Using Seismic Techniques to Characterize Fracture in Rock (original) (raw)
Related papers
Nondestructive ultrasonic testing is commonly used to assess damage in infrastructure mostly based on elastic wave velocity. This study focuses on understanding the effects of a thin fracture not only on ultrasonic elastic wave velocity but also on attenuation. Experiments are performed to quantitatively assess the effects of a thin fracture within polymethylmethacrylate (PMMA) specimens. Wave velocity and attenuation are measured across the width of these homogeneous specimens using the ultrasonic pulse velocity method. Seventeen specimens are tested for three different conditions (intact, with a hole, and with a fracture) for two different thicknesses. First, specimens made of two PMMA blocks with an intact fused interface are tested; then, specimens with a small hole (created for generating stress concentration) perpendicular to the interface and milled ends are tested; and, finally, specimens with an induced fracture at the fused interface are tested. Four additional specimens, two with fused (but weak) interfaces between blocks and two solid blocks, are tested during fracture growth under uniaxial strain-controlled test conditions. In fact, wave attenuation can cause the first arrival to be undetected and overestimated by up to 10 %. This error in the selection of the first arrival could be misinterpreted as a change in wave velocity when fractures are present in the material. Although wave velocity shows marginal reduction, less than 4 %, when a thin fracture is present, wave energy attenuates by up to 60 %. This work demonstrates quantitatively that wave attenuation measurements from selected frequency bands in the Fourier spectra can be used to identify the presence of thin fractures using ultrasonic testing.
Earth, Planets and Space, 2009
We analyzed temporal changes in the velocity and amplitude of P waves transmitted through a granite sample during a triaxial compression test, with the goal of monitoring the fault formation process associated with open and shear cracking. We used newly developed transducer assemblies for the broadband recording, and we continued to record transmitting waves even after the peak stress occurred. For transmitting P waves with paths parallel to the maximum compressive axis, we found that both the first wave amplitude and the velocity decreased after dilatancy started, and they kept decreasing even after the peak stress. In addition, the large nonlinear decrease in amplitude was associated with a rapid decrease in differential stress, whereas the rate of decrease in velocity remained almost constant. Thus, before the rapid decrease of differential stress, when both the amplitude and the velocity gradually decreased, open cracking was indicated to be dominant. Thereafter, shear cracking was indicated to become dominant in synchronization with the rapid decrease in differential stress. It is suggested that a main fault started to grow around the sample surface and then progressed into the sample interior; this corresponded to the rapid stress decrease. This fault acts as a strong scatterer for P waves that are parallel to the maximum compressive axis.
Stress-induced ultrasonic wave velocity anisotropy in fractured rock
Ultrasonics, 1988
The closure of cracks in rock under an applied compressive stress can significantly affect the permeability of the rock. Crack closure may be monitored using ultrasonic wave velocities, since these are significantly reduced in the presence of open cracks. When a non-hydrostatic compressive stress is applied to a rock, an initially isotropic distribution of cracks will become anisotropic and the rock will display an elastic anisotropy determined by the orientation distribution of those cracks remaining open. The crack orientation distribution function gives the probability of a crack having a given orientation with respect to a set of axes fixed in the rock. The coefficients W,,,,, of a series expansion of this function in generalized Legendre functions can be obtained to order /=4 from the angular variation of the elastic wave velocity. This allows construction of microfracture pole figures, which specify the orientation distribution of open cracks. The theory is applied to the measurements of Nur and Simmons, who applied a uniaxial compressive stress to a sample of Barre granite. Cracks with normals aligned along the stress direction are closed preferentially in agreement with the theory of Walsh. However, for crack normals perpendicular to the applied stress there is some evidence of crack opening that is not predicted by the theory. This is also observed in the electron microscope study of Batzle et al. and a possible mechanism is discussed.
Laboratory measurement of elastic waves in Basalt rock
Measurement, 2017
This paper reports the laboratory measurement of compression and shear waves in Basalt rock. Compression or primary waves were produced using a standard ultrasonic pulse velocity tester. However, it is well accepted that the production of pure shear waves in rock is difficult because of which suitable assumptions are made in the testing. Since bender elements have traditionally been used to produce pure shear at very small strains in soft materials, their use has been extended in this study to Basalt rock. The fabrication and setup of bender element tests is first discussed. The transducers were accommodated in pre-drilled slots at the two flat ends of the samples, the effect of which was carefully investigated using experimental and numerical studies. For 10 to 12 mm long piezoceramic plates, the shear wave velocity was found to reduce by about 5% because of the slots. The shear wave velocity was estimated using the first arrival method after removing the near field and the crosstalk effects from the output signal. The shear wave velocity ranged between 1.15 and 3.31 km/s and was nearly equal to one-half of the primary wave velocity. The results also show that the ratio of the shear wave velocity and the compression wave velocity was independent of the density and porosity of the Basalt rock. Because most of the rock was unweathered and compact and, its cavities filled with secondary minerals such as zeolites, calcites and silicates, it is unlikely that the high overburden pressures would affect the observed wave velocities.
