Velocity anisotropy in dry and saturated rock spheres and its relation to rock fabric

Abstract

The effects of water saturation on compressional wave velocity, V p , and elastic property anisotropy are investigated omnidirectionally in dry- and water-saturated spheres of Barre Granite and Tennessee marble by the ultrasonic pulse method. Barre Granite is found to be strongly anisotropic in V p with orthorhombic symmetry at both dry and saturated conditions. Tennessee marble is only weakly anisotropic in V p and changes in symmetry from axial at dry, to orthorhombic at saturated conditions. Anomalously low velocity in these rocks in the dry condition, compared with estimated intrinsic velocity, and substantial increases in V p with water saturation of about 30 per cent for the granodiorite and 12 per cent for the marble indicate the presence of pre-existing microcracks in these rocks. Compressional wave velocity anisotropy indicates that the microcrabks have strong preferred orientation in the Barre Granite and weak preferred orientation in the Tennessee marble. Petrofabric investigations of the preferred orientation of structural elements in these rocks support this conclusion. The scanning electron microscope is used to examine typical pore structure characteristics and determine some crack aspect ratios in these rocks. The technique of determining V p in dry and saturated rock spheres is shown to provide information about (1) the presence of microcracks and their effect on the elastic constants, (2) the anisotropy in elastic as well as other physical properties, and (3) the dominant structural cause of anisotropy. The mean V p is saturated rock moreover is useful for predicting the velocity in rock at shallow depths below the water table. The velocity difference method for detecting microcracks and their preferred orientation might be used to assess the effectiveness of certain fragmentation techniques and, moreover, may have widespread application in rock mechanics research because of the great effects microcracks can have on the elastic, strength and other properties of rock.