The application of ground penetrating radar for mapping fractures in plutonic rocks within the Whiteshell Research Area, Pinawa, Manitoba, Canada

FFFLIED I EIDFII IICS Journal of Applied Geophysics 33 (1995) 125-141

ELSEVIER

The application of ground penetrating radar for mapping fractures in plutonic rocks within the Whiteshell Research Area, Pinawa, Manitoba, Canada K.M. Stevens, G.S. Lodha, A.L. Holloway 1, N.M. Soonawala Atomic Energy of Canada Ltd., Whiteshell Laboratories, Pinawa, Manitoba ROE ILO, Canada

Received 4 January 1993; accepted 18 December 1993

Abstract Ground penetrating radar (GPR) has been applied in the Canadian Nuclear Fuel Waste Management Program to map structural features in the granitic and gneissic rock of the Canadian Shield. Field results confirm the theoretical analysis, which shows that useful signal penetration in excess of 60 m is possible in near-surface rocks of low electrical conductivity containing fresh and dilute groundwater. Effective probing distances are drastically reduced (less than 20 m) at greater depth ( > 400 m) because of increased electrical conductivity associated with saline groundwaters. GPR has been applied successfully on profiles on outcrops and from boreholes. Examples of mapping low-dip fracture features in granitic and gneissic outcrops in the Whiteshell Research Area and in the tunnel and boreholes of the Underground Research Laboratory are presented. Results of a near-surface crosshole tomography survey in combination with surface radar reflection profiling are discussed in terms of combined effectiveness in mapping the continuity of fracture features below an overburden-covered area around and between boreholes. Hydraulic transmissivity values and water flow rates determined from hydraulic pump tests at discrete fracture intervals in the boreholes support evidence for the fracture interconnectivity interpreted from the radar surveys.

1. Introduction Most countries producing nuclear-generated electricity have a national program for the management and disposal of the resulting nuclear fuel waste. Furthermore, most countries have opted for "geological disp o s a l " , a scheme that involves burial of the nuclear waste deep underground in excavated openings. The Canadian Nuclear Fuel Waste Management Program ( C N F W M P ) , formally initiated in 1978, is a multidisciplinary program in which the concept of disposal of nuclear fuel waste in the intrusive igneous rock of t Now at Varanidex Inc., Box 40 Cloyne, Ontario KOH 1K0, Canada

SSDIO926-9851(94)OOO25-J

the Canadian Shield has been developed (Dormuth and Nuttall, 1987). Soonawala et al. (1990) described some of the geological features of interest for the C N F W M P , particularly those suitable for mapping by geophysical methods, and the role of geophysics in the CNFWMP. M o d e m geophysical equipment and data processing methods are now being used in the C N F W M P . Our work includes GPR surveys over low-dipping fracture zones in the upper 30 to 100 m of granitic rock, borehole radar surveys, a crosshole radar tomographic survey, and radar profiles, at levels 240 and 420 m below the surface. In this paper we report some of our recent experiences in the application of GPR in the Whiteshell Research Area ( W R A ) . The applications we describe

126

K.M. A'teven.s et al. /.Iournal <~['Applied Geophysic~ 33 (1995) /25 t41

may be useful in many other geological programs involving waste disposal.

2. Geology of the Whitesheli Research Area All GPR work reported in this paper was carried out over the Lac du Bonnet granite batholith in the WRA. the geology of which has been described by Brown et al. c 1989). The batholith lies in the southern part of the English River subprovince of the Superior Province of the Canadian Shield. It trends ENE. with about 1500 km 2 exposed east of the Paleozoic rock cover. Fig. [ shows the extent of the batholith, and the areas where geological and geophysical investigations, including

Manitoba

Whiteshell Research Area

I

I

Fig. 1. Location of the Whiteshell Research Area, Whiteshell Laboratories and AECL permit areas A - J in relation to the local geology of the Lac du Bonnet batholith and surrounding area, Manitoba. Canada~ (Simplified after the Regional Geology Section, Applied Geoscience Branch, AECL. )

