Microcracks In Rock Research Articles

The three processes of brittle crystalline rock creep

Three elementary processes (elastic, plastic and dilatant deformations) in brittle rock under sustained load are considered. Though these occur independently and in parallel with each other, their contributions to the creep strain of brittle rock are not equal and vary with the stages of creep. The elastic creep governs the primary stage; whereas, visco-plastic flow dominates at the secondary stage; and the third stage; i.e. the accelarating creep, is associated with the dilatant strain caused by the process of microfracturing. Besides these, the instantaneous strain at the moment of loading plays a significant role too. The strain is a function of the magnitude of the load. When the latter approaches the failure level, the instantaneous strain is a sum of the elastic deformation and instantaneous plastic deformation as well. A rheological model of such a strain is described in the paper. The primary creep duration is estimated to be three times the relaxation time of the Kelvin substances modelling elastic creep. An analysis of experimental creep curves indicates that the relaxation time for the same rock is not constant and depends upon the magnitude of the sustained load, increasing with it, via a power function. The boundary time between the secondary and tertiary creep is found from the intensity of the process of brittle rock microfracturing. A rheological model as a massive dash-pot is proposed for visualizing the accelerating creep. A method of decomposing experimental creep curves is developed to facilitate the estimation of the essential parameters of creep of the rock under testing. This allows one to design a creep curve for the same rock at an arbitrary magnitude of sustained load up to the moment of failure.

ＤＳＣＡによる岩石内微小き裂の評価

In the first part of this study, strains under hydrostatic pressure in an elastic and isotropic body containing a number of penny-shaped microcracks were theoretically investigated to clarify the physical meaning of the parameters used in Differential Strain Curve Analysis (DSCA) and then, a new parameter was proposed to reduce the effects of anisotropy in the intrinsic linear compressibility on the evaluation of crack density.In the second part of this study, the effects of loading methods were experimentally investigated first and the results in the third loading cycle were proposed to be used to evaluate microcracks in rock. Then the parameters characterizing microcracks were determined for some rocks by DSCA. The results showed that better reproducibility was obtained by the new parameter, (Δβ/βl) I, and that microcracks in a soft and porous sedimentary rock can be evaluated by DSCA if the maximum pressure is chosen so that the pressure does not cause new microfracturing in the rock.

Seismic signals in geothermal areas of active volcanism: a case study from ‘La Fossa’, Vulcano (Italy)

Since the last eruption (1888–1890) volcanism at Vulcano, Aeolian Archipelago, southern Tyrrhenian Sea, has taken the form of persistent fumarolic activity. The gas-vapour phases of the geothermal systems are mainly discharged within two restricted areas about 1 km apart from each other, in the northern part of the island. These areas are ‘La Fossa’ crater, and the beach fumaroles of the ‘Baia di Levante’. Fluids released at the two main fumarolic fields display quite different chemical and temperature characteristics, implying different origins. The local seismicity essentially takes the form of discrete shocks of shallow origin (depth≤1 km) at ‘La Fossa’, usually with energy < 1013ergs. They are thought to be related to the uprise of pressurized hot gases and vapours discharged at the crater fumaroles. The present investigation points to the existence of two principal categories of seismic events (called ‘M-shocks’ and ‘N-shocks’). These are short events (normally < 10 s). M-type shocks are thought to be due to resonance vibrations within the interior of the volcano, probably driven by the excitation of shock-waves within cavities deeply affected by deposition and alteration of self-sealant hydrothermal minerals. N-type events display features that resemble those of volcano-tectonic earthquakes, but have no recognizable S-phases. Here they are tentatively attributed to microfracturing of rocks which have been extensively hydrothermally altered. Results of the present study permit a preliminary conceptual model of the local shallow seismic processes in the framework of geochemical modelling of fumarolic activity and geological inferences from geothermal drilling.

