Anomalous seismic events observed at the CSM HDR project, U.K.

Int. J. Rock ~lech Min. Sci & Geomech ,4b~tr. Vol. 26. No. 3 .t. pp 257-269. 1989

Printed in Great Britain

0148-9062 89 $3.00 4-000 Pergamon Press pie

Anomalous Seismic Events Observed at the CSM H D R Project, U.K. R. BARIAt~ A. S. P. GREENt R. H. JONES," Seismic events detected during fluid injection into a crystalline jointed rock (granite) with an anisotropic stress regime are predominantly due to shear dislocation at the joints. Although seismic signals from shear dislocation are the most commonly recognised, it is becoming evident that there are other processes at work. Data from the Camborne School of Mines (CSM) hot dr)' rock (HDR) geothermal project site are presented in this paper, to show that there are microseismic events which are not caused by shear dislocation. These events vary from long period events detected during a gas lift test to high-frequency events detected during a massive viscous stimulation at a depth of 2300m. These events do not show the clear P and S waves associated with shear failure. The long period events look very similar to the events detected during boulder splitting experiments in the laboratory and those detected at the Los Alamos HDR site during hydraulic injection. Tube waves detected during the circulation of water between two wells indicates that water was squirted in and out of the well. Mechanisms for the generation of these events are not clear, but their presence does suggest that the modelling of rock joint behaviour during fluid injection will have to take these anomalous events into consideration in order to produce a representative model.

INTRODUCTION The extraction of energy from a hot dry rock (HDR) geothermal reservoir requires the creation of a heat exchange region between an injection and production well. The engineering of this reservoir consists of dilating existing joints by hydraulic stimulation to provide easier access for fluid flow. Rocks with a low natural permeability, such as granite, are used to confine the reservoir and to reduce water losses. In order to understand, design and model hydraulic stimulations, the stress field and joint orientations need to be evaluated. Diagnostic techniques must be used to provide data to help in the construction of the reservoir model. Some of the diagnostic techniques used are the monitoring of wellhead pressures, borehole logging of flows and temperatures, water sampling and microseismic monitoring. Monitoring of microseismicity generated during stimulations is the only method which gives some indication of the pressure disturbance in the joint apertures ~Camborne School of Mines Geothermal Energy Project. Rosemanowes Quarry. Penryn. Cornwall. U.K. ~British Geological Survey. 257

away from the wellbore. Microseismicity has been used by various researchers to understand and model rock joint failure mechanisms during hydraulic stimulations. It has been shown that downward microseismic growth observed during water injections is caused by an increase in the anisotropy of the horizontal stresses with depth [1]. The injection wellhead pressure at the onset of microseismicity in a virgin rock mass can be used to estimate the pore pressure required to reduce the effective normal stress and cause a joint to shear. Analysis of the coda and fault-plane solutions show that reservoir growth is predominantly by shear failure. Many of the observed features of microseismicity induced by fluid injections has been simulated using the numerical model FRIP (Fluid Rock Interaction Program) [2]. The above introduction shows that the information obtained from microseismic data is invaluable for understanding and modelling the mechanism of growth of a HDR reservoir. The information predominantly used is the analysis of the coda of microseismics and their locations. There are events detected which do not fit a shear mechanism but these are not normally considered because of the difficulty in interpreting them. The theme

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of this paper is to show that not all the microseismic events caused by fluid injection are a result of shear dislocation and that these non-shear events have often been systematically rejected because of a lack of understanding of the mechanism of their generation.

beginning of Phase 2B (1983-1986) to provide access to the microseismically stimulated zones below the existing two wells. Its position was selected to penetrate through the microseismically defined zones. A flow profile taken

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BACKGROUND INFORMATION 400

The H D R research project in the U.K. is located at Rosemanowes Quarry (Cornwall) on the Carnmrnrllis granite pluton. The in situ stress field in the Carnmenellis granite was measured from near-surface to 2610m using hydrofracture stress measurements and overcoring. The results show a significant stress anisotropy [3]. The principal stress axes are approximately vertical and horizontal. The variation in magnitudes of the principal stresses is shown in Fig. 1 and the direction of the horizontal stresses is shown in Fig. 2. The strikes of the natural joint system are approximately orthogonal, with two main sub-vertical and one sub-horizontal sets. The azimuths of the two major vertical sets are 320-340'N and 240-270"N (Fig. 2). The spacing of vertical joints varies from 1-5 m at the surface to 1-10 m at 2500 m depth. During Phase 2A (1980-1983), two deviated wells (RHII and RHI2) were drilled to a depth of 2100 m [4] with a vertical separation of about 300 m. Hydraulic injections were carried out which produced an extensive microseismic zone below the two wells [5]. However, there was not a sufficient increase in permeability to sustain a viable HDR system. A third well (RHI5) was drilled in 1984 at the

