Annals oJGlaciology 24 1997
International Glaciological Society
©
Interpretation of borehole itn.pulse tests at Haut Glacier
d'Arolla, Switzerland
BERND KULESSA, BRYN HUBBARD
Centrefor Glaciology, Institute of Earth Studies, University of Wales, Aberystwyth, Cemligion SY23 3DB, Wales
ABSTRACT.
One hundred and sixteen impulse tests were conducted in a dense borehole array at Haut Glacier d'Arolla, Switzerland, duringJuly and August 1995. Impul setest response pressure oscillations reveal marked variations in three parameters: relative
amplitude decay, osci llation frequ ency and net water-level displacement. Analysis of response signals from water-filled boreholes indicates that (i) responses from individual
boreholes are reproducible over some tens of minutes; (ii ) responses are similar, but not
identical, for slug insertion ("slug tests" ) and removal ("bail tests" ); (iii ) signal amplitude
decay varies with the depth at which the recording pressure transducer is located in a
borehole; and (iv) coherent minima in all three response parameters coincide with the
location of a known preferential, subglacial water-flow pathway. This correspondence suggests, first, that water-filled and non-fluctuating boreholes have established a link to th e
subglacial aquifer at Haut Glacier d'Arolla and, secondly, that locally transmissive basal
sediments may be idel1lified by relatively low response-signal frequencies, response-signal
decays and net water-l evel displacem em s. Impulse testing water-filled bO~'e hole s ther~fore
has th e capacity to provide information relating to th e local hydrogeologlcal propertI es of
subglacial aquifers.
In 43 cases, water levels in borehol es adjacent to those being tested were recorded in
order to identify possible subglaciallinkages. No such connections were detected .
INTRODUCTION
Subglacial meltwater drainage systems m ay be classified as
either arborescent or non-arborescent. Arborescent dra inage via
subglacial channels is hydraulically effective a nd cha racterised by an inverse relation between steady-state meltwater discharge a nd pressure (Rothlisberger, 1972; Nye,
1973). In contrast, non-arborescent or distributed systems are
relatively ineffective, lubricate extensive areas of the glacier
bed a nd are generally characterised by a positive relationship between stead y-sta te disch a rge a nd pressure. Nonarborescent drainage config urations include meltwate r
film s (e.g. Weertman, 1957) and linked cavities (e.g. Walder,
1986) at hard-bedded glaciers, and pore-water flow (e.g.
Clarke, 1987) and flow via "canals" within or above the sediment
body (Wa lder and Fowl e l~ 1994) at soft-bedded glaciers. In
reality, many valley glaciers m ay be underlai n by patches
of sediments interspersed with bedrock, and drained by
temporally varying hydrological configurations (e.g. H arper
and Humphrey, 1995). Elucidating the hydraulic chara ctel~
structure and evolution of such subglacial drainage systems
represents a research priority, since these charac teristics
may have direct implications for the motion of temperatebased glaciers and their response to climate change. Borehol e
response testing may provide one means of deriving such inform ation.
Analysis of glacier borehole response tests is based on the
ass umption th at water in a borehole drill ed to th e glacier
bed is in communication with the subglacial hydrologi cal
system (considered as an aquifer). Impulse testing involves
a rtificially disrupting a steady-stat e borehole water level
a nd monitoring its recovery to equilibrium. The nature of
this response is regulated by, and therefore reflects, the
hydrogeological properties of the subglacial aquifer to
which th e borehole is connected. Water-level disruption
may conveniently be induced by the rapid insertion (slug
test ) and removal (bail test ) of a sealed pipe, com monly
referred to as a slug. Borehole response tests therefore provide a relatively simple and effective m eans of investigating
glacial hydraulic system s.
