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PURPOSE: This study aims to confirm the usefulness of active acoustic emission (Active AE) for reproducible and non-invasive generation of physical external force which is required for conventional AE. METHODS: Experiment 1: A root dentin-resin adhesive interface was observed. The post space was filled with a dual-cure resin composite core material with and without adhesive. The vibration characteristics of the data obtained from the time-frequency analysis were evaluated. Experiment 2: A crown-abutment tooth adhesive interface was observed. Adhesive resin cement was used for luting the crown and adhesion states in the same specimen over time were analyzed with three measurements: at trial-fitting, immediately after luting, and 2 weeks after luting. Data were subjected to time-frequency analysis and relationships between amplitude (indicating loudness) and frequency (indicating the sound component) were analyzed. RESULTS: Experiment 1: Time-frequency analysis confirmed multiple peak frequencies for each specimen without adhesive and monomodal peak frequency in all specimens using adhesive. Experiment 2: Two weeks after luting, all specimens showed a single major peak except one which showed multiple weak peaks. The three-dimensional visualization of time-frequency analysis revealed one specimen with multiple weak peaks while all others displayed a single, low-amplitude band at 2 weeks after luting. CONCLUSION: The state of the adhesive interface can be evaluated using active AE. This basic technique may prove useful to evaluate changes in the adhesive interface of prostheses over time. ispartof: JOURNAL OF PROSTHODONTIC RESEARCH vol:66 issue:2 pages:236-242 ispartof: location:Japan status: published

Detection and analysis of small magnitude events is valuable for better characterization and understanding of reservoirs in addition to developing strategies for mitigating induced seismicity. Three-component (3C) receivers, which are now widely used, are commonly deployed in boreholes to provide continuous seismic data amenable to novel and powerful analysis. Using multicomponent continuous records of ground motion, we utilize two principal features of the direct P-wave arrival: (1) linearity and (2) polarization in the direction along the ray path to the source region to detect small magnitude events undetectable by conventional methods. We evaluate the linearity of polarization and direction of arrival in the time and frequency domains by introducing the spectral matrix analysis method and combine them into a scalar characteristic function that is thresholded for event detection purposes. We boost the signal-to-noise ratio by stacking characteristic functions obtained at different 3C receivers along an empirical moveout of a master event known to have occurred in an area of interest. This allows us to detect smaller events and spatially tie them to a relatively small area around the large event. We apply our method to field data recorded at the Groningen gas field in the Netherlands. Our method detects all catalog events as well as several previously undetected events.

Identification and clustering of similar acoustic emission (AE) events are important to determine source locations precisely and to evaluate subsurface cracks. An AE cluster analysis method using phase-only correlation (POC) is proposed to identify and hierarchically classify similar AE waveforms. The POC of time-varying spectral representations is used to evaluate the similarity between two waveform images in the time–frequency domain, and cluster analysis is used to classify the waveforms into groups according to a distance measure. The method is applied to waveforms from local earthquakes in Japan to assess its ability to identify similar waveforms perturbed by white noise.

A highly-stressed shaft pillar is prone to large seismic events, falls of ground and rockbursts, which may cause injuries and loss of production, especially in weak geotechnical zones. It is thus important to identify weak geotechnical zones in order to mitigate risks. In this study, we present integrated studies (underground mapping, petrology , rock mechanics and high-resolution microseismicity analysis) to understand the different geotechnical zones in the shaft pillar of Cooke 4 mine in South Africa. The footwall of the remnant shaft pillar comprises the Upper Elsburg reef of the Mondeor Formation, while the Ventersdorp Contact Reef (VCR) of the Venterspost Formation and soft/weak lavas of the Westonaria Formation form the hangingwall. Results from underground mapping and microscopic analysis show that the shaft pillar is composed of quartzites , pebbly quartzites, argillaceous quartzites and conglomerates. Underground mapping further shows that the shaft pillar is characterized by several discontinuities , which vary from minor to macro scale fractures. Laboratory uniaxial compressive strength (UCS) tests indicate that quartzite has the strongest strength, followed by pebbly quartzite, argillaceous quartzite and lastly, conglomerate. Analysis of high-resolution acoustic emissions (AEs) clusters indicates that the majority of AEs are associated with the mining stope faces. The clusters show the formation of Ortlepp shears ahead of the stope, which is caused by the excavation-induced stress field. Microseismic data further reveal that the fracture turning-point occurs in the soft strata (weak hangingwall lavas). The integration of these datasets has allowed us to develop the fracture model for different geotechnical zones, which concurs with previous models developed for the similar underground environment (i.e., weak/soft lava hangingwall and quartzite/conglomerate footwall). This has major implications for future mining, support, production and safety.