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in RHI5 during a gas lift test showed that flow into the well was contained within the section of the well intersecting the microseismically active region. However, the overall impedance between RHI2 and RHI5 was still high (about 2MPa/l/sec) and it was decided to stimulate RH 15 using a viscous fluid with a viscosity of 50 cP [6]. Microseismic monitoring was undertaken during the viscous stimulation [7] and showed that the predominant mechanism of joint failure was shear on a NW-SE joint set. The stimulated reservoir was subsequently circulated in increments rising from 51/see to a maximum of 37 l/see. Microseismic events were located using a combination of downhole and sub-surface sensors. The seismic network consisted of a hydrophone string, a single hydrophone and six sub-surface accelerometers. The layout of the seismic network in relation to the wells is shown in Fig. 3. The string, consisting of three hydrophones, was normally deployed in RH11 and the separations between the top and the middle, and middle and bottom hydrophones were 90 and 102.6m, respectively. A single hydrophone was deployed in RHI2. Details of the sensors, data transmission, data acquisition and processing have been described by Baria et al. [5].

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MICROSEISMIC EVENTS (a) Shear erents In an anisotropic stress regime, shear failure occurs on joints when the pore pressure reduces the effective normal stress to a level at which the shear stress on the joint causes failure. Models of joint failure by shear dislocation are well documented and will not be discussed here [8-10]. Events from shear failure appear to predominate during hydraulic stimulations. However, this may well be caused by the higher seismic energy output from shear failure rather than their more frequent occurrence. (b) Shear and long-period events during the gas lift test RHI5 was drilled into the microseismic structure below the existing wells R H I I and RHI2. Flow meter logs were run during a gas lift test in RHI5 which showed that flow into the well was contained within the section of the wells intersecting the microseismically active region [7]. The gas lift test was carried out (30 December 1984) for approx. 2 days as part of an investigation related to hydraulic conditions around the well before any further disturbance took place. Nitrogen at a maximum pressure of 16.7 MPa was injected into the drillpipe at flow rates up to 85m3/min. Liquid nitrogen was heated to vapourisation and injected into the dritlpipe by positive displacement pumps. Microseismicity was monitored using a single hydrophone unit deployed in RHII at 2340m measured depth (MD) and the six remote sub-surface stations. The hydrophone gain was set at 25V/mbar giving a sensitivity of about 180 nbar. A total of 250 events were captured on the hydrophone sensors and categorised either as shear or long period events. About 22 had both P and S waves which were timeable. None of the events captured were large enough to reach the sub-surface sensors, which were about 2-3 km from the source. The P-S time difference for each event was used to calculate the distance between the source and the hydrophone using a VP:VS ratio of 1:75. A large proportion of the events were about 310 m from the hydrophone, about the same distance as the flowing zone in RH15. Figures 4 and 5 show some of the shear events and their amplitude spectra. The spectra were calculated using a 120msec window covering trace lengths as shown in Fig. 4. The amplitudes of some of the traces were very low and have caused discretisation problems, but it was felt that this occurs only on a few traces and therefore does not affect the main conclusion. The main energy for P waves is at about 800 Hz. The main energy for the S waves is between 400 and 600 Hz (for events 16, 17 and 18) and about 400Hz (for events 24, 31, 55, 56 and 97). An approximate estimate of the source radius using the P wave frequency is about 2 m [10]. The shear events in Fig. 4 show that the P-S time difference for these events are similar. This suggests that these events are from the same distance and may origi-

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A N O M A L O U S SEISMIC EVENTS AT THE CSM H D R P

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nate from the same areas. These events may represent repeated shear dislocations on the same joint. In Fig. 5 the amplitude spectra of events 24 to 97 show a monochromatic characteristic in the shear wave signatures. This is represented as a spike of about 400 Hz which could be an indication of resonance. An increase in resonance appears to be time dependent. The mechanism for this is not understood and requires further work. During the gas lift test some long period events were

also captured. The traces and amplitude spectra are shown in Figs 6 and 7. The spectra of the traces were calculated using 1.2 sec time window covering trace lengths as shown in Fig. 6. The spectral peak at 50 Hz was caused by the 50-cycle pick-up from the mains power supply. These occurred at about the same time as the shear events. Some of these events show similar characteristics to those identified at the L A N L H D R site [ll]. At LANL, signatures consisted of a high-frequency signal superimposed on a monochromatic low-frequency