Stone (1993) a nd Stone and Clarke (1993) used borehole
response tests to investigate th e hydrogeological properties
of unconsolidated sediments beneath Trapridge Glacier,
Canada. These autho rs modellecl impulse-test records in
terms of aquifer transmissivity, compressibility and hydraulic conductivity. While these parameters could not be investigated in isolation, fi eld data suggested the prese nce of a
highly transmissive, but spatially and temporally variable,
subgl acial water-flow system at Trapridge. In the present
stud y we adopt a more empirical approac h to impul se testing, reporting on 116 tests conducted in a dense borehole
array overlying a melt-season, subglacial channel at Haut
Glacier d'Arolla, Switzerland. Each impulse test compri sed
slug insertion (slug test) a nd removal (bail test). Testing was
conducted duringJul y a nd August 1995, fo cusing on (i) the
form s ofimpulse-tesl responses monitored in different borehol es; (ii ) method-related influences on impulse-test
responses; and (iii ) spatial variability in response sig nals
relative to a known subglacial channel.
FIELD SITE AND METHODOLOGY
H a ut Glacier d 'A rolla is a small valley glacier located at th e
397
Kulessa and Hubbard: Borehole impulse tests at Haut Glacier d'Arolla
- - - ........
93000
Bouquetins
3838 ....
92000
MonlCollon
3637 ....
91000
L' Eveque
3716 ....
90000
I km
89000
605000
606000
607000
608000
Fig. 1. The location qf the borehole array at Haut Glacier
d'Arolla, Switzerland.
head o[Val d'Herens, Valais, Switzerland (Fig. I). It has a
surface area of ~6.3 km 2 and extends rrom ~2560 m a. s.L
at its snout to ~3500 m a.s.L at its headwalL In 1995, a dense
array of bore holes was drilled ~L5 km from the terminus in
an area considered to be underlain principally by unconsolidated sediments (Hubbard, 1992; Fig. 2). The borehole
array spans a major melt-season subglacial drainage channel, identified on the basis of hydraulic reconstructions by
Sharp and others (1993) and Hubbard and others (1995).
Boreholes drilled to the glacier bed are frequently classified as either "connected" or "unconnected" to the subglacial
hydrological system (e.g. Hodge, 1976, 1979; Fountain, 1994;
Smart, 1996). The former are generally identified by diurnally fluctuating water levels that are assumed to connect
with hydraulically active regions of the g lacier bed. "Unconnected" boreholes are characterised by constant, highstanding water levels and are assumed to be isolated from
the subglacial drainage system. However, boreholes that
penetrate the beds of temperate glaciers must be connected
to some form of drainage system, since these bases are predominantly at th e pressure-melting point. Indeed, Murray
and Clarke (1995) recently recorded and analyzed pressure
fluctuations in sealed, high-standing boreholes at Trapridge
Glacier, Canada, and suggested that pressures (if not levels )
in these boreholes responded to changes in subglacial hydraulics at a diurnal time-scale. Thus, since apparently "unconnected" boreholes may, in fact, have established a link to
the subglacial drainage system (however poor), we refer to
them in the present study as water filled. For consistency we
refer to apparently "connected" boreholes as fluctuating. A
variety of methods was used to ascertain borehole status,
including in situ automated sensor records, regular electrical
conductivity profiling and visual checking of high-standing
water levels. The spatial distribution of fluctuating and
water-filled boreholes, along with those that were impulse
tested, is presented in Figure 2.
Impulse-test response signals are generally characterised by a series of oscillations that decay to the initial, or
a new, equilibrium water level (Stone and Clarke, 1993).
Responses were monitored in the present study by sensors
placed (unless otherwise stated ) ~5 m below the water surface at a frequency of 64 H z on Campbell Scientific CRlO
data loggers. In order to characterise these signals, three response parameters have been identified: (i) amplitude
decay, (ii ) oscill ation frequency, and (iii ) net water-level
displacement (Fig. 3). Net water-level displacement is considered to be related to aquifer storativity, although imprecisely (e.g. Harvey, 1992). Patterns of amplitude decay in
different boreholes were too diverse to be described by an
exponential function without overinterpreting record complexity. Thus, we propose a simple means of comparing amplitude decays in different boreholes, by defining relative
amplitude decay (Arel) as:
A
r el
=
Al - A3
Al
(1)
where Al and A3 are respectively the amplitudes of the first
a
Max(1) ________
80
E
60
~
E.
'"
40
Cl.
E
20
c
0
'C
Relative amplitude decay
Oscillation frequency
Max(3)
"
E
'"
.Q
Net water·level displacement
~
'u -2 0
0'"
-40
0
5
10
15
20
25
30
35
40
45
b
E
E.