特集 応用地質における画像処理・解析 画像処理による微細な割れ目の抽出と評価 風化した花こう岩と一軒圧縮試験を行った安山岩における微細な割れ目の解析

特集 応用地質における画像処理・解析 画像処理による微細な割れ目の抽出と評価 風化した花こう岩と一軒圧縮試験を行った安山岩における微細な割れ目の解析

Comment on “Crack models for a transversely isotropic medium” by C.H. Cheng

Journal of Geophysical Research: Solid EarthVolume 98, Issue B8 p. 14211-14213 CommentariesFree Access Comment on “Crack models for a transversely isotropic medium” by C.H. Cheng C. M. Sayers, C. M. SayersSearch for more papers by this author C. M. Sayers, C. M. SayersSearch for more papers by this author First published: 10 August 1993 https://doi.org/10.1029/93JB00700Citations: 4AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Birch, F., The velocity of compressional waves in rocks to 10 kilobars, 1, J. Geophys. Res., 65, 1083– 1102, 1960. Birch, F., The velocity of compressional waves in rocks to 10 kilobars, 2, J. Geophys. Res., 66, 2199– 2224, 1961. Bruner, W. M., Comment on “Seismic velocities in dry and saturated cracked solids” by R. J. O'Connell and B. Budiansky, J. Geophys. Res., 81, 2573– 2576, 1976. Budiansky, B., R. J. O'Connell, Elastic moduli of a cracked solid, Int. J. Solids Struct., 12, 81– 97, 1976. Cheng, C. H., Crack models for a transversely isotropic medium, J. Geophys. Res., 98, 675– 684, 1993. Crampin, S., I. Bush, C. Naville, D. B. Taylor, Estimating the internal structure of reservoirs with shear wave VSPs, Leading Edge, 5, 35– 39, 1986. Henyey, F. S., N. Pomphrey, Self-consistent moduli of a cracked solid, Geophys. Res. Lett., 9, 903– 906, 1982. Hoenig, A., Elastic moduli of a non-randomly cracked body, Int. J. Solids Struct., 15, 137– 154, 1979. Hudson, J. A., Overall properties of a cracked solid, Math. Proc. Cambridge Philos. Soc., 88, 371– 384, 1980. Hudson, J. A., Wave speeds and attenuation of elastic waves in material containing cracks, Geophys. J. R. Astron. Soc., 64, 133– 150, 1981. Hudson, J. A., A higher order approximation to the wave propagation constants for a cracked solid, Geophys. J. R. Astron. Soc., 87, 265– 274, 1986. Kachanov, M., Continuum model of medium with cracks, J. Eng. Mech. Div. Am. Soc. Civ. Eng., 106, 1039– 1051, 1980. Kranz, R. L., Microcracks in rock: A review, Tectonophysics, 100, 449– 480, 1983. O'Connell, R. J., B. Budiansky, Seismic velocities in dry and saturated cracked solids, J. Geophys. Res., 79, 5412– 5426, 1974. Paterson, M. S., Experimental Rock Deformation-The Brittle Field, 254, Springer-Verlag, New York, 1978. Sayers, C. M., M. Kachanov, A simple technique for finding effective elastic constants of cracked solids for arbitrary crack orientation statistics, Int. J. Solids Struct., 12, 81– 97, 1991. Vakulenko, A. A., M. Kachanov, Continuum theory of medium with cracks, Mehanika Tverdogo Tela, 6, 159– 166, 1971. Citing Literature Volume98, IssueB810 August 1993Pages 14211-14213 ReferencesRelatedInformation

Methods for estimating the size of microcracks in rock

Methods for estimating the size of microcracks in rock

A data collection and analysis system for research on rock fracturing

We have made a data collection and analysis system for recording microcracks in rock to study the three dimensional pattern of fracture and the dynamic properties of rock during pressurization. This system, with 8 channel data detection, records in real-time, from the initial microcracks to the final vibration waveform excited by the master fracture. The detected data are recorded digitally at the hard disk of an IBM computer (or 286, 386 personal computer) as well as transmitted to disk. The sampling rate for each channel is 10 MHz.

