The geometry of the conduit is a "dike (or plate) shaped", with the length as large as several hundred meters and the thickness of 10-20 m. Although conduits are often visualized as pipes, this is in fact a misconception. Exposed sections of arc volcanoes show that even at shallow depth the feeders are plate-like rather than pipe-like. The horizontal length of the Unzen conduit is several hundreds m. Drilling is scheduled normally to the dike. The probability to hit the conduit is very high.
The above figure shows seismic tomography below the summit area of Unzen volcano. Inversion was carried out using the result of a seismic experiment in 1995. Numbers represent seismic velocity (P wave) in km/s. The white arrow indicates the location of conduit piercing into the plain of sea level. Hs is the top of lava dome.(@copyright of K. Nish, Kyoto University)
In order to determine the detail position and structure of the conduit, the seismic experiment using vibrators was carried out along the survey line passing the volcano in a SN direction in December 2001. The analytical result suggested a portion with a lower seismic reflection, suggesting the position of the conduit about 2 km west of the dome and at the depth of the sea level. Seismic reflections suggesting the base planes of the Unzen volcanic graben and the head of pressure source deduced in the ground leveling survey during eruption were also detected.
Experiment image: Seismic experiment to find the precise position of the conduit, scheduled in the end of 2001. Conduit shown here represents only the thickest part.
Result (1): Seismic record of the exploration (horizontal axis is time in s, covering 0 to 6 s) (left) and the survey line (about 10 km long) on the map (right). The seismic exploration was carried out in December 2001, in order to detect the detail target (conduit). A star in the middle of the survey line shows the location of vibrator (seismic source). Arrows plotted in 1 s (about 1 km depth) in the seismic record indicate the existence of a reflector, whose estimated horizontal distribution is shown in the map (shaded area). The area confirmed the possible conduit site determined already by seismic tomography (Nishi et al., 2000). The precise target can be focused through further analysis of the data obtained in geophone networks with 25 m-intervals.(@copyright of Inst. Seis. Volc., Kyushu University)
Result (2) (interpretation): seismic depth-section in NS direction showing the cross section of the magmatic conduit, obtained after the migration procedure. Circles are earthquakes during eruption. (@copyright of Inst. Seis. Volc., Kyushu University)