We investigated the activities of very small (−5.10 ≤ MW ≤ −2.35) repeating earthquakes that occurred during 14 months on a geological fault at a 1 km depth in the Cooke 4 gold mine in South Africa. Out of 4214 acoustic emissions occurring on the fault, 1785 (42.36% of the total) were identified as repeaters on the basis of the waveform similarity and the proximity of hypocenters. Quite a few large groups, having up to 57 members, were recognized. Activities of some groups certainly continued throughout the 14 months, but we also found some repeater groups that newly emerged or disappeared during this period. Both types often coexisted within a distance as small as ~0.5 m, within which background fault creep should be coherent. Some disappearing groups showed a decrease in the event magnitude with time. These temporal changes of repeater activities may imply the formation and dissipation of topographical asperity contacts on the fault. Our results suggest that ultrasensitive observation of acoustic emissions can reveal the physical evolution of asperities.

Forecasting the shape of a microseismic cloud is essential to pre-design an energy extraction system. The microseismic cloud produced after hydraulic stimulation is empirically known to extend to t...

We established a method for installing a methyl group at the β-position of a butenolide ring. The methylated position is located at the congested ring juncture of a 5,6,5-tricyclic lactone, which is common to neurotrophic

The presence of fault gouge and the generation of wear material between two sliding rock surfaces plays a critical role in slip weakening and propagation of ruptures along underground brittle shear fractures forming ahead of tabular excavations in deep and high stress gold mining. We performed two types of friction experiments: one with a fault gouge layer between two sliding surfaces, and the other without a fault gouge layer ‘rock-on-rock’, both under room dry conditions at slip velocities ranging from ~1.0 mm/s to 1200 mm/s. These friction experiments revealed a remarkable difference in the frictional weakening behaviour, e.g., rock-on-rock friction experiments show weakening behaviour at lower slip velocity (~5 mm/s) and generally has lower frictional strength than those with the intervening fault gouge between sliding surfaces. This study shows that the existence of the fault gouge layer between sliding rock surfaces delays the onset of fault weakening (i.e., slip weakening displacement of gouge layer experiments is larger compared to rock-on-rock experiments). It is proposed that flash heating may be the main active weakening mechanism within both our gouge and rock-on-rock experiments, and provides a feasible account for the observed weakening. The observed slip weakening displacement (Dc) differences may be attributed to the presence of a gouge layer between sliding surfaces, which has many more contacts during sliding compared to rock-on-rock experiments, thus reducing the average slip velocity per contact, consequently, the potential for activation of flash heating which delays the onset of weakening. Here we suggest that we may be able to describe brittle shear fracture rupture propagation process along underground brittle shear zones by conducting low, intermediate and high slip velocity friction experiments with and without an intervening fault gouge between sliding rock surfaces. These findings should have important implications for the modelling of rupture propagation processes in underground shear zones, a phenomenon that influences the severity of rockbursts, and hence the safety of mine workers and mining operations.

