1) Objectives

Based on the goals set for the entire MIU Project and its Phases, the objectives of geological investigations are set as follows⁷⁾:

Data acquisition on geology and geological structures from the ground surface to deep underground in the Shobasama site
Construction of the geological model by examining the obtained data and verifying its validity
Developing methodology for systematic investigations and analyses of geology and geological structures

2) Overview of the investigations

In the Phase I-a, as the first step to achieve the above-mentioned objectives, it is necessary to summerize the geology and geological structures in the Shobasama site, and to construct the geological model. The geological models are constructed for two areas. One is set to improve the accuracy of groundwater flow simulation (ca.4kmca.6km). The other is the Shobasama site. The knowledge obtained through the geological investigations forms the basis of the models for hydrology, hydrochemistry and rock mechanics (For models for hydrology and rock mechanics, see Chapters 4.2 and 4.4).

In order to examine the correlation between (a) varieties/amounts of investigations and (b) accuracy of the results, two models are constructed for each area. First model is constructed using data obtained from literature survey and geoscientific researches except for the MIU Project. Second model is constructed using the data obtained from Phase I-a in addition to the data used for the first model.

3) Construction of geological model

Knowledge used in the construction of geological model are thought to form the basis of the models for hydrology, hydrochemistry and rock mechanics. Therefore, methodologies of constructing above-mentioned other models must be taken into consideration upon constructing geological model.

To give an example, there are two kinds of models conceivable when constructing hydrogeological models. Specifically, a continuum model (postulating the rock mass as a porous and continuous medium) and a discontinuous body model (characterized by fractures in the rock mass). For the groundwater flow simulation in the Phase I-a is adopted the continuum model due to information shortage on the distribution of fractures and permeability as well as constraints on time required for analyzing the several-kmseveral-km rock mass as a discontinuous body. Supposing that hydrogeological model is constructed as a continuum model, construction of geological model requires following conditions.

Divide sedimentary rocks into geological units on the assumption that physical properties are uniform in the same sedimentary formations.
Specify the location, continuity and width of discontinuity such as faults that extend throughout the whole area.
Divide crystalline rocks into geological structureswhich have statistical significant differences in fracture density/direction, filling and so on.

Geological model is constructed in the following order.
Extract the geological units based on the investigation results
Prepare the data sets regarding boundaries between geological units
Construct the model using 3-D visualization software (See Chapter 4.1.1 4))

4�jVisualization of geological model

Geological model forms the basis of the models for hydrology, hydrochemistry and rock mechanics. Thus, 3-D visualization of the geological model through computer graphics makes it easy to share the necessary information for constructing above-mentioned models.

EarthVision, 3-D visualization software produced by Dynamic Graphics Inc., is used for visualizing the geological model. The software forms a 3-D geological model by estimating shapes of discontinuity planes such as boundaries between geological formations and faults, and combining these discontinuities by taking these relationship such as positions and development process into consideration^16),17).

Furthermore, this software is used to construct models in overseas major projects for geological disposal, such as Sellafield (Nirex), Wellengerg (Nagra), HRL (SKB) and Yucca Mountain (USGS, USDOE)¹⁸⁾.

Minimum tension theory based on the spline interpolation, one of the functions of the EarthVision, is applied to estimate the ground surface, geological boundaries and fault planes. This method is to interpolate between the adjacent data with the smoothest curved surface by n-dimentional polynomial formula based on the input data of positions and directions¹⁹⁾.