Porosity of rocks is one of the most fundamental physical properties and is required to quantitatively evaluate the characteristics of rocks in fault zone drilling projects. In the drilling project of Futagawa fault, which ruptured during the 2016 Kumamoto earthquake mainshock, although porosities of intact rock core samples were measured, there was not a 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 about 300–660 m except 383–399 m in the 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 for 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 showed better estimations for sedimentary rocks, but a previous one called Li et al.’s equation gave closer estimations for volcanic rocks. The porosities estimated by our new equation for sedimentary rocks were approximately 50% at the depths of about 300–330 m and approximately 20–40% at about 330–350 m and 510–660 m. The porosities by Li et al.’s equation were approximately 15% for volcanic rocks (massive lava) at the depths of about 380–460 m, and approximately 30–40% for volcanic rocks (autobrecciated lava) of about 350–380 m and 460–510 m. Obviously, the porosity derived from a sonic log of volcanic rocks was higher than those measured using intact core samples due to fracture porosity existing and alteration. Therefore, the derived porosity profile might reflect a reasonable in-situ state in the borehole of the Futagawa fault drilling project.
