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Mizunami Underground Research Laboratory Project
Results from 1996-1999 Period
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| 4. Main results of the Phase I-a |
| 4.5 Investigation techniques and equipment |
The MIU Project also plays a role for applying newly employed investigation techniques and equipment to the in-situ investigations. Some techniques and equipment are developed by TGC. If investigation results do not meet expectations of individual study field (data precision and reliability of equipment), the techniques and equipment are aimed to improve. Besides, techniques and equipment of investigations needed in and after the Phase II are also developed.
1) Techniques and equipment for borehole investigations
The MIU Project maintains a rule to use fresh water instead of mud water as drilling fluid so that original permeability of the rock mass and hyrdochemical properties of groundwater are not disturbed. However, boreholes are apt to collapse when fresh water is used. Therefore, drilling techniques to minimize the borehole collapse and techniques to prevent the partial collapse are developed.
Drilling system using reverse aerated wire-line method
In order to minimize the disturbance of hydrogeological properties of the rock mass and hydrochemical properties of groundwater, it is desirable to use fresh water as drilling fruid. However, it tends to incur collapse of boreholes. As a countermeasure, the reverse aerated wire-line method is developed. The method leads both of the fresh water and mud water into boring rods to prevent them from contacting with the borehole wall and minimizes collapse of the wall (Fig.4.71).
This method is designed in 1997 FY86). Its overall drilling system is designed in 1999 FY87).
Partial casing insertion equipment
Partial casing insertion equipment is developed to cope with partial collapse of boreholes. This equipment consists of a partial reaming bit, a casing insertion unit and a partial casing88). It can be used for drilling of new boreholes as well as keeping and reaming of existing boreholes (Fig.4.72).
The partial reaming bit is manufactured in 1996 FY89). And application test was carried out86) and the casing insertion unit is manufactured in 1997 FY. Also in 1997 FY, the whole equipment is assembled and an operational application test on the ground is carried out86). In 1999 FY, the overeall application test is carried out using all of the partial reaming bit, the casing insertion unit and the partial casing. As a result, BTV investigations find out that the lower end of the partial casing set at the borehole bottom is bent to inner side of the borehole to prevent the casing from fitting the rock mass adequately. In the future, the way to cut off the above-mentioned bent casing is expected to develop87).
2) Technique and equipment for geological investigations
Seismic tomography
The development of seismic tomography using 1,000m-class boreholes is in progress to understand the extent of discontinuity planes deep underground. The present work consists of the development of an in-borehole nondestructive seismic source (henceforth, "sparker") with little effect on the surrounding borehole and the development of data analysis techniques.
The in-borehole sparker is designed and assembled in 1997 FY. An application test of the assembled equipment is carried out in 1998 FY88), 90). Subsequently, an application test is carried out in the MIU-1 and MIU-2 aiming at down to the depth 1,000m in 1999 FY87). In addition, the development of a data analysis technique called "full-wave inversion" is carried out with the purpose of improving the resolution.
Details of data acquisition in the application tests are shown in Tab.4.38. Results of the application tests on the Toki Granite are as follows.
Using the sparker, tomography data are obtained with inter-borhole intervals of about 100 m and with a target depth of 1,000m.
Based on the previous application test, a distance that allows separating an initial P-wave generated by the sparker from noises is estimated at some 260m. However, the distance drops to about 150m due to too large noises in the test.
In order to improve the maximum distance that allows recognizing the initial P-wave, a stacking test by several oscillations is carried out. This test results in an improvement of S/N ratio. Fig.4.73 exemplifies an initial P-wave made recognizable by 32-times stacking (the distance between the sparker and the receiver is some 240m).
