Comparative Study of Geological, Hydrological, and Geophysical Borehole Investigations

摘要

Understanding the permeability of the bedrock can be improved by supplementing the results of water injection tests with information from core mapping, TB-inspection and borehole geophysics. The comparison between different borehole investigations encompasses core mapping, TV-inspection and various geophysical borehole measurements. The study includes data from two different study areas, Kraakemaala and Finnsjoen, in which extensive geological, hydrological and geophysical investigations have been carried out. The fractures and microfractures in crystalline rock constitute the main transport paths for both ground water and electric currents. They will therefore govern both the permeability and the resistivity of the rock. In order to get a better understanding of the influence of fractures on permeability and resistivity, a detailed comparison has been made between the hydraulic conductivity, respectively, and the character of fractures in the core and the borehole wall. The fractures show very large variations in hydraulic conductivity. Microfractures and most of the thin fractures have no measurable hydraulic conductivity (in this case < 2 x 10 exp -9 m s exp -1 ), while test sections which contain a single isolated fracture can have no measurable to rather high hydraulic conductivities (> 10 exp -7 m s exp -1 ). Wide fracture zones often have hydraulic conductivities which vary from very low (less than 2 x 10 exp -9 m s exp -1 ) to high values (10 exp -5 m s exp -1 ), indicating that the hydraulic conductivity is governed by a few discrete fractures. The resistivity shows a continous variation in the range 1,000- 100,000 ohm-m and a relatively poor correlation with hydraulic conductivities. The observed difference is considered to the effect of restriction of water flow on a few channels, while electric surface condition, i.e. current transport through thin water films, makes current transport possible through fractures with very small aperatures. (Atomindex citation 16:072457)