GPR, have been carried out under the CNFWMP. The site of the Underground Research Laboratory (URL), where the subsurface work described in this paper was implemented, is also shown in Fig. 1. The principal petrological units are a pink, porphyritic granite-granodiorite, a similar but biotite-rich and gneissic phase, a xenolith-bearing granite, and a grey granite-quartz monzonite. The structural features of greatest significance for GPR are the low-intermediate-dipping (LID) fractures with dips of 10-30 ° (Brown et al., 1989). They are common and scattered at the surface, but reduce to well defined fault zones at depth. Fig. 2 shows the typical structure of such fractures when they occur at the surface. They form asymmetric valleys with a steep cliff on the downdip (hanging-wall side) of the fiacture and a gently dipping outcrop with a dip roughly equal to that of the fracture zone on the foot-wall side. This shape is evidently caused by the intersection of the dipping fracture with smaller-scale vertical fractures present near the surface. This figure also shows the logistics of surface and borehole radar surveys with respect to the location of the fracture zone. The presence of saline groundwater at depth in the Lac du Bonnet batholith is of interest for GPR studies since the associated low resistivities severely restrict the penetration of radar energy. Gascoyne et al. ( 1987 ) have presented groundwater data from throughout the Canadian Shield, including the Lac du Bonnet batho= lith. The data show a general presence of saline groundwaters below a depth of about 300 m. Freely circulating surface waters found above that depth are generally not saline. One of the highest values reported for the Lac du Bonnet batholith was 15 g/1 total dissolved solids in groundwater from a depth of 390 m. Gascoyne et al. (1989) reported the presence of readily soluble chloride salts in the rock matrix of unaltered grey granite of the Lac du Bonnet bathotith with concentrations ranging from 30 to 48 mg/kg. The altered pink-red granite, which occurs at the surface and in the vicinity of deeper fracture zones has much lower chloride concentrations. Fig. 3 shows the distribution of the total dissolved solids concentration vs. depth for groundwater in the WRA (Gascoyne and Kamineni, 1992). The open circles indicate samples of in situ groundwater whereas the solid circles represent samples contaminated with surface water. The inset in the figure shows conductiv-

K.M. Stevens et al./ Journal of Applied Geophysics 33 (1995) 125-141

LINEAR VALLEY

outcro p with S l Oontal b •fractures z

Typical surface radar survey

127

Schematic of a borehole radar profile

Cliff

Fig. 2. Typical structure of low-to-intermediate-dipping (LID) fracture features of outcrops in the WRA (modified after Soonawala et al., 1990).

ity as a function of the total dissolved solids. Electrical geophysical logs show a decrease in resistivity with depth (Soonawala et al., 1990). A typical resistivity log (borehole WB-1 from permit area B, in Fig. 1 ) is also shown in Fig. 3 to illustrate this observation.

3. Theoretical considerations for radar wave propagation in granite The electric field generated by an antenna radiating radio-frequency energy can be described by Maxwell's equations. One formulation is:

Eq. ( 1 ) contains an implicit harmonic time dependance function e -i'°t suppressed in the equation. This expression is valid for a lossy but homogeneous medium where we assume constant magnetic permeability (/~), electrical conductivity (or), and permittivity (e). For most geologic media, the magnetic permeability (/x) may be considered to be close to the value in a vacuum. For a plane wave (which is characteristic of the radar wavefront in the radiated far field) in a general homogeneous medium, the solution of Eq. ( 1 ) is: E = Eoe- ikz

(2)

where V2E + to2el~E + io~lz(r E = 0

( 1) k = ( to2 etx + itolxtr ) l/2

where E is the electric field vector ( V / m ) , to is the angular frequency (radians / s) = 27r~, ( v = frequency in Hz), • is the electrical permittivity of medium ( e = ~o × ~r, F / m ) , eo is the permittivity of free space, 8.854 × 10-12 ( F / m ) , Er is the relative dielectric constant of the medium,/x is the magnetic permeability of free space, 47r× 10 - 7 ( H / m ) , o- is the electrical conductivity ( S / m ) , i = ( - 1 )1/2, and t is time (s).

The symbol k is the propagation parameter or the wave number. The real part of k is associated with the phase factor (/3, r a d / m ) and the imaginary part is associated with the attenuation constant (a, d b / m ) of the wave. The real and imaginary parts of the wave number, k, can be rewritten to give expressions for the phase factor and the attenuation constant:

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K.M. Stevens et al. /Journal of Applied Geophysics 33 (1995) 125-141

medium with a velocity of propagation independent of frequency. The calculation of the skin depth can then be simplified to (Olsson et al., 1987):

k=13+io~

,8=2rr/a=oJ{~.l~/2[(l +

0"2/0)2e--2) 1/2-} - 1] }1/2

and

Eq, (5) will apply for ground penetrating radar frequencies in the megahertz range, which are used in a medium of very low conductivity (e.g. high-resistivity granite). The skin depth thus depends only upon the electrical permittivity (E) and the electrical conductivity (~r). Fig. 4 shows a log-linear plot of the skin depth (m) as a function of resistivity (O. m) for dielectric constants (~,) typical of the Stripa granite in Sweden (Olsson et al., 1987) and of the Lac du Bonnet batholith.

(4)

~=1/6=¢o{~1~/2[(1+o'2/oa2~)1/2-1]} '/2

(5)

3= (21o-)(el/~) j/2

(3)

If we rearrange Eq. (4), then we have an equation that defines the skin depth (6) or the distance over which the initial amplitude of the electromagnetic pulse decreases by a factor of 1/e. When the operating frequency is high enough such that the term cr2/o~2~ is much less than unity (i.e. o'2/ ~oZe2 << 1), the radar energy propagates through the 10 s ,

G ® .-I ®

10 4 E r~

.'g_ o CO "10

_> 0

.~ 10 3 a

~tTo~O~m~la~ ~c°~t~d~.rDaMvnmO

o

~

102 0

oEn" E ,~oo ,4 ~" =ooo

o It

200

400 600 800 Depth ( rn )

~

--'2 1000

,moo "

-

£7

0

Fig. 3. Log-linear plot of measured distribution of total dissolved solids (TDS) in ground waters as a function of bedrock depth from boreholes in the WRA. The inset shows the measured electrical conductivity of the ground water as a function of TDS Concentration. A typical electrical resistivity log from a WRA borehole is also shown.