We observed very small repeating earthquakes with −5.1 ≤ Mw ≤ −3.6 on a geological fault at 1 km depth in a gold mine in South Africa. Of the 851 acoustic emissions that occurred on the fault during the 2 month analysis period, 45% were identified as repeaters on the basis of waveform similarity and relative locations. They occurred steadily at the same location with similar magnitudes, analogous to repeaters at plate boundaries, suggesting that they are repeat ruptures of the same asperity loaded by the surrounding aseismic slip (background creep). Application of the Nadeau and Johnson (1998) empirical formula (NJ formula), which relates the amount of background creep and repeater activity and is well established for plate boundary faults, to the present case yielded an impossibly large estimate of the background creep. This means that the presently studied repeaters were produced more efficiently, for a given amount of background creep, than expected from the NJ formula. When combined with an independently estimated average stress drop of 16 MPa, which is not particularly high, it suggests that the small asperities of the presently studied repeaters had a high seismic coupling (almost unity), in contrast to one physical interpretation of the plate boundary repeaters. The productivity of such repeaters, per unit background creep, is expected to increase strongly as smaller repeaters are considered (∝ Mo −1/3 as opposed to Mo −1/6 of the NJ formula), which may be usable to estimate very slow creep that may occur on intraplate faults.

We applied advanced mapping techniques to 291 230 acoustic emission (AE) events as small as around M −4 that were recorded over 50 days by an ultra-high resolution network close to the active front of a tabular mining stope being advanced northward at 1 km depth in the Cooke 4 Gold Mine in South Africa. We first applied joint hypocenter determination (JHD) to improve absolute locations, and then applied the double-difference relative location algorithm to the JHD output. These steps resolved the seemingly continuous, dense cloud of AEs that extend about 20 m ahead of the stope front into several discrete, steeply dipping tabular clusters a few meters thick and 10–30 m in dip extent, separated by quiet intervals a few meters thick. The clusters have a strike parallel to the stope face and a dip of about 65°, resembling commonly observed large shear fractures along the plane of maximum shear (Ortlepp shears). In general, the activity of the clusters changed in similar ways as the stope face advanced, but each cluster remained stationary and the gaps between clusters were impressively quiet. This study demonstrates that high-resolution AE mapping can delineate the formation of large structures of localized damage in the highly stressed intact rock mass ahead of the stope face, a process that may culminate in hazardous seismic events.

Acoustic Emissions (AE) down to M W ~ –4 were recorded at a site 1 km beneath the surface in the Cooke 4 Mine, South Africa. Several planar AE clusters with lateral extent of 10–100 m were identified. Most of them were located several tens of meters away from the mining front, and exhibited steady activity during the analysis period of about two months. Some of the clusters coincided with mapped faults. The planar-cluster AEs were sharply aggregated within a thickness of several decimeters, likely delineating the fracture interface of the fault and its higher-order morphology such as branches, bends, and stepovers. The composite focal mechanism evaluated for each cluster was consistent with slip events on the fracture interface. These results imply that numerous shear microfractures occur steadily on a natural fault surface subjected to a mining-related stress increase. The planar clusters consist of very small AEs (99.7% were smaller than M W –2), exhibiting high b -values much exceeding unity. This contrasts with the more usual b -values of the stope-cluster AEs, which were aggregated within 20 m of the mining front and exhibited a more scattered distribution. The size distribution of microfractures on a fracture interface may directly reflect fine-scale irregularities of the interface. On the other hand, many other mapped faults near the planar AE clusters were not accompanied by AE activities, despite the fact that these quiet faults were subjected to a similar stress history. The presence or absence of AE activities on a fault may reflect different states of the fault, including stress and strength.

Three months of acoustic emission (AE) monitoring in a South African gold mine down to Mw −5 revealed a newly emergent planar cluster of 7557 events −3.9 ≤ Mw ≤ −1.8 (typical rupture radius of 6–70 cm) that expanded with time to reach a size of 20 m on a preexisting geological fault near an active mining front 1 km beneath the ground. It had a sharply defined, planar configuration, with hypocenters aggregated within a thickness of only several decimeters. We infer that the zone defines an aseismic slip patch on the fault, wherein the individual AEs represent failures of very small asperities being loaded by the aseismic slip. Additional support for the interpretation was obtained by analyzing composite focal mechanisms and repeating events. The patch expansion over 2 months was likely quasistatic because all individual AEs ruptured much smaller areas than the cluster size at the corresponding time. The b values dropped gradually from 2.6 to 1.4, consistent with a significant increase in shear stress expected of the mining style. Another cluster with similar characteristics emerged later on a neighboring part of the same fault and grew to a 10 m extent in the last weeks of the study period. The quasi-static expansion of inferred localized slow-slip patches to sizes of 10–20 m suggests that the critical crack length on natural faults can be at least as large, much exceeding the decimeter range derived from laboratory stick-slip experiments on saw-cut rocks.

