JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B4, PAGES 7481-7495, APRIL 10, 1999
Subglacial electrical phenomena Erik W. Blakex and Garry K. C. Clarke Department of Earth and Ocean Sciences,University of British Columbia, Vancouver, British Columbia, Canada
Abstract. The subglacialelectrical environment is surprisinglycomplex owing to the variety of processesthat promote spatial and temporal variability. Possible sourcesof variability are the chemical evolution of subglacial water and the structural and morphologicalevolution of the drainage configuration. Such changes contribute to changesin the apparent resistivity of the glacier bed. Less obvious, but possiblymore interesting, are those electrical phenomena associatedwith the natural transport of water and chargein the subglacialdrainage system. Streaming potentials are generated by water flow through subglacial sediment, and these potentials are closelyassociatedwith the subglacialwater pressurefield. To explore these effects, we have monitored induced and natural electrical responsesin the deformingmaterial beneath Trapridge Glacier, Yukon, Canada. Several arrays of electrodeswere placed at the ice-bed interface, and data were recorded over a period of severalyears. Data from theseexperimentsindicate that natural potentials can be used to monitor water pressuregradients and that the bulk resistivity of the glacier bed can be altered by changingthe hydraulic regime. We present evidencethat temporal variations in streaming potentials can indicate fluctuations in subglacial water flow rate and that spatial variations can be related to variations in bed elevation, water pressure,and hydraulic connectivity.
1.
Introduction Geoelectric
methods
have received
scant
attention
from those interested in the study of glaciers and ice sheets.R6thlisberger [1967]reviewedthe subject,andin severalaccompanying papers[RSthlisberger and VSgtli, 1967;Hochstein,1967; V6gtli, 1967]it is demonstrated
that DC (directcurrent)resistivitysoundingcanyield
The electrical environment beneath glaciersis likely to be complex and rich in physical phenomena. Our purposein applying electricalmethodsto the study of gl-•ciersis to investigate the properties and processes of subglacial materials. Of particular interest are the structure
and behavior
of deformable
water-saturated
subglacialsediment. Assuming that the bed material
is unfrozen,a two-phase mixtureof solids(bedrockand
ice thicknessinformation. Few wouldarguethat this apunlithifiedsediments) andwatershouldprevailandelecproach is superior to standard methods such as radar or trolytic conduction is likely to be the dominant mechaseismicreflectionsounding. Somewhatmore promising nism for charge transport. For deformableglacier beds applicationshavebeenexploredby Bentley[1977]and both the solid and liquid phasesare mobile, and a mulShabtaieandBentley[1979]in the courseof geophysical titude of processescan conspire to produce temporal investigations of the Ross Ice Shelf, West Antarctica. and spatial variations in the bulk electrical properties. Their aim was to examine the anisotropy,temperature, A variety of drainage configurationscan be imagined. and density structure of the ice shelf and to infer conAt the ice-bed contact an areally distributed interface ditions at the lower boundary where ice and seawater hydrology can develop, whereaswithin the bed material are in contact.More recently,HaeberliandFisch[1984] volumetric water transport processeswill operate. Posand Brand et al. [1987]haveplacedelectrodesin boresible morphologiesassociatedwith an interface network holes in order to measure the resistivity of the glacier include ice-walledconduits[RJthlisberger, 1972;Shreve, bed. 1972], channelsdowncutinto bedrock[Nye, 1973], an
XNow at Icefield Instruments Inc., Whitehorse, Yukon, Canada.
areally distributedsystemof linked cavities[Walder, 1986; Kamb, 1987],canalsdowncutinto soft sediment [Walderand Fowler, 1994],and a thin macroporous horizon of clasts from which a preexisting matrix of fine
sedimenthas been removed[Stone, 1993; Stone and Clarke, 1993].
Copyright 1999 by the American GeophysicalUnion.
Volume-distributed drainage configurationscan exist within the bed material. For these the assumption
Paper number 98JB02466.
0148-0227/ 99/ 98JB02466509.00 7481
7482
BLAKE
AND
CLARKE:
SUBGLACIAL
ELECTRICAL
PHENOMENA
of Darcian flow [Boulionel al., 1974]may be appro- presentthemselvesas temporal changesin the apparpriate, but there are additional complicationsbecause ent resistivityof the glacierbed and couldbe observed sedimenterosion,deposition,and deformationprocesses usingstandardmethodsof DC electricalprospecting. can causethe permeability structure to evolvetempo- Suchapproachesare regardedas "active"becausethey rally and spattally [Clarke, 1987; Boulion and Hind- involve injection of electric currents into the bed and observationof the associatedvoltageresponse. marsh,1987]. A less obviousapproach to studying the electrical A further causeof variability in electricalproperties is that the conductivity of subglacialwater can vary. propertiesof the subglacialenvironmentis to observe Meteoric water from rainfall and melt is comparatively naturally occurringelectriccurrents.Of greatestinterresistivebut water that has experiencedprolongedcon- est are the electrokineticpotentials, known as streamtact with subglacialsolidswill becomechemicallysat- ing potentials,whichresult when water flowsthrough urated and electrically conductive. Thus subglacially an electricallyactive substancesuchas subglacialsedrouted meteoricwater that followsa fast-flowpathway iment. This phenomenonwill be discussed in greater will remain resistive,whereaswater followinga slow- detail in section2.2 but, for now, a simpleexplanation flow pathwaywill becomeconductive.There is strong will be advanced. A characteristic featureof the subglacialenvironment observationalsupportfor this assertion[Stoneel al., is that water and mineral surfacesare in closeproxim1993;Stoneand Clarke,1996]. Figure la shows a schematic representationof the ice-bed contact for a glacier that rests on a deformable sedimentarybed. Electrical propertiesof the substrate are expected to evolve because of changesin water chemistry,changesin morphologyof the interface drainage system, and changesin morphologyof the volume-distributedsystem. These changeswould
ity. Dissolution reactions in this mixture ensure that
the water phase,thoughelectricallyneutral, is ionized. Figure lb showsan idealized representationof a water pathwayboundedby a solid havingan electrically chargedsurface.An electricallyneutral mineralgrain will developa chargedsurfacesimply becausethe surface introducesa disruption of the orderedsymmetry of the crystal lattice. If one supposesthat the surface