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K.M. Stevens et al. / Journal of Applied Geophysics 33 (1995) 125-141

to

ac ac cx a s c K

®

.

.

.

.

.

.

.

.

.

.

.

.

.

(m) Fig. 4. Log-linear plot of skin depths (8 [ m ] ) calculated from Eq. (5) as a function of electrical conductivity (o'20MHz= conductivity at 20 MHz; o'oc = DC-conductivity [ S/m] ) with static dielectric constants (¢r) and magnetic permeability (p~) for the Stripa granite in Sweden (O; Olsson et al., 1987) and the Lac du Bonnet granite, Canada ( × ). P2oMHz= resistivity at 20 MHz, Poc = DC-resistivity [/2. m] ; - - = range of values for undeformed Stripa granite at 20 MHz measured from samples (near surface; Olsson et al., 1987); - = estimated range of values for Lac du Bonnet granite from borehole resistivity logs (for depths of 0 to 300 m). The dielectric constants used are 5.5 and 6.5, respectively. Typical DC and 20 M H z resistivity/conductivity ranges of bulk rock for each location are also shown in Fig. 4 by means of vertical bars alongside the y-axis. The simplified expression plotted in Fig. 4 suggests that the skin depth (or effective G P R probing depth) at a centre frequency of 20 M H z should be as high as 100 m for the highly resistive near-surface Lac du Bonnet granite, whereas typical skin depths range from 20 to 40 m for the less resistive Stripa granite. In the case of the Lac du Bonnet batholith, the above discussion applies to about the top 200 to 300 m. The skin depth is much less at greater depths, where saline groundwaters occur. This has implications for GPR surveys in workings at possible vault depths. It should be noted that approximating the useful depth of penetration of the G P R signals by the skin depth is an empirical limit which is dependent upon the G P R system transmitted power, receiver sensitivity, antenna efficiency, and variations in the electrical properties of the rock.

4. Equipment, methods, and procedures The surface G P R surveys described in this paper were performed with using a Pulse E K K O IV system manufactured by Sensors and Software Inc. o f Canada. The RAMAC system, purchased from A B E M Limited of Sweden, was used for the borehole surveys. Both are short-pulse systems with fully digital data acquisition and fibre-optic communications for data acquisition. Portable computers with appropriate software were used to control the GPR system and to record the field data. The RAMAC system was operated at centre frequencies of 22 and 60 MHz, and the Pulse E K K O IV system was operated at 25 and 100 MHz. In surface surveys, the transmitting and receiving antennas are generally separated at a fixed distance, with the axes of the antennas perpendicular to the profiling direction. A profile traverse is surveyed by moving the antenna pair in steps of 0.5 to 2 m along the survey line. For single-hole reflection surveys, the transmitting and receiving antennas are housed in

K.M. Stevens et al. / Journal of Applied Geophysics 33 (1995) 125-141

130

cylindrical probes separated at a fixed distance apart, usually 6 to 10 m, and this downhole probe assembly unit is moved in 0.5 to 2 m intervals. In the crosshole radar tomography surveys, either the transmitting probe or the receiving probe is fixed in one borehole while the other probe is moved in discrete intervals of about 2-5 m in the second hole. Traveltime and amplitude data are used to construct tomographic images of the variations in electrical properties between the boreholes. Subsurface GPR surveys at the URL follow a procedure similar to that for the surface surveys. 4.1. Data presentation

Data from surface profiling are generally shown in an equispaced wiggle trace format, with positive amplitudes shaded. Data from borehole reflection surveys

a)

WARR/CMP Surface Reflection Profile Tx to Rx Separation (n't) 5 10 15 20 25 30 35

are generally shown as composites of wiggle traces in a grey-scale variable-intensity plot. Depth scales are computed from an average near-surface wave propagation velocity of 120-125 m//zs. A wide-angle reflection and refraction technique ( W A R R ) is used on outcrops to estimate the near-surface velocity in that area. Fig. 5a shows a typical W A R R profile and the near-surface velocity as calculated from the GPR pulse wavelet moveout. Borehole-to-borehole radar transmission traveltimes were also used to estimate the wave velocity in relatively homogeneous rock in the upper 300 m of the Lac du Bonnet batholith. Fig. 5b shows the crosshole traveltime vs. transmission distance for 706 independent measurements taken between boreholes WB-1 and WB-2 of permit area B. The average wave velocity was calculated from the slope of a bestfit line through the data. Topographic corrections have been applied in some cases. Variable gain is applied to compensate for frequency-dependent attenuation that typically ranges from 40-60 dB/100 m at the 25 MHz profiling frequency in surface granite and gneisses.

waw Wae~ 0.3 rn/ns

5. Field examples Wave - 0.120

b)

~

2.~

~

2.28

Vedoclty

rnlr~

5.1. GPR surveys on outcrop WB-1 fo WB-2 ~ t e

Traveltlme Vs Raylength

1,9"2

86

Several selected examples of the application of GPR are presented to illustrate the detection of fractures at depth and the effects of saline groundwaters on GPR surveys.

