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The tube-wave method of estimating in-situ rock fracture permeability in fluid-filled boreholes
Geoexploration, Elsevier Science
22 (1984) 245-259 Publishers B.V., Amsterdam
THE TUBE-WAVE METHOD FRACTURE PERMEABILITY
CF.
HUANG
245 - Printed
in The Netherlands
OF ESTIMATING IN-SITU ROCK IN FLUID-FILLED BOREHOLES
and J.A. HUNTER
Atomic Energy of Canada Limited, c/o Geological Survey of Canada, Ottawa, Ontario KlA OE8 (Canada) Geological Survey of Canada, Ottawa, Ontario KlA OE8 (Canada) (Accepted
for publication
March 23, 1984)
ABSTRACT Huang, C.F. and Hunter, J.A., 1984. The tube-wave method fracture permeability in fluid-filled boreholes. Geoexploration,
of estimating in-situ 22: 245-259.
rock
Studies on subsurface fracture permeabilities in igneous and/or metamorphic rock bodies are in progress as part of the AECL (Atomic Energy of Canada Limited) Nuclear Fuel Waste Management Program. A multichannel borehole hydrophone array has been used to detect open fractures and rock property changes circumjacent to deep boreholes in granitic rock. The la-channel array, with equispaced hydrophones of approximately 1 m, is lowered at intervals in the borehole. The seismic source consists of a small dynamite charge detonated in a shallow shot hole near the well head. A 12-channel digital engineering seisomograph system is used to record high resolution data (0.2 ms sample rate) over a recording period of 200 ms. In addition to first-arrival compressional waves, high-amplitude low-velocity later events were observed, which appeared to be generated at various intervals within the borehole. These events have been identified as “tube waves”, which are interpreted as being generated by the incidence of compressional waves from the surrounding rock body onto fluid-filled fracture zones intersecting the borehole. Correlations of positions and amplitudes of tube waves with open fractures observed from TV logging, core logging and hydrogeology studies suggest that tubewave events can be used as reliable indicators of fractures open to fluid flow. The relative peak-to-peak amplitudes of tube waves appear to be directly related to the measured hydraulic conductivities in a borehole.
INTRODUCTION
For several years, Atomic Energy of Canada Limited (AECL) and the Department of Energy, Mines and Resources have been actively investigating the feasibility of using crystalline rock bodies as possible sites for nuclear fuel waste repositories. The principal objective in such investigations is to locate pathways in the rock mass and determine the resultant groundwater movement. Since the permeability of most unfractured plutonic rocks is very low, the main thrust of investigation is confined to the study of faults and fracture systems within such rock formations.
0016-7142/84/$03.00
o 1984 Elsevier
Science
Publishers
B.V.
246
100
200
E
300
V
I
c P
w
a
400
500
600
-i .-r
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As a part of the program to determine the in-situ fracture permeability in a crystalline rock body, several methods have been used for identifying and characterizing open fractures which intersect a borehole. These include core logging, TV logging, standard logging, hydraulic conductivity measurements and downhole seismic surveys referred to as the tube-wave method. Detailed correlations of occurrences of tube-wave generating points with open fractures, as detected by other geological methods, have suggested that the tube-wave method can be used as a reliable indicator of fractures open to fluid flow. In addition, the relative amplitudes of tube waves appear to be correlatable with measured hydraulic conductivities (Huang and Hunter, 1981a,b). This paper summarizes the result of in-situ seismic tube-wave measurements in boreholes that are currently being studied by AECL in the concept assessment phase of its Nuclear Fuel Waste Management Program. The boreholes utilized for this study are located on the grounds of the proposed site of Canadian underground research laboratory (URL) and the AECL Pinawa site. Both sites are situated on a mineralogically rather homogeneous granitic batholith, the Lac du Bonnet batholith. Tube-wave
method
Tube waves have been the subject of investigations in borehole seismic surveys for many years, mainly because of their interfering effects in the identification of later arrivals (shear waves and reflections). Tube waves are low-velocity, large-amplitude later events propagating along the interface between the borehole wall and the borehole fluid. They can be generated by the incident compressional wave energy from a surface source near the borehole, a source within the borehole, or a compressional wave in the surrounding solid passing any major discontinuity in the borehole (White, 1981). The initial intention of the borehole seismic survey was to investigate the possibility of utilizing in-situ seismic wave measurements for detecting and mapping cracks and fracture zones in a borehole. During the first-arriIal compressional wave studies in boreholes at Chalk River and Pinawa, we found that the record suites displayed many large-amplitude, low-velocty events which appeared to be generated at various intervals within boreholes. These later events were subsequently interpreted as “tube waves”, which are generated at fluid-filled fracture zones intersecting the borehole. Fig. 1 is a typical example of the record suite from a borehole at the URL research site showing “tube waves” of varying amplitude radiating up and down the hole from fracture zones. Similar results have been obtained in Fig. 1. Tube-wave record suite in borehole URL-1, Lac du Bonnet, Manitoba, (the end of record is at 670 m).
