Microfissures of thermal origin were investigated in granitic rock specimens, taken from the area of a heater test performed at the Stripa Test Site in Sweden. Optical and electron microscope techniques were simultaneously applied to obtain a complete information of the rock microfractography related to the texture and mineralogy. This microfractography network was quantified by digital image processing. Original microphotographs were digitized by a flat microdensiometer. Then, segmentation, thinning and line-following algorithms were successively applied for enhancing, detecting and measuring the microfissures. The described technique was applied to material subjected to one, slow in situ heating cycle reaching a maximum temperature of about 2200C and to non-heated material taken from the same area. No significant differences in the fissuration density were detected when comparing heated and non-heated material.
A Swedish-American co-operative research program on radioactive waste storage in mined caverns is currently in progress in the Stripa Mine in Sweden (Whitherspoon and Degerman, 1978). The field experiments of the program were initiated 1977, and among the early activities was an in situ, pilot heater test, performed by the Department of Rock Mechanics, University of Luleå, under sponsorship of the KBS Project (Carlsson, 1978). The pilot heater test was followed by a laboratory investigation of the heated material. The investigation concerned possible, chemical and mechanical, micro-scale effects of the artificial heating cycle applied to the rock. Special attention was paid to the state of microfissuration of the material (Leijon et al, 1980). This paper presents the technique applied for the observation and quantification of the micro-crack features of the Stripa Granite. The microcrack density, as observed in both heated and non-heated specimens is evaluated and compared. Several methods have been considered in the past for evaluation of microfissuration of rock. The different manual and automatic procedures are so far tedious and sometimes, when applied to difficult situations under petrographic microscopy, of a very low viability. Among those of automatic character stands out those commercial available; from the 60', analogic systems, in which the microscope is connected to a black and white TV system.
The microfissures have to be electronically discriminated on the TV screen, by means of brightness and contrast and later evaluated by analog methods. In the particular case of the Stripa Granite, with a very light microfissure network, those methods could not render satisfactory results on account of their low sensitivity and the lack of a precise procedure to enhance and delimite the microfissures. For that reason a recently developed automatic method, based on digital processing techniques on microscope images could be more successfully applied. A detailed description of this method, especially applied to the difficult microfissuration network of the Stripa Granite, is given in this paper.
A crosscut of the heater test area, from which all studied rock specimens were taken, is shown in Fig. 1. A central main heater and three peripheral heaters were positioned, remote from excavations, in downwards oriented boreholes.
