Estimating the permeability of the Nojima Fault Zone by a hydrophone vertical seismic profiling...

Thematic ArticleEstimating the permeability of the Nojima Fault Zone by a hydrophone

vertical seismic profiling experiment

TSUTOMU KIGUCHI,*,† HISASO ITO, YASUTO KUWAHARA AND TERUKI MIYAZAKI

Geological Survey of Japan, 1-1-3, Higashi, Tsukuba, Ibaraki 305-8567, Japan (email: [email protected])

Abstract A multi-offset hydrophone vertical seismic profiling (VSP) experiment was donein a 747 m deep borehole at Nojima Hirabayashi, Hyogo prefecture, Japan. The boreholewas drilled to penetrate the Nojima Fault, which was active in the 1995 Hyogo-ken Nanbuearthquake. The purpose of the hydrophone VSP is to detect subsurface permeable frac-tures and permeable zones and, in the present case, to estimate the permeability of theNojima Fault. The analysis was based on a model by which tube waves are generated whenincident P-waves compress the permeable fractures (or permeable zones) intersecting theborehole and a fluid in the fracture is injected into the borehole. Permeable fractures (orpermeable zones) are detected at the depths of tube wave generation, and fracture perm-eability is calculated from the amplitude ratio of tube wave to incident P-wave. Severalgenerations of tube waves were detected from the VSP sections. Distinct tube waves weregenerated at depths of the fault zone that are characterized by altered and deformedgranodiorite with a fault gouge, suggesting that permeable fractures and permeable zonesexist in the fault zone. Tube wave analysis shows that the permeability of the fault gougefrom 624 m to 625 m is estimated to be approximately 2 ¥ 10-12 m2.

Key words: active fault, hydrophone vertical seismic profiling, permeability, tube wave.

distinguish permeable fractures from imperme-able ones and to evaluate crack properties, such aspermeability.

Analysis of tube waves recorded by hydrophonevertical seismic profiling (VSP) is considered oneof the most promising methods for detecting andcharacterizing permeable fractures that intersectboreholes (Beydoun et al. 1985; Hardin et al. 1987;Toksöz et al. 1992; Li et al. 1994; Kiguchi et al.1995). This analysis is based on a model by whicha tube wave is generated when an incident P-wavecompresses the permeable fracture intersectingthe borehole and fluid in the fracture is injectedinto the borehole. The depth of the tube wave gen-erated indicates the existence of the permeablefracture or the permeable zone, and the amplitudeof the tube wave is related to permeability.Beydoun et al. (1985) developed the tube wave gen-eration theory and estimated permeability fromthe ratio of the tube wave to P-wave amplitudes(AT/AP). Hardin et al. (1987) extended the method

INTRODUCTION

Mapping and characterization of subsurface frac-ture systems are very important in many fields,including earthquake fault mechanics, enhancedrecovery in oil and gas, and site characterizationfor hazardous waste disposal. Fracture permeabil-ity is one of the most important parameters tostudy because it controls subsurface fluid flow. Ithas become possible recently to measure fracturesin boreholes using imaging tools, such as the ultrasonic borehole televiewer (BHTV) and theFormation MicroImager (FMITM; Schlumberger,Houston, TX, USA). However, it is still difficult to

*Correspondence.†Present address: GSJ/AIST, Tsukuba Central 7, 1-1-1, Higashi, Tsukuba,

Ibaraki 305-8567, Japan.

Received 3 March 1999. Accepted for publication 4 December 2000.© 2001 Blackwell Science Asia Pty Ltd.

The Island Arc (2001) 10, 348–356

Permeability of the Nojima Fault 349

to analyze multi-offset source data and estimatedthe dip, strike, and permeability of permeablefractures. Li et al. (1994) modified the theoreticalmodel for estimating the permeability of fracturezones.

In the present study, a multi-offset hydrophoneVSP experiment was carried out in a 746.7 m bore-hole of the Geological Survey of Japan at NojimaHirabayashi (Hirabayashi GSJ borehole). Thisborehole was drilled to penetrate the NojimaFault, which slipped during the 1995 Hyogo-kenNanbu earthquake (Ito et al. 1996). We tried todetect subsurface permeable fractures and per-meable zones and to estimate the permeability ofthe Nojima Fault.