~'"
Cl.
600
450
300
150
E
os
c
0
~
'u
-150
H ~I,..L.
0
'"
0
...
- - - - - . . - . - . - - - .. .-
-300
0
10
15
20
25
30
Time [s]
35
40
45
91680 +-----~----~----~----~----~------~
606570
606620
606670
606720
606770
606820
606870
Easting (glacier bed)
Fig. 2. The 1995 borehole array. Open and closed circles respectively represent fluctuating and waterfilled boreholes.
Impulse-tested boreholes are listed in Table 1.
398
50
Time [s]
Fig. 3. 1jpical impulse-test responses from (a) waterfilled
boreholes and (b) fluctuating boreholes. Note the less welldifined responses from the fluctuating borehole. Relative
amplitude decay, oscillation frequency and net water-level
displacement are difined in the text.
50
Kulessa and Hubbard: Borehole impulse tests at Haut Glacier d'Arolla
a
Table 1. Boreholes tested in the study
Rollllve Impllludo doCly
0.9
Borehole No.
Number of lesls
Dale (Julian day)
c:
0 .6
~
0 .5
g.
0.4
o
I
5
5
5
6
6
7
7
8
8
8
8
9
9
10
10
14
14
16
20
22
23
25
4
4
2
2
2
10
2
4
4
4
2
8
4
4
4-
8
2
4
4
20
6
8
4-
222
212
227
230
230
239
221
229
221
222
230
237
230
239
230
239
229
230
224
220
229
224227
RESULTS
COInparison of respon se signals froITl water-fi lled
and fluctuati n g boreh oles
Typical impulse-test responses recorded in water-filled and
flu ctuating boreholes are given in Figure 3. As expected, signals generally take the form of a series of damped oscill ations aro und the new water level. However, whi le relative
a mplitude decay and oscill ation frequency are clearly identified in water-filled boreholes, the signals reco rded fro m the
fluctuating holes a re less clearly defined and more irregular.
I n the following analysis we therefore treat impulse-test
responses from water-filled and fluctuating holes separately.
Water-level d isplacements in fluctuating boreholes are typi-
y
= 1.1681x . 0.1791
A2 = 0 .7575
,--'--"\ ----- ,
•
,
a: 0.3
Slug t••t
(ob .. rvod)
Ball t ••t
(ob.erv.d)
- - Sl ug t ••t
( regr ••• lon,
y = 0.8842x . 0.011
0 .2
---- san
t•• t
(regr••• lon:
R' = 0.7131
0 .1
O +--+--+--+--r--+--r-~~~~~
o
0.1
0.2
0.3
0.4
0.5
0.6
0 .7
0 .8
0 .9
1
Initial test
b
2 .8
Ooc lllotl on lraquoncy 1Hz)
2 .4
y = O.9664x • 0.0315
c
~ 1.6
!
~
\
1.2
0 .8
y
0 .4
O. B
0 .4
1.2
2.4
Slug 1..1
(obo.,.od)
•
Sail t ••t
(ob.arv.d)
- - Slug t••t
(reSlr••• lon)
---- a.1I t•• t
= 0.7426X • 0.2065
A' = 0.904
1.6
o
(.. g .... lon)
2.B
Initial1es1
c
c
and third major oscillations following disturbance (Fig. 3a).
The first and the third oscillations were chosen in order to
cater for the irregularity commonly observed in the first two
signal-response cyc les, whilst eliminating the impact of increasing noise from later, lower-amplitude osci llations. Frequency was a lso determined over the first three responsesignal oscill ations. Since oscillations induced by an impulse
test usually ceased within 10 s of disturbance, net water-level
displacement is defined as the mean water leve l 10- 15 s after
the bail test, minus the steady-state water level recorded
prior to the corresponding slug test.
One hundred and sixteen tests were cond ucted in the
present study (Table 1). Of these, 43 were also monitored
simu ltaneously in nearby (secondary ) boreholes in orde r to
investigate potential subglacial connections between neighbouring bore holes. Ni nety-one of the tests were conducted
in water-filled boreholes, largely reflecting the logistical difficulties involved with testing low-standing water levels.
Much of our analysis is therefore based on tests conducted
in water-filled boreholes. Test responses in fluctuating boreholes are the subject of ongoing research a nd will be published in a later paper.