develops a negativecharge(asindicatedin Figurelb), then positive ions in the water phasewill tend to collect
near mineral surfaces.Becausethe water is electrically neutral,therewill be a corresponding excess of negative ionsat somedistancefrom the mineralsurface;a double layeris formed. If a waterpressuregradientis appliedto this system,then Poiseuilleflow will occurin the capillary tube. Near the capillary walls the water flow velocity will vanish,and alongthe capillary axis it will reach a maximum. Becausepositiveand negativechargesare not uniformlydistributedwithin the capillarytube, any water flow will lead to a net chargetransport and can produce an observableelectric current. The complete picture is considerablymore involvedas adsorbed,hydrated, and nonhydratedions danceat the solid-liquid
interface[Stern,1924;Grahame,1947]. Figure 2 showsthe results of some simple calculationsbasedon the Gouy-Chapmanmodel[Gou!t,1910; Chapman,1913]as described by Hunter[1981,pp. 2227]. This modelprovidesthe simplest,thoughperhaps not the mostrigorous,explanationof streamingpotentials. If the mineralsurfaceis assumedto be negatively charged, a positive charge density is concentratedin the electrolytenear the capillary wall. This positive chargedensity decreasesmonotonicallywith distance from the wall. The electricalpotential corresponding Figure 1. Electrical phenomenaassociatedwith sub- to this chargedensityhasa negativesign,and its magglacialwaterandsubglacial physicalprocesses. (a) Hy- nitude decreaseswith distancefrom the capillary wall. potheticalinterfacehydrologyand sedimentsubstrate. At the capillary wall, positive chargesfrom the elecAnnotation "C" indicates a hydraulically connected channelerodedinto the substrate. "U" indicatesa hy- trolyte are tightly bound to negativechargesfrom the
draulicallyunconnected regionofthe bed. (b) Poiseuille mineral surface, and this iramobilizes water near the
flow distributionin a capillary tube showinga Gouy- wall. If a fluid potential gradient is imposed,a shear Chapmanmodel chargedistribution. surfaceforms at some distance from the capillary wall
BLAKE
0.8 i• 0.6
It ."•
AND
CLARKE:
SUBGLACIAL
ELECTRICAL
PHENOMENA
7483
configurationsdiffer dependingon the application, but, in general, an expressionof the form
...•
(2)
,0'"'/,•
0.4I-•i•:
can be constructed, where pa is termed the "apparent
0.2
resistivity"and k is a geometricfactor [e.g.,Keller and FrischknecM,1966, p. 94]. Equation(2) is analogous
o
to Ohm's law for electrical circuits and provides a basis for discussinggeoelectricobservations. For the case of -0.4 an infinite uniform conductor the apparent resistivity pa and true resistivity p coincide. -0.6 The depth of penetration of electric current into the -0.8 substrate is related to the scale of the array. If elec-1.0 trode spacing is small, then penetration is also small. 0 5 lO 15 20 Increasing the spacing in a systematic fashion yields Distancefrom CapillaryWall (l/K) resistivity soundings that provide information on the Figure 2. Normalizedfluid velocity,chargedensity, vertical stratification of electrical resistivity and hence and voltage (calculated using the Gouy-Chapman the structure of the subglacialmaterial. We have taken model)adjacentto capillarywall. The ( potentialis the suchmeasurements,but they are of far lessinterest than potential at the hydrodynamic shear surface. Distance from the capillary wall is normalized to the Debye- observationsof temporal variability in apparent resistivH/ickel parameter •c. There is no implied relation be- ity. As explained,pa - pa(t) can resultfrom temporal tween the location of the shear surface and changesin water chemistry,flow rate, and drainage system morphology.
:•-0.2
2.2.
Natural
Potentials
that marks the boundary between immobilized water and flowing water. The electrical potential obtained at this shear surfaceis referred to as the zeta potential. In summary, there are strong arguments that support the existence of electrical effects associated with water flow, as well as with evolution of the structure and morphologyof the subglacialwater system. To explore these possibilities, we launched a field study of electricalphenomenabeneathTrapridge Glacier, Yukon
Sourcesfor natural potentials include streaming potentials, thermoelectric potentials, electrochemicalpotentials, and telluric potentials induced by disturbances of Earth's magnetosphere. Time-varying thermoelectric potentials are not expected in these experiments because the ice-bed interface at the experimental site
and Blake, 1991; Blake et al., 1992; Blake, 1992; Fischer and Clarke,1993]. Our geoelectricstudy commenced in
tentials. Apart from streaming potentials we believe that the only significant sourcesof time variability arise from electrochemical potentials associated with electrode aging. Becausethe concept of streaming potentials is new to glaciology, we present a self-contained overview of the relevant physics. More detailed analyses are given
is at the meltingpoint [Clarke and Blake, 1991]. Telluric potentials were monitored using an on-site mag-
netometer,but the telluric signalwas small (approxTerritory, Canada. The glacieris surge-type[Clarke et imately250nVm- znT- z) andat relatively highfreal., 1984]andunderlainby deformable sediment[Clarke quency, so it cannot be confused with streaming posummer
1987 and continued
until
the end of summer
1990.
2.
2.1.
Theoretical
Basis
DC Resistivity
,t
[t75].
[tOSa].
:Uoga,
For an infinite homogeneous mediumhavingelectrical [1989].Streamingpotentialsarisewhenchargeis transconductivitycr and resistivityp - 1/or, the electrical ported by flowing water. The total charge flux potential V in the vicinity of a current electrode of strength I is written
J--Je+Ja
(3)
comprises a diffusive electron flux J e and an advective
flux Ja of charge mobilized by water flow. In the absenceof net sourcesand sinks of chargethe principle of where r is the spatial position of the voltage electrode charge conservationrequires that
and r0 is the positionof the current electrode[e.g., Keller and FrischknecM,1966].A standardarray comprisestwo current electrodeshaving equal but opposite strength and two voltage electrodespositionedto measure the resulting potential difference AV. Electrode
V.J -- V. (Je +Ja) -- 0.
(4)
In a conductingisotropic medium, electron flux is governed by Ohm's law
7484
BLAKE
AND
CLARKE.
SUBGLACIAL
ELECTRICAL
PHENOMENA
(5)
is electrically neutral; thus if we substitute the approx-
where cr is the scalar electrical conductivity. Advective charge transport is assumedto be directly coupledto water flux q; thus
substitutethe resultinto (9) while letting r • - R- r (wherer • is distanceawayfromthe shearplane),we get
Je - -•VV,
imation(R2 - r 2) m 2R(R- r) for R m r into(8) and
Ja - xq,
,
where X is a coupling coefficient.