104

122 140 Tx to RX Ray4englh (m)

158

Fig. 5. (a) Results of a typical WARR radar profile from surface outcrop near the URL. Near-surface direct rock path velocity calculated from Tx to Rx separation (m) and the radar pulse traveltime (ns). (b) Averageradar wave velocity (linear least-square best fit) in the top 300 m of typical near-surface Lac du Bonnet granite as derived from crosshole radar transmission traveltimes (WB-I to WB-2, numberof plotted measurements 706).

Fig. 6 shows a radar profile collected at an outcrop location in the northwest part of the URL lease area. These data were collected using 25 MHz centre frequency antennas. The fracture tog of borehole M-10, a vertical borebole collared near the northwest end of the profile line, is also shown in this figure. The inset of the figure shows the location of the subcropping of FZ2, a prominent local LID fracture zone, deduced from geological mapping and its known locations in the URL shaft (Brown et al., 1989). The most significant feature in this radar profile is a large continuous reflection, 40 m deep at the northwest end of the profile and about 50 m deep at the southeast end. This reflection corresponds to the location of fracture zone FZ-2 (logged in borehole M-10) which is estimated to have a southeasterly

K.M. Stevens etal./Journal of Applied Geophysics 33 (1995) 125-141

Borehole M10 Fmclumo/m 0 5

NW

Dlstance (m)

o

s

~o

I

I

I

131

SE

~5

o.

25

3o

36

4o

45

6o

I

I

I

I

I

I

I

I

I

.27O

lo '250

2030-

,230

40-

,220

| 210

Velocity= 0.120 mlns Time Scale 0 to 1100 ns (two way travel lime)

URL AGrrld ea

Location of Section Fig. 6. Radar profile results from an outcrop survey in the vicinity of the URL shaft and the subcropping of the LID feature Fracture Zone 2 (FZ-2). The surface location of borehole M10 is shown in the inset and the fracture log is shown relative to the radar profile.

dip of 20-25 °. Another reflection event, which is probably a fracture feature, extends from about 30 to 55 m along the profile at a depth ranging from 15 to 20 m. The weak curved radar reflection event from the midprofile termination of this fracture to a depth of about 40 m at the northwest end of the profile is probably a wave diffraction event caused by partial signal reflection from the area of termination of the fracture. The large continuous event very near the surface of the radar profile is the direct arrival of the air-ground wave, and can be observed in all profiles.

Fig. 7 shows a radar reflection profile from an outcrop location on permit area J, which is in a gneissic terrain just south of the Lac du Bonnet batholith. The inset shows the relative locations of the survey line, whose radar profile is shown (Line 2), other survey lines, the surface projection and dip of an LID fracture zone, and the approximate orientation and plunge of borehole WJ-1. A prominent dipping reflector can be noted between a depth of 10 m on the southeast end of the profile and a depth of 35 m at the 45 m position on the profile. A horizontal reflector feature intersects the

t32

K.M. Stevens et al. / Journal of Applied Geophysics 33 (1995) 125-/41

NW

Distance (m)

Collar Bo~¢oo~

i

l

8o

SE 90

i

i

w J1

70 J

i

T

I

t

dipping reflector does not correlate with any mapped open fracture feature in borehole W J-1 at that approximate depth. Therefore, it is suggested that this fracture feature terminates at its intersection with the dipping fracture zone. This type of fracture termination at a fracture/fracture intersection is common at shallow depths within the granitic and gneissic rocks of the area (A. Brown, pers. commun. ). v

£

5.2. Subsurface radar surveys at the URL

Velocity = 0.I 20 mlns Time Scale 0 to 1800 ns (two w a g travel tlme) Radar Proflllng Lines Permlt Area "J' i W J1 ...... - ' "

N

t t

f /

/ , ~ . . Une 2

./ I

Om

i

\

•- . . < ,.q,,,/up

-/7

~,

97r

Fra ,'o

\.\

f~'ehole

60m

Fig. 7. R a d a r reflection profile f r o m an outcrop area at permit area J in gneissic lithology. Borehole WJ-1 and its fracture log are shown in relative position to the radar profile.