Besides, an initial travel time tomography analysis is carried out by reading the propagation time of initial P-wave using the obtained data. Fig.4.74 shows a result of P-wave velocity structural analysis. The grid size for analysis is 2.5m (horizontal)
3m (vertical). The tomography section is divided into the shallow area with a higher velocity (5.3km/sec.) and the deeper area with a lower velocity (4.9km/sec.) at about 850m in depth. This shows a trend coincident with the results of seismic wave velocity logging. The Tsukiyoshi Fault estimated to cross the MIU-2 at around 900m in depth is not recognized as a structure with a sharp velocity contrast against the surrounding rock mass.
Table 4.38 Details of seismic tomography
Besides, as part of the development of analysis techniques of seismic tomography data, applicability study of full-wave inversion analysis to actual data is carried out. The data (Tab.4.38) obtained by sparker application tests in 1999 FY is used for this study.
Contents of the analysis are as follows.
Digital simulation for verifying the validity of a series of analysis codes
Study on preliminary processing of actual data used for the full-wave inversion analysis
Implementation of the full-wave inversion analysis and extraction of future tasks
The results of full-wave inversion analysis are shown in Fig.4.75. The grid size for analysis measures 25cm25cm, featured by a higher resolution than the conventional initial travel time tomography analysis with a resolution of several meters. Though vertical variations in velocity show a similar trend to the results of seismic wave velocity logging, only results obtained by a nearly-horizontally-installed pair of sparker-receiver are usable for analysis due to too large noises. As a result, a horizontal structure is dominant in the result. In the future, examinations on the applicability limit against noise in the full-wave inversion analysis are expected. Also, the possibility of improvement in the analysis method is needed to be examined.
3) Techniques and equipment for hydrogeological investigations
Hydraulic test equipment for depths of up to 1,000m
In order to understand groundwater hydrology deep underground, it is necessary to clarify hydrogeological properties of the highly impermeable rock mass less than 10-8m/sec. in permeability. Such a rock mass is traditionally classified into an impermeable layer in civil engineering. Also, the equipment is required to be able to acquire accurate data under a high temperature/pressure because the target depth exceeds hundreds of meters. TGC is developing equipment allowing in-situ permeability tests on the highly impermeable rock mass down to 1,000m in depth (Fig.4.76)91), 92). This equipment is assembled in 1997 FY, and is being used in the MIU Project and the RHS Project.
A pipe system (where an inner probe connected with a rod moves up and down) is adopted as a basic structure of the equipment as a precaution against in-borehole collapse. Permeability tests feasible by the equipment include slug method (non-steady method), pulse method (non-steady method, devised for the highly impermeable rock mass) and pumping test (steady method). A simultaneous application of these test methods allows measurement for a wide range of permeability (10-6
10-12m/sec.).
The equipment is featured by a five-unit-multipackers set in a row and a BTV mounted at its tip. The multipacker allows not only changing the test section between 2m and 14m but also confirming the sealing effect of packers by measuring and comparing pore pressure and temperature of test/non-test intervals. This function is extremely effective to guarantee the quality of the test results.
In addition, the BTV allows real-time observations of the rock mass conditions on front and lateral sides of the equipment. It enables to keep the packer from being stuck in boreholes. Also, verifying the rock mass conditions to set up a packer, and furthermore, installing the equipment with a high precision (e.g. specific fracture zone) become possible.
Furthermore, the equipment is improved for coping with a bent borehole, for example, by adding a centerlizer since 1998 FY 88). This improvement is shown in Fig.4.77.
4) Techniques and equipment for hydrochemical investigations
Water sampling equipment for depths of up to 1,000m
For fast and precise understanding of hydrochemical properties of groundwater between the ground surface and deep underground, water sampling equipment is being developed92), 93). Its target depth is 1,000m and aims to minimize disturbance of the properties of geological environment caused by boreholes excavation. This equipment is developed until 1997 FY and is used for the MIU Project and the RHS Project.
The equipment is composed of a ground section, a connector and an underground section (Fig.4.78). All functions related to water sampling are concentrated in the underground section. A pipe system is adopted as the basic structure of the equipment as a precaution against borehole collapse like in the above-mentioned hydraulic test equipment for depths of up to 1,000m.