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signal and without the clear P and S waves associated with a shear event. Long-period events have been observed by many researchers studying volcanoes [12, 13]. These have been interpreted as oscillations in magma chambers [14,15], oscillations in fluid conduits along which magma is transported [16] and tensile opening (e.g. jacking) of fluid filled cracks [17, 18, 11]. Interpretation of long-period events by Aki et al. [I 7] and Bame and Fehler [11] may not be applicable to these events. In this model the fluid behaves passively as a

cushion and there is no acoustic pressure source within the fluid. Hence, there is no acoustic resonance in the crack and the motion of the crack wall represents a purely passive motion of the fluid in that case. A more realistic model of the physics of the fluiddriven crack is that discussed recently by Chouet [19, 20] in which a solution is obtained for the fluid dynamics along with the elastodynamics in the solid. These numerical results show the well-developed acoustic resonance of a crack in response to a pressure transient

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applied in the fluid. Resonance is sustained by a very slow wave trapped in the fluid-filled crack. This guided wave, called the crack wave by Chouet, is similar to the tube wave propagating in a fluid-filled borehole; it is inversely dispersive, showing a phase velocity that decreases with increasing wavelength. The wavespeed of the crack wave is always lower than the acoustic velocity of the fluid and decreases rapidly as the crack stiffness C increases [C = (b/l)(L/d), where b is the bulk modulus of the fluid, I is the rigidity of the solid, and L and d are the crack length and crack thickness, respectively].

This behaviour is well demonstrated in Fig. 17 of Chouet [19]. For a hydraulic fracture the aspect ratio may be in the range 104-105 [21] and for a water-filled crack in granite, b/# ~ 0.1 so that the crack stiffness C is of the order of 10~-I0~. Chouet [20] discusses the effect of various parameters on the far-field spectrum radiated by the excitation of a fluid-filled crack. Although Chouet's models do not include cracks with stiffnesses C t> 103, the model with the stiffness C = I00 he discusses can be used to obtain an upper bound on the crack length from spectral content of Fig. 7 of

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ANOMALOUS SEISMIC EVENTS AT THE CSM HDRP

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Fig. 7. Amplitude spectra of long period events during the gas lift test in RHIS. Chouet [20]. There is a considerable variation in the spectral content of these signatures. If it is assumed that the above model can be used and taking event 101 with its two spectral peaks of about 10 and 100Hz, and applying this data to the dominant far-field resonant frequencies for a crack with the stiffness C = 100 (see Figs 6e-h and Table 3 of Chouet [20]) and assuming a compressional wave velocity in the solid P = ~/3, S---6.1 km/sec (S = 3.5 km/sec), one obtains estimates of crack lengths in the range of 3-19 m for a signal RMMS

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with the dominant frequency of 100 Hz, and 30-190 m for signal with the dominant frequency of 10 Hz. The range of lengths in each case depends on which mode of the crack dominates the far-field spectrum; the lower estimate is based on the assumption that the fundamental longitudinal mode of the crack dominates, and the upper estimate is obtained from the assumption that the dominant spectral peak represents one of the overtones of the source. The crack length derived for a crack with the stiffness C >/103 will of course be smaller

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than that obtained for a crack with stiffness C = I00, since the wavespeed of the crack wave decreases with increasing stiffness (see Fig. 17 of Chouet [19]). Therefore the expected crack lengths inferred from the data would be somewhere in the lower range of the 3-190m derived here. The source parameters calculated from shear events detected during the subsequent viscous stimulation show source radii varying from 5 to 40m. with an average of about 10m. This would appear to be in rough agreement with the source radii of 5 to 40 m for the shear events. On the other

hand, cross-hole seismics carried out between the wells at depth do show reflectors which represent features up to 250 m long [22]. Unfortunately the long-period events were only detected during the gas lift test and not during any subsequent injections. It is important to note that these events appear to have a very low amplitude, and therefore may have been systematically rejected during other injection tests where the gain on the sensors were reduced in order not to saturate data when recording large amplitude shear events.