The full situation is summarir.ed by the Onsager The nature of the electrical double layer ensuresthat equations of irreversible thermodynamics for electro- p,(r') is smallfor moderateand large valuesof r', so
kinetic phenomenaIOnsager, 1931a, b; Nourbehecht, we canapproximate(10) by 1963,p. 25; de Grootand Mazur, 1984,p. 364]
J q --
-LllVVL12V•b --Z21•7F'-- Z22•7•,
(7a) (7b)
whereq•- P + p•gz is the fluid potential(P is fluid pressureas measuredby a pressuresensor,p•ois fluid density, g is gravitational acceleration, z is elevation
I.-_ wR2 (0•)•R The effective thickness of the diffuse layer, or more precisely the "center of gravity" of the layer, can be
approximatedby l/n, wheren is the Debye-Hfickelparameter
abovedatum), and Lij are phenomenological coefficients. We have ignored other possible contributions to nekT these fluxes such as osmosis,thermo-osmosis,Seebeck and where n0 is the volume density of charge,e0 is the potential, and diffusion potential. The reciprocityconditionLij -- Lji expresses com- chargeon a proton, k is Bokzmann's constant, and T is plementarity between the processescausingelectro-os- the absolute temperature. For aqueousunivalent molar
mosis and streaming potentials. By Ohm's law, L• -
or,and by Darcy's law, L22 - K/p•g, where K is the hydraulic conductivity. We also define the "streaming potentialcoefficient"C - L•2/L•i. Someearlyworkon streamingpotentials suggestedthat the phenomenonis nonlinear[Wyllie, 1951],but this nonlineartryis probably associatedwith turbulent flow [Boumans,1957a, b; Kurtz et al., 1976].
concentrations of 10-2 10-4 and 10-• the approxi-
matevalueof 1/n in metersis 10-•, 10-s, and 10-7 respectively. The layer thickness increasesas concen-
tration decreases.Levine et al. [1975]treat the case wherethe pore radiusis not large comparedwith although a small pore size will affect the magnitude of the streamingpotential, its sign will remain unchanged.
Substituting theexpression V2• - -p,/e (wheree is
That Ja can be nonzero in the presenceof sources the electrical permittivity and • is the electrical potenand sinks of electrically neutral water is explained by tial distributionin the boundarylayer) into (11) and the presenceof an electrical double layer at fluid-grain integrating by parts gives
boundaries[Gouy, 1910; Chapman,1913; Stern, 1924; Grahame,1947]. An expressionfor C can be derived usingthe Helmholtz-Smoluchowskitheory of Poiseuilledistributed laminar flow through a capillary tube and introducing corrections for porosity and tortuosity. Consider the laminar flow through a single cylindrical I•+ (13b) capillary tube. Expressedin a cylindrical polar coordinate system, the axial velocity v• at a distance r from the axis of the tube is given by Poiseuille'sequation where < is the potential ß at the hydrodynamic shear
r•d• =•
• d•
boundary(r • - 0). This potential,commonlyreferred
-
4*7 •zz'
as the "zeta potential" is a physical property that (S) to relates to both the mineralogy of the solid phase and
where R is the radius of the capillary at the hydrodynamicshearplane and *7is the dynamicviscosityof the fluid. The streamingcurrentI, is foundby integrat-
ing the productof volumetricchargedensityp,(r) and
the chemistryof the liquid phase[e.g.,Hunter, 1981]. The definiteterm in (13a) vanishes because d•/dr •- 0 at r • - R. Normalizing(13b) by the cross-sectional area of the pore gives
velocity over the cross-sectionalarea of the tube
Ja -- +--
- f0
V •
'
(0) The Kozeny-Carmanrelation[e.g.,Bear, 1972;Ber-
We are only interested in the charge distribution near the shear boundary becausethe bulk fluid in the tube
ryman and Blair, 1987] followsfrom a microphysical model of permeability that is consistentwith Darcy's
BLAKE
AND
CLARKE:
SUBGLACIAL
law. As with the Helmholtz-Smoluchowski theory, the derivation of the Kozeny-Carman relation assumesa
ELECTRICAL
PHENOMENA
only way this conflicting requirement can be reconciled
with (19) is if J - 0, that is,
Poiseuilledistributionof flow qp within a cylindrical pore. To arrive at the bulk fluid flow q, an empirical tortuosity correctionco - 8/5 is applied to the flow within the pore giving
q -- conqp,
JR--+
Je -- -Ja -- --xq, which
C0T/,•(
V•.
(16)
Matchingtermsof (3) with thosein (7a) leadsto the associations
-L• VV -- -rrVV, -Lx2Vd- -CcrVd,
(17a) (17b)
conE(
-CVc) -- v4.
In realistic geologicalconditions of pH and electrolyte concentrationthe ( potential is in the range of-20 to
-100mV [Hunter, 1981; Ishido and Mizutani, 1981; Morgan et al., 1989]. From (21), assuming( is negative, we infer that the streaming potential should in-
creasewith decreasing fluid potential(i.e., asonemoves downflow).This leadsto a rule of thumb upon which we shall base subsequentinterpretation: the negative streaming potential gradient is a proxy for positive fluid potential gradient.
Equations(18)and (21)indicate that estimationof C and ( from measurements of V'V requires a knowledgeof r/, e, and n. This may not be as straightforward as we would
Je : JR :
(20)
leads to
V'V --(p•ogC/K)q-
(15)
wheren is the porosity(typically0.35 for a dilatingtill [Clarke,1987]). Making a similar adjustment for porosity and tortuosity in (14) and assumingthat preferredelectrical currentflow paths are parallel to the pressuregradient (becausecurrentflowsby surfaceconductionand bulk conduction,this is equivalentto assumingthat water flowpathsare parallelto the pressure gradient),weget
7485
like:
there
can be increases
in the effect-
ive viscosity of the fluid due to electrokinetic interac-
tions within the doublelayer [Elton, 1949],the dielec-
tric constant can be significantly altered in the boundary layer, and there may be variation in e close to an C -- -coned/err 1. (18) interface as a result of poorly understood interactions Solving(7b) for Vd and substituting into (17b) gives with the solid surface. Notwithstanding these qualifications, the values listed in Table 1 yield the estimate
and thus
J•,- (p•gCcr/K)q whenwe ignorethe negligiblecontribution
of the electro-osmosis
term.
It follows that
X -- P•,gCrr/K. For an infinite region having uniform electrical and hydraulic properties, water flux q and electron flux Je are coalignedand proportional. Thus Je - 7J•, where 7 is a proportionality factor, and it follows that
J -(1 + 7)JR- (1 + 7)xq. We have commented
that
V.
J -
(19)
0 in the absence of
chargesources.If water sourcesare present,as they are in the caseof subglacialhydraulics,then V.q -f=0. The
ICI .-- 2.2 x 10-sV Pa- x. 3.
