In this study, the ultrasonic wave tomography scanning on artificial fractured and inhomogeneous rock, as well as on a homogeneous cement mortar specimen under compressive loading cycles are conducted in laboratory scale. First-arrival times of ultrasonic wave through fractured and inhomogeneous rock are recorded using a pair of 150 kHz transmitter and receiver. Along a typical cross section, up to 400 scanning line measurements are performed and velocity tomographic images are developed. The velocity tomography distinctly identifies the fracture and the inhomogeneous rock material in the interior of specimens. The tomography scanning also demonstrates that as the strength of rock decreases under repeated cyclic compressive loading, the ultrasonic wave velocity decreases accordingly. In addition, ultrasonic wave propagation through inhomogeneous rock is simulated using ABAQUS, a general-purpose finite element program capable of simulating acoustic wave propagation. A finite element model is built for the cylindrical specimen of the inhomogeneous rock. Acoustic pressure wave of 150 kHz is applied on a small area at the center of the cylindrical surface and the wave propagation through the rock is simulated using the transient dynamics analysis procedure. The simulated first arrival time agrees with the experimental result. In addition, the finite element modeling provides more detailed characteristics of the wave propagation through the rock, such as the attenuation of the wave as it propagates through the rock and the wave frequency shifting through the rock as observed in some laboratory experiments.
Rock physical properties, stress, fracture and mechanisms of failure can be studied using seismological techniques at locations where seismic events are generated. Seismic travel time and amplitude data can be used to determine the nature of the rock between the source and the receiver. Tomographic imaging and seismic monitoring have been used to characterize rock in the laboratory and rock masses in situ. Ultrasonic imaging in the laboratory (20 kHz -2 MHz frequency) and seismic imaging in situ (1 ? 10 kHz frequency) provide a combined approach to study rock properties and stress environment.
Tomography is an application of nondestructive testing to view the interior of a body without penetrating its surface by physical means. In tomography, the radiation carries information about the physical properties in the transmitted medium when radiation is passed through a material along a straight line [1]. This process is repeated with different locations of transmitting and receiving radiation to achieve the desired resolution. This method of imaging is used widely in the medical field using ultrasound waves or x-rays to create a cross-sectional image. The data are then inverted using a transform technique. Tomographic inversion techniques are derived from the Radon Transform. The transform was named after Radon in 1917 and was described in depth by Nolet [2]. There are in existence a variety of algorithms that can construct tomographic images, such as filtered back projection, algebraic reconstruction tomography (ART) and simultaneous iterative reconstruction technique (SIRT). Tomography has been used in the field for many mining and geological applications, including geologic characterization, void detection, and stress analysis. Structural changes in geology are detected by measuring anisotropy [3], high resolution 3D tomography [4], or by noting velocity characteristic changes across a rock mass [5, 6]. High stress detection is also an important application of ultrasonic tomography.