248
more than 20 boreholes at the four AECL research sites*’ in granitic rocks. White (1965) and Kitsunezaki (1971) show that a fracture zone in rock behaves as an effective source of tube waves when a compressional wave is incident onto it. Fig. 2. shows the mechanism for generation of tube waves in a borehole. When the compressional wave energy from the surface explosive source impinges onto the fracture zone, the water contained in this zone is squeezed out and pushed into the borehole, thus generating a tube wave that radiates up and down the hole. Based on this model, if tubewave generation zones can be mapped in the borehole, zones of high permeability (and open fractures) can be mapped.
Fractured zone P 8
: :
Compressional
Wait rock
Fig. 2. Schematic of p-wave.
wave
Tube wave
:
Granite
diagram showing tube waves generated at a fracture zone by incidence
**The four AECL research sites are near: (1) Chalk River, Ontario; (II) Atikokan, tario; (III) Pinawa, Manitoba; (IV) URL, Lac du Bonnet, Manitoba.
On-
249
INSTRUMENTATION
Borehole seismic records are taken by using a crystal cable array. This 12-hydrophone array (Mark Products Ltd., p-27), designed for high resolution surveying, is a reduced-scale version of that used in the oil industry. Instead of an array with 15 m between hydrophones, the interval is reduced to 1 m giving an 11-m “live” section. The frequency response of the hydrophone is relatively broad-band, between 2 Hz and 1000 Hz. The centre frequency of seismic energy currently used in this study is about 200 Hz. The pressure-sensitive hydrophones, with built-in preamplifiers, are connected in line with the interconnectors and the 1500-m cable. The 28.conductor steel-cored cable is stored on a winch and is used in conjunction with a well head tripod (see Fig. 3). Data from the hydrophone array are recorded on a Nimbus ES 1210-F, 12.channel seismograph using a record length of 200 ms (0.2 ms sampling rate) and then stored on the Nimbus G-724s digital tape recorder for permanent data storage and future analysis.
Fig. 3. The field assemblage of seismic cable, winch research site (URL-l), Lac du Bonnet, Manitoda.
Surveying
and well-head
triDod at the AECL
methods
In surveying, the array of hydrophones is lowered at intervals in the borehole (Fig. 4). At each array position, a small dynamite charge (75 N 150 g of Forcite 75%) is detonated in a shallow shot hole at offset dis-
260
tances varying between 10 and 40 m from the well head. Experience has shown that the best energy transmission is obtained when the shot hole is drilled into outcrop and fully tamped with water. After each shot, the hydrophone array is moved to a new position with some overlap of the previous position. The amount of overlap varies with each hole record according to the number of active hydrophones available in each survey. Amplifier gains are adjusted so that the entire seismic wave train from the 12-channel array can be adequately recorded for tube-wave studies. Records from each shot location are played back and edited using a microcomputer (Hunter, 1980) to form a continuous record suite displaying seismic traces from all hydrophone positions in the hole. Figs. 1, 5 and 6 are examples of such record suites showing tube waves with varying amplitudes radiating from fracture zones in three different holes. To
Nimbus
IZIOF
Seismic
Recorder
\
Fig. 4. The crystal cable array configuration showing the overlap. RESULTS
The record suites recorded in the two holes (URL-1 and URL-2) at the URL site and one hole (WN-1) at the Pinawa site are analysed for tube-
251
h
E v
I I-
a. Y
a
Fig. 5. Tube wave record suite in borehole URL-2, of record is at 1100 m).
Lac du Bonnet, Manitoba (the end
252
Fig. 6. Tube wave record
suite in borehole
WN-1,
Pinawa,
Manitoba.
253
wave generation points (see Figs. 1, 5 and 6). Results are displayed in Figs. 7, 8 and 9, respectively. In each of these three figures, bar-graphs showing the integrated information from TV, core, hydrogeology and tube-wave data for each borehole are constructed. These composite bar-graph figures not only clearly show correlations with open fractures but also provide the most complete information about fracture characteristics and/or permeabilities in each borehole. The data shown in Figs. 7 through 9 (from right to left) are the following. (1) Tube-wave amplitudes. The identified tube waves are visually estimated and subdivided into small (S), medium (M), large (L) and very large (VL) according to their relative peak-to-peak amplitudes. (2) Hydraulic conductivity measurements. Borehole WN-1 is the only one in which this type of information is shown in this paper. Presently hydraulic conductivity me~~ements are selectively done over certain UNDERGROUND RESEARCH LAB. CURL #l)
Fig. 7. Correlations of open fractures observed from tube-wave studies, core logging, and TV logging in borehole URL-1.
depth intervals which are marked by solid triangles (b) on the left side of the depth axis in the tube-wave log. The hydraulic conductivity of each testing interval is represented in terms of equivalent single aperture in micrometres (,um) (Davison, 1980). These aperture figures are calculated UNDERGROUND RESEARCH LAB. CURL X2>
---
Fig, 8. Correlations of open fractures and TV logging in borehole URL-2.
observed from tube-wave studies, core logging
by assuming that there is only one open fracture within each test interval of 2.97-5 m since the injecting technique could not effectively isolate individual fractures in each testing interval. (3) Core log data. Total numbers of naturally open fractures (including both open and possibly open to fluid flow) per metre from inspection of the core log. (4) TV log data. Total number of open and partly open fractures per metre interval are computed from the TV log.
