Hirokazu Moriya

Multiplet-Clustering Analysis Reveals Structural Details within the Seismic Cloud at the Soultz Geothermal Field, France

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.

Precise source location of AE doublets by spectral matrix analysis of triaxial hodogram

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.

Detailed Fracture System of the Soultz-sous-Forêts HDR Field Evaluated Using Microseismic Multiplet Analysis

Abstract — 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.

Current status of seismic and borehole measurements for HDR/HWR development

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.

Precise detection of a P-wave in low S/N signal by using time-frequency representations of a triaxial hodogram

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...

Pore pressure migration during hydraulic stimulation due to permeability enhancement by low‐pressure subcritical fracture slip

Understanding the details of pressure migration during hydraulic stimulation is important for the design of an energy extraction system and reservoir management, as well as for the mitigation of hazardous induced seismicity. Based on microseismic and regional stress information, we estimated the pore pressure increase required to generate shear slip on an existing fracture during stimulation. Spatiotemporal analysis of pore pressure migration revealed that lower pore pressure migrates further and faster, and that higher pore pressure migrates more slowly. These phenomena can be explained by the relationship between fracture permeability and stress state criticality. Subcritical fractures experience shear slip following smaller increases of pore pressure and promote migration of pore pressure because of their enhanced permeability. The difference in migration rates between lower and higher pore pressures suggests that the optimum wellhead pressure is the one that can stimulate relatively permeable fractures, selectively. Its selection optimizes economic benefits and minimizes seismic risk.

Quasi‐static slip patch growth to 20 m on a geological fault inferred from acoustic emissions in a South African gold mine

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.

Spectral matrix analysis for detection of polarized wave arrivals and its application to seismic reflection studies using local earthquake data

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 using local earthquake data. Reflectors (Bright spots) are known below the Nagamachi-Rifu fault, which caused an M 5.0 class event in 1998. 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 Z-parameter was also used to image the reflectors by using a migration technique that assumes the P×P and S×S reflection waves travel through a multi-layered velocity structure. Distinct reflectors were detected at depths of around 10 km, 14 km, 17 km, 21–26 km, 35 km and 40 km, that is, from deeper than the fault and the Moho. This study demonstrated the feasibility of using the spectral matrix of three-component seismic signal to detect polarized waves and to image reflectors in the earth’s crust and upper mantle.

Unexpectedly frequent occurrence of very small repeating earthquakes (–5.1 ≤ MW ≤ –3.6) in a South African gold mine: implications for monitoring intraplate faults

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.

Ultrasonic measurement of bone thickness for spinal surgery

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. Pearsons correlation coefficient showed a strong and positive relationship between the two measurements (r = 0.97, n = 22, P <; 0.0001).