The batch water sampling function is adoptedto the equipment so that sampling in a confined inert state is possible. Also, continuous drainage function by pump is added to enhance the sampling efficiency.
Furthermore, the equipment is improved for coping with a bent borehole, for example, by adopting flexible joints since 1998 FY88). The improvement is shown in Fig.4.79.
Geochemical logging unit
The geochemical logging unit is developed for in-situ acquisition of physicochemical parameters of groundwater, including pH, redox potential, electric conductivity, sulphide ion concentration and water temperature.
This unit is mounted on water sampling equipment for depths of up to 1,000m, allowing real-time data acquisition of the physicochemical parameters of groundwater during the continuous water sampling. Thus, it allows confirming the replacement process of water in borehole by groundwater flowing into the sampling interval partitioned by double packers. Also, it enables the accurate timing of water sampling93).
5) Techniques and equipment for rock mechanical investigations
Initial stress measuring equipment for depths of up to 1,000m
Initial stress of the rock mass acts as a boundary condition indispensable not only for optimized designing and stability assessment of the MIU Project's research drifts but for numerical analysis of shaft excavation.
Initial stress measuring techniques of general-use are classified into borehole-using methods (e.g. hydraulic fracturing test and stress analysis, etc.) and core-using methods (e.g. AE test and DRA test, etc.). The respective techniques are in different stages ranging from a practical use stage to a research/development stage. Therefore, limiting conditions are unavoidable for measurement and analysis. Also, there is a depth limit for application. Thus, there is no highly reliable technique that allows measuring 3-D initial stress at 1,000m in depth94).
Based on the above-mentioned situation, the development of initial stress measuring equipment for depths of up to 1,000m is expected with the purpose of establishing a measuring technique of 3-D initial stress as deep as 1,000m in depth. This equipment is to measure in-situ initial stress by a stress relief method (Fig.4.80).
Upon the development of this equipment, literature surveys are carried out on domestic and overseas studies. In 1996 and 1997 FY, recommended techniques and conceptual design of the equipment are extracted. Also, future tasks are extracted86), 89), 94). Based on the results, designing of the measuring equipment and manufacturing of parts are being carried out since 1998 FY88). These parts are strain gauge cell (with a thermometer), recorder and pressure vessel to house batteries and an azimuth-inclination measuring device88). The remaining devices (azimuth-inclination measuring device and recorder) will be manufactured and their function tests will be carried out in and after 2000 FY.
6) Techniques and equipment for in and after the Phase II
Continuous-wave radar investigation techniques
The radar tomography can be used for two boreholes with the distance of up to several tens of meters or less. It is presumably possible to expand the distance by employing continuous waves as transmitting-receiving signals, especially, in crystalline rocks such as granite characterized by a large resistivity and a small energy loss. As an improvement of the borehole radar investigation technique that had been used in the Kamaishi Mine, the method of continuous wave radar investigation is developed. It aims to expand the survey depth and raising the resolution. In the long run, this development aims both much higher space resolution and much longer survey distance than the current radar exploration, by putting the ACROSS (accurate regulation of steady signal system) technology95) to practical use.
In 1998 FY, an experimental device of continuous wave radar is manufactured. In 1999 FY, with the purpose of understanding the heterogeneity near the ground surface and using this technique as a tool for understanding the frequency-dependence of ground permittivity, the input impedance in air and on water are measured using three kinds of antennas (targeted frequency bands: 15, 550, 50200MHz). As a result, basic data on the resultant impedance of antenna and medium is obtained87).
A long-term monitoring system using boreholes
The MP system for observing groundwater pressures has no measuring capability under a high differential pressure environment caused by shaft excavation in the Phase II and a large-scale pumping test. Consequently, a long-term monitoring system is being developed to cope with such an environment.