PINAUA BOREHOLE #I (UN-I) Fig. 9. Correlations of open fractures observed from tube-wave analyses, core logging, TV logging and hydrogeology studies in borehole WN-1.
Borehole URL-1, Lac du Bonnet Fig. 7 shows the comparison among tube-wave studies, the core log and the TV log. The following observations can be made. (1) All open fractures detected by the TV camera are associated with tube waves. (2) Both core and TV logs indicate a closed fracture*1 at 62 m depth. However, well-defined tube waves are observed radiating up and down the hole at this location (see Fig. 1) suggesting that the indicated fracture should be open. A further hydraulic conductivity test at this location can clear up this uncertainty. (3) The relative amplitudes of tube waves appear to be correlatable with fracture frequencies in the TV and core logs. Those open fractures (or open fracture zones) having higher fracture frequencies are all associated with tube waves having greater than small amplitudes. Open fractures located at 80-90 m, 110-120 m and 325-331 m are such examples. (4) In general, the three borehole logs correlate well as regards positions of open fractures. Examples are the open fractures located at 25-27 m, *’ According to the TV and core logs, a closed fracture is a fracture which was once open and is now sealed. Therefore, it is not open to fluid flow.
256
72-90 m, 97 m, 110-120 m and 325-331 m. However, the tube wave analysis and the core log appear to detect more open fractures than the TV log.
Borehole URL-2, Lac du Bonnet This hole has a total length of 1100 m, and is the deepest one studied by the tube-wave method. The bar-graphs showing correlations of open fractures observed from the TV log, core log and tube-wave analysis are shown in Fig. 8. The following observations can be made. (1) As in URL-1 all open fractures detected by the TV camera are associated with tube waves- The relative amplitudes of tube waves appear to be correlatable with fracture frequencies of the TV and core logs. These open fractures (or open fracture zones) with high fracture frequencies are all associated with medium to large amplitude tube waves. The open fractures located at 48-52 m, 90-93 m, 125-132 m and 162 m are such examples. (2) Comparative studies amongst the TV log, the core log and the tubewave analysis suggest that there is a good correlation between the occurrence of tube-wave generating points and open fractures shown from the TV and core logs. However, more open fractures are detected by the corelog and the tube-wave analysis than by the TV log. (3) All three borehole logs show that the top 165 m of the host granitic rock is highly fractured while the rock body below 165 m is almost a completely solid unit.
WiV-1. Pinawa Though the WN-1 borehole is located where the overburden thickness is 22 m, the record quality is the same as that from boreholes URL-1 and -2. However, the seismic frequencies seem to be slightly lower than those of the latter two holes (see Figs. 1, 5 and 6). Fig. 9 shows the comparison amongst the tube-wave, core, TV and hydrogeology logs. The following observations have been noted from the composite bar-graphs. (1) In general, there is a good correlation with the occurrences of the tube-wave generating points and open fractures shown from the TV log, the core log and the hydrogeology study. In other words, all four methods of fracture ch~acte~zation appear to give good indications for open fractures or open fracture zones. The open fractures located at 41 m, 127 m, 397 m and 416 m are such examples. (2) Presently, hydraulic conductivity tests are selectively carried out over certain depth intervals which check those open fractures or open *I Unpublished, unrestricted report available from Research, Co., Chalk River, Ontario KOJ lJ0, Canada.