HYDROPHONE VERTICAL SEISMIC PROFILING DATA

The borehole that was used as a receiver well in the present VSP experiment was situated atNojima Hirabayashi on the north-western coast ofAwaji Island, Japan. The drilling site was 74.6 mfrom the surface break of the fault. The boreholewas cased to a depth of 152 m and was uncasedbelow that depth when we conducted the presentVSP experiment.

Figure 1 shows the geometry of the wells usedin the present experiment. We used dynamite as aseismic source in the four offset wells, which weredistributed azimuthally around the receiver well.

Horizontal distances between the receiver andshot wells were 96.1 m to 253.6 m. A hydrophonearray with an interval of 1 m was set betweendepths of 139 m and 732 m. Table 1 summarizes thespecifications of the VSP experiment.

Figure 2 shows the VSP sections for the SH-A,SH-B, SH-C, and SH-D shots. Each trace in Fig. 2is scaled relative to its maximum value and, there-fore, the relative amplitudes cannot be seen in thesection. The dominant frequency of the P-wavewas from 100 Hz to 200 Hz. We applied a band-passfilter from 40 Hz to 400 Hz. The symbol ‘P’ in theVSP sections indicates P-wave arrivals.

At several depths, tube waves were gener-ated from the P-wave and propagated upward and downward with a velocity of approximately1300 m/s, which corresponded to tube wave veloc-ity. These tube wave generations suggest that per-meable fractures and permeable zones exist atthese depths. Arrows in the VSP sections indicatedistinct depths of tube wave generation. BecauseP-waves and generated tube waves overlap at thedepths of tube wave generation, P-waves at thesedepths have rather large amplitudes. At somedepths, for example 669 m to 673 m, tube wavesseemed to be generated for a range of depths. Inthese cases, the depth ranges are considered to bepermeable zones.

COMPARISON BETWEEN DEPTHS OF TUBE WAVEGENERATION AND CORE INSPECTION

Cores from the receiving well were recovered foralmost the entire depth interval from 152 m to746 m (Ito et al. 1996). The borehole was drilledthrough 300 m of granodiorite at the surface. Theborehole then penetrated layers of protolith gran-odiorite and porphyry and the fault zone from426 m to the bottom of the hole, which was char-

Fig. 1 Geometry of the wells for the multi-offset hydrophone verticalseismic profiling (VSP) and the surface trace of the Nojima Fault. GSJ,Geological Survey of Japan.

Table 1 Specification of the hydrophone vertical seismicprofiling experiment

Source type Dynamite (50 g, 100 g)

Source position SH-A (150 m depth)SH-B (48 m depth)SH-C (48 m depth)SH-D (47 m depth)

Receiver type ITI DH-6 hydrophone array

Receiver interval 1 m

Measured depth 139–732 m

Sampling rate 0.25 ms

350 T. Kiguchi et al.

acterized by deformed and altered rocks (Ito et al.1996; Tanaka et al. 1999). The coaxial zone of the Nojima Fault, which consists of a fault gouge,was found at 623.3–625.1 m. In the fault zone(426–746 m), fault gouges and a cataclasite were

detected. Fault gouges appeared at depths of525.5–525.7 m, 623.3–625.1 m, and 707.8–707.9 m,and the cataclasite appeared at depths of669.5–672.7 m.

In order to correlate geological features with

Fig. 2 Hydrophone vertical seismic profiling (VSP) sections. The symbols ‘P’ indicate P-wave arrivals. Arrows indicate the depths of tube wave gen-erations. Each trace is scaled relative to its own maximum value in each section. (a) SH-A shot; (b) SH-B shot; (c) SH-C shot; and (d) SH-D shot.


the tube wave generated, we compared the depthsof tube wave generation with the results of coreinspection. Figure 3 shows the geological structureof the borehole and the VSP section for the SH-Dshot, which has the shortest offset distance.