dipping feature at this point and obscures the trace of the dipping feature below it. Borehole W J- 1 was drilled after the radar survey, and its fracture log was used to confirm the results of the radar survey. The correlation between the open fractures logged in the hole at 32 to 36 m and the dipping radar reflector is good. A similar analysis of reflection data from the other survey lines revealed that Line 2 is approximately perpendicular to the strike of this local fracture feature. Locally the apparent dip of this feature has been determined to be 25-30 ° to the northwest, and it most likely subcrops beneath the overburden to the southeast of the outcrop location. The LID fracture has been mapped on other lines to a depth of 60 m below the outcrop. An extrapolation of the horizontal reflector, Line 2, across the

Numerous GPR surveys have been performed underground, primarily at the 240 m level, to map the structural features in the vicinity of the URL. The geological interpretation of the structure is shown in Fig. 8 (after Everitt and Read, 1989). The approximate orientation of this section through the URL is shown in the inset. This cross-section shows the locations of the shaft, the fracture zones FZ-2, FZ-2.5, and FZ-3, the 240 m and the 420 m levels, and boreholes HG-3, HG-4, and PH5. Fig. 9 (Everitt and Read, 1989) shows a plan view of the 240 m level development overlaid with the projected structure contours for the top of FZ-2 (solid contours, below 240 m level) and the bottom of FZ2.5 (dashed lines, above 240 m level). The broken line in room 205 is the location of one of many radar reflection surveys conducted at this level for the sublevel characterization of FZ-2. Although the profile line is at an oblique angle to the projected dip of FZ-2, the fracture zone should still occur at 10-20 m below the room. Fig. 10 (after Soonawala et al., 1990; Holloway. 1992) shows in detain the estimated structure in the vicinity of room 205 of the 240 m level located in the central part of Fig. 8. Fig. 10 also shows a 120 MHz radar profile obtained along the floor of room 205. A fracture log of borehole HC- 14, drilled from room 205, is also shown. The radar profile clearly shows FZ-2 ( Reflecting Zone 2) and a small splay of FZ-2 (Reflecting Zone 1 ). The apparent orientation of FZ-2 was determined from the radar profiling to be 020/15, which compares well with that determined by geologic interpretation (Fig. 9). Other examples of radar reflection profiling on the 240 m level have been reported by Holloway and Mugford (t990) and Holloway (1992). Borehole radar reflection profiling has also been carried out extensively at various levels within the URL. Fig. 9 shows the orientation of the horizontal borehole,

133

K.M. Stevens et al. / Journal of Applied Geophysics 33 (1995) 125-141

NW

2oo-I 0

SE

Shaft

Pink

,--300 Pink

~ Granite

Granlte

loo--

E 0-

i

~k to Grey Granite __ 0

HG~I

-100 --

~)

Chiefly Massive Grey Granlte

LM -200 --

HG-,3

-300 m

-200

-,300

Location of Section Fig. 8. A rough cross-section of gross scale geological and structural features across the URL lease area through the URL shaft location and the underground workings. Fracture zones FZ-2, FZ-2.5, FZ-3 and the locations of boreholes HG-4, PH-5, and HG-3 are shown (after Everitt and Read, 1989). PH-5. The structure contours suggest that this borehole is approximately parallel to the strike of the top of FZ2. The cross-sectionin Fig. 8 suggests that this borehole also intersects FZ-2.5 at a distance of approximately 100-130 m downhole. This has been confirmed by fracture logging. Radar profiling was carried out in this borehole in an attempt to detect these fracture features and their orientation in the vicinity of borehole PH-5. Fig. 11 shows a variable-intensity grey-shaded profile (i.e. large amplitudes result in darker shades) collected at a centre frequency of 22 M H z in borehole PH-5. The vertical axis shows the calculated radial distance from the borehole, and the horizontal axis shows the midpoint position of the downhole tool string. The upper half of this figure shows only the radar profile; interpretive annotations have been added in the lower half. Many hyperbolic reflection events from localized point sources and a few weak planar reflections can be seen. The weak planar event at radial distances of 80 m to I l 0 m is interpreted to be the bottom of FZ-3 (Fig. 8). Its apparent dip interpreted from the radar profile matches the dip deduced from geological mapping in the direction of PH-5. The broad hyperbolic reflectors in the central part of the radar profile (radial distance 5 0 - 7 0 m) are believed to correspond to reflections from the undulating top surface of FZ-2, which is below borehole PH-5. The apexes of the hyperbolas (the closest point to the borehole) form

Fig. 9. A plan view of the 240 m level development of the URL and the estimated structure contours (masl) for the top of FZ-2 and the bottom of FZ-3. Borehole PH-5 is also shown (after Everitt and Read, 1989).

K.M. Stevens et al. /.Iournal ~/ Applied Geophx'sics 33 (1995) 1 2 5 / 4 /

134

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..... . , ::" .,. ; "";":" ~ "~'~"';" ;'~;°;'";'; ;",,., : ' ' ". .: '.". ~. .". ."";: "~ : ' - : ' -~; ~ T ~ : ~~; ~ m ~

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Legend

5

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Fig. 10. Estimated vertical structure of FZ-2 and FZ-2.5 in the vicnity of room 205 of the 240 n1 level ( top ). A t 20 MHz radar profile obtained from room 205 and annotated with reflector interpretations (bottom) ( after Holloway, 1992 ~.