SDDO
Atomic
Energy
of
Canada
257
fracture zones that have already been detected by the geological and/or geophysical methods. Hence a complete comparison between the tubewave analysis and the hydrogeology study over all depths in WN-1 is not possible. However, for those intervals which have been hydrologically tested, the relative amplitudes of tube waves correlate well with hydraulic conductivity measurements and suggest that the amplitudes of tube waves are directly related to fracture permeabilities. Examples are those open fractures located at 41 m, 127 m, 135 m, 397 m, 416 m, 444 m and 451 m. (3) Comparative studies amongst the TV log, the tube-wave log and the hydrogeology study indicate that some open fractures which are not indicated by the TV log are detected by both the tube-wave analysis and the hydrogeology study. Open fractures located at 22 m, 58 m, 92 m, 149 m and 303 m are such examples. Similar comparison amongst the core, tube-wave and hydrogeology logs shows that more open fractures are detected by the tube-wave analysis and the hydrogeology study than by the core log. Examples are the open fractures located at 22 m, 30 m, 32 m, 35 m, 58 m and 92 m. (4) Comparison amongst the TV log, core log and tube-wave analysis indicate that more open fractures are identified by the tube-wave analysis and the core log than by the TV log. Such examples are the open fractures located at 98 m, 102 m, 111 m, 115 m, 136 m, 149 m, 164 m, 191 m, 303 m, 381 m and 387 m. (5) A study of the section between 380 and 460 m illustrates some interesting features. At certain locations there are corresponding increases amongst the tube-wave amplitudes, the hydraulic conductivities and the frequencies of open fractures. These are at 395-400 m, 416 m, 431 m, 442 m and 451 m. These positive correlations may possibly provide evidence that the relative amplitudes of tube waves are associated with the degree of permeability of open fractures. This assumption is clearly justified when the following two fracture zones are compared; the one at 395-400 m (denoted as zone A) and the one at 416 m (denoted as zone B). By comparing the number of open fractures per metre in these two zones, one would consider zone A (37 naturally open fractures by the core log and 9 open fractures by the TV log) has much higher fracture permeability than zone B (6 naturally open fractures by the core log and 1 partly open fracture by the TV log). In spite of the heavily fractured nature of zone A with respect to that of zone B, the hydraulic injection tests, however, show the reverse result in permeabilities, i.e., 205-210 prns in aperture for zone B and 83-86 prns for zone A. This differential in fracture permeability is supported by tube wave studies which show a very large increase in amplitude for zone B in comparison with much smaller amplitude for zone A (see Figs. 6 and 9). These characterizations by relative amplitudes of tube waves do suggest that zone B is more hydrologically significant than any other open fracture in borehole WN-1. As a result, the tube wave response seems to be more directly related to permeability than fracture data obtained from the TV and core logs.
258
In general, the combined borehole data in these three holes can be summed up as follows: Each method is more or less limited in its own application; however, the inte~ation of different groups of borehole data could probably provide the most constructive and complete information about fracture permeabilities. CONCLUSION
Our experimental work suggests that the identification of tube wave events on borehole seismograms can be used as reliable indicators of fractures open to fluid flow. As well, the relative amplitudes of tube waves can be utilized to characterize fracture permeabilities. Moreover, in cases where doubt exists from the geological logging, the tube-wave method can serve to clarify whether a fracture is open to fluid flow. The tube-wave method is fast and efficient in field operation; over 600 m can be logged in a day by two men. With this capability of providing fast data acquisition and inte~re~tion of tube wave events, it is possible to provide a fast means of identifying zones of large fluid conductivity prior to detailed hydrogeologic tests. So far, we have succeeded in recording tube waves down to 1100 m in one of the holes. The seismic record quality at this depth is unchanged, which implies that it is possible to employ the tube wave method to even greater depth. Because of the relatively low frequency employed in this seismic study, it is suggested that tube wave events are effectively sampling a volume of rock within several metres of the borehole. At least a macroscopic bulk property of the rock mass (containing open fractures) is monitored at the time of incidence of compressional wave onto a fluid-filled fracture zone which intersects the hole. Hence, the result obtained from this new method can possibly provide the information of fracture permeability with substantial depth penetration into the immediate vicinity of the borehole wall (Huang and Hunter, 1980a,b). To obtain a better understanding of subsurface fracture permeability it is suggested that borehole data obtained by different methods be combined to provide the best means of establishing the fracture characteristics in a borehole. ACKNOWLEDGEMENTS
The authors wish to thank Atomic Energy of Canada Limited and Department of Energy, Mines and Resources for permission to publish this paper. Special thanks are extended to H.A. MacAulay and R.M. Gag& of the Terrain Geophysics Section at the Geological Survey of Canada for their assistance in proofreading and drafting this publication.