•
O.B
0.7
·20
15
i
i'
a:
20
-•
•
0
· 40
y
-60
= 0.1049x . 21.766
R2 = 0 .0112
·80
~
·100
· 70
·60
·50
· 40
·30
·20
· 10
10
20
30
40
Initial le.l
Fig. 4. Repeated impuLse-test responses /Jlotted against initiaL
responses: (a) relative amplitude decay, (b) osciLlationfrequency, and ( c) net water-level displacement. Note that (c)
is less stable than (a) 01' (b). Least-squares linear regression
statistics are presented.
cally an order of magnitude greater than those in waterfilled holes (Fig. 3).
Response-signal reproducibility
Repeated impulse tests in water-filled boreholes indicate
that relative amplitude decays and oscill ation frequencies
a re reproducible over some tens of minutes (Fig. 4). Linear
regression of initial test signals against repeat test signals
reveals co rrelation statistics (R2 ) of 0.75 (slug) and 0.71
(bail) for relative amplitude decay (Fig. 4a) and 0.95 (slug)
and 0.90 (bail ) for osci ll ation frequency (Fig. 4b). M arked
temporal vari ations in these response parameters from a
given borehole may therefore renect temporal variabi lity
in the hydraulic system to which that borehole is connected.
In contrast, only a weak co rrelation (0.01) is reco rded
between repeated net water-level displacements (Fig. 4c).
Comparison of slug and bail responses for each test series
indicates consistent, but not identical, relative amplitude decays (R2 = 0.57; Fig. 5a) and oscillation frequencies (R2 =
0.88; Fig. 5 b). We therefore treat slug- and bail-test response
parameters separately.
Method-related variability in respons e signals
T h ree test series were carried out with pressure transduce rs
399
Kulessa and Hubb ard: Borehole imjJUlse tests at Haut Glacier ditrolla
a
a
I
0.9
0 .7
y
5'
•
e
0 .8
= 1.0902x - 0.1807
0.6
~ O.B
20
22
,,=_~
7
~
.~
R' = 0.568
0. 0.6
0.5
E
e
0.4
0 .3
0 .2
O. I
14
•
Relative amplitude decay
•
•
'"~
e
~
e
£
•
o +---~---+----r---+----r---+---1----r---~--~
O. I
0.2
0.3
0 .4
0.5
0.6
0 .7
0.8
0 .9
o
0.4
_ S l u g 'e.'
--.. -- ean 'e.'
0 . 2 +-------~--------~------~------~
606570
606645
606720
606795
606870
Slug test
b
b
2.2
_Slug 'e.'
2 .8
Oocillotlon frequency [Hz]
_ 1. 8
2.4
•
N
:Eo
2
j
ij'
y = 0.8228x + 0.18
1.6
~
e
1.4
"
1.2
I
0.8
,g
0 .4
~ 0.6
A' = 0.8817
~
10
--.. -- ea" 'a.'
c:
~
0 +-----r----1-----+----~----~----+_--__1
o
0 .8
0 .4
1.2
2.4
2
1. 6
2 .8
0.2 +--------.---------r--------.--------.
606795
606870
606570
606645
606720
Slug test
Fig. 5. Comparison if ( a) relative amplitude decays and (b)
oscillation fre quencies, recorded during slug and bail tests.
Least-squares linear regression statistics are presented.
C 40
E 20
~
cCl>
E
located at different depth s in (water-fill ed ) borehol es in
order to investigate the influence of transducer depth on test
responses. Tests were carri ed o ut at incrementa l sensor
depths of 10 m, both as the senso r was lowered to the glacier
bed a nd as it was subsequently raised back to the ice surface.
22
20
0
~
.
0.-20
'6
Qj
~ ~ 40
~
a;
~ -60 +-----~-----.----~-------,
606570
606645
606720
606795
606870
Easting (glacier bed) [Swiss grid)
a
y
~
e
0.8
~
A
~ 0.6
'is.