Chapter 3: Elastic waves in isotropic, homogeneous rocks
IFP Energies nouvelles eBooks, 2014
This book can be considered as a natural continuation of the book entitled 'Acoustics of Porous Media', co-authored by Thierry Bourbié, Olivier Coussy and Bernard Zinszner, and issued by our laboratory in 1986 for the French version, and in 1987 for the English version. However, here the clear guideline is experimentation. In contrast to previous books, all the techniques, from the most conventional (using piezoelectric transducers) to the most recent spaceage methods (as laser ultrasonics) are detailed. Furthermore the book is mainly based on experimental data allowing to select the most appropriate theories for describing elastic wave propagation in rocks. Emphasis on Nonlinear elasticity and Seismic anisotropy are also originality of the book. A part of the book also focuses on the history of the different sub-fields dealt with, having in mind that the knowledge of the history of a field contributes to understanding the field itself. For instance, in spite of the clear anteriority of their work the names of the Persian mathematician, physicist and optics engineer Ibn Sahl, and of the English astronomer and mathematician Thomas Harriot are unfairly not, or rarely, associated with the law of refraction, compared to the names of the Dutch astronomer and mathematician Willebrord Snell van Royen, known as Snellius, and of the French philosopher and writer René Descartes, as detailed in the first chapter.
On wave and fracture propagation in Rock Media
Experimental Mechanics, 1975
Transient deformations and wave and fracture propagation were studied in marble and granite plates loaded explosively, by means of photoelastic coating methods, and moire'and high-speed photographic techniques by I. M. Daniel and R. E. Rowlands ABSTRACT-~Experimental-stress-analysis techniques were used to study wave and fracture propagation in rock media. Marble and granite plates were loaded explosively. Wave propagation was observed through isochromatic-fringe patterns on bonded photoelastic coatings and moire-fringe patterns. These patterns were recorded with a Beckman and Whitley camera operating at rates from 250,000 to 1,000,-000 frames per second. Dilatational, shear and Rayleigh wave velocities were determined. The leading part of the pulse is compressive and shows appreciable attenuation. The trailing part goes into tension, causing widespread tensile fracture. The velocity of propagation of this fracture zone in marble was nearly equal to the theoretical terminal velocity. In the case of induced cracks in marble and granite, the velocities of crack propagation were appreciably lower than the theoretical terminal values. Experimental results obtained were discussed and interpreted for their relevance to the rapid-excavation process in rock.
Acquisition of Complete Acoustic Emission Amplitude Records during Rock Fracture Experiments
2014
This paper presents the results from a triaxial deformation experiment where acoustic emission (AE) waveforms were continuously recorded at two different gain levels. The purpose of this work is to quantify the extent of missing amplitude data, which is typically lost at critical points during rock fracture experiments due to waveform clipping. A cylindrical sample of Westerly granite was axially loaded until failure at a constant displacement rate and 25 MPa of confining pressure. AE was monitored by 18 piezoelectric sensors mounted on the cylinder ends and around its circumference. AE data was continuously acquired and digitized at 10 MHz and 12-bits for the duration of the experiment where four channels were amplified 6 dB and the rest 40 dB. Two large stress-drops occurred in the post peak-stress regime resulting in large amplitude bursts of AE. Channels amplified 40 dB showed complete amplitude saturation, making event analysis difficult whereas those channels amplified 6 dB re...
Elastic waves in fractured rocks under periodic compression
International Journal of Mechanical and Materials Engineering
Background: One of the current problems in studying the mechanical properties and behavior of structurally inhomogeneous media with cracks is the characterization of acoustic wave propagation. This is especially important in Geomechanics and prognosis of earthquakes. Methods: In this work, the authors propose an approach that could simplify characterization of wave propagation in medium with cracks. It is based on homogenization procedure performed at a set of equations characterizing acoustic wave propagation in media weakened by fractures under condition of external distributed loading. Such kind of loading in most cases is close to the real one in case of consideration of Geomechanics problems. Results: On the basis of the proposed homogenization technique, we performed characterization of elastic properties and plane acoustic waves propagation in a pre-loaded linear elastic medium weakened by a large amount of cracks. We have investigated two special cases of loading: uniaxial compression and complex compression. We have also studied how the wavespeeds depend on averaged concentration and distribution of craks. Conclusions: Effective elastic properties were theoretically characterized for fractured media under external loading. The results revealed high dependency of the longitudinal wave propagation speed on the relation between stresses reasoned by an external loading.
Ultrasonic Velocities, Acoustic Emission Characteristics and Crack Damage of Basalt and Granite
Pure and Applied Geophysics, 2006
Acoustic emissions (AE), compressional (P), shear (S) wave velocities, and volumetric strain of Etna basalt and Aue granite were measured simultaneously during triaxial compression tests. Deformation-induced AE activity and velocity changes were monitored using twelve P-wave sensors and eight orthogonally polarized S-wave piezoelectric sensors; volumetric strain was measured using two pairs of orthogonal strain gages glued directly to the rock surface. P-wave velocity in basalt is about 3 km/s at atmospheric pressure, but increases by > 50% when the hydrostatic pressure is increased to 120 MPa. In granite samples initial P-wave velocity is 5 km/s and increases with pressure by < 20%. The pressure-induced changes of elastic wave speed indicate dominantly compliant low-aspect ratio pores in both materials, in addition Etna basalt also contains high-aspect ratio voids. In triaxial loading, stress-induced anisotropy of P-wave velocities was significantly higher for basalt than for granite, with vertical velocity components being faster than horizontal velocities. However, with increasing axial load, horizontal velocities show a small increase for basalt but a significant decrease for granite. Using first motion polarity we determined AE source types generated during triaxial loading of the samples. With increasing differential stress AE activity in granite and basalt increased with a significant contribution of tensile events. Close to failure the relative contribution of tensile events and horizontal wave velocities decreased significantly. A concomitant increase of double-couple events indicating shear, suggests shear cracks linking previously formed tensile cracks.