Nine tube wave generations were detected forall VSP sections. As shown in Figs 2 and 3, tubewaves are generated both inside and outside thefault zone. In the fault zone, tube waves are clearlygenerated at the depths where fault gouges and cataclasite are detected, suggesting that the fault gouges at 525.5–525.7 m, 623.3–625.1 m, and707.8–707.9 m and the cataclasite at 669.5–672.7 mare permeable. It is noteworthy that only specificdepth intervals in the fault zone are permeable;that is, fault gouges and cataclasite zone. The tubewaves detected outside of the fault zone corre-sponded to fractures that were detected by core

inspection and the BHTV and FMI images, imply-ing that the tube waves outside of the fault zonecorresponded to permeable fractures in the intactgranodiorite.

PERMEABILITY ESTIMATED BY THE VERTICALSEISMIC PROFILING DATA IN THE FAULT ZONE

In order to estimate the permeability of theNojima Fault, we analyzed the amplitudes of thetube waves generated at the depths of fault gougesin the fault zone. To calculate permeability, weapplied the model of Beydoun et al. (1985) for per-meable fractures and that of Li et al. (1994) forpermeable zones. In the model of Beydoun et al.(1985), tube waves were generated at isolated per-meable fractures. In the model of Li et al. (1994),

Fig. 3 Geological structure of the Hirabayashi Geological Survey of Japan (GSJ) borehole (modified from Ito et al. 1996) and vertical seismic profil-ing section for the SH-D shot.


the source of the tube wave generation corre-sponded to the permeable zone, which has a widthof the order of 10 cm to 1 m. The main differencebetween the two models is the oscillation of theincident P-wave within the width of the fracturesor permeable zones. In the fracture model ofBeydoun et al. (1985), the fracture displacementdue to an incident P-wave was equal to the parti-cle displacement of the P-wave in the vicinity of thefracture. In the permeable zone model of Li et al.(1994), the oscillation within the width of the per-meable zone was given as the phase difference ofthe P-wave between the upper and lower bound-aries of the permeable zone. We can calculate thetheoretical curves of tube wave to P-wave ampli-tude ratio (AT/AP) as a function of frequency forseveral permeabilities for each model. Figure 4shows the theoretical curves for the two models.The two models show clearly in the theoreticalcurves that they are both frequency-dependent.For the fracture model, the value of AT/AP

decreases with frequency. Conversely, for the permeable zone model, the amplitude ratioincreases with frequency. To estimate permeabil-ity, we determined which model should be appliedby means of the frequency dependence of theamplitude ratio obtained from the VSP data. Tubewave and P-wave amplitudes at several frequen-cies were determined by applying a band-pass filter that showed the amplitudes were notcontaminated.

We will now explain one example of an estima-tion of the permeability in the fault zone at depthsof 623.3–625.1 m. A fault gouge at a depth of623.3–625.1 m is the coaxial zone of the NojimaFault. Weakly deformed and altered granodioriteappears just below the fault gouge.

First, we determined which model should beapplied. We applied the permeable zone modelbecause the amplitude ratio obtained from theVSP data increases with frequency and corre-

sponds to the theoretical curves of the model forthe permeable zone. Next, we needed to estimatethe width of the permeable zone. Figure 5 showsthe VSP section for the SH-D shot around the faultgouge. In the VSP section, the tube wave going inan upward direction was clearly generated at adepth of 624 m, but the tube wave going in a down-ward direction was not clear because of contami-nation by another strong phase. From this VSPdata, it was difficult to determine the lower bound-ary depth of the permeable zone, so we applied twoother methods for estimating the width of this per-meable zone. One method uses tube wave attenu-ation in the VSP section, and the other usesStoneley wave attenuation obtained from soniclogging.

Fig. 4 Theoretical curves of tube wave toP-wave amplitude ratios as a function of fre-quency for several permeabilities using (a)the permeable fracture model of Beydounet al. (1985) and (b) the permeable zonemodel of Li et al. (1994). (a) P-wave veloc-ity, 5000 m/s; S-wave velocity, 2900 m/s;tube wave velocity 1400 m/s. (b) P-wavevelocity, 5000 m/s; S-wave velocity,2900 m/s; tube wave velocity, 1400 m/s. P-wave velocity in permeable zone, 3000 m/s;width of permeable zone, 3 m.