BARIA et al.:

ANOMALOUS SEISMIC EVENTS AT THE CSM HDRP

(c) E,'ents from hydrofracturing of granite blocks Small cube-shaped granite boulders, about 50 cm per side, were used to carry out hydrofracturing (splitting) using a small rubber packer. A more detailed account is given elsewhere [23]. A 27-mm dia hole was drilled to a depth of about 14 cm. A hand pump was used to inject fluid into the hole. Injected pressure and seismicity generated were tape-recorded initially and subsequently digitised and stored on a VAX 11/780 computer for analysis. In the majority of the experiments fluid was injected until a crack migrated to a face of the block without splitting the boulder into two. Seismicity was monitored using a small ball hydrophone and a three axis accelerometer. The ball hydrophone, with a bandwidth in excess of 280 kHz, was deployed in a small hole in the rock sample filled with water. A three axis accelerometer unit was clamped to a plate glued to the rock sample. Figure 8 shows the pressure attained and the seismic signature detected during rock failure. The top trace in the figure shows the pressure behaviour followed by signatures from a hydrophone and an accelerometer. The bottom two traces show an enlarged portion (50 msec) of the hydrophone signature with its amplitude spectrum. The spectrum of the enlarged portion was calculated using 50msec time window covering the whole of the expanded trace. The pressure curve shows a drop in pressure followed by the breakdown. The slight drop in the pressure may be a function of leakage in the packer or an enlargement of the hole before breakdown. There is a good correlation between breakdown pressure and onset of the main seismic event. The seismic signature consists of a high frequency modulated by a low frequency. The amplitude spectra show this to be between about 300 and 1600 Hz. The mechanism of failure during the splitting was tensile. The seismic signature is similar to those observed in the gas lift test (Fig. 6).

(d) High-frequency events During Phase 2A, hydrofracturing was carried out between the two wells (RH ! 1 and RH 12) using water at high pressure and flow rates. A total of 6500 seismic events were captured and about 550 events were located [5]. Most of these events were from shear failure, with an average frequency of about 350Hz. During this experiment a proportion of the total captured events showed frequencies in excess of 3000 Hz. These were detected on the downhole hydrophone string deployed in the production well. In a similar stimulation during Phase 2B using viscous gel, high-frequency events were again detected [7]. These events were detected during the latter part of the viscous stimulation. Figure 9 shows the signature and the spectral content of a typical high-frequency event. The spectrum of the trace was calculated using 200 msec time window covering the whole trace as shown in the figure. The frequency content of these events was in excess of 3000 Hz, the upper frequency being limited by the

265

response of the data transmission system. These events were too small to be located using a combination of downhole and sub-surface sensors but appear to originate from close to the top hydrophone in RH I I. Shear waves could not be identified on the signature and this may be a function of the event being too close to the sensors. The mechanism by which these high frequency events were generated is not clear, but Fig. 9 shows that they are very quickly attenuated. The separations between hydrophones 1 and 2 and 2 and 3 were 102 and 90 m respectively. These events were detected towards the end of the injection period when the production well (RHI2) was shut in. A possible explanation could be that these events are associated with the pressure front moving ahead of the shearing activity, causing some asperities to be crushed as stress adjustment takes place.

(e) Tube wares Tube waves are generated in the well at the intersection of permeable fractures with the well. They are low-velocity, large amplitude events propagating along the interface between the borehole wall and borehole fluid [24]. They can be generated by incident compressional wave energy from a source within a well, a source on the surface, or a compressional wave in the surrounding solid passing any major discontinuity in the well [25]. The mechanism of the generation of tube waves has been illustrated by Huang and Hunter [24] and shows that when compressional wave energy from a seismic source impinges onto a fracture zone the water contained in this zone is squeezed out of the fractures and pushed into the well. This generates a tube wave that radiates up and down the well. Tube waves were detected on the hydrophones during a number of circulation tests. Figure i0 shows such a tube wave detected on a string of three hydrophones deployed in RH! l, but not on a single hydrophone deployed in RH 12. Approximately 70 tube wave events were detected during this test and were associated with a previously flowing joint zone at about 2288 m MD in RH 11. Spectral analysis of the tube wave showed that the first higher frequency wavelet was about 90 Hz, with the subsequent oscillation at about 17 Hz. These tube waves were generated during the first circulation test after the main viscous stimulation in Phase 2B. Water was injected into RHI2 at about 21 l/sec and the wellhead pressure rose to a maximum of about I 1 MPa. RHI5 was shut-in for about 3 days and the wellhead pressure rose to about 8.7 MPa when the well was allowed to flow at about 7 l/sec. The hydrophone string was deployed in R H l l which was also shut-in. The generation of a tube wave requires an external source to compress a permeable zone linked with a fracture in a borehole. This could be an active seismic source such as an explosive, or a passive seismic source such as a microearthquake. The majority of the tube waves generated during this test period at the project do not show any seismic source associated with them. The existence of tube waves in the absence of any

