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The porosity of rocks is one of the most fundamental physical properties and is required to quantitatively evaluate the characteristics of rocks in drilling projects in fault zones. In the drilling project of the Futagawa fault, which ruptured during the 2016 Kumamoto earthquake mainshock, although the porosities of intact rock core samples were measured, there was no continuous porosity profile because core samples could not be obtained in fractured zones. Therefore, we estimated a vertical, continuous porosity profile for a depth interval of approximately 300–660 m, except for 383–399 m in borehole FDB-1 of the project, using sonic log data. First, we tested several different empirical equations proposed in previous studies for both sedimentary and volcanic rocks and proposed a new equation considering the effects of compaction and lithology on sedimentary rocks. Second, we compared the estimated porosities with the core porosities at the depths of the measured core samples. As a result, our new equation provided better estimates for sedimentary rocks, but a previous equation called Li et al.'s equation provided closer estimates for volcanic rocks. The porosities estimated by our new equation for sedimentary rocks were approximately 50% at depths of approximately 300–330 m and approximately 20–40% at approximately 330–350 m and 510–660 m. The porosities estimated by Li et al.'s equation were approximately 15% for volcanic rocks (massive lava) at depths of approximately 380–460 m and approximately 30–40% for volcanic rocks (autobrecciated lava) at approximately 350–380 m and 460–510 m. Obviously, the porosities derived from the sonic logs of volcanic rocks were greater than those measured using intact core samples due to existing fracture porosity and alteration. Therefore, the derived porosity profile might reflect a reasonable in situ state in the borehole of the Futagawa fault drilling project.
Shibutani et al. (Thu,) studied this question.