Field
Measurements
From summer
1987 to summer
1990 we recorded
ural potentials and DC resistivity beneath Trapridge Glacier. To provide some context for our study of the subglacial environment, we also measured DC electrical properties of the deglaciated region in the glacier forefield. As recently as 1980, this region was ice-covered, and thus forefield sediments are believed to be representative of those beneath the glacier. These meas-
Table 1. Propertiesfor Estimation of StreamingPotential Coefficient Physical Property Gravity acceleration, g Density of water, p• Dielectric permittivity of free space, e0 Dielectric constant, Dielectric permittivity, e = •Ee0 Viscosity of water, •7 Zeta potential, ½ Porosity, n Geometrical parameter, co
nat-
Value 9.80
Units ms
--2
1000
kgm-3
8.85415 x 10 -•2
Sm -1
80
7.0833 x 10 -•ø
Sm -1
1.787 x 10 -a
Pas
-100 0.35
8/S
mV
7486
BLAKE AND CLARKE: SUBGLACIAL ELECTRICAL PHENOMENA
urements indicate typical resistivities of 1700fl m at a
depth of •2m, increasingto 2250fim at a depth of
structed from narrow-mouthed porous ceramic pots
(Figure 3c) containingapproximately40mL of copper sulphate crystals and a metallic copper slug; the slug was solderedto a wire leading through the sealedlid of the electrode pot to the surface electronics.
On the glacierwe installedfive electrodearrayseach comprisingfour to six current electrodesand eight to Metal-metal salt electrodes surround the metal elecnine potentialelectrodeswhichwereorganizedin rectitrode with an equilibrium solution of metal salt thereby linearor L-shapedlayouts(Figure3a). Electrodes were stabilizing any potential across the solid-liquid interplaced50cm abovethe ice-bed interfacein boreholes face. The concentration gradient of metal salt surdrilled to the bed using a hot-water drill, and for the rounding the electrode will produce a diffusion potenyears1988-1990the true positionof eachelectrodewas tial, but interelectrode and temporal variations in difdeterminedusing a boreholeinclinometer[Blake and fusion potential tend to be small; when differential poClarke,1992].Fromthesemeasurements we discovered tential measurements are made, the diffusion potentials that nonverticalityof the boreholesproducesa haphazwill cancel [Petiau and Dupis, 1980]. Time-varying elecard electrodepattern at the bed, so that the true array trode noise was estimated by placing two electrodes adgeometriesare far from conventional. jacent to each other in a single borehole; after an aging Current electrodes in all years and potential electrodes in 1987 were 10-cm lengths of nominal 0.5-inch period of 2-3 days, electrode noisestabilized at +1 mV.
(1.27cm)copperdomestic waterpipe(Figure3b). For the years 1988-1990, potential electrodeswere con-
Data for all electrode arrays were collected using Campbell ScientificCR10 data loggersattached to custom-built potential and current electrode multiplexers
(Figure3d). The potentialelectrodemultiplexersused during 1988-1990 were capable of connectingall combinations of two electrodes, chosen from as many as 32 potential electrodes,to the differential input channel of the CR10; for each pair, one electrode served as Earth groundfor the data logger. The current electrode multiplexers used during the same period were capable of connectingall combinationsof two electrodes,chosen from as many as eight electrodes, to an electrically isolated current-limited 250-V power supply. The polarity of the power supply output could be reversed so that polarization-compensatedDC resistivity measurements
could be made.
For these observations
the
applied current was •10mA, and this was measured to an accuracy of -t-200pA using an optically isolated frequency-modulation circuit.
I SWITCHING CIRCUIT
4.
P1 i V V i P2
I
P3
Cu
V
V
V
I
V
V
I
I
V
V
I
I
V
V
P4
•s
s• s3 [•
50 rnrn
I
I 1
d
We attribute
2
3
V
V
V
V
4
5
observed variations
in natural
electrical
potentials to subglacialstreaming potentials associated with I
I V 'V I
S2
Results
I
water
flow
at and beneath
the ice-bed
contact.
In order to discussresults, we first establishsome principlesfor interpreting streaming potential data. As previously noted, the expression
I 6
7
8
ELECTRODE
vv: - co**l½l
(22)
Figure 3. Schematicdiagramsshowingfield measure- where ( is assumedto be negativeand the signis explicment setup and electrodes. (a) A multielectrodear- ifiy indicated, invites the generalization VV cr -V•b. ray in an L-shapedconfiguration. Voltage and current electrodesare placedin boreholesand positionedin water-filled cavitiesimmediately abovethe ice-bed con-
tact. (b) Currentelectrodeconstructed from a copper pipe. (c) Voltageelectrode.(d) Exampleof programcontrolledmeasurementsemployingsharedcurrent and
voltageelectrodesas usedin 1987 (P-type configurations were for profiling, and S-type configurationswere for depthsounding).
In practice, VV is approximated by the macroscopic voltage gradient
,
(23)
where r z and r2 are measurementlocationsand t is time. In conventional electrical circuit measurements,
BLAKE
AND
CLARKE:
SUBGLACIAL
voltagesare referred to Earth ground, but for our application there is no obvious choice of a reference potential. This is an important issue, and in the absence of an obvious
choice
we seek a reference
that
has a
ELECTRICAL
4.1.
PHENOMENA
Secular, Episodic, and Diurnal
7487
Variations
Figure 4 presents pressure, natural potential, and apparent resistivity records for an 18-day period in
glaciologicaljustification. From many years of data summer1988. The plan map (Figure 4d) showsthe collection we have learned that hydraulically uncon- locationsof the natural potential electrodes(labeled nected regionsof the glacier bed tend to be associated "plus"and "minus"),the DC resistivityarray (quadriwith high subglacialwater pressureand low temporal lateral outline with solid circlesindicating current elecvariability relative to hydraulically connectedregionsof trodesand open circlesindicatingvoltageelectrodes),
the bed [Stone,1993; Stone and Clarke, 1993;Murray and Clarke, 1995]. A voltage electrodepositionedin an unconnectedregion might therefore be expected to maintain a comparatively steady electrical potential as comparedto a voltage electrodein a connectedregion. Thus, where possible,we take such an electrode as our voltage reference.
a water pressuresensor("P"), and regionsof the bed that werehydraulicallyconnected (unshaded)or unconnected(shaded)to the subglacialdrainagesystemat the time of drilling. A number of noteworthy features
appearin theserecords:(1) The apparentresistivityis slowlydroppingoverthe interval.(2) BeginningonJuly 25, there is a series of pressure pulses that are clearly
A second problemin using(22) is that an appropriate correlated with pulses in the natural potential record. valueof V'•b• [•b(r2,t) - •b(rz,t)]/Ir2 - rz I cannotbe (3) FromAugust4, diurnalfluctuationsin natural poestimated
from
our data.
Because we chose to reduce
the risk of introducing stray current paths, we avoided placing pressuresensorsin holes that contained electrodes. Thus pressure is measured at a third location
r• = (•,y•,z•).