Accelerated variation of earth-resistivity as a short-immediate precursor of earthquake

From the viewpoint of geophysics, if you want to study the variations of a physics parameter, you should first research the variations of its amplitude, direction and the rate of chagnes. For the variations of ps which is most important in geoelectricity, a lot of researches are about the amplitude and direction. The following research is mainly about the rate of changes,especially about the sudden change generated by acceleration variations. Rikitake et al. ( 1 9 7 6 ) reported a kind of sudden change that almost occurred at the same time with earthquake. However , this kind of change is difficult to apply in earthquake prediction. People pay attention to the sudden changes which can be used as earthquake precursors. The reasons causing sudden changes can be divided into two types. One is man-made sudden change due to abnormal work of an instrument, line leakage,environmental change, etc. The other is non man-made sudden change and is dependent on the variation of state in the earth medium. The change of stress will be able to change structure of electrical profiling and electric property. The sudden stress drop may cause fast change and made the sudden change. The rock microcrack,acceleration crack and sew up crack can generate sudden change that is caused by fast distribution again of hole liquid. Elevation and subsidence of ground-water may cause the sudden change that is caused by fast variation of electrical thickness in the shallow layer. And once the deep and shallow water containing different salinity are linked each other , i t can also cause the sudden change etc. Now we select all earth-resistivity data of the geoelectric network of Yunnan during the time period from 1979-1987 . The network consists Yuanmou, Tengchong,Chuxiong and Tonghai four geoelectric stations. In the data processing,first of all , we calculate all the daily means of the earth resist ivi ty, then the daily variation rate of the daily relative change of p ,

Thermal stress cracking in granite

We have developed a comprehensive experimental capability to study thermally induced microfractures in rock. We count acoustic emissions and measure compressional wave velocities while a sample is heated to a maximum temperature of 300°C under confining pressures up to 55 MPa. After a heating cycle we measure crack porosity and crack compressibility. We count the cracks under an SEM, sorting them by grain boundary and intragranular types and by mineral pair occurrence. Our observations reveal that (1) significant thermal cracking occurs in granites heated above 200° or 250°C under 28 and 55 MPa and above 100°C in granites heated under 7 MPa, (2) most newly created thermal cracks close at pressures below 40 MPa (1.5 km depth), (3) grain boundaries between quartz grains or between quartz and another mineral are preferentially cracked, (4) the threshold temperature for acoustic emissions is a positive, linear function of pressure, (5) total acoustic emission counts in Westerly granite heated to 300°C are slightly lower under 55 than 28 MPa confining pressure and are substantially lower under 28 than 7 MPa, (6) the compressional velocity decreases 22% as Westerly granite is heated to 300°C at 7 MPa, 15% at 28 MPa, and 10% at 55 MPa, and (7) greater hysteresis in compressional velocities occurs in samples heated at lower pressures. The acoustic emission counts in Westerly granite during heating show one or more peaks of activity as a function of temperature, with more peaks at lower pressures. We interpret this result to mean that additional crack populations are able to open at lower confining pressures. The curves of acoustic emission count rates can be fit by summing one to three Gaussian density functions.

Fractal structure of spatial distribution of microfracturing in rock : Hirata, T; Satoh, T; Ito, K Geophs J R Astr SocV90, N2, Aug 1987, P369–374

Fractal structure of spatial distribution of microfracturing in rock : Hirata, T; Satoh, T; Ito, K Geophs J R Astr SocV90, N2, Aug 1987, P369–374

Fractal structure of spatial distribution of microfracturing in rock : Hirata, T; Satoh, T; Ito, K Geophys J R Astr SocV90, N2, Aug 1987, P369–374

Fractal structure of spatial distribution of microfracturing in rock : Hirata, T; Satoh, T; Ito, K Geophys J R Astr SocV90, N2, Aug 1987, P369–374