K.M. Stevens et al./ Journal of Applied Geophysics 33 (1995) 125-141

135

100

1.5

o 0

E

=

1.0 50 "0

~- 0.5

0

0

50

100

FZ-3

d)

E

il

O

FZ-2,

50 •

borehole

100

" 150 FZ-2.5

length ( m e t r e s )

Fig. 11. A non-annotated (top) and an annotated (bottom), 22 MHz borehole radar profile from borehole PH-5 at the 240 m level, URL. A fracture log from borehole PH-5 indicating fracture frequency and the intersection of FZ-2.5 is also shown.

136

K.M. Stevens et al. /Journal of Applied Geophysics 33 (1995) 125 141

II)

~5 t

"10

.g

?~

50

100

150

Q

g

FZ-2

50

100

FZ-2.5

1,50

310-~ Fig. 12. A non-annotated (top) and an annotated (bottom), 60 MHz borehole radar profile from borehole PH-5 at the 240 m level, URL. A fracture log from borehole PH-5 indicating fracture frequency and the intersection of FZ-2,5 is also shown.

K.M. Stevens et al./ Journal of Applied Geophysics 33 (1995) 125-141

a slightly dipping line, suggesting that the borehole is more or less parallel to the strike of FZ-2. The presence of hyperbolic reflectors rather than a continuous planar reflector suggests that the fracture zone is undulating (or has minor fracture splays) and that certain localized areas reflect more of the radar energy. The undulating nature of FZ-2 has been determined independently by geological mapping ( Everitt and Read, 1989). The variable electrical properties could be caused by varying degrees of fracturing and/or areas of higher hydraulic permeability. Reflections resulting from FZ-2.5 have been interpreted closest to the axis of borehole PH-5. The borehole fracture log in Fig. 11 shows that FZ-2.5 intersects the borehole at about 120 m. A weak reflection from this zone can be traced at a radial distance of about 15 m in the radar profile. However, since the zone is very close to the borehole, its reflection is somewhat obscured by the direct arrivals. The origin of isolated hyperbolic reflections at a radial distance of 10 to 50 m cannot be determined in the absence of borehole data, but may correspond to isolated zones of fracturing within the rock mass. Borehole PH-5 was surveyed using a GPR centre frequency of 60 MHz as shown in Fig. 12 to improve the resolution of the near-field events. In this case the depth of penetration is reduced significantly in comparison with the penetration at 22 MHz. The approximate radial distance of investigation using the 60 MHz antennas is less than 60 m. Fig. 12 shows the results in a grey-shaded format as in Fig. 11. Fault FZ-3 is outside the range of investigation, but it is evident that the reflections resulting from the other two fracture zones are sharply defined. The top of FZ-2 is again characterized by a slightly dipping zone of hyperbolic reflectors, and only the section from 100 m to 160 m downhole is within the range of investigation. The planar reflection from FZ-2.5 is more clearly defined, but is not completely resolved. However, its intersection with the borehole accurately matches that shown in the fracture log. In addition, the apparent dip of the reflector correlates well with the predicted dip of the bottom of FZ-2.5.

5.3. Combination of crosshole and surface radar surveys Detailed surface radar reflection surveys were conducted at 25 MHz on an outcrop in the northern part of

137

permit area B. A crosshole radar transmission tomography survey at 22 MHz was also conducted in two roughly co-planar boreholes, WB-1 and WB-2, close to the reflection survey area. The inset in Fig. 13 shows the dashed edges of the outcrop areas, the survey lines, and the traces of the two boreholes. The boreholes are collared = 75 m apart and the overburden-covered area between these boreholes has restricted the use of surface radar in this area. Fig. 13 shows data from the radar reflection surveys on Lines 1-4 for the top 70 m combined with the tomographic reconstruction of the crosshole data from the boreholes for the same interval. The fracture logs showing relative degrees of open (shaded) and total (white) fracturing are also plotted on the profiles. Lines 1-3 of the profile reflection data shown in Fig. 13 indicate a continuity of several reflector features, notably a reflector at a depth of 40-45 m and a reflector at a depth of 65-70 m below the outcrop. Two nearsurface ( 10-15 m) reflectors can also be traced along these northwest profiles. As indicated in the fracture log for borehole WB-2, these reflectors correlate well with open fracture zones at their corresponding depths. Two major reflector features can be seen on Line 4 to the southeast of the area, notably one reflector at a depth of 15-20 m and a second reflector characterized by a horizon of broad hyperbolas at a depth of 35-40 m. These reflectors also correlate well with open-fracture features logged at corresponding depths in borehole WB-1. Other reflection profiles in the area suggest that these fracture features are spatially horizontal ( < 5° dip; Holloway et al., 1992; Stevens and Holloway, 1992). The crosshole tomographic survey between boreholes WB- 1 and WB-2 was conducted at a depth-interval spacing of two metres down each borehole to a depth of 250 m (Holloway and Stevens, 1990). Traveltime data from the top 70 m of this survey (429 measurements) have been reprocessed to provide a residual traveltime tomogram for this interval for comparison with the results from surface profiling. Residual traveltime information was extracted following the procedure outlined by Olsson et al. (1987). The residual traveltime data were then processed with a simple linear backprojection reconstruction algorithm (Ivansson, 1984) and the resulting tomogram (2 × 2 m pixel resolution) is presented in the central part of the profile in Fig. 13. The zones of negative residual traveltime indi-