269
REFERENCES Biot, M.A., 1952. Propagation of elastic waves in cylindrical bore containing a fluid. J. Appl. Phys., 23 (9): 997-1005. Cheng, C.H. and Toksoz, M.N., 1981. Elastic wave propagation in a fluid-filled borehole and synthetic acoustic logs. Geophysics, 46 : 1042-1053. Davison, C.C., 1980. Physical Hydrogeologic Measurement in Fractured Crystalline Rock - Summary of 1979 Research Programs at WNRE and CRNL. AECL Tech. Rec., TR161. Hardage, B.A., 1981. An examination of tube wave noise in vertical seismic profiling data. Geophysics, 46 (6): 892-903. Hauge, P.S., 1981. Measurements of attenuation from vertical seismic profiles. Geophysics, 46 (11): 1548-1558. Huang, C.F. and Hunter, J.A.M., 1980a. A Progress Report on the Seismic Downhole Survey. AECL Tech. Rec., TR-31. Huang, C.F. and Hunter, J.A.M., 1980b. Identification and Correlation of Tube Wave Events on Borehole Seismic Records. AECL Tech. Rec., TR-32*‘. Huang, C.F. and Hunter, J.A.M., 1981a. The correlation of tube wave events with open fractures in fluid-filled boreholes. Current Res., Part A, Geol. Surv. Can., pp. 361376, Pap. 81-1A. Huang, C.F. and Hunter, J.A.M., 1981b. A seismic “tube wave” method for in-situ estimation of rock fracture permeability in boreholes. Presented at the 51st Annual International SEG Meeting, Los Angeles, October, 1981. Hunter, J.A.M., 1980. Mating the digital engineering seismograph with the small computer - Some useful techniques. Presented at the 50th Annual International SEG Meeting, Houston, Texas, 1980. Kitsunezaki, C., 1971. Field Experimental Study of Shear Waves and the Related Problems. Geophysical Institute, Kyoto University, No. 11, 1971, pp. 103-177. Ording, J.R. and Redding, V.L., 1953. Sound waves observed in mud-filling well after surface dynamite charges. J. Acoustic Sot. Am., 25: 719-726. Paillet, F.L., 1980. Acoustic Propagation in the Vicinity of Fractures which Intersect a Fluid-filled Borehole. Society of Professional Well Log Analysts, 21st Annual Logging Symposium, Lafayette, Louisiana, Translations, 33 pp. Paillet, F.L., 1982. Acoustic Characterization of Fracture Permeability at Chalk River. U.S. Geol. Surv., Denver, Colorado, unpublished manuscript. Riggs, E.D., 1955. Seismic wave types in a borehole. Geophysics, 20 (1): 53-67. White, J.E., 1965. Seismic Waves Radiation, Transmission and Attenuation. International Series in the Earth Sciences, McGraw-Hill Book Company, New York, N.Y., 380 pp. White, J.E., 1981. Tube Waves Due to Fluid-filled Cracks. Colorado School of Mines, Golden, Colo., unpublished manuscript.
22 (1984) 245-259 Publishers B.V., Amsterdam
THE TUBE-WAVE METHOD FRACTURE PERMEABILITY
CF.
HUANG
245 - Printed
in The Netherlands
OF ESTIMATING IN-SITU ROCK IN FLUID-FILLED BOREHOLES
and J.A. HUNTER
Atomic Energy of Canada Limited, c/o Geological Survey of Canada, Ottawa, Ontario KlA OE8 (Canada) Geological Survey of Canada, Ottawa, Ontario KlA OE8 (Canada) (Accepted
for publication
March 23, 1984)
ABSTRACT Huang, C.F. and Hunter, J.A., 1984. The tube-wave method fracture permeability in fluid-filled boreholes. Geoexploration,
of estimating in-situ 22: 245-259.
rock
Studies on subsurface fracture permeabilities in igneous and/or metamorphic rock bodies are in progress as part of the AECL (Atomic Energy of Canada Limited) Nuclear Fuel Waste Management Program. A multichannel borehole hydrophone array has been used to detect open fractures and rock property changes circumjacent to deep boreholes in granitic rock. The la-channel array, with equispaced hydrophones of approximately 1 m, is lowered at intervals in the borehole. The seismic source consists of a small dynamite charge detonated in a shallow shot hole near the well head. A 12-channel digital engineering seisomograph system is used to record high resolution data (0.2 ms sample rate) over a recording period of 200 ms. In addition to first-arrival compressional waves, high-amplitude low-velocity later events were observed, which appeared to be generated at various intervals within the borehole. These events have been identified as “tube waves”, which are interpreted as being generated by the incidence of compressional waves from the surrounding rock body onto fluid-filled fracture zones intersecting the borehole. Correlations of positions and amplitudes of tube waves with open fractures observed from TV logging, core logging and hydrogeology studies suggest that tubewave events can be used as reliable indicators of fractures open to fluid flow. The relative peak-to-peak amplitudes of tube waves appear to be directly related to the measured hydraulic conductivities in a borehole.
INTRODUCTION
For several years, Atomic Energy of Canada Limited (AECL) and the Department of Energy, Mines and Resources have been actively investigating the feasibility of using crystalline rock bodies as possible sites for nuclear fuel waste repositories. The principal objective in such investigations is to locate pathways in the rock mass and determine the resultant groundwater movement. Since the permeability of most unfractured plutonic rocks is very low, the main thrust of investigation is confined to the study of faults and fracture systems within such rock formations.
0016-7142/84/$03.00
o 1984 Elsevier
Science
Publishers
B.V.