~
0 .4
~
a;
0 .2
I
= -0.0049x + 0.7939
A' = 0.8711
A
•
•
Slug lesl
(observed)
•
Ball teat
(ob.orvod)
- - Slug test
(,og,o•• lon)
y
a:
0
0
= -0.003x + 0.6782
A' = 0.1608
20
40
---- Ball to.t
•
60
80
(,eg,eo.lon)
lOO
Pressure Iransduce, depth [m]
b
1.6
y
1.4
'N
~ 1.2
g
":>
C"
!
c
0
8
+ 1.0254
•
A'.= 0.0271
0.6
•
A'
Ball lell
(observed)
--Slug lesl
(,egre.. lon)
• •
------ Ball test
(,eg,e.slon)
y = 0.0003x + 0.7946
0.4
Slug t •• t
(obse,vod)
A----. ----,----;;----------i.----... ----.----:
. •
~ 0 .8
~
= -0.0003x
/
= 0.0032
0 .2
0
20
40
60
80
lOO
P,essure transducer depth [m]
Fig. 6. (a) Relative am/Jlitude decay and ( b) oscillation frequency plotted against transducer depth for re/Jeated resjJonse
tests conducted in bore/wle 20. Least -squares linear regression
statistics are presented.
400
Fig. 7. (a) Relative amplitude decay, (b) oscillation fre quency, and (c) net water-level displacement, plotted against
Easting (Swiss grid) for the impulse-test responses ana(yzed
in the present study.
Readings for matchi ng depths were consistent (R2 > 0.9),
and observed trend s were similar in a ll three boreholes.
The most comprehensive da ta series was recorded in borehol e 20. Test responses from thi s borehole indicate that relative amplitude decay decreases with increasing transducer
depth for both slug (R2 = 0.87) a nd bail (R 2 = 0.16) tests
(Fi g. 6). In contras t, oscill ation frequencies remain stable
throughout th e test series, for both slug and ba il tests
(R2 < 0.1) (Fig. 6b ). No system atic change in net water-level
displacement was recorded with sensor depth.
Spatial variability in response signals
In order to assess spatia l variations Itl th e responses recorded in water-filled boreholes, parameters were ave raged ,
separately for slug a nd bail tes ts, for all impulse tests carri ed
out in individu al boreholes (Table I). Analysis of the results
indicates a region of decreased relative amplitude decay,
oscillation frequency and net water-level displacement (a
maximum negative di splacement ) centred on Easting 606820
(Fi g. 7). In each case these response minima are less tha n
Kulessa and Hubbard: Borehole impulse tests at Haut Glacier dilrolla
ha lf th e va lues recorded in borehol es 20 and 22, located
close to th e g lacier centre line. Th is zone of min im um impulse-test response parameters coincides with the location
of a major melt-season drainage chann el reconstructed from
glacier morpholog y (Sharp and others, 1993) a nd analysis of
patterns of diurn al water-l evel vari ability late in the 1993
melt season (Hubba rd a nd others, 1995).
Secondary borehole-response signals
Compa rison of impulse-test response records from primar y
a nd seconda ry boreho les shows that, of the 43 du a l impulse
tes ts conducted , only one, boreholes I and 2, was characteri sed by a simulta neous secondary response (borehole 2).
H owever, borehole inclinometer surve ying indicates that
these two boreholes crossed within a few metres of ea ch
other at depth. In this case, th er efore, we interpret th e seconda r y sig nal as a direct englacia l connection rather than a
subglacia l communication between th e two boreholes.
SUMMARY AND DISCUSSION
An alysis of relati ve a mplitude decay, oscill ati on frequency
a nd net water-l evel displacement for 11 6 impul se-test resp onses a t H a ut G lacier d 'Aroll a has all owed investigation
of both method-related influences on response signa ls a nd
spati a l vari ati ons in the hydraulic sys tem of th e glacier.
R elative amplitude decay a nd oscill ation frequency have
different cha rac teri stic patterns for water-fill ed a nd fluctuating boreholes. Initia l wa ter-level di spl acements in waterfill ed bore holes are typically a n order of m ag nitude sm all er
t ha n those recorded in flu ctuating boreholes. Thi s suggests
th at the two borehole types refl ect d ifferent subglacia l hydraulic conditi ons, with well-connected drain age path ways
cha racterised by la rger initial test responses.