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K.M. Stevens et al. /Journal of Applied Geophysics 33 (1995) 125-141

5.4. Effects of saline groundwater on GPR

cate areas of slowed wave propagation, which would be expected in areas of intense fracturing and/or open fractures. The tomogram indicates a zone of low residual traveltime spanning the area between the fracture feature at a depth of 40 m to the northwest, with the fracture zone at a depth of 20 m to the southeast. Crosshole hydraulic pump testing carried out subsequent to the drilling has shown that these open fracture zones are indeed conduits for water flow and are probably hydraulically connected. Hydraulic transmissivity values on the order of 10 - 6 m2/s and flow rates of 10 1/ min have been measured for the fracture zone at 17-24 m in borehole WB-1, and similar values occur for the fracture zone at 55-60 m in borehole WB-2. In comparison, transmissivity and flow rates for the predominantly closed fracture zone at 51-71 m in borehole WB-1 are 10 -8 m2/s and 0.1 l/min (D.R. Stevenson, pers. commun.). The tomography data, therefore, indicate a possible structural connection of these two fracture features. This 350 m long cross-section over the outcrop and overburden thus shows the value of a judicious combination of surface and borehole radar techniques with other geotechnical investigations. ResistivityO h m - m 0J

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The presence of saline groundwaters at depth and their effect on the depth of penetration of the radar energy has already been discussed (see Fig. 3). In general, groundwaters and rock types typical of the WRA have salinities that range from 0.5 g/1 for the shallowest layers to 50 g/l for the deepest layers. These values are consistent with observations of other Canadian Shield environments (Gascoyne et al., 1987). Borehole resistivity and single-borehole radar reflection surveys have also indicated this increase in bulk rock conductivity with depth. Fig. 14a shows the results of a 40 cm normal resistivity log in borehole WG-1 (permit area G in Fig. 1) in chiefly featureless grey porphrytic granite. The approximate vertical depth spanned is 750 m. The shaded zone at the top of the log is associated with highly fractured rock. The high resistivity near the surface and the decreasing resistivity values with depth are typical of logs collected from all of the WRA boreholes. With increasing salinity, an associated decrease in the amplitude of the direct arrival radar energy can be expected. Fig. 14b shows the peak amplitudes of radar energy from a radar log of borehole

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K.M. Stevens et al. / Journal of Applied Geophysics 33 1995) 12.5- 141

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6. Conclusions The ground penetrating radar method has proved to be very useful in mapping structure in granitic and

gneissic rock. With modern equipment, the useful depth of penetration of radar energy is well in excess of 60 m in near-surface resistive gramtic and gneissic rocks. However. the reduced resistiwty associated with saline waters at depths greater than 300-400 m in the Whiteshell Research Area rapidly diminishes the effective depth of penetration of the radar energy. Estimated values of effective probing range based upon skin depth calculated from theoretical data closely match the typical penetration depths observed in the field profiles. Single-hole radar reflection has been employed to map structures at radial distances up to 80-90 m from boreholes at depths to 300 m below surface. Crosshole tomography in near-surface depth intervals can be used to obtain data from under overburden-covered areas where the use of surface profiling is limited by the conductive overburden prevalent in glaciated areas of the Canadian Shield. Granitic rock fracture zones are often not truly planar but can be undulating with the aperture of the fracture zone somewhat variable. Similarly, the water content in the fracture zones can also be spatially variable The GPR data from such situations appear as localized reflections rather than as planar reflections. A careful processing and interpretation of such data, as in the case of borehole PH-5, will need supplementary geological and hydrogeological information. GPR surveys, both surface and borehole, have been successfully applied on outcrops and in the Underground Research Laboratory, However, because of the increasing conductivity of the bulk rock and the presence of saline groundwaters at depth (i.e. at subsurface depths greater than 300-400 m), the radar method suffers from an inherent limitation in effective probing range. The useful depth of penetration of the radar signal is likely to be much reduced beyond these depths, as indicated by skin depth calculations based on resistivity logs ( < 10,000 ~(2.m ) and increased attenuation of the direct-arrival radar wave amplitudes at increasing depths in the rock mass.

