The occurrence of depression on the ground surface due to lignite pit has been observed at central southern area of Gifu prefecture, Central Japan. In order to prevent the ground surface depression caused by collapse of the tunnel due to the shaking of the Tokai and Tonankai earthquakes, which is expected to occur near future, the work on filling fluent earth materials to the underground cavity is proceeding. To clarify the distribution of lignite pits and to monitor the filling condition of fluent earth materials in the cavity, the seismic tomography is planned between ground surface and the borehole several meters below and parallel to the lignite layer, which is drilled by the directional drilling technology. We report the current status of the site, drilling and survey plan, and outline of the 3D simulation results of seismic tomography.

1 Introduction
1.1 Background

Since 2000, CRIEPI (Central Research Institute of Electric Power Industry) has been conducting the project on the directional drilling and logging/measurement technologies in its boreholes (Kiho et al. 2009). This project was implemented as part of the development of an effective investigation method at the site selection stage of geological disposal in Japan. This project is almost completed in 2013 as planned and was scheduled to contribute to the site selection afterwards, but the geological disposal project has not started yet.

Until starting the geological disposal project, we have established a CD (Controlled Drilling) work-shop consisting of members including authors in order to inherit this technology and started searching for the application of this technology, in order to apply this drilling method to civil engineering investigation and civil engineering construction.

On the other hands, currently, there are many artificial underground cavities such as abandoned mines and shelters and natural underground cavities such as limestone caves, nationwide. The ground surface depression and the ground facility disasters such as settlement and inclination are frequently occurring due to the modification of the ground surface such as the reduction of the soil covering thickness by excavation, the increase of the load by the embankment, the degradation and aging of the rock, the external forces such as the earthquake. In particular, cases where the abandonment of the lignite mine causing depression due to rock deterioration and earthquakes has become conspicuous. In Mitake Town, Gifu Prefecture, surveys and countermeasures are being carried out in order to suppress the risks of abandoned lignite mines ahead of the whole country, but a fixed method has not been established for the method of investigation and the evaluation of countermeasure effect.

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