4-5 Unzen Scientific
Drilling Project (USDP)
The
Unzen Scientific Drilling Project (USDP) is a six-year term project started in
April 1999 following a one-year feasibility study. The project, named
"Unzen Volcano: International Cooperative Research with Scientific
Drilling for UnderstandKyushu University, etc., sponsored by Ministry of
Education, Culture, Sports, Science and Technology (MEXT) of Japan. Geological,
geophysical and geochemical researches on the growth history, subsurface
structure and manner of magma ascent at Unzen Volcano are the principal
subjects of study. USDP consists of two phases (three years each). Phase I
included drilling in two places at the flanks of Unzen Volcano and conducting
associated research to reveal the three-dimensional structure and the growth
history of the volcano. Scientific and drilling strategy for the conduit
drilling and the design were also completed, and a pilot hole for the conduit
penetration was drilled on the northern slope of the volcano. Phase II is drilling
into the conduit of the 1990-95 magmas in order to clarify the ascending and
degassing mechanism of magmas and to evaluate eruption models for the 1990-95
activity. Phase II is being undertaken as a joint research venture with the
International Continental Scientific Drilling Program (ICDP).
In the
last one kilometer of the ascent, magma is subjected to an order of magnitude
decrease in solubility of water in melt, which is the main reason that
effective degassing occurs in this level (Fig. 1). Degassing of the SiO2-rich
melt and crystallization resulting from the melt's dehydration cause great
changes in magmatic physical properties. Occurrence of swarms of low-frequency
(long-period) earthquakes and evidence for build-up of pressure at shallow
levels just before or during eruption, indirectly help to interpret phenomena
related to the magma's degassing at shallow depth; i.e., in the 1990-1995
eruption at Unzen, isolated tremor events (1.5-0.5 km deep), low-frequency
earthquake events (0.5-0 km deep ), and pressure sources and hypocenters of
vulcanian explosions at 0.8-0.6 km and 1 km-depths, respectively. Direct
drilling into these levels is the best way to, in situ, investigate the
phenomena which accompanied degassing during eruption.
The
geometry of the target conduit is considered to be "dike- (or plate-)
shaped", with a length as large as several hundred meters and a thickness
of 10-20 m. It probably is east-west trending, considering the distribution of
volcanotectonic earthquakes just before eruption, the shape of the lava spine
at the dome-top and the N-S extentional tectonic stress surrounding the Unzen
volcano field. Drilling was started in the northern slope normal to the dike.
The probability to hit the conduit is very high. The seismic experimental data
suggest the location of the dyke in a region having low seismic velocity and no
seismic reflection from horizontally-stratified strata. The normal drilling in
which drilling starts vertically and increases in inclination with depth is the
best choice to drill the hot and challenging target with the lowest risk (Fig.
2). The temperature of the conduit center is estimated as high as 600 °C. However, such
a high temperature is limited to the area close to the conduit. The hole bottom
temperature during operation of conduit drilling can be controlled at rather
low values with a special casing program and mud circulation system, so that we
can use the logging tools (temperature probe, bore hole televiewer, etc) even
at considerable depth in the conduit.
Fig 1
Image of lava dome and upper part of the conduit at Unzen. Effective
degassing from foamy magma occurred during its fragmentation and annealing in
the upper part of conduit. The conduit condition will be investigated in
drilling main hole (USDP-4) in 2003, and continuous coring will be performed by
drilling sidetrack hole (USDP-4a) in 2004.
Fig. 2 Trajectories of
conduit drilling. New RS-3 and RS-3 are drilling sites of main-sidetrack holes
and a pilot hole, respectively.