Fractal structure of spatial distribution of microfracturing in rock

A constant stress fracture experiment of Oshima granite was carried out at the confining pressure of 40 MPa. Hypocentres of 2064 acoustic emissions were located during the experiment. Using the ‘correlation integral’, we found that the spatial distribution of hypocentres of acoustic emission is a fractal, and that the fractal dimension decreases with the evolution of rock fracturing. The spatial distribution of earthquake's hypocentres reveals fractals ranging from regional to worldwide distribution. If we extrapolate from laboratory measurements, it is possible to predict the occurrence of large earthquakes by the decrease in the fractal dimension.

Comparison between core log and hydraulic and geophysical measurements in boreholes

The fractures and microfractures in crystalline rock constitute the main transport paths for both groundwater and electric currents. They will therefore govern both the permeability and the resistivity of the rock. In order to get a better understanding of the influence of fractures on permeability and resistivity, a detailed comparison has been made between the hydraulic conductivity and resistivity, respectively, and the character of fractures in the core and the borehole wall. The fractures show very large variations in hydraulic conductivity. Microfractures and most of the thin fractures have no measurable hydraulic conductivity (in this case < 2 · 10 −9 m s −1), while test sections which contain a single isolated fracture can have no measurable to rather high hydraulic conductivities (> 10 −7 m s −1). Wide fracture zones often have hydraulic conductivities which vary from very low (less than 2 · 10 −9 m s −1) to high values (10 −5 m s −1). This indicates that the hydraulic conductivity is governed by a few discrete fractures. The resistivity shows a continuous variation in the range 1,000–100,000 ohm-m and a relatively poor correlation with hydraulic conductivities. The observed difference is considered to be the effect of restriction of water flow on a few channels, while electric surface conduction i.e. current transport through thin water films, makes current transport possible through fractures with very small apertures.

Memory, relaxation and microfracturing in dilatant rock : Holcomb, D J J Geophys Res, v86, NB7, 10 July 1981, P6235–6248

Memory, relaxation and microfracturing in dilatant rock : Holcomb, D J J Geophys Res, v86, NB7, 10 July 1981, P6235–6248

Memory, relaxation, and microfracturing in dilatant rock

When Palisades diabase and Westerly granite are made dilatant by cyclic differential stress not exceeding ≅ 85% of fracture strength, they behave as nonelastic, energy‐absorbing media. However, in some ways, their properties show reversibility more typical of elastic media. The quasi‐elastic behavior is characterized by the property ‘memory’. Memory in dilatant rock is manifested by the closure of material property hysteresis loops when the sample is returned to certain previous differential stress states. Simple examples of memory have been observed before in Westerly granite. The present work confirms the existence of memory for strain and compressional wave velocity in Westerly granite and Palisades diabase at confining pressures between 250 and 1500 bars. Memory of maxima or minima of the stress difference can be developed, with multiple points remembered simultaneously. A theory of memory based on reversible tensile Griffith cracks was developed which predicts a variety of phenomena, many of which have been observed. A property that is not predicted is the healing which occurs when unloading is stopped and the load is held constant. A time‐dependent increase in velocity and decrease in sample volume is observed, consistent with the healing of cracks. Relaxation ceases after approximately 2 min. After relaxation, a further decrease in load produces an initial elastic response followed by a return to the previous nonelastic behavior. Perfect memory is lost when microfracturing occurs as a result of either exceeding the previous absolute maximum stress or extensive cycling. Acoustic emissions are observed for conditions where memory is not perfect and are not observed when memory is perfect. Using travel time data, a new, low value was found for the dependence of dilatancy onset on pressure.