266

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seismic source has been taken to indicate the rapid expulsion of water into the well from a joint. A possible mechanism could be that the pressure near the bottom of R H I I had built up in excess of 10MPa (above hydrostatic) to overcome the minimum horizontal stress for joints in set 1, causing them to open momentarily and squirt water into the well. This seems unlikely, as the pressure near the bottom of R H ! ! was just over 1 MPa above hydrostatic during this test, and flows into R H I 1 from the surrounding rock mass were modest through-

out Phase 2B. The origin of the tube waves in this test is uncertain and requires further examination. Similar tube waves were again detected during a later 24 I/sec circulation test. These were captured on a single hydrophone unit deployed in R H I I and therefore the position in the well where they occurred could not be identified. The generation of tube waves requires water to be injected into the well, which would appear as a compressional first motion. Figure I1 shows a number of tube waves with a compressional first motion, except

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ANOMALOUS SEISMIC EVENTS AT THE CSM HDRP

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Fig. I0. Tube wave signatures detected on hydrophones in RHII during a 21.5 l/sec into RHI2. for event I i which shows a dilational first motion. This dilational first motion can be interpreted as fluid being squirted out into the formation from the well. Again the mechanism or source which would cause this in a well which is flowing is not clear, as it suggests that the pressure in the well must be greater than in the formation for this to occur. DISCUSSION AND CONCLUSIONS The data presented in this paper indicate that there may be processes occurring during the stimulation of

jointed rocks in an anisotropic stress field other than shearing. It is clear that microseismic events other than those by shear failure are sometimes captured, but these events are often systematically rejected and only events associated with shear dislocation processed. A lack of interest in the non-shear events can be associated with two basic problems: the lack of observed data and lack of appropriate models to explain the various mechanisms. The absence of observed data can, to a large extent, be a function of poor instrumentation used for capturing

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A N O M A L O U S SEISMIC EVENTS .AT THE CSM H D R P

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the data. For example, long-period events captured during the gas lift test had sensor gains set at a very high level, which made it possible to detect these events. During other hydraulic injections the sensor gains were reduced to stop saturation by the larger amplitude shear events. Therefore, any long-period low-amplitude waves generated during hydraulic injections would be systematically filtered out. Similarly, the high-frequency events (3000 Hz) detected during viscous stimulation would have been missed, had it not been that the sensor and

data transmission had the capability to detect and transmit signals with a frequency content of up to 5 kHz. The other important factor is that a sensor must be close to a source in order to be able to detect high-frequency seismic events because attenuation and scattering losses makes them difficult to detect over short distances of about 500 m. Even at the CSM site, where data from the hydrophones have been of good quality, it has become apparent that to get further information from these events requires the use of a three axis clamped tool

BARIA et ul.: ANOMALOUS SEISMIC EVENTS AT THE CSM HDRP

in order to obtain the particle motion. A three axis clamped tool with a wide bandwidth (5-5000Hz). a large dynamic range (80 dB), high sensitivity, a downhole compass and without tool resonance would be an ideal downhole seismic sonde. The development of a model normally, but not exclusively, follows an observation, or in other words a model is produced to explain the data and subsequent data refine the model. Therefore. it is also necessary to document and publish anomalous events in order to provide data for future researchers to generate models, and further the understanding of behaviour of jointed rock under dynamic conditions. Acknowledgements--This work was carried out at the Camborne School of Mines under contract to the EEC and the UKAEA contract numbers EG-D-2-UK(N), E 5A/CON 125/960 and E 5A/CON137/ 1593. The support and encouragement of the staff of these organisations and the advice of the reviewers is gratefully acknowledged.

8. 9. I0. 11. 12. 13.

14. 15. 16. 17.

