The metamorphic rocks in the contact metamorphic aureole around the Cretaceous Tono Granodiorite, Central Kitakami Mountain land are originated from the Late Paleozoic formation composed chiefly of black shales and fossiliferous limestones. The contact metamorphic aureole is divided into the four progressive mineral zones based on the mineral assemblages of the pelitic rocks. The texture of the rocks gradually changes from the slates (very-low grade metapelites) to the hornfelse (high grade metapelites). The textures of metapelite are closely related to the types of the weathering process and physical characteristics of the rocks. For example, the area where composed of slates have thick type weathered zones, while the area where composed of hornfelse have thin weathered layer caused of different weathering prosses. The studied area is recognized as an elevated peneplane. The flat top of the mountain seems to be a peneplane and various erosional forms are observed. The remarkable difference of the topography is observed between low and high grade metapelites. For example, the V-shaped valley dominantly develop in the area of low grade metapelites, whereas the flat-flood valley develop in the area of high grade metapelites. The origin of these rocks is same, and the historical development of landformes and the climate are not different from each area. Therefore the difference in topography is considered to be a structural relief developed under the affection of the contact metamorphic grade. The structural relief in studied area is systematically explained as expression of the gradual change in texture of the rocks.
A measurement system was assembled for easily and rapidly detecting full features of the fissures buried in the ground, and the applicability of the system was tested in the field. Geological features analyzable by the system are: distribution of formations or rock layers, locations of their boundaries, locations of fault fissure zones, degrees of fissuring along such zones, locations of open fissures and the amount of radon-222 accumulated in the surface layer. This amount reflects the scale of fissures, and an abnormal increase in the amount can be a significant indicator in prospecting subterranean heat, uranium, natural gas and oil resources and in earthquake and landslide prediction studies. Direct object of measurement by the system is the dose of the primary gamma rays emitted by three radionuclides, 214Bi, 208Tl, and 40K contained in the surface layer within a cone about 30cm in maximum depth and about 1m in basal radius. The gamma-ray dose is measured by means of a 5-channel pulse-height analyzer equipped with a NaI (Tl) crystal detector 12.7cm in diameter and height. This system can shorten the measurement time to a half as that practiced in the method of Adams and another. The primary gamma-ray dose of each of the three nuclides indicates the concentration of each nuclide in the surface layer, as a principle. This dose, however, has a difficulty for practical application as it changes with geometrical conditions of ground surface, various man-made factors and diverse meteorological factors. A new technique was then proposed to make use of the ratio of primary gamma-ray doses of two different nucliedes as an indicator for overcoming this difficulty. This technique could thus effectively eliminate most of the said hindering factors and enabled quantitative analyses of needed geological features. A steep slope mountain area and a crowded city area, which both involved difficult conditions for application, were subjected to field applicability test. The existence of active faults inferred in those areas was examined. The existence of open fissures, their locations, changes of fissure openings and local changes of fissuring degree could be detected by the new system in both areas. The existence of accompanying closed fissures developed in parallel and local variations of their state of fracturing could also be detected.
New electrical prospecting techniques utilizing effective surface and subsurface solid electrode arrays, large continuous data processing, high density computer simulation and visualization have become important useful exploration tools increasingly for detecting deeply located mineral or low mineralized deposit, energy and groundwater reservoirs, geological fracture zones and rock interior structures, and also for monitoring groundwater in rock mass and contermination, nuclear waste repository and various changes of underground structures. An important objective of electrical methods including resistivity tomography techniques, is to maximize, extract and enhance the response due to target inhomogeneities using surface electrodes on the various ground conditions and buried electrodes in boreholes, tunnels and other underground conditions. Recently, new computerized resistivity section construction techniques has been mainly developed with emphasis on improvement of the reliability to extract the subsurface informations for the domains of groundwater and environmental explorations. For an example, a future work in improving the reliability is to design effective surface and solid electrode array combinations under the restricted field conditions by new prediction and feedback techniques. This paper begins with a review of the recent activities about development of elements as an aid to new surface and subsurface electrode array techniques. Next, the basic concepts of resistivity array methods including high accuracy forward modeling, high resolution inversion, sensitivity distribution and image reconstruction from computerized tomography procedure, are reviewed. And also, some results on the improvement of reliability of electrical methods have been discussed by using the examples of subsurface signal extraction and enhancement, investigated by the authors.