Acknowledgements The authors wish to thank Mel Gascoyne, D. Tomsons, T. Iwanowsky and S.H. Whitaker for reviewing the manuscript and John Hayles for providing support for the tomographic imaging of the crosshole radar

K.M. Stevens et al./ Journal of Applied Geophysics 33 (1995) 125-141

data. M e l G a s c o y n e a n d P a u l Street p r o v i d e d s o m e o f the u n p u b l i s h e d salinity a n d b o r e h o l e r e s i s t i v i t y data w h i c h s u p p o r t e d o u r analysis. D o u g S t e v e n s o n h a s prov i d e d e s t i m a t e s o f h y d r a u l i c c o n d u c t i v i t y v a l u e s for d i s c r e t e f r a c t u r e z o n e s in b o r e h o l e s W B - 1 a n d W B - 2 . T h i s w o r k h a s b e e n c o n s t a n t l y e n c o u r a g e d b y C.C. D a v i s o n for m a n y years. T h e w o r k w a s c o n d u c t e d u n d e r the C a n a d i a n N u c l e a r Fuel W a s t e M a n a g e m e n t P r o g r a m , w h i c h is f u n d e d j o i n t l y b y A t o m i c E n e r g y o f C a n a d a L i m i t e d a n d O n t a r i o H y d r o u n d e r the a u s p i c e s o f the C A N D U O w n e r s G r o u p .

References Andersson, P., Falk, L., Niva, B. and Olsson, O., 1987. Results from borehole radar investigations at URL, Canada, during April and May 1987. Swed. Geol. Co. SGAB Rep., # SWP/SKB: 514520. Brown, A., Soonawala, N.M., Everitt, R.A. and Kamineni, D.C., 1989. Geology and geophysics of the Underground Research Laboratory Site, Lac du Bonnet, Manitoba. Can. J. Earth Sci., 26: 404-425. Dormuth, K.W. and Nuttall, K., 1987. The Canadian Nuclear Fuel Waste Management Program. In: Radioactive Waste Management and the Nuclear Fuel Cycle, 20(2/3), pp. 93-104. Everitt, R.A. and Read, R.S., 1989. Geology of the 240 level of the Underground Research Laboratory, 1: General geology. AECL Techn. Rec., TR-491-12. Gascoyne, M. and Kamineni, D.C., 1992. Groundwater chemistry and fracture mineralogy in the Whiteshell Research Area: Supporting data for the geosphere and biosphere transport models. AECL Techn. Rec., TR-516 a. Gascoyne, M., Davison, C.C., Ross, J.D. and Pearson, R., 1987. Saline groundwaters and brines in plutons in the Canadian Shield. In: P. Fritz and S.K. Frape (Editors), Saline Water and Gases in

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Crystalline Rocks. Geol. Assoc. Can. Spec. Pap., 33: 53--68. Gascoyne, M., Ross, J:D., Watson, R.L. and Kamineni, D.C., 1989. Soluble salts in a Canadian Shield granite as contributors to groundwater salinity. In: D.L. Miles (Editor), Proc. 6th Int. Symp. Water-Rock Interaction. Balkema, Rotterdam, pp. 247249. Holloway, A.L., 1992. Fracture mapping in granite rock using ground probing radar. In: J. Pilon (Editor), Ground Penetrating Radar. Geol. Surv. Can. Pap., 90-4: 85-100. Holloway, A.L. and Mugford, J.C., 1990. Fracture characterization in granite using ground probing radar. Can. Inst. Min. Bull., 83(940): 61-70. Holloway, A.L. and Stevens, K.M., 1990. Fracture characterization in granite using borehole radar. In: J. Lucius, G. Olhoeft and S. Duke (Editors), Proc. 3rd Int. Conf. Ground Penetrating Radar. US Geol. Surv., 90-414, p. 32. Holloway, A.L., Stevens, K.M. and Lodha, G.S., 1992. The results of surface and borehole radar profiling from permit area B of the Whiteshell Research Area, Manitoba, Canada. In: P. Hanninen and S. Autio (Editors), Proc. 4th Int. Conf. Ground Penetrating Radar. Geol. Surv. Finl. Spec. Pap., 16: 329-337. Ivansson, S., 1984. Crosshole investigations - Tomography and its applications to crosshole seismic measurements. Stripa Proj. Rep., IR-84-08, SKB. Olsson, O., Falk, L., Forslund, O., Lundmark, L. and Sandberg, E., 1987. Crosshole investigations - Results from borehole radar investigations. Stripa Proj. Techn. Rec., 87-11, SKB. Soonawala, N.M., Holloway, A.L. and Tomsons, D.K., 1990. Geophysical methodology for the Canadian Nuclear Fuel Waste Management Program. In: S.H. Ward (Editor), Environmental Geophysics. Soc. Explor. Geophys., 1, pp. 309-331. Stevens, K.M. and Holloway, A.L., 1992. Surface radar reflection profiling results from permit area B of the Whiteshell Research Area, a report of field work, 1990. AECL Techn. Rec., # TR548 2. 2 Unrestricted file report available from SDDO, AECL Research, Chalk River Laboratories, Chalk River, Ontario K0J 1J0, Canada.