We take the local elevationof the
tential and pressureare evident. These features will be discussedseparately below. 4.1.1. Secular change. Neither the water pressure nor the natural potential record displaysany longterm trend. Figure 4c showsa progressivedecreasein
apparent resistivity, of roughly 15%, from July 23 to August 10. One possibleexplanation is that there is implicitly,sealevel (wherez - 0) is the referencefor a progressiveincreasein electrical conductivity of subfluid potential. Becausedifferent referencesare used for glacial water, suggestingan increasedsubglacial residelectricaland fluid potentialmeasurements, (22) has, encetime and reducedeffectivenessof the drainage sysglacier bed as the water pressure datum so that the correspondingfluid potential is •b• = p•oz• -]- P•, and,
at best, qualitative usefulness,and even the polarity of pressurefluctuations relative to those of voltage fluctuations
cannot
be foreseen.
d
50•
•o 25-
©
tem. Another possibility is that the electrical connectivity of the substrate is slowly increasingwith time in conjunctionwith slow evolution of the drainage system morphology. 4.1.2. Episodic phenomena. Pressure records sometimesexhibit large sudden increasesor decreases in subglacialwater pressurethat typically decay over a period of a day or so. Changesof this sort can commonly be attributed to drilling activity, although we have also observednaturally occurringpressurepulses. During drilling a borehole is usually completely water-
PRESSURE
---.-_f-'r'"'"r• -
b
POTENTIAL
filled. 1.9
C
If the borehole
reaches a connected
zone at the
RESISTIVITY
bed, water drains rapidly into the subglacialhydraulic system. Conversely, if the borehole reaches an unconnected zone, the water column in the borehole drains 1.6 JULY •1• AUGUST 10 much more slowly into the glacier bed. In either case, 1988 local water pressurerises,and temporally varying water Figure 4. Pressure,natural potential, and apparent re- pressuregradients are developedwhich could produce sistivitydata. (a) Subglacialwater pressurerecordfor temporally varying streaming potentials. We expect sensorat positionP on plan map. (b) Natural potential measured between electrodes at positions marked that pressurerises are more localized and that pressure "plus"and "minus"on plan map. (c) Appareniresis- gradients are more extreme in the case of an uncontivity calculated using electrode array sketchedin ac- nectedglacierbed [Murray and Clarke, 1995]. Figure 5 focuseson the time interval from July 24companyingmap. (d) Plan map showingthe surface location of the L-shaped electrode array. The quadri- 28, where Figure 4 showsevidencefor sudden changes lateral indicates the position of the DC resistivity ar- in water pressure, natural potential, and apparent reray (solidcirclesrepresentcurrentelectrodes, and open sistivity induced by drilling. Holes 88H35 and 88H36, ,
i
i
,
,
i
,
,
,
,
,
i
,
,
,
23
circlesrepresent voltageelectrodes).The locationof the
pressuresensoris indicated by P. Shading indicates a region of the bed that was hydraulically unconnectedto the subglacialwater systemat the time the instrument holes were drilled.
denotedU and C, respectively, on the plan map (Figure 5d), were drilled during the observationinterval; their identifications and completion times are marked by arrows on the time axis of Figure 5c. Note that al-
7488
BLAKE AND CLARKE' SUBGLACIAL ELECTRICAL
55
E
PRESSURE
40 20.
POTENTIAL
PHENOMENA
conductionpathwaysrather than a temporary change in pore water conductivity.
Hole88H36(denotedC) connected stronglyat 1358 LT on July 25. Suddenpronouncedincreasesin water
pressure (Figure5a) andnaturalpotential(Figure5b) were noted at 1400 LT. Within the time resolution of our
15'
measurementsthe sensorresponsesto hole completion 1.00,
RESISTIVITY
wereimmediate.(Note that a 1-mV risein potentialis observedabout 40 min before hole completion; we be-
0.95'
lievethis rise to be naturally occurring.)Comparison of Figures5a and 5b revealsthat for at least 48hours
after the hole was completed,the natural potential reFigure 5. Pressure,natural potential, and apparent resistivity recordsillustrating responsesinducedby hot- sponseis virtually identicalto that for water pressure. water drilling. Two holeswere drilled during the obser- The quickresponseof the pressuresensorto the com-
vationinterval. The completiontime for theseholes(denotedU and C) are indicatedby arrowsalongthe time axis. (a) Subglacialwater pressurerecordfor sensorat positionP onplanmap. (b) Naturalpotentialmeasured between electrodesat positionsmarked "plus" and "mi-
pletionof hole 88H36 demonstrates that the connected patch is very efficientat distributingpressureat the bed. We suspectthat reference electrode("minus")for the natural potentialmeasurement alsoexperiences this pressureincreasebecausethe electrodewasplacedin a
nus"on plan map. (c) Apparentresistivitycalculated connectedhole. Connection of hole 88H36 had little, if using electrode array sketchedin accompanyingmap. onapparentresistivity(Figure5c). Keep(d) Plan map showingthe surfacelocationof the L- any,influence ing in mind that resistivityis sensitiveto localchanges shaped electrode array. The C indicates the location of connected hole 88H36 and the U indicates the location in water chemistryor drainagesystemmorphology,but of unconnected hole 88H35. See the caption to Figure not to changes in pressure, it is likelythe connected hole 4 for an explanation of other features on the map. is too far and/or isolatedfrom the resistivityarray to affect it.
though the results appear similar, the resistivity array usedto calculate apparent resistivity in Figure 5c differs from that used to calculate apparent resistivity in Figure 4c. Note also that water pressurewas measured at 2-min intervals, natural potentials were measuredat 10min intervals, and apparent resistivity was measuredat 5-min intervals. Relative positions of electrodes,boreholes, and a pressuresensorare indicated on the plan
map (Figure5d). Hole 88H35 (denotedU), completedat 1812LT on July 24, did not connect, but a nearby water pressure sensorindicates a modest pressure increase at 2004 LT
(Figure5a), and the natural potentialexhibitsa small pulse(Figure 5b). Theseeventsappearto be associated with the slow outward diffusion of a water pressure wave. At 1805 LT, roughly the time at which the
drill encounteredthe glacierbed, there is a sudden4% decreasein apparentresistivity(Figure 5c). The bottom location of this hole is 1.1 m from one of the potential electrodes in the resistivity array, and we interpret the sudden change as an indication of reorganization of nearby subglacial conductionpaths. In the vicinity of an unconnectedhole the basal water pressure exceedsice overburden pressure, and this overpressure could initiate an adjustment of the subglacial drainage system, for example, by mobilizing sedimentat the icebed contact. Examination of Figure 5c showsthat the apparent resistivity did not recoverfrom the suddendecrease. This providesstrong evidencethat the causeof the changewas somepermanent changein the electrical
4.1.3. Diurnal phenomena. During the summer melt seasonthere is a strong diurnal cycle in surface melt rate which can drive diurnal subglacialresponses. Figure 6 isolatesa portion of Figure 4 that exhibitsdi-
45-
PRESSURE
e
©
E
40
a 75-
E
3O 25-
POTENTIAL
C
10
, RESISTIVITY
1.7
d 3
4
5
AUGUST
1988
8
9
10
Figure 6. Pressure,natural potential, and apparentre-
sistivitydata from a diurnalcyclingepisode.(a) Subglacial water pressurerecord for sensorat position P on the plan map. (b) Subglacialwater pressurerecord for sensorin hole 88H39, beyond the map boundaries
androughly115m ENE of P on the planmap. (c) Natural potentialmeasuredbetweenelectrodesat positions marked"plus"and "minus"on planmap. (d) Apparent resistivitycalculatedusing electrodearray sketchedin accompanying map. (e) Plan map showingthe surface location of the L-shaped electrodearray. See the caption to Figure 4 for an explanationof other featureson the map.
