JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 88, NO.B9, PAGES 7373-7380, SEPTEMBER 10, 1983 MICROCRACK STUDY OF GRANITIC CORES FROM ILLINOIS DEEP BOREHOLE UPH 3 Bart J. Kowallis Department of Geology, Brigham Young University, Provo, Utah 84602 Herbert F. Wang Department of Geology and Geophysics, University of Wisconsin-Madison, Madison , Wisconsin 53706 Abstract. Microcracks were examined in five granite samples ranging in depth of origin from 696 to 1572 m by optical and scanning electron microscope observations and by elastic property measurements. The samples were obtained from the continuous crystalline core from Illinois deep borehole UPH 3 located in Stephenson Coun- ty, Illinois. The core is a fine-grained gran- ule aL •ne cop, an• 1• becomes progressively coarser with depth. Cracks and fractures can be seen with the unaided eye in most of the core. Many of the larger fractures are coated or filled with secondary minerals. Long, narrow, tapered cracks which are open have not been ob- served in other midcontinent granites. These narrow cracks have sharp, matching edges and appear to be fresh. They are attributed to stress relief due to removal of the core by drilling. This interpretation is supported by measurements of velocity and strain as a func- tion of confining pressure. The velocity and static bulk modulus at 1-bar pressure were smaller for samples originating from greater depths, indicating progressively greater stress relief cracking for deeper samples. The crack spectrum derived from the strain measurements showedthat the majority of the crack porosity was for cracks with closure pressures between 150 and 225 bars. The conclusion that stress relief microcracks occur with greater concentra- tions in cores from greater depths implies that cautious interpretation must be made of labora- tory measured physical properties which are sensitive to open microcracks. Introduction Cracks are nearly ubiquitous in surface rocks. Most physical properties important to geophysics such as seismic velocity, electrical conductivi- ty, or thermal conductivity are strongly affec- ted by the presence of microcracks. However, it is necessary to establish the presence or ab- sence of microcracks at depth in order to apply laboratory data to the upper crust. Further- more, the existence of microcracks at depth has a large implication for fluid transport in the upper crust. Direct examination of crystalline rocks in the upper crust is subject to an uncertainty principle limitation. The process of drilling and recovering core may introduce microcracks not originally present in the crust. The Copyright 1983 by the American Geophysical Union. Paper number 2B1956. 0148-0227/83/002B-1956505 . 00 Illinois Deep Hole UPH 3 in Stephenson County, Illinois, is an important opportunity for a microcrack study of midcontinent basement be- cause both velocity logs and continuous core are available for a large section (670-1650 m) of relatively uniform granite. The history of microcrack formation, healing and sealing processes and the in situ state of microcracking in midcontinent crystalline rocks have been the subject of several previous studies. Simmons and Nur [1968] found that the velocity variation with pressure found in labora- tory measurements on dry granite cores from wells in Wyoming and Oklahoma contrasted sharply with the nearly constant velocities measured as a function of depth in the wells. The compari- son suggested that the cores were either satura- ted or crack-free in situ. Richter and Simmons [1977] and Feves et al. [1977] used differential strain analysis (DSA) and scanning electron microscope (SEM) observations to characterize crack distributions and determine the crack his- tory for several granites from quarries and out- crops in Wisconsin and Missouri. They showed that most of the open cracks in these granites closed with a load of less than 2 kbar. Their results implied that much of the Precambrian basement in the central United States contains few or no open microcracks and that at shallow depths the physical properties of these rocks should approach the intrinsic values of the con- stituent minerals. Wang and Simmons [1978] studied gabbroic core from 5.3-km depth in the Michigan Basin. Froma combination of velocity data, DSA measurements, and microscopic observa- tions, they concluded that the gabbro wascrack- free at depth and that cracks found in the core weredue to stress relief. An important working hypothesis in the present paper is that stress relief contributes to the formation of the major- ity of microcracks observed in the Illinois gran- itic core. Engelder[1981] has reviewed the outcropand near-surface observations of strain relaxation andaccompanying changes in rock physical proper- ties. Several of the observations are consis- tent with the production of microcracks as boundary stresses are relaxed. Strickland and Ren [1980] have used DSA measurements to relate crack porosity in deep (829-3632 m) sandstones and shales to in situ stress under the assump- tion that the microcracks are the result of stress relief. Crack Pet rography The crystalline core from UPH 3 varies from fine-grained granite at the top (about 670-m 7373