Fig. 5 Vertical seismic profiling section for SH-D shot around thecoaxial zone (623.3–625.1 m). The tube wave going in an upward direc-tion is clearly generated at a depth of 624 m.


Figure 6 shows several tube waves going in adownward direction in the VSP section for the SH-D shot. These tube waves attenuate very stronglyat a depth of 624 m. The attenuation occurs withinthe 1 m sensor spacing. We concluded that thisattenuation of tube wave indicates that this depthhas a high permeable zone and the width of thepermeable zone is less than 1 m (Kuwahara et al.1998). Figure 7 shows the results of Stoneley waveanalysis (Ito et al. 1996). The shaded zone in thecolumn labeled ‘Stoneley attenuation’ indicates ahigh permeable zone estimated from the Stoneleywave attenuation. The width of the high permeablezone is approximately 1 m in this fault gouge.

From the analyses of tube wave attenuation and Stoneley wave attenuation, we assumed thatthe width of this permeable zone is approximately1 m. We consider the accuracy of the width is of the order of 10 cm. Figure 8 shows a comparisonbetween theoretical curves and the amplituderatios from the VSP data for the depth of 624–625 m. The model for calculating the theoreticalcurves is a permeable zone with a 1 m width. Thepermeability of this zone is estimated to be approx-imately 2 ¥ 10-12 m2.

DISCUSSION

Although we estimated the permeability of faultgouge from the VSP data, there is no other directlyobtained data with which to compare the perme-ability in the fault zone. Instead of direct meas-urements, Ito et al. (1996) tried to evaluate thepermeability in the fault zone from acoustic wave-

forms. The analysis is based on the fact that theStoneley wave obtained by sonic logging is sensi-tive to fractures and formation pore fluid mobility(Brie et al. 1988; Hornby et al. 1989; Winkler etal. 1989; Tezuka et al. 1997). Stoneley wave re-flections, Stoneley attenuation, and an increase in Stoneley slowness were observed at severaldepths of the fault zone. Figure 7 shows the resultsof Stoneley wave analysis (Ito et al. 1996). Theseeffects on Stoneley waves suggest that permeablefractures and permeable zones exist in the faultzone. The depths of tube wave generation obtainedfrom the VSP data correspond to regions of per-meable fractures and permeable zones detected by the Stoneley wave analysis. Therefore, we canconfirm that permeable fractures and permeablezones can be detected from tube waves generatedin the VSP data. Because permeability was notquantitatively estimated from the Stoneley waveanalysis, we were unable to directly compare thepermeability value calculated from the tube waveanalysis with the results of the Stoneley waveanalysis.

Ito et al. (1996) estimated the permeability ofthe fault gouge by using the same method and thesame data. Ito et al. (1996) assumed that the widthof the permeable zone is 10 m and estimated the permeability as approximately 1 ¥ 10-13 m2. Weanalyzed the width of the permeable zone in moredetail and estimated the width to be 1 m. There-fore, we think the present value of permeability(2 ¥ 10-12 m2) is more accurate than the value esti-mated by Ito et al. The width of the permeablezone is an important parameter when estimatingpermeability by using VSP data. In order to esti-

Fig. 6 Vertical seismic profiling section forthe SH-D shot showing wave attenuationmoving in a downward direction at a depth of624 m.


mate the permeability more accurately, we mustimprove the evaluation of the width of the perme-able zone from the borehole data.