We investigated frequency-magnitude distribution (FMD) of acoustic emissions (AE) occurring near an active mining front in a South African gold mine, using a catalog developed from an AE network, which is capable of detecting AEs down to M W −5. When records of blasts were removed, FMDs of AEs obeyed a Gutenberg−Richter law with similar b values, not depending on post-blasting time from the initial 1-min interval through more than 30 h. This result denies a suggestion in a previous study (Richardson and Jordan Bull Seismol Soc Am, 92:1766–1782, 2002) that new fractures generated by blasting disturb the size distribution of background events, which they interpreted as slip events on existing weak planes. Our AE catalog showed that the GR law with b ∼ 1.2 was valid between M W −3.7 and 0 for AEs around the mining front. Further, using the mine’s seismic catalog, which covers a longer time period of the same area, we could extend the validity range of the GR law with the same b value up to M W 1.

We measured the thickness of the transverse structures associated with the bovine coccygeal transverse processes (bone specimen) by using ultrasonic waves and examined the reliability of this measurement for use in spinal surgery. We first measured the velocity of ultrasonic waves propagating in the spinous process. We then made a hole in the transverse process with an air drill and placed an ultrasonic transducer with a center frequency of 10 MHz in the hole. The time of reflection of the ultrasonic wave from the underside of the transverse process was detected to estimate the remaining bone thickness. The thickness estimated by using ultrasound was compared with the thickness measured by microscopic examination. We could detect reflection waves from the underside of the transverse process in 91.7% of cases (i.e., 22 of 24 measurements using 6 bones from 3 cows). The thickness of the transverse processes in which we detected the waves varied from 0.24 to 6.8 mm. The 95% limit of agreement between ultrasonic and histological measurement was 0.71 mm. Pearson's correlation coefficient showed a strong and positive relationship between the two measurements (r = 0.97, n = 22, P

Identification of similar seismic events is important for precise estimation of source locations and for evaluation of subsurface structure. Phase-only correlation is well known as a real-time image-matching method for fingerprint identification. I applied the phase-only correlation in a geophysical context to identify similar waveforms among microseismic events. The waveforms were first transformed into time-varying spectral representations to express frequency content in the time-frequency domain. The phase-only correlation function is calculated between two time-varying spectral representations and similarity is evaluated using the peak value of the phase-only correlation function. This method was applied to arbitrarily selected waveforms from aftershocks of an earthquake in Japan to assess its ability to identify similar waveforms perturbed by white noise. The detection of similarity of the proposed algorithm was compared to the similarity as detected by a 2D crosscorrelation function of the time-varying spectral representation and a 1D crosscorrelation of the raw waveform. This showed that the phase-only correlation function exhibits a sharp peak that quantifies similarity and dissimilarity over a wide range of signal-to-noise ratio (S/N) and remained unaffected by the length of the time window used to estimate time-varying spectral representations. Phase-only correlation may also have applications in other geophysical analyses and interpretations that are based on waveform and seismic image data.

I present a statistical method for detecting the arrival times of polarized seismic waves on three-component seismic observations in the time and frequency domains. In this method, the polarization, which is a representation of the 3D particle motion of seismic waves, is evaluated on the basis of a spectral matrix in the time and frequency domains and the statistical parameters are defined by using the eigenvalues of the spectral matrix for detecting the arrival times of linearly and elliptically polarized waves, where the idea of a statistical test of hypothesis is introduced. An evaluation of a synthetic signal revealed that the method can detect the arrival times of linearly and elliptically polarized waves within 10 sampling points at signal-to-noise ratios of −7 dB. Application of the method to an earthquake suggested that it can be used to detect the arrival times of both linearly and elliptically polarized waves, which are difficult to identify manually.

