4.1.3 Construction of geological model in the RHS Project

Mizunami Underground Research Laboratory Project
Results from 1996-1999 Period

4. Main results of the Phase I-a

4.1 Geological investigations

4.1.3 Construction of geological model in the RHS Project

1) Construction of geological model using data from literature survey and other geoscientific researches

(1) Extraction of geological unit

For both sedimentary rocks and granite, lithofacies are extracted as primary elements whereas fractures and faults are extracted as secondary elements generated after their formation.

As for sedimentary rocks, Seto Group, Oidawara Formation, Akeyo Formation, Toki Lignite-bearing Formation (Upper and Lower) and the Tsukiyoshi Fault are extracted as geological units.

As for granite, the weathered part, the rest of the granite and the Tsukiyoshi Fault are extracted.

(2) Model construction

Using the data shown in Tab.4.9, boundaries of geological units are defined. Afterwards, topographical features, each element's boundary and shape of fault plane are estimated using the minimum tension theory (Fig.4.18). The results of borehole investigations in the AN-1 and AN-3 are used to extract the geological units. However, these results are not used when defining the above-mentioned boundaries as the granite are only divided into three parts in these boreholes : weathered part, the rest of the granite and the Tsukiyoshi Fault. Thus, results of borehole investigations in the AN-1 and AN-3 are not included in Tab.4.9.

Uncertainties in the geological model are as follows.
Data on the heterogeneity of the granite deep underground are scarce. As a result, the granite could be divided only into two elements : the fractured zone including the weathered part and the rest of the granite.
Information on the Tsukiyoshi Fault is not available in the granite. Thus, it is postulated that the fault in the granite has the same properties as that in the sedimentary rocks.

Table 4.9 Data used to set geological structural boundaries of geological model
(Study area : ca.4kmca.6km)
(except for the result of the Phase I-a)

Boundary	Data
Ground surface	Digital elevation data (20m mesh)^*
Upper boundary of Oidawara Fm.	59 boreholes for uranium exploration AN-8, SN-14,TH-6,TH-8 Seismic survey in the Shaft Excavation Effect Experiment^*
Upper boundary of Akeyo Fm.	53 boreholes for uranium exploration AN-8, SN-14,TH-6,TH-8 Geological map (PNC,1994)
Upper boundary of Toki Lignite-bearing Fm.(Upper)	135 boreholes for uranium exploration AN-6,AN-8, SN-14,SN-6,TH-14,TH-68 Seismic survey in the Shaft Excavation Effect Experiment^*
Upper boundary of Toki Lignite-bearing Fm.(Lower)	141 boreholes for uranium exploration AN-6,AN-8,SN-14,SN-6,TH-14,TH-68 Seismic survey in the Shaft Excavation Effect Experiment^*
Upper boundary of weathered granite	120 boreholes for uranium exploration AN-6,AN-8, SN-24,SN-6 TH-15,TH-78, HN-1 Topographical map of basement rock^20),31) Seismic survey in the Shaft Excavation Effect Experiment^*
Upper boundary of intact granite	(Calculated by assuming the thickness of weathered granite as 20m)
Tsukiyoshi Fault	13 boreholes for uranium exploration TH-13 Topographical map of basement rock ³¹⁾ Seismic survey in the Shaft Excavation Effect Experiment^*

^* : 1/25,000"Toki" and "Mizunami"published by Geophysical Survey Institute

2) Construction of geological model using data including the result of the Phase I-a

(1) Extraction of geological unit

As for sedimentary rocks, Seto Group, Oidawara Formation, Akeyo Formation, Toki Lignite-bearing Formation (Upper and Lower) and the Tsukiyoshi Fault are extracted as geological units.

As for granitic rock, the weathered part, "Upper fracture zone", "Moderately fracture zone", "Fracture zone along the fault" and the Tsukiyoshi Fault are extracted as geological units. (See Chapter 4.1.4 2))

(2) Model construction

Using the data shown in Tab.4.10, the boundaries among the above-mentioned geological units are set. The drilling result of the AN-1 is included into the data.
Uncertainties in the geological model are as follows.

Table 4.10 Data used to set geological boundaries of geological model
(Study area : ca.4kmca.6km)
(including the result of the Phase I-a)

Boundary	Data
Boundary	Data obtained by literature study / geoscientific researches except for the MIU Project	The result of the Phase I-a
Ground surface	Digital elevation data (20m mesh)^*	-
Upper boundary ofOidawara Fm.	59 boreholes for uranium exploration AN-8, SN-14,TH-6,TH-8 Seismic survey in the Shaft Excavation Effect Experiment^*	-
Upper boundary of Akeyo Fm.	53 boreholes for uranium exploration AN-8, SN-14,TH-6,TH-8 Geological map (PNC,1994)	-
Upper boundary of Toki Lignite-bearing Fm. (Upper)	135 boreholes for uranium exploration AN-6,8, SN-14,SN-6,TH-14,TH-68 Seismic survey in the Shaft Excavation Effect Experiment^*	-
Upper boundary of Toki Lignite-bearing Fm. (Lower)	141 boreholes for uranium exploration AN-6,AN-8,SN-14,SN-6,TH-14,TH-68 Seismic survey in the Shaft Excavation Effect Experiment^*	-
Upper boundary of weathered granite	120 boreholes for uranium exploration AN-6,AN-8, SN-24,SN-6, TH-15, TH-78, HN-1 Topographical map of basement rock ^20),31) Seismic survey in the Shaft Excavation Effect Experiment^*	-
Upper boundary of "Upper fracturezone"	AN-1 ³⁴⁾	MIU-13 ^39),40,41)
Upper boundary of "Moderately fracturezone"	AN-1 ³⁴⁾	MIU-13 ^39),40,41)
Tsukiyoshi Fault	Geological map ¹³⁾	MIU-23 ^40,41)
Upper and Lowerboundary of "Fracture zone alongthe fault"	-	MIU-13 ^39),40,41)

^* : 1/25,000"Toki" and "Mizunami"published by Geophysical Survey Institute
Bold : Data used to renew geological model

The MIU-13 are nearly vertical. Therefore, information on highly-inclined fractures is insufficient.
The methodology to specify permeable fractures needed to form an equivalent continuum model are not sufficiently verified. As a result, the specification is unreliable.