In‐plane propagation of shear microcracks in brittle rocks under triaxial compression

The localized separation of the two crack faces near the tip of a shear microcrack, which is otherwise kept closed by a normal pressure, is suggested as a possible mechanism for the propagation of microcracks in rocks loaded in triaxial compression. Finite element calculations show that when a shear crack runs along a surface of elastic discontinuity (interface between different minerals or between differently oriented domains of an anisotropic mineral), it can remain open at its tip while a normal pressure acts across that part of the crack which has reclosed. Such a separation bubble allows the shear crack to propagate in plane without frictional sliding taking place. It is speculated that dilatancy could be the result of a residual separation of the reclosed crack faces. On the basis of this mechanism a heuristic failure model is derived, which can correlate the published triaxial failure data of Westerly granite up to a confining pressure of 2000 MPa (20 kbar).

Dilation Brecciation--Proposed Mechanism of Fracturing, Petroleum Expulsion, and Dolomitization in Monterey Formation, California: ABSTRACT

The Monterey Formation has been selectively replaced by dolomite and subsequently fractured, brecciated, and relithified with several generations of dolomite cement. The two dolomite types are distinctive in morphology, color, stoichiometry, ^dgrC13, ^dgrO18, and insoluble and trace-element content. Dilation breccias evidently originate in embrittled rocks through a distinctive sequence of steps induced by tectonism: dilatancy, fluid expulsion, natural hydraulic fracturing, brecciation, hydroplastic flow, injection, and dolomite precipitation. Development is most abundant in, but not restricted to, areas of strike-slip faulting. Initially, breccia clasts are angular, large, and closely fitted. In advanced development, smaller clasts appear unsupported and volumetrically subordinate to fracture-filling dolomites. Complex examples contain a very wide range of unsorted clasts and cement, similar to a slurry. They appear to be injected under pressure into swollen bedding planes and terminal fractures. Tectonic stresses cause an initial compression and subsequent dilation (elastic) of rock microcracks and imperfections. With continued stress, the cracks are propagated inelastically and develop into major fracture networks. Fracturing associated with excess pore-fluid pressures triggers an instantaneous flow of connate fluids across several hundred feet of newly fractured strata. The resulting sharp drop in fluid pressure and temperature causes rapid precipitation of fracture-healing dolomite. The relithified rock is then subject to renewed dilatancy and rupture. The dilatancy is pervasive and sufficient in magnitude to cause the expulsion of indigenous petroleum held initially in the organic matrices of the relatively impervious Monterey Shale. Several periods of petroleum migration are recorded in breccia paragenesis. Dilation breccia is a distinct form of nondepositional breccia. It probably occurs in many tectonic provinces. End_of_Article - Last_Page 1586------------

Microfractures in rocks from two geothermal areas

Core samples from the Dunes, California, and Raft River, Idaho, geothermal areas show diagenesis superimposed on episodic fracturing and fracture sealing. The minerals that fill fractures show significant temporal variations. Sealed fractures can act as barriers to fluid flow. Sealed fractures often mark boundaries between regions of significantly different physical properties. The fracture porosities measured on several samples are less than 0.1%. This low value indicates that fractures are effectively sealed or that fracturing is confined to the relatively few large fractures visible within the samples. Fracture sealing and low fracture porosity imply that only the most recently formed fractures are open to fluids.

Earthquake prediction and oriented microcracks in rocks : 1F, 8R. Nature, V251, Oct 4TH, 1974, P405–406

Earthquake prediction and oriented microcracks in rocks : 1F, 8R. Nature, V251, Oct 4TH, 1974, P405–406

Earthquake prediction and oriented microcracks in rocks

VARIATIONS in the ratio of compressional wave velocities and shear wave velocities, presage some earthquakes1. The ratio first decreases, but just before the earthquake it returns to, or rises above, the original value. An attractive model has been proposed by Nur2. Briefly, he suggested that anelastic expansion of the rock (dilatancy) reduced the velocities. With the subsequent rise of the pore pressure of water, the velocity increases again. The increased pore pressure also weakens the rock, resulting in an earthquake. All that effects the velocity of compressional waves more than the velocity of shear waves, giving the apparent variation in their ratio.