246
100
200
E
300
V
I
c P
w
a
400
500
600
-i .-r
_~_
~----------=~ mpZZTFZF=Z_
247
As a part of the program to determine the in-situ fracture permeability in a crystalline rock body, several methods have been used for identifying and characterizing open fractures which intersect a borehole. These include core logging, TV logging, standard logging, hydraulic conductivity measurements and downhole seismic surveys referred to as the tube-wave method. Detailed correlations of occurrences of tube-wave generating points with open fractures, as detected by other geological methods, have suggested that the tube-wave method can be used as a reliable indicator of fractures open to fluid flow. In addition, the relative amplitudes of tube waves appear to be correlatable with measured hydraulic conductivities (Huang and Hunter, 1981a,b). This paper summarizes the result of in-situ seismic tube-wave measurements in boreholes that are currently being studied by AECL in the concept assessment phase of its Nuclear Fuel Waste Management Program. The boreholes utilized for this study are located on the grounds of the proposed site of Canadian underground research laboratory (URL) and the AECL Pinawa site. Both sites are situated on a mineralogically rather homogeneous granitic batholith, the Lac du Bonnet batholith. Tube-wave
method
Tube waves have been the subject of investigations in borehole seismic surveys for many years, mainly because of their interfering effects in the identification of later arrivals (shear waves and reflections). Tube waves are low-velocity, large-amplitude later events propagating along the interface between the borehole wall and the borehole fluid. They can be generated by the incident compressional wave energy from a surface source near the borehole, a source within the borehole, or a compressional wave in the surrounding solid passing any major discontinuity in the borehole (White, 1981). The initial intention of the borehole seismic survey was to investigate the possibility of utilizing in-situ seismic wave measurements for detecting and mapping cracks and fracture zones in a borehole. During the first-arriIal compressional wave studies in boreholes at Chalk River and Pinawa, we found that the record suites displayed many large-amplitude, low-velocty events which appeared to be generated at various intervals within boreholes. These later events were subsequently interpreted as “tube waves”, which are generated at fluid-filled fracture zones intersecting the borehole. Fig. 1 is a typical example of the record suite from a borehole at the URL research site showing “tube waves” of varying amplitude radiating up and down the hole from fracture zones. Similar results have been obtained in Fig. 1. Tube-wave record suite in borehole URL-1, Lac du Bonnet, Manitoba, (the end of record is at 670 m).
248
more than 20 boreholes at the four AECL research sites*’ in granitic rocks. White (1965) and Kitsunezaki (1971) show that a fracture zone in rock behaves as an effective source of tube waves when a compressional wave is incident onto it. Fig. 2. shows the mechanism for generation of tube waves in a borehole. When the compressional wave energy from the surface explosive source impinges onto the fracture zone, the water contained in this zone is squeezed out and pushed into the borehole, thus generating a tube wave that radiates up and down the hole. Based on this model, if tubewave generation zones can be mapped in the borehole, zones of high permeability (and open fractures) can be mapped.
Fractured zone P 8
: :
Compressional
Wait rock
Fig. 2. Schematic of p-wave.
wave
Tube wave
:
Granite
diagram showing tube waves generated at a fracture zone by incidence
**The four AECL research sites are near: (1) Chalk River, Ontario; (II) Atikokan, tario; (III) Pinawa, Manitoba; (IV) URL, Lac du Bonnet, Manitoba.
On-
249
INSTRUMENTATION
Borehole seismic records are taken by using a crystal cable array. This 12-hydrophone array (Mark Products Ltd., p-27), designed for high resolution surveying, is a reduced-scale version of that used in the oil industry. Instead of an array with 15 m between hydrophones, the interval is reduced to 1 m giving an 11-m “live” section. The frequency response of the hydrophone is relatively broad-band, between 2 Hz and 1000 Hz. The centre frequency of seismic energy currently used in this study is about 200 Hz. The pressure-sensitive hydrophones, with built-in preamplifiers, are connected in line with the interconnectors and the 1500-m cable. The 28.conductor steel-cored cable is stored on a winch and is used in conjunction with a well head tripod (see Fig. 3). Data from the hydrophone array are recorded on a Nimbus ES 1210-F, 12.channel seismograph using a record length of 200 ms (0.2 ms sampling rate) and then stored on the Nimbus G-724s digital tape recorder for permanent data storage and future analysis.
Fig. 3. The field assemblage of seismic cable, winch research site (URL-l), Lac du Bonnet, Manitoda.
Surveying
and well-head
triDod at the AECL
methods
In surveying, the array of hydrophones is lowered at intervals in the borehole (Fig. 4). At each array position, a small dynamite charge (75 N 150 g of Forcite 75%) is detonated in a shallow shot hole at offset dis-
260
tances varying between 10 and 40 m from the well head. Experience has shown that the best energy transmission is obtained when the shot hole is drilled into outcrop and fully tamped with water. After each shot, the hydrophone array is moved to a new position with some overlap of the previous position. The amount of overlap varies with each hole record according to the number of active hydrophones available in each survey. Amplifier gains are adjusted so that the entire seismic wave train from the 12-channel array can be adequately recorded for tube-wave studies. Records from each shot location are played back and edited using a microcomputer (Hunter, 1980) to form a continuous record suite displaying seismic traces from all hydrophone positions in the hole. Figs. 1, 5 and 6 are examples of such record suites showing tube waves with varying amplitudes radiating from fracture zones in three different holes. To
Nimbus
IZIOF
Seismic
Recorder
\
Fig. 4. The crystal cable array configuration showing the overlap. RESULTS
The record suites recorded in the two holes (URL-1 and URL-2) at the URL site and one hole (WN-1) at the Pinawa site are analysed for tube-
251
h
E v
I I-
a. Y
a
Fig. 5. Tube wave record suite in borehole URL-2, of record is at 1100 m).