In water-filled boreholes, relati ve am plitude decay a nd
oscill ation frequency are reproducible over a short timescale. In co ntrast, cauti on m.ay need to be applied when a naIyz ing net water-l evel displacements, since these a re relati vel y irreg ul a r under appa rently simil a r hyd rol ogical
conditions. Our resul ts th erefo re sugges t th at a nalysis of net
water-l evel d isplacement does not p rovide a n acc urate indicatio n of aq uifer storati vity beneath H aut G lacier d 'A roll a.
Slug-test responses a re consistent with, but not identical
to, bail-test responses. This m ay refl ect a number of causes,
including (i) m ethod-related vari abilit y in the speed of slug
insertion and r emoval, (ii ) sm a ll changes in the hydrogeological properti es of the local subglacial laye r caused by the
wa ter fl ows induced by the testin g, o r (iii ) hydraulic differences betwee n water flow away from th e borehole base (slug
tes t) compa red wi th water fl ow towa rd s it (ba il test ). Slug
and bail tests m ay therefore refl ect sli ghtly different aquifer
p ro perti es. This should be borne in mind when ave raging
slug- a nd bail-t est res ults.
Of the three impulse-tes t res ponse cha racteristics a nalyzed , oscill ati on frequency has th e hi ghest regression coeffi cient for both short-term reproducibility (R 2 = 0.95) a nd
slug- a nd ba il-test compatibility (R2 = 0.88). O scillation
frequency is not influ enced by the depth of th e pressure
transducer in water-filled boreholes (above). Our results
therefore suggest that oscillation frequency is the most reliable a nd stable impulse-test response pa ra meter at H a ut
Gl acier d 'Aroll a.
The depth at which a pressure sensor is located within a
wa ter-fill ed borehole influences th e relative amplitude decay of test response signals, but not th eir oscillation frequencies. Th e form er, as defin ed in Equat ion (I), decreases with
depth. Consequentl y, comparisons of relative amplitude decays in impulse-test response records from different borehol es should take th e depth of transducer pl acement into
a cco unt.
Spatial variations in response pa ra meters from waterfi ll ed boreholes coincide coherently with the location of a
subgl acial channel at Haut G lacier d'Arolla. This region is
cha racteri ed by marked minima in a ll three signal parameters, suggesting that impul se-t est responses from waterfill ed boreholes may well refl ect local subglacial hydrogeological conditi ons. Ass um ing th at the water-fill ed boreholes a na lyzed in the present stud y have established a link to
the subglacia l drainage system, it is likely that their
impulse-tes t responses refl ec t highly localised transmi ssivity vari ati ons within a reas of th e glacier bed that have not
ye t beco me inc orporated into the a rea directl y influenced
by the nearby m elt-season chann el. Thi s interpretation is
consistent with the nature of the development of this channel d uring the 1993 melt season (evolving from a spatially
heterogeneo us drainage pattern into one cha racterised by
a high deg ree of spati al coherence; Gordon a nd others, in
press) a nd th e fac t that cha nnel g row th was pa rticul a rl y late
d uring th e incl ement 1995 summer. The co nsistency of th e
correspondence between th e response-para meter minim a
a nd th e location of this seaso na l channel strongly suggests
a direct associati on. We therefore believe tha t explana tions
involving variations in englacia l conditi ons or in borehole
cha rac ter a re unlikely. Instead, loca l variati ons in hydra ulic
tra nsmissivity caused by m ulti-seaso na l eluviation of fin es
from a subglacial debri s layer close to the inferred cha nnel
m ay acco un t for the observed resp onse-pa rameter minim a
at H a ut G lacier d 'Arolla.
ACKNOWLEDGEMENTS
Th e a uthors would like to th ank M . Sh a rp, I. Willi s, A.
Hubba rd , C. Pyle, B. Rule, A. Seagren, a nd S. Gordon fo r
assisting in drilling a nd tes ting bo reholes. J. H arbor and L.
Copla nd kindly provided horehole inclinome tr y data. Th e
I-escarch was supported by gra nts from th e University of
"Vales, Abe rys twyth, a nd th e lIfTi cld Foundati on. B. KlIlcssa
is supported by a University of Wales Ph.D. studentship. We
tha nk two a nonym ous reviewers for helpful comments on
the m a nuscript.
REFERENCES
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