REFERENCES I. Pine R. J. and Batchelor A. S. Downward migration of shearing in jointed rock during hydraulic injections, int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 21, 149-263 (1984). 2. Pine R. J. and Cundall P. A. Application of Fluid-Rock Interaction Program (FRIP) to modelling of hot dry rock geothermal energy systems. In ISRM Int. Syrup. on the Fundamentals of Rock Joints, Bjorklinden. Lapland, Sweden (September 1985). 3. Pine R. J., Ledingham P. and Merrified C. M. In situ stress measurement in the Carnmenellis granite: 2--hydrofracture tests at Rosemanowes Quarry to depths of 2000 m. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 20, 63-72 (1983). 4. Beswick A. J. The drilling of two deep geothermal wells in the Carnmenellis granite. Camborne School Mines J. 82, (1982). 5. Baria R.. Hearn K. C. and Batchelor A. S. Induced seismicity during the hydraulic stimulation of a potential hot dry rock geothermal reservoir. Proc. Fourth Conf. on Acoustic Emission / Microseisrnic Actirit.v in Geological Structures and Materials, Pennsylvania State University, October 1985 (Edited by R. H. Hardy). Trans. Tech. Publication. Clausthal. Germany (1986). 6. Pine R. J., Baria R.. Pearson R. A.. Kwakwa K. and McCartney R. A technical summary of Phase 2B of the Camborne School of Mines HDR project, 1983-1986. Expanded version of paper presented to EEC/US Workshop on Hot Dry Rock Geothermal Energy, Brussels (May 1986). 7. Baria R. and Green A. S. P. Seismicity induced during a viscous stimulation at the Camborne School of Mines hot dry rock

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21. 22.

23. 24.

25.

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geothermal energy project. In 8th International Acoustic Emission Syrup., Tokyo, Japan (October 1986). Honda H. Earthquake mechanism and seismic waves. J. Phys Earth 10, 1-97 (1962). Haskell N. A. Total energy and spectral density of elastic wave radiation from propagating faults. Bull. Seismol. Soc. Am. 54, 1811-1841 (1964). Brune J. N. Tectonic stress and the spectra of seismic shear waves from earthquakes. J. Geophys. Res. 75, 4997-5009 (1970). Bame D. and Fehler H. Observations of long period earthquakes accompanying hydraulic fracturing. Los Alamos National laboratory (1986). Guerra I., Bascio L., Luongo G. and Searpa R. Seismic activity accompanying the 1974 eruption of Mt Etna. J. Volcan. Geotherm. Res. 1, 347 (1976). Fehler M. and Chouet B. Operation of a digital seismic network on Mount St Helens volcano and observations of long period seismic events that originate under the volcano. Geophys. Res. Letts 9, 1017 (1982). Crosson R. S. and Bame D. A. A spherical source model for low frequency volcanic earthquakes. J. Geoph.vs. Res. 90, 1023 (1985). Shimozuru D. Volcanic micro-seisms-----discussion on the origin. Bull. Volcanol. Soc. Japan 5, 154 (1961). Chouet B. Excitation of a buried magmatic pipe: a seismic source model for volcanic tremor. J. Geophys. Res. 90, 1881 (1985). Aki K., Fehler M. and Das S. Source mechanism of volcanic tremor: fluid driven crack models and their application to the 1963 Kilauea Eruption. J. Volcan. Geotherm. Res. 2, 259 (1977), Chouet B. Ground motion in the near field of a fluid-driven crack and its interpretation in the study of shallow volcanic tremor. J. Geophys. Res. 86, 5985 (1981). Chouet B. Dynamics of a fluid-driven crack in three dimensions by the finite difference method. J. Geophys. Res. 91, 13,967 (1986). Chouet B. Resonance of a fluid-driven crack: radiation properties and implications for the source of long-period events and harmonic tremor. Proc. Int. Workshop on Forced Fluid Flow Through Strong Fractured Rock Masses, Garchy, France. Commission of the European Communities EUR 11164/1, Brussels, pp. 21-78 (1987). McFarland R. D. and Murphy H. D. Extracting energy from hydraulically-fractured geothermal reservoirs. Report LA-UR-761324, Los Alamos Scientific Laboratory (1976), Green A. S. P. and Baria R. Active seismics to determine reservoir characteristics of a hot dry rock geothermal system. In Stanford Unit'ersity 12th Workshop on Geothermal Engineering, Stanford, Calif. (January 1987). Read P. J. Hydrofracture technique and analysis in granite boulders. B.Se. Thesis No. 36/85, Camborne School of Mines (1985). Huang C. F. and Hunter J. A. The tube wave method ofestimating in situ rock fracture permeability in fluid-filled boreholes. Proc. Workshop on Geophysical Investigations in Connection with Geological Disposal o f Radioactire Waste, Ottawa, Canada (1982). White J. E. Seismic waves: radiation, transmission and attenuation. International Series in the Earth Sciences, p. 380. McGrawHill. New York (1965).