Lac du Bonnet, Manitoba (the end
252
Fig. 6. Tube wave record
suite in borehole
WN-1,
Pinawa,
Manitoba.
253
wave generation points (see Figs. 1, 5 and 6). Results are displayed in Figs. 7, 8 and 9, respectively. In each of these three figures, bar-graphs showing the integrated information from TV, core, hydrogeology and tube-wave data for each borehole are constructed. These composite bar-graph figures not only clearly show correlations with open fractures but also provide the most complete information about fracture characteristics and/or permeabilities in each borehole. The data shown in Figs. 7 through 9 (from right to left) are the following. (1) Tube-wave amplitudes. The identified tube waves are visually estimated and subdivided into small (S), medium (M), large (L) and very large (VL) according to their relative peak-to-peak amplitudes. (2) Hydraulic conductivity measurements. Borehole WN-1 is the only one in which this type of information is shown in this paper. Presently hydraulic conductivity me~~ements are selectively done over certain UNDERGROUND RESEARCH LAB. CURL #l)
Fig. 7. Correlations of open fractures observed from tube-wave studies, core logging, and TV logging in borehole URL-1.
depth intervals which are marked by solid triangles (b) on the left side of the depth axis in the tube-wave log. The hydraulic conductivity of each testing interval is represented in terms of equivalent single aperture in micrometres (,um) (Davison, 1980). These aperture figures are calculated UNDERGROUND RESEARCH LAB. CURL X2>
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Fig, 8. Correlations of open fractures and TV logging in borehole URL-2.
observed from tube-wave studies, core logging
by assuming that there is only one open fracture within each test interval of 2.97-5 m since the injecting technique could not effectively isolate individual fractures in each testing interval. (3) Core log data. Total numbers of naturally open fractures (including both open and possibly open to fluid flow) per metre from inspection of the core log. (4) TV log data. Total number of open and partly open fractures per metre interval are computed from the TV log.
PINAUA BOREHOLE #I (UN-I) Fig. 9. Correlations of open fractures observed from tube-wave analyses, core logging, TV logging and hydrogeology studies in borehole WN-1.
Borehole URL-1, Lac du Bonnet Fig. 7 shows the comparison among tube-wave studies, the core log and the TV log. The following observations can be made. (1) All open fractures detected by the TV camera are associated with tube waves. (2) Both core and TV logs indicate a closed fracture*1 at 62 m depth. However, well-defined tube waves are observed radiating up and down the hole at this location (see Fig. 1) suggesting that the indicated fracture should be open. A further hydraulic conductivity test at this location can clear up this uncertainty. (3) The relative amplitudes of tube waves appear to be correlatable with fracture frequencies in the TV and core logs. Those open fractures (or open fracture zones) having higher fracture frequencies are all associated with tube waves having greater than small amplitudes. Open fractures located at 80-90 m, 110-120 m and 325-331 m are such examples. (4) In general, the three borehole logs correlate well as regards positions of open fractures. Examples are the open fractures located at 25-27 m, *’ According to the TV and core logs, a closed fracture is a fracture which was once open and is now sealed. Therefore, it is not open to fluid flow.
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72-90 m, 97 m, 110-120 m and 325-331 m. However, the tube wave analysis and the core log appear to detect more open fractures than the TV log.
Borehole URL-2, Lac du Bonnet This hole has a total length of 1100 m, and is the deepest one studied by the tube-wave method. The bar-graphs showing correlations of open fractures observed from the TV log, core log and tube-wave analysis are shown in Fig. 8. The following observations can be made. (1) As in URL-1 all open fractures detected by the TV camera are associated with tube waves- The relative amplitudes of tube waves appear to be correlatable with fracture frequencies of the TV and core logs. These open fractures (or open fracture zones) with high fracture frequencies are all associated with medium to large amplitude tube waves. The open fractures located at 48-52 m, 90-93 m, 125-132 m and 162 m are such examples. (2) Comparative studies amongst the TV log, the core log and the tubewave analysis suggest that there is a good correlation between the occurrence of tube-wave generating points and open fractures shown from the TV and core logs. However, more open fractures are detected by the corelog and the tube-wave analysis than by the TV log. (3) All three borehole logs show that the top 165 m of the host granitic rock is highly fractured while the rock body below 165 m is almost a completely solid unit.
WiV-1. Pinawa Though the WN-1 borehole is located where the overburden thickness is 22 m, the record quality is the same as that from boreholes URL-1 and -2. However, the seismic frequencies seem to be slightly lower than those of the latter two holes (see Figs. 1, 5 and 6). Fig. 9 shows the comparison amongst the tube-wave, core, TV and hydrogeology logs. The following observations have been noted from the composite bar-graphs. (1) In general, there is a good correlation with the occurrences of the tube-wave generating points and open fractures shown from the TV log, the core log and the hydrogeology study. In other words, all four methods of fracture ch~acte~zation appear to give good indications for open fractures or open fracture zones. The open fractures located at 41 m, 127 m, 397 m and 416 m are such examples. (2) Presently, hydraulic conductivity tests are selectively carried out over certain depth intervals which check those open fractures or open *I Unpublished, unrestricted report available from Research, Co., Chalk River, Ontario KOJ lJ0, Canada.
