerial fumaroles whose energy estimation is often accompanied by large uncertainties. Our monitoring technique of a crater lake provides information on the subsurface hydrothermal system beneath it, for which any in-situ measurements are practically impossible

Significance of Investigations on the Structure of Kirishima Volcanoes

霧島火山群の構造に関するミニシンポジウムが1994年1月に行われた.本論に続く論文はその際の発表をまとめたものである.本論はその前置きとして,霧島火山群を紹介すると共に,同火山群の構造を研究するに際して目標とすべき3つの課題を示している.第1に,霧島火山群は複数の火山が同時に活動している火山群であって,そのマグマがどこからどのように供給されているかを明らかにすることが重要である.第2に,なぜ火山群を作るのかを応力場や南九州のテクトニクスとの関係から検討する必要がある.第3に,帯水層と考えられる低比抵抗層が,霧島の広い範囲に見られる.帯水層の分布状況を正確に把握し,噴火の様式や噴火の前兆現象との関係を明らかにしていく事が重要である.In recent years, investigations on the structure of volcanoes have been noteworthy for the further understanding of volcanic processes including locations of magma reservoir, magma rising process before eruptions and causes of related phenomena. The following seven papers including the present one were presented in the mini-symposium on the structure of the Kirishima Volcanoes held at the Earthquake Research Institute in January, 1994