Application tests are carried out down to 200m in depth using the system manufactured in 1999 FY. As a result, its overall function is ascertained. However, noises (chattering) occurred in the depth sensor hinder connection with measuring port at the precise depths. Oil fouling inside the casing is found out to be the main cause, thus the inner surface of casing is cleaned by organic solvent. It turns out to prevent chattering87).
An investigation system of research drift walls
The wall of research drifts are observed during excavation. The information obtained by the observations is important not only to verify the validity of predictions made in the Phase I but to understand geological environments around the research drifts. So far, engineering information on tunnel facing survey systems is gathered and following future tasks are extracted88).
It is important to extract the distribution of discontinuity planes (e.g. fractures), water inflow conditions and weathering. Extraction of fractures requires a high resolution, whereas extraction of water inflow and weathering requires color information. Thus, survey system employing a digital camera is thought to be effective.
When the wall of a 6m-across shaft is covered by 46 photographs using a digital camera with the highest resolution (six million pixels as of 1998 FY), resolution as small as some 2mm is expected.
Design and cost of the investigation system depend largely on a degree of automation of photographing by digital cameras.
Extraction of discontinuity planes by image processing is put to practical use. However, automatic extraction of water inflow conditions and weathering has yet to see a practical example.
7) Data base construction and development of data analysis/visualization system on geological environments
Data base construction
The MIU Project is planning to obtain a huge amount of data from investigations. Introduction of data base systems begins for management and utilization of the data in 1996 FY89). One of them is GEOBASE, an integrated underground resources data base system developed by Geothermal Technology Development Co., which is being improved on the following points86), 88).
Addition of a table for the information management to guarantee the data quality, including data acquisition/analysis techniques
Addition of a table for the data management on core description in boreholes and chemical analysis
Addition of a table for time management data in boreholes
Addition of a function to refer to and display the information extracted by BTV investigations
Development of a PC-based simple reference software
System renovation so that GEOBASE can be accessible to every researcher using intranet.
In the data base system are registered the location data of boreholes, geophysical logging, hydraulic tests, geological columnar sections and surface water hydrology.
Data analysis/visualization system on geological environments
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 developed 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 consideration16),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). This system is used for the construction of the geological model in the Phase I-a in the MIU Project (See Chapter 4.1). 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-dimensional polynomial formula using the input data of positions and directions19).
In addition, this visualization system not only secures a close connection with the above-mentioned GEOBASE but also introduces FRAC-AFFINITY17), a 3-D saturated/unsaturated seepage flow analysis code by finite difference method. The FRAC-AFFINITY has the following characteristics.
It is a hybrid medium that allows simultaneous handling of porous medium part and fractured medium part.
The fractured medium part can take both deterministic fractures and stochastic fractures into consideration.
Physical properties in porous medium part and fractured medium part (deterministic) could be set either homogeneously for every geological formation/ structure or heterogeneously using statistical technique based on fractal theory.
Fractured medium part (stochastic) could be set to develop either at random or at fixed positions. Physical properties could be set to develop either homogeneously or stochastically based on normal distribution.
Data-interface environments from the EarthVision used for the construction of geological model are already systematized. Therefore, it is easy to form input data and differential meshes.
Improvements are expected in and after 2000 FY. For example, a function to take anisotropic permeability of fractures into consideration is expected to be added to the saturated/unsaturated seepage flow analysis code of the existing systems.
8) Techniques and equipment for information disclosure
In order to explain geoscientific studies carried out in the MIU Project to the general public more effectively than ever, the virtual reality (VR) technology is investigated. Also, its applicability to the MIU Project is examined. Specifically, software is newly developed. This software enables users to feel as if they walk around in the ground facilities and research drifts which are in the planning stage. This software is improved to allow visitors more realistic experience by using a head mount desplay (HMD)96). Besides, models86) for explaining the outline of the MIU Project and drilling techniques are manufactured. These are placed in the community plaza in the Shobasama site for the general public.