BLAKE
AND CLARKE:
SUBGLACIAL
urnal cyclesin water pressure,natural potential, and apparent resistivity. Interestingly, these separate diurnal responsesare not in phase with each other. The pressurerecordshownin Figure 6a is that for the
ELECTRICAL
PHENOMENA
7489
a possibleincreasein fluid permeability[Clarke, 1987; Murray, 1990]. Archie [1942]proposedthe relation
p- an-'•S-apo
(24)
sensorlabeledP on the plan map (Figure 6e), whereas to describethe resistivity of porousrock, where n is the the pressurerecord shown in Figure 6b is from hole fractionalpore volume(porosity),S is the saturation water), m is a 88H39 which lies roughly 115 m ENE of P and well be- (fractionof the porevolumecontaining yond the map boundaries. The two pressurerecords are dissimilarin many respects,and we shall argue that the signalfrom the distant sensormore closelyindicates the state of the subglacialhydraulic system. Pressure fluctuationsin Figure 6b are muchlarger than thosein Figure 6a, and major peaksof Figure 6b, which tend to occur in late afternoonor evening,seemto align with troughsof Figure 6a. Thesecharacteristicsstronglyimply that the distant sensoris hydraulically connected and that sensor P has become unconnected.
Stress cou-
pling through the glacier from distant connectedzones may cause fluctuations in overburden pressure at P
cementationfactor, d is the saturationexponent,and p0 is the resistivity of the pore water. From this relation
we wouldexpecta decreasein resistivitywith increasing porosity.
As an aside, we note that dilation producespressure gradients that drive pore fluid toward dilatant zones. Dilation-induced streaming potentials may be observed if the influenceof these zonesis large enoughcompared with electrode spacing. Apparent resistivity fluctuations could also be caused by changesin water conductivity. Surface meltwater can reach the bed through crevasses and englacialchan-
thereby producingthe inversepressuresignal. Close nels [e.g.,Seaberget al., 1988;Hookeet al., 1988]. For examination of Figure 4a suggeststhat the transition Trapridge Glacier this water input is commonlygreatfrom connected to unconnected
behavior
occurred late
on August 1 when the character of the pressurerecord changesfrom a sawtooth pattern to a jittery pattern. This kind of switchingbehaviorhas been previouslyde-
scribedby Murray and Clarke[1995]. Figure 6b shows peak pressure occurring between 2100 LT and midnight during periods of diurnal pressure cycling. The natural potential variations in Figure 6c are consistent with streaming potentials arising from diurnal cycling of pressurein the connected zone. Diurnal fluctuation in streaming potential may also result from the input of resistive meltwater. Mor-
gan et al. [1989]demonstratedan increasein ( potential magnitude with increasingfluid resistivity in their
rock samples. (Note that changesin cr cancelout in (17b) and (18); changesin ion concentration affectthe valueof (directly.) Whether this effectis similarfor clay-rich deforming sedimentsis not certain but could
explainthe largerstreamingpotentialsduringthe afternoon and evening. The diurnal input of water can also causediurnal changesin the subglacialwater pressure; shouldthesepressurechangesintroducelateral pressure gradients,they may be reflectedin changes in streaming potential. Diurnal fluctuationsin resistivity might be causedby cyclic changesin the structure and morphology of the sedimentary substrate. Diurnal variations in the hydraulic connectivity of the ice-bed interface drainage systemof Trapridge Glacier have been observedby Murray and Clarke [1995]and providea ready explanation for diurnal changesin electrical connectivity. Alternatively, cyclic deformationof the sedimentarysubstrate, as has also been observedbeneath Trapridge Glacier
[Blakeand Clarke,1992;Blake,1992],couldproducea similar electricalresponse.Sheardeformationof glacial till is accompaniedby a dilatant increasein porosityand
est in the afternoon or evening. Surfacemeltwater from glaciersis resistive compared to subglacial water that has spent time in contact with bed minerals. Thus we would expect the resistivity of water at the ice-bed interface
to increase
in the afternoon
as resistive
melt-
water reachesthe bed and then to decreaseovernight as this meltwater
mineralizes
or mixes
with
conduct-
ive water deeper in the subglacial sediments. The unknown transit time between water leaving the surface
(whereupona near-immediate risein pressureresults) and reachingthe electrodes(at which time resistivity will change)is in part responsible for the observed phase differencebetween the two signals. Perhaps short-term movement measurements and meteorological data could be used to resolve the relative contribution of deformation and freshwater input. Unfortunately, thesedata are not available. As pressure and resistivity are not in phase, we suspect that more than one driving force is at play. 4.2.
Polarity
Reversals in Apparent
Resistivity
Figure 7a showsa remarkable example of time-varying changesin apparent resistivity. It is important to appreciate that in 1987 we did not perform inclinometry measurements in our boreholes, so that the true spatial positions of electrodes is unknown. Nevertheless, it is also important to remember that the relative position of the electrodes will remain fixed over time, no matter what their absolute position. We therefore take the surveyedmap positionsof the borehole collars, which were arranged to correspond to a conventional
Wenner array [e.g., Keller and Frischknecht,1966], as the nominal map positions of the electrodes. This assumption introducesuncertain errors in the geometrical factor k, and the vertical axis in Figure 7a is therefore a relative one. That the true electrode positions differ substantially from the nominal positions is immediately
7490
+150
BLAKE
AND CLARKE:
SUBGLACIAL
a
-150
88
PRESSURE
ELECTRICAL
PHENOMENA
Figure 7c showsthe plan view of a cross-shaped electrode configurationthat can produce polarity reversals for slight changesin the path taken by the electric current. If current flow follows the path indicated by the solid curve, a positive resistivity will be recorded;the reverse holds true if current flow follows the path indicated by the dashed curve. Thus our observations provide very strong evidence for temporal changesin the subglacialhydraulic regime, possiblyas a result of sediment deformation or adjustments of the interface
drainagenetwork. Muvvayand Clarke[1995]havede-
84
27
JULY1987
I
I
AUGUST1987 4
+v
ß
scribeddiurnal changesin subglacialhydraulic connectivity, and such changescould readily explain the observedpolarity reversals. 4.3.