Shimazaki et al. (1998) conducted a water inject-ion test in the Disaster Prevention Research Institute, Kyoto University (DPRI) 1800 m bore-hole at Nojima–Ogura. This borehole is 4 kmsouth-west of the borehole that was used in thepresent VSP experiment. Shimazaki et al. (1998)estimated the hydraulic conductivity (k · h, k: per-meability, h: width of permeable zone) of the deeppart of the borehole from the water injection test.Assuming the width of the permeable zone is 1 m,Shimazaki et al. (1998) estimated the averagedpermeability of the fault zone as 1 ¥ 10-13 m2. Com-pared with the permeability estimated from the water injection test done by Shimazaki et al.(1998), the value of permeability obtained from theVSP data, 2 ¥ 10-12 m2, is very high. One of thereasons for the discrepancy in permeabilitybetween the two boreholes is considered to be thedifferent property of the fault zone. The presentStoneley wave analysis for the GSJ Hirabayashi

Fig. 7 The results of Stoneley wave analysis with the geological structure of the borehole. The shaded zones in the Stoneley wave analysis indicatepermeable zones. FMI, Formation MicroImager.

Fig. 8 Comparison between theoretical curves and vertical seismicprofiling (VSP) data for the depth of 624–625 m (width 1 m). The per-meability of this permeable zone in the fault gouge is estimated to beapproximately 2 ¥ 10–12 m2.


borehole shows that the fault gouge at 624–625 mcorresponds to the localized and high permeablezone (Fig. 7). In contrast, it is not sure that theNojima–Ogura borehole penetrates the localizedhigh permeable zone or fault coaxial zone andwhether the water injection test was conducted inthe region that includes both the localized and highpermeable zone (Kobayashi et al. 1999) such as thefault gouge in the GSJ borehole.

Table 2 shows four methods for evaluating per-meability and their advantages and disadvantages.From hydrophone VSP data, we can estimate per-meability quantitatively, but hydrophone VSP datahave a low resolution compared to logging data.Stoneley wave analysis has the ability to show thedistribution of permeability, but it is still difficultto estimate permeability quantitatively, especi-ally for the fractured medium. These featuresimply that we need to integrate the results ofhydrophone VSP and Stoneley wave analysis inorder to clarify the fine permeability distributionof the active fault.

In the present study, when we estimated thepermeability of the zone at a depth of 624–625 m,it was difficult to obtain the width of the perme-able zone from the tube wave generated in the VSPsection. However, Stoneley wave analysis showedthe detailed permeability distribution around thisdepth interval, enabling us to determine the widthof the permeable zone. Also, the detailed coreinspection suggests that there is fine structure inthis zone. Hence, it is important to integrate theresults of hydrophone VSP, Stoneley wave analy-sis, and core inspection.

CONCLUSION

From the present hydrophone VSP experimentdone in the borehole drilled to penetrate theNojima Fault, we observed several tube wave gen-erations. The depths of tube wave generations in

the fault zone corresponded to regions of perme-able zones identified by the Stoneley wave analy-sis. As a result of detecting the depths of tube wavegeneration, we have concluded that only specificregions of the fault zone, such as the fault gougeand cataclasite, are permeable. It is important thatonly restricted parts of the fault zone are perme-able, which suggests that the whole fault zone wasnot uniformly damaged in the 1995 Hyogo-kenNanbu earthquake and only a limited narrow zonewas damaged.

We estimated the permeability of the narrowzone of the fault gouge in the coaxial zone bymeans of comparing the ratios of the tube wave toP-wave amplitudes (AT/AP) obtained from the VSPdata using the theoretical curves. The permeabil-ity of the zone (624–625 m) in the coaxial zone isestimated to be approximately 2 ¥ 10-12 m2.

In order to analyze the fine permeability struc-ture of the active fault, it is necessary to integratethe results of hydrophone VSP, Stoneley waveanalysis, and core inspection.

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HARDIN E. L., CHENG C. H., PAILLET F. L. & MENDEL-SON J. D. 1987. Fracture characterization by meansof attenuation and generation of tube waves in frac-tured crystalline rock at Mirror Lake, New Hamp-shire. Journal of Geophysical Research 92,7989–8006.

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Table 2 Four methods for evaluating permeability

Method Advantages Disadvantages

Packer test Conventionally used direct measurement Not continuous in depthHydrophone vertical seismic Dip/strike estimation, quantitative analysis Rather low resolution compared

profiling to loggingStoneley wave analysis Continuous estimation of permeable zone Qualitative estimation for fractured

mediumCore measurement Matrix permeability Difficult to estimate fracture

permeability, related to in situconditions


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