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fracture zones that have already been detected by the geological and/or geophysical methods. Hence a complete comparison between the tubewave analysis and the hydrogeology study over all depths in WN-1 is not possible. However, for those intervals which have been hydrologically tested, the relative amplitudes of tube waves correlate well with hydraulic conductivity measurements and suggest that the amplitudes of tube waves are directly related to fracture permeabilities. Examples are those open fractures located at 41 m, 127 m, 135 m, 397 m, 416 m, 444 m and 451 m. (3) Comparative studies amongst the TV log, the tube-wave log and the hydrogeology study indicate that some open fractures which are not indicated by the TV log are detected by both the tube-wave analysis and the hydrogeology study. Open fractures located at 22 m, 58 m, 92 m, 149 m and 303 m are such examples. Similar comparison amongst the core, tube-wave and hydrogeology logs shows that more open fractures are detected by the tube-wave analysis and the hydrogeology study than by the core log. Examples are the open fractures located at 22 m, 30 m, 32 m, 35 m, 58 m and 92 m. (4) Comparison amongst the TV log, core log and tube-wave analysis indicate that more open fractures are identified by the tube-wave analysis and the core log than by the TV log. Such examples are the open fractures located at 98 m, 102 m, 111 m, 115 m, 136 m, 149 m, 164 m, 191 m, 303 m, 381 m and 387 m. (5) A study of the section between 380 and 460 m illustrates some interesting features. At certain locations there are corresponding increases amongst the tube-wave amplitudes, the hydraulic conductivities and the frequencies of open fractures. These are at 395-400 m, 416 m, 431 m, 442 m and 451 m. These positive correlations may possibly provide evidence that the relative amplitudes of tube waves are associated with the degree of permeability of open fractures. This assumption is clearly justified when the following two fracture zones are compared; the one at 395-400 m (denoted as zone A) and the one at 416 m (denoted as zone B). By comparing the number of open fractures per metre in these two zones, one would consider zone A (37 naturally open fractures by the core log and 9 open fractures by the TV log) has much higher fracture permeability than zone B (6 naturally open fractures by the core log and 1 partly open fracture by the TV log). In spite of the heavily fractured nature of zone A with respect to that of zone B, the hydraulic injection tests, however, show the reverse result in permeabilities, i.e., 205-210 prns in aperture for zone B and 83-86 prns for zone A. This differential in fracture permeability is supported by tube wave studies which show a very large increase in amplitude for zone B in comparison with much smaller amplitude for zone A (see Figs. 6 and 9). These characterizations by relative amplitudes of tube waves do suggest that zone B is more hydrologically significant than any other open fracture in borehole WN-1. As a result, the tube wave response seems to be more directly related to permeability than fracture data obtained from the TV and core logs.
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In general, the combined borehole data in these three holes can be summed up as follows: Each method is more or less limited in its own application; however, the inte~ation of different groups of borehole data could probably provide the most constructive and complete information about fracture permeabilities. CONCLUSION
Our experimental work suggests that the identification of tube wave events on borehole seismograms can be used as reliable indicators of fractures open to fluid flow. As well, the relative amplitudes of tube waves can be utilized to characterize fracture permeabilities. Moreover, in cases where doubt exists from the geological logging, the tube-wave method can serve to clarify whether a fracture is open to fluid flow. The tube-wave method is fast and efficient in field operation; over 600 m can be logged in a day by two men. With this capability of providing fast data acquisition and inte~re~tion of tube wave events, it is possible to provide a fast means of identifying zones of large fluid conductivity prior to detailed hydrogeologic tests. So far, we have succeeded in recording tube waves down to 1100 m in one of the holes. The seismic record quality at this depth is unchanged, which implies that it is possible to employ the tube wave method to even greater depth. Because of the relatively low frequency employed in this seismic study, it is suggested that tube wave events are effectively sampling a volume of rock within several metres of the borehole. At least a macroscopic bulk property of the rock mass (containing open fractures) is monitored at the time of incidence of compressional wave onto a fluid-filled fracture zone which intersects the hole. Hence, the result obtained from this new method can possibly provide the information of fracture permeability with substantial depth penetration into the immediate vicinity of the borehole wall (Huang and Hunter, 1980a,b). To obtain a better understanding of subsurface fracture permeability it is suggested that borehole data obtained by different methods be combined to provide the best means of establishing the fracture characteristics in a borehole. ACKNOWLEDGEMENTS
The authors wish to thank Atomic Energy of Canada Limited and Department of Energy, Mines and Resources for permission to publish this paper. Special thanks are extended to H.A. MacAulay and R.M. Gag& of the Terrain Geophysics Section at the Geological Survey of Canada for their assistance in proofreading and drafting this publication.
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