PlanView
•-V
Figure 7. Apparent reversal in polarity of apparent
Hydraulic
Shutdown
During the 1988-1991 field seasonswe witnesseda dramatic pressure-relatedevent which occurredin late
resistivity.(a) The apparentresistivityrecordfor one of the 1987 electrodeconfigurations.Note the diurnal
7O
PRESSURE
32OO
RESISTIVITY
changesin resistivitypolarity in early August. (b) Water pressure record for sensor in hole 87H09 located
~27 m southof the electrodearray center.(c) An electrode configurationthat can produce polarity reversals in apparent resistivity. If the current flows along the solid curve, a positive potential differencewill be recorded; if the current flows along the dashed curve, the potential differencewill be negative.
confirmed by the fact that the calculated apparent resisitivity is negativeover the interval July 28 to August 1. However, the most significant features in Figure 7a are the excursions,in early August, which indicate reversalsin the polarity of apparent resistivity. For a homogeneousisotropic medium with accurately positioned electrodes,apparent resistivity pa is alwayspositiveand polarity reversalsare impossible. We are confidentthat the observed reversals
are true ones and are not caused
E
r'
1200
c WATER CONDUCTIVITY
--24
i i ii ii i ii i i ii ii id i 1
10
20
AUGUST
1989
by equipment malfunction becausethe transitions from positive to negative resistivity are smooth and because no modifications to the apparatus or electrode array were made during this interval.
¸
Water pressuremeasurements (Figure 7b) taken in the immediate vicinity of the resistivity array shed little light on the situation. Pressure is consistently
high (substantiallyabovethe ice flotationpressureof • 65mH•.O),and the fluctuationmagnitudeis lessthan 4-2 m. Such behavior
is characteristic
of a sensor that is
meters I .... o
P[--•-•
I lO
e
Figure 8. Subglacialconditionsat the end of the sum-
mer. (a) Subglacialwater pressurerecordsfor sensors hydraulically unconnected. There is some correspond- P1 (solidcurve)and P2 (dashedcurve). (b) Natural encebetween variations in apparent resistivity and variations in water pressure,but no clear narrative presents itself.
potential record obtained by differencing the voltages
measuredat holes"plus"and "minus". (c) An apparent resistivityrecordfrom the array. (d) A subglacial
water conductivity record at C. Note that the calibraThe only viable explanation of the observedpolarity tion scaling for this sensoris believed to be incorrect. reversalsis that they result from temporal changesin (e) A plan viewof the electrodearrayshowingthe relaelectrical conduction paths and are therefore an indic- tive surfacelocationsof the electrodearray (solidline) ation of changesin subglacialstructure or morphology. and the various sensors.
BLAKE
AND CLARKE:
SUBGLACIAL
July or early August. Within a few days of each other, pressuresensorsthat had previouslyindicated vigorous diurnal fluctuations abruptly switched from an active mode to a quiescentone. We interpret this cessation of activity as a partial shutdownof the subglacialhydraulic system. Commonly, this shutdown is followed by a temporary resumptionof diurnal pressurecycling in late August and early September,possiblydriven by water input from melting of early-winter snowfall. Figure 8 presentsdata from one of the 1989 electrode arrays. Difficulties with the apparatus and reconfiguration for over-winter operation on August 9-10 have resultedin severaldata gaps. Figure 8a showsthe pres-
ELECTRICAL
PHENOMENA
7491
conductivitycell locatedat C (a connected hole). Note that no diurnal fluctuation in water conductivity is discernable either before or after August 1. The lack of a diurnal conductivity signal suggeststhat changesin subglacialwater conductivity are not responsiblefor the natural potential fluctuations. Note that the calibration of the conductivity cell probably has an incorrect scaling factor, so the absolute conductivity values cannot be trusted.
It is difficult to imagine how natural potentials and apparent resistivity can continue to exhibit diurnal os-
cillationsin the absenceof a diurnal hydraulicforcing. We therefore interpret Figure 8 in terms of a series of
surerecords for sensors P1 (solidcurve)andP2 (dashed shutdown steps. The termination of diurnal pressure curve)overa 30-dayperiodbeginningon July 29, 1989. cyclingon August 2 is attributed to the shutdownof the SensorP1 is located 66 m upflow from sensorP2. The onset of hydraulic shutdownis evident on August 2. Figure 8b showsthe natural potential differencerecorded between the electrodesmarked "plus" and "minus"
subglacialhydraulic systemin the immediate vicinity of the pressuresensorsand the electrodearray. The continued electrical cycling which terminates around August 10 we attribute to mechanicalforcing of the subin Figure8e. (The positiveelectrodeand P2 are in the strate. This might occurif diur.nally oscillatingstresses same hole; two isolated data loggerswere used for the were transmitted from a hydraulically active region of two sensorsin order to avoid groundloop problems.) the glacier to the study area. We have no short-term Despite the apparent hydraulic shutdown in the vicin- movement measurementsthat might help clarify this isity of the two pressure sensorsthe natural potential sue,but we believethat terminationof electricalcycling showsa strong diurnal signal from August 1-10 before indicates the hydraulic shutdown of a more distant rejoining the pressuresensorsin a quiescentmode. The gion of the bed. The sequenceof events is consistent electrical shutdown around August 10 is indicated by with a progressiveareal shutdownof the subglacialhymany other potential electrodesin that region of the draulic system.
glacier(Figure 9). Apparentresistivitymeasurements display a similar pattern, oscillatingstrongly over the period August 1-10 and switching to a quiescentmode around August 10. Figure 8d showsthe record from a AB
C
D
11'
ii
89H15 89H16
.......
"
",
. _.•-. _..•-_• ..... :..•w-.
89H18 89H19
r.,--89H20 89H21
...•,,,•
.
'
--
.;..' ........... i...-•-L-•...... •...•................ 89H22
•
__
. . 8.9H5.3... ! 89H68
i
'A•i'l'0 ' ' ' ' ' 'D B ' ' ' ' ' ' ' '•" C 2•0' AUGUST 1989 Figure 9. Potential recordsfor one of the 1989 electrode arrays. The electrode in hole 89H14 is the referenceelectrode, and hence there is no graph for this electrode. To effect a sign switch, we take the reference
4.4.