A series of injection-induced slip experiments using pre-fractured granitic rock specimens was conducted to investigate the dynamic hydraulic responses of existing fractures during hydraulic stimulation. The results show a characteristic stepwise slip and temporal increase in fluid flow in the fractures during slip. This increase is considered to result from significant shear dilation that is sufficient to change the flow properties of rough fractures with large asperities. Our laboratory data show that the stepwise slip is caused by temporal drops in pore fluid pressure in the fractures associated with shear dilation, which in turn indicates that the geometry of fracture surfaces plays a significant role in terms of both slip behavior and changes in fluid flow during induced slip. These results raise the possibility that stepwise slip and temporal permeability changes occur in actual rough fractures. This hydro-mechanical coupling behavior of such fractures during fluid injection suggests that dynamic (temporary) permeability changes may occur in geothermal reservoirs subjected to hydraulic stimulations.

SUMMARY A small change in traveltime and shear-wave splitting in elastic waves transmitted through a pressurized single fracture has been detected through downhole triaxial cross-hole seismic measurements. A field experiment was carried out at Higashi‐Hachimantai Field of Tohoku University, Japan, where an artificial single fracture had been created at 369 m by hydraulic fracturing in intact welded tuff. P and S waves transmitted through the pressurized single fracture were measured by using a downhole three-component seismic detector installed in a borehole near the fracture. In our previous studies, a small change in traveltime of about 0.1 ms was detected by using the cross-spectral moving window technique. In this recent study, the arrival times of split shear waves and the polarization directions were detected by using the wavelet transform of a three-component signal. The traveltime difference between the first and second shear waves was 0.05‐0.12 ms, when the wellhead pressure was increased up to 3.4 MPa and the incident angle of waves into the main fracture was changed from 12 ◦ to 20 ◦ . A fracture model of a single fracture and a microcrack zone in the vicinity of the main fracture was used to explain the shear-wave splitting phenomenon. The width of the reopening microcrack zone and the crack density were quantitatively evaluated by using the fracture model and shear-wave splitting analysis. The best-fit theoretical curve to the observed data suggested that the width of the reopening zone was 0.8 m and the crack density was 0.39.

Hydraulic fracturing experiments were carried out at the Bernburg salt mine in Germany. The induced acoustic emission (AE) events were detected by accelerometers surrounding the sources, and the observed AE events had a frequency component of over 40 kHz. Many similar waveforms were identified in the detected events, and their source locations were determined with an error of less than 6 cm. The principal direction of the source distribution changed with depth according to the rotation of the principal stress direction, and the shape of the fracture was determined to be elliptical. The source locations were distributed as an ellipse with a radius of about 1 m in both fracturing and re-fracturing tests at six different levels. A theoretical examination of fracture growth showed that macroscopic fractures extend with an aspect ratio which is a function of normal and shear stresses, and suggests that shear stress leads to the formation of elliptical fractures and that the elliptical shape of macroscopic fractures is controlled by the ratio of the shear stress to the normal stress.

