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| Crystalline_Rock_Strategies.pdf (794.09 KB) | 794.09 KB |
A general strategy for the exploration of crystalline rock masses in the<br/>eastern United States for the identification of potential sites for high-level<br/>radioactive waste repositories has been generated by consideration of the<br/>Department of Energy (DOE) Siting Guidelines, available information on these<br/>crystalline rocks, and the capabilities and limitations of various exploration<br/>methods. The DOE has recently screened over 200 crystalline rock masses in 17<br/>states by means of literature surveys and has recommended 12 rock masses for more<br/>intensive investigation including field investigations. The suggested<br/>strategy applies to the next stage of screening where the objective is to<br/>identify those potential sites that merit detailed site characterization<br/>including an exploratory shaft and underground study.<br/>The DOE Siting Guidelines are reviewed to determine the types of<br/>information that are both important and suitable for collection prior to site<br/>characterization; that is, the area phase of exploration, in the terminology<br/>used by DOE in the screening process. The single most critical need for<br/>information concerns the issue of whether or not the hydrology of a potential<br/>site can be adequately characterized. There is almost no information on<br/>hydrology at repository depths in the areas being screened by DOE.<br/>Early in the exploration of an area, a preliminary delineation of the<br/>regional and local ground water flow systems should be made. The locations of<br/>fracture zones, faults, shear zones, large dikes, large inclusions of country<br/>rock in intrusive rocks, and major changes of rock type need to be determined<br/>to begin to characterize the hydrologic system and to avoid such features, if<br/>possible, in siting the repository. Major fracture zones are significant<br/>conduits for water flow in some areas as shown by a summary of deep holes and<br/>excavations in the eastern United States and Canada. Details of these holes<br/>and excavations, which unfortunately are very limited in number, are presented<br/>in an Appendix. The survey of deep holes shows that subhorizontal fracture<br/>zones will probably be present in the subsurface in most areas, but these<br/>zones appear to be widely spaced below depths of 200 m.<br/>Exploration methods that may be used at the area phase include<br/>reconnaissance hydrology, remote sensing, geologic mapping, seismic<br/>monitoring, mineral resource studies, potential field geophysics, electrical<br/>and electromagnetic techniques, active seismology, drilling, borehole logging,<br/>core studies, sampling for hydrochemistry, hydrologic testing, and in situ<br/>stress measurements. Three phases of exploration are suggested:<br/>1) reconnaissance, which makes use of existing information and includes<br/>reconnaissance hydrology, remote sensing, and potential-field geophysics;<br/>2) surface study, which involves detailed ground studies of low cost, such as<br/>surface water and spring sampling, geologic mapping, potential-field<br/>geophysics, electromagnetic methods, and mineral resource potential studies;<br/>and 3) drilling, which directly investigates hydrologic conditions at depth by<br/>means of numerous logging and testing techniquyes. The first two phases have<br/>high potential for discovering steeply dipping fracture zones and faults; but<br/>if subhorizontal fracture zones and faults are present, they may be detectable<br/>only by drilling. This uncertainty regarding the presence of subhorizontal<br/>fracture zones at a site dictates a drilling strategy in which the number,extent, and depth of such zones are delineated as early as possible in the<br/>exploration program; encountering them unexpectedly at a late stage could be a<br/>serious problem. Depending on the distribution of subvertical and<br/>subhorizontal fracture zones, a multilevel repository may be more feasible<br/>than a single level one.