Mapping
Natural
Electrical Potentials
Detailed examination of data from the 1989 array reveals that all the potential electrodesexhibited strong variation with respect to the electrode in hole 89H14
(the "plus"electrodein Figure 8) but little variation with respect to each other. This would single out the electrodein hole 89H14 as being a suitable voltage reference VR. We take this reference to be the positive, rather than negative, voltage so that A V = VR- •, where V• is the voltage of the ith electrode in the array. By adopting this convention, we invert the sign of AV which will facilitate subsequentinterpretation; recall that we expect the electric and pressuregradients to be of opposite sign. Figure 9 shows natural potential variations, relative to the reference electrode in 89H14, for 10 electrodes in the 1989 array. The previously discussedelectrical shutdown that occurred around August 10 is conspicuously evident as is the strong synchronousdiurnal cycling prior to the shutdown. A less apparent but truly remarkable feature of the data is revealed by selecting snapshottimes and examining the spatial variations of natural potential. If we record potential difference val-
uesat specificobservation times(denotedA, B, C, and
electrodeas the high potential so that relative voltages are calculated using A V - VR- V rather than in the D) and plot thesedata as potential surfaces,a striking correlation with subglacialtopography becomesapconventional manner A V- V- VR. The marked times (A, B, C, and D) correspond to the potentialsurfaces parent. Figure 10a showsa contour map of subglacial in Figure 10. topography, oriented so that northward distance in-
7492
BLAKE
AND
CLARKE:
SUBGLACIAL
ELECTRICAL
PHENOMENA
creasesto the right, together wi•h true map positionsof potential electrodes. Bed topography was determined from the inclinometer-correctedhole depths. The refer-
Figures 10d-10g showsurfaceplots of natural potentials measured at the observation times A, B, C, and D. For all plots the correlation between potential surence electrode at 89H14 is located in the southeast corfaces and basal topography is striking. How one inner of Figures 10a-10c. To simplify the task of compar- terprets this observationdependson how one imagines ing the bed topographywith plots of potential surfaces, subglacialstreamingpotentials are generated. In terms the bed surface is also rendered as a three-dimensional of the subglacialhydrologicalsystemthere are two lim-
surface(Figure10b). A conventionally orientedmap of
iting cases: (1) an interfacesystemexistingat the
subglacialtopography is included as Figure 10c.
ice-bed contact and characterized by water transport
SubglacialTopography -' •"-•l ' ' '/1I .... I'"'l'"'•
.... I .... I'/'"l"l"l''"l
=80H18 e80H15
•
Ze•9H19•
-,....
820
810
•
•
830
....
840
•
850
.... I''"q
•
•
•
d
....
860
870
NORTH (m)
'•' t • 2295 2294 LLI 2293-r LLI 2292,.• 810 b
•9
ß e• • 'Illill
750
- co 830 840 850 860 870 /VO/•TH
760
770
EAST (m)
Electrical Potential
.,oo
-11o] •.
5 820830 ....•f• 850860 870 •
-170• e
•
820
830
840
NOnTH(m)
850
860 870
850
860
ao
I--
•-' o
O -15'-•
•'
•10 f
820
o 03
830
840
NOt•TH(m)
85'0
860
870
• •J•
e
810 820 830 840 •
NORTHS)
870
••
Figure10. Basaltopography andpotential surfaces corresponding to thetimemarks(A, B, C, andD) in Figure9. (a) Contour mapofbedtopography. Contour interval is 0.5m. (b) Surface plot of bedtopography.(c) Contourmapof bedtopography orientedsothat northis upward. (d) Surface plotof naturalpotentialat timeA. (e) Surface plotof naturalpotentialat timeB. (f) Surface plotof naturalpotentialat timeC. (g) Surface plotof naturalpotentialat timeD.
BLAKE
AND
CLARKE:
SUBGLACIAL
ELECTRICAL
PHENOMENA
7493
that is tangentialto this boundaryand (2) a volume- approachto examining the interplay betweensubglacial distributed system developed in permeable bed material and characterized by bed-normal water transport at the ice-bed contact. We shall discussthese possibilities separately,keepingin mind that the potential gradients in Figure 10 are plotted in reversepolarity. 4.4.1. Interface transport. If streaming potentials are generated by water migration tangential to the ice-bed contact, then the correlation between bed topography and the potential surfaces could be explained by water transport from topographic highs to topographic lows. Such flow could be driven by higher ice overburden pressure on elevated areas of the bed. This would, in turn, imply that the interface was not hydraulically well-connected. For a well-connected region the fluid potential c) - P + p•ogz would approximate an equipotential, and there would be no appreciable pressurepotential gradients to drive water from topographic highs to topographic lows or to generate streaming potentials. 4.4.2. ¾olume[ric [ranspor[. Streaming potentials generated at depth can be observed at the icebed boundary even if it is a surface of constant fluid potential; this is, in fact, the norm for measurements
of streamingpotentialsin geophysical work [e.g., Bogoslovsky and Ogilvy,1973; Cotwin and Hoover,1979]. The theoretical derivation of these surface responsesis considerablymore complex than that presented above
[e.g.,Fitterman, 1979, 1984],but the practicalresultis that for the caseof negative ( potential elevatedsubsurface pore pressurewill result in a lower surface natural potential. Followingthis line of thought, potential highs would correspondto regions of elevated pore pressure within the bed and would imply that topographic highs experience greater ice overburden pressure than topographic lows. Although pressurerecord P2 in Figure 8 showsthat the reference electrode
89H14
is in an unconnected
zone
at high water pressure, the streaming potential data also affirm this. The potential difference between electrode
89H14
and the other electrodes
is at a maximum
in the early morning, and the polarity of the difference implies that the referenceelectrode is at a higher pressure potential. Either 89H14 is at a constant high pressure and the pressure at the other electrodes fails in the early morning, or the other electrodesare at a low pressure and the pressure in 89H14 rises in the early morning. Since we normally see low water pressurein the early morning, the former scenariorings true. Electrode
89H14
would
then be located
in an unconnected
zone, and the remaining electrodeswould be located in a connected zone; the generation of streaming poten-
tials is thereforedominatedby volumetrictransport(in this particularcase). 5.
Conclusions
Long-term observationsof natural and induced electrical potentials beneath wet-basedglaciersoffer a new
mechanical and hydrological processes. Our results demonstratethat the subglacialenvironmentis electrically heterogeneousand that this heterogeneity varies with time. It would be extravagant to claim that geoelectricmeasurementsare superior to direct meas-
urementsof water properties(e.g., pressure,turbidity, and conductivity)or of sedimentproperties(e.g., shearstrain rate and ploughingresistance)for eachoffers a useful perspective. DC resistivity measurements present a volume-averagedprojection of the structure of the bed and morphology of its drainage system. In this respect such measurementsare complementary to highly localized measurementsof water pressure,water quality, and bed properties. One of the frustrating characteristics of the subglacial environment, at least that of Trapridge Glacier, is the extreme spatial variability of the water pressure and stressfields. For this reason the averaging that necessarilyaccompaniesresistivity measurementsmight be viewed as helpful. To date, no satisfactory method has been developed to measure subglacial water velocity. Tracer injection is labor intensive and may not yield a representative result. Water turbidity can be used to indicate whether water is ponded or flowing, but