SUMMARY A high-rate injection of 20 000 m3 of water into granite between 2.8 and 3.4 km depth at the Soultz hot dry rock (HDR) test site in France in 1993 September led to a 200-fold increase in borehole transmissivity and produced a subvertical cloud of microseismicity of dimensions 0.5 km wide, 1.2 km long, 1.5 km high and oriented 25°NW. The resulting data set is unusually complete and well suited to studying permeability creation/enhancement processes in crystalline rock and the utility of microseismic data for revealing them. Although the microseismic cloud defined using joint hypocentre determination (JHD) locations was diffuse and showed little structure, application of the collapsing method showed it to be composed largely of discrete tubes and planes that propagated coherently. One prominent structure that extended 350 m downwards from the vicinity of a flow inlet early in the injection and that appears to contain a major flow path was subjected to detailed investigation to establish its hydrogeologic nature and the mechanisms underpinning its inferred permeability enhancement. High-resolution microseismic mapping techniques (i.e. multiplets and clustering) showed it to be a subvertical, NNW–SSE striking, fracture zone of width 10–20 m. The strike and scale of the structure identifies it as a member of a family of hydrothermally altered, cataclastic shear structures that constitute the primary permeable paths for fluid migration within the rock mass, both under ambient and forced fluid flow conditions. The microseismicity occurred on subvertical, small-scale fractures within the cataclastic shear zone whose azimuths scatter within 22° of parallel to the parent structure. Although the structure is likely to have been naturally permeable to some degree, its permeability appears to have been significantly enhanced as a consequence of the injection. The most likely mechanism of permeability enhancement, which is in accord with the strong preference for the microseismicity to grow downwards, involves strike-slip shearing, which produced the opening of vertical tubes at along-strike jogs in the fault (the so-called Hill mesh). Seismic moment release averaged over the structure suggests shear displacements of at least 0.3 mm occurred, which are sufficient to generate aperture changes that are hydraulically significant. The preponderance of discrete structures within the microseismic cloud after collapsing suggests that significant flow and permeability enhancement (i.e. stimulation) within the rock mass is largely confined to the interiors of shear zones that appear to have a spacing of approximately 100 m.

Multiplet-clustering analysis is a method for precise determination of microseismic event locations and is used to identify subsurface fractures and fracture networks. A multiplet is a group of microseismic events with very similar waveforms, despite different origin times, and it is likely the expression of stress release on the same structure. The relative source locations of similar events can be determined with high resolution and accuracy by using the moving-window cross-spectrum analysis technique. Deduced seismic clusters, called multiplet clusters, are indicative of seismically activated structures, and the orientations of these structures can be estimated using the seismic clusters even though the absolute locations of the multiplet clusters cannot themselves be determined. We examine methods of determining the relative locations of multiplet clusters and introduce the concept of clustering analysis. The clustering analysis method is used to estimate the relative location of multiplet clusters by detecting phase differences between similar stacked events. We describe the procedure for multiplet-clustering analysis, estimate relative locations of multiplet clusters, and apply the method to induced microseismic data from the Soultz Hot Dry Rock field, France. We show that a fracture network can be delineated through multiplet-clustering analysis, whereas it is difficult to identify detailed structures on the basis of source locations estimated by the joint hypocenter determination method. Manuscript received 1 March 2002.

— The reservoir structure of the Soultz HDR field has been investigated by examining induced microearthquake multiplets. Microseismic events with similar waveforms have been selected from microseismic data obtained during a 1993 hydraulic fracturing experiment. Precise relative arrival times and source locations have been determined by cross-spectrum analysis. The cross-spectrum analysis decreased the residual from 0.75 ms to 0.1 ms. The estimated orientations of the multiplet planes are consistent with fracture orientations detected in Soultz boreholes. A comparison between the stress field and the orientation of structural planes suggests that the structural planes were under a critical condition of frictional slip.

A multi-disciplinary study of the response of the rock mass to mining and mining-induced earthquakes has been conducted in six gold mines in South Africa at sites deeper than 3 km, or with equivalent stress. More than 80 holes (with a total length of more than 2.8 km) were drilled in earthquake-prone areas to locate faults and install instruments. Microfracturing activity associated with an ML2.1 event at 3.3 km depth in Mponeng Mine and a highly stressed rock mass at 1 km depth in Ezulwini Mine were finely delineated and analysed. A hole drilled through the hypocentre of the ML2.1 event at Mponeng Mine allowed the stress and strength in the seismogenic area to be constrained. Previously published in situ stress measurements had been limited to depths or stresses smaller than 2.7 km or 100 MPa, respectively. We successfully measured stress at depths and stresses up to 3.4 km and 146 MPa, respectively, at four mines. These in situ measured stresses were used to calibrate elastic stress modelling and yielded better estimates of stress and strength on the rupture planes of seven earthquakes (ML2.1 to 4). Comparison of the elastically modelled stress with the constrained stress in a hole drilled across the ML2.1 fault and the strain change monitored in situ at a close distance showed that the elastically modelled stress was smaller but correctable. The South African National Seismograph Network was enhanced by installing 10 surface strong-motion seismometer stations in the Far West Rand mining district. Two other such dense surface networks are currently operated in mining areas by the Council for Geoscience: 25 stations in the Klerksdorp region, supported by the Mine Health and Safety Council; and 17 stations in the Central Rand to monitor fluid-induced seismicity. Research on the routine processing of spectral parameters was carried out using seismicity recorded by these 52 new surface stations.

Seismic and borehole measurements provide significant information about HDR/HWR reservoirs that is useful for reservoir development, reservoir characterization, and performance evaluation. Both techniques have been widely used during all HDR/HWR development projects. Seismic measurements have advanced from making passive surface measurements during hydraulic fracturing to making passive observations from multiple boreholes during all phases of HDR/HWR development, as well as active seismic measurements to probe regions of the reservoir deemed to be of interest. Seismic data provide information about reservoir extent, locations and orientations of significant fractures, and areas of thermal drawdown. Recent advances include the ability to examine structures within the seismically active zone using statistics-based techniques and methods such as seismic tomography. Seismic method is the only means to obtain direct information about reservoir characteristics away from boreholes. Borehole measurements provide high-resolution information about reservoir characteristics in the vicinity of the borehole. The ability to make borehole measurements has grown during the course of HDR/HWR development as high temperature tools have been developed. Temperature logging, televiewer logs, and electrical property measurements have been made and shown to provide useful information about locations of fractures intersecting wellbores, and regions where water leaves and enters injection and production wellbores, respectively.

We have developed a signal processing technique for three‐component microseismic data that allows the precise determination of P‐wave arrival times. The method is based on a time‐frequency representation of the signal that allows the evaluation of the 3-D particle motion from seismic waves in both time and frequency domains. A spectral matrix is constructed using the time‐frequency distributions. A crosscorrelation coefficient for the three‐component signal is derived through eigenvalue analysis of the spectral matrix. The P‐wave arrival time is determined through a statistical test of hypotheses using the crosscorrelation coefficient. This signal processing method is evaluated using a synthetic signal and it is compared to the local stationary autoregressive method for determining P‐wave arrival times. We also show that the proposed method is capable of determining the arrival time of a synthetic P‐wave to within 1 ms (five points in the discrete time series) in the presence of a signal‐to‐noise ratio of −5dB. The method can detect the arrival time of different frequency components of the P‐wave, which is a possibility for the evaluation of velocity dispersion of the seismic wave. We demonstrate the feasibility of the method further by applying it to microseismic data from a geothermal field.

We have developed a precise relative source location technique using acoustic emission doublets (AE doublets) in the triaxial hodogram method to evaluate the direction and distance of subsurface extension cracks. An AE doublet is a pair of acoustic emissions with similar waveforms and adjacent locations on the same crack but which occur at different times. The relative source location is estimated by an analysis in the frequency domain. The relative distance between two AE sources is determined from the difference of P-S arrival time delays by cross‐spectrum analysis. The relative direction is derived using a spectral matrix from the difference in P‐wave polarization directions. We also propose a method to optimize the estimated relative location by using a group of AE doublets. The accuracy of the estimated source location was confirmed by performing field experiments. The relative locations of artificial wave sources about 150 m from a triaxial detector can be estimated with distance errors of less than 1 m, and direction errors of less than 3.8 degrees in both azimuth and inclination. Results of the application of this analysis on AE doublets in a geothermal field demonstrate its ability to evaluate deeper subsurface fractures.

Identification and clustering of similar seismic events are important for precise estimation of source locations and for evaluation of subsurface structure. Phase-only correlation is well known as a real-time imagematching method for fingerprint identification. In this paper, I propose a method of cluster analysis using phase-only correlation (POC) to both identify and hierarchically classify similar seismic waveforms. The POC value of time-varying spectral representation is used to evaluate similarity between two images of a waveform, and cluster analysis is used to classify waveforms into groups according to a distance measure. The waveforms are first transformed into time-varying spectral representations to express frequency content in the time–frequency domain. The phase-only correlation function is calculated between two time-varying spectral representations and similarity is evaluated using the shape and peak value of the phase-only correlation function. This method is applied to arbitrarily selected waveforms from aftershocks of an earthquake in Japan and assessed its ability to identify similar waveforms perturbed by white noise. The results show that the phase-only correlation function exhibits a sharp peak that quantifies similarity and dissimilarity over a wide range of S/N. A matrix is defined using vectors consisting of the peak values of the POC functions, and hierarchical cluster analysis is applied based on the city-block distance and a Ward criterion to define groups. The feasibility of the method is demonstrated by applying it to aftershocks at an active fault.

In this paper, we discuss the relationship between the accurate hypocentral distribution and three-dimensional (3-D) geological structure in and around the Horonobe area in northern Hokkaido, Japan. The multiplet-clustering analysis was applied to the 421 micro-earthquakes which occurred from 1 September, 2003 to 30 September, 2007. The 3-D geological structure model was mainly constructed from previous seismic reflection profiles and borehole data. As a result of this analysis, although with slight differences in depth between them, the hypocenters were found to be distributed in the NNW-SSE direction and become deeper from the west toward the east. The distributed pattern of the hypocenters is similar to that of the geological structure. These results indicate that the hypocentral distribution may represent existence of active zone related to the geological structure, and provide effective information which can contribute to establishing methods for estimating the future evolution of the geological environment.

A new method for detecting the arrivals of linearly or elliptically polarized waves by using a spectral matrix has been developed and the method has been applied to reflection imaging of the earth’s crust. In this paper, the theory for the detection of polarized wave is described and the detectability of polarized waves has been discussed using the synthetic three-component signal, where the dependence of detectability on time window length and the center of frequency for analysis have been quantitatively evaluated. The local earthquakes observed at Sendai, Japan, were analyzed to confirm the validity of a method of polarization analysis using the spectral matrix of seismic wave and its application to seismic reflection studies of the crust. Reflectors are known below the Nagamachi-Rifu fault, which caused an M5.0 class event in 1998, and the polarization analysis was applied to earthquake data in and around the fault. Use of the Z-parameter, which is defined using the eigenvalues of the spectral matrix and a statistical value representing the confidence level for the detection of the arrival of polarized waves, allowed detection of linearly and elliptically polarized waves in coda waves. The Zparameter was also used to image the reflectors by using a migration technique that assumes the PxP and SxS reflection waves travel through a homogeneous velocity structure. Distinct reflectors were detected at depths of around 15-21 km, 20-22 km, 32 km, and 37 km, that is, from deeper than the fault. This study has demonstrated the feasibility of using the spectral matrix of threecomponent seismic signal to detect polarized waves and to image reflectors.

The model experiments which simulated shear slip induced by injection of pore fluid have been conducted under triaxial compressive stress condition using a roughened bare fault. The objective is to understand characteristics of the induced slip behaviour, especially after the slip initiation obeying Coulomb failure criterion and pore fluid behaviour associated with the induced slip. Experimental results showed that intermittent slip occurred in all the injection‐induced slip experiments. Moreover, it was also found that the pore fluid flow into the fault increased instantaneously accompanying with dynamic slip, which represents a part of higher slip velocity (V > 0.05 mm/s, Vmax of 0.4 mm/s) in the intermittent slip. Quantitative estimation on the volume of the inflow of pore fluid during the dynamic slip suggested that a shear dilatancy would cause the instantaneous increase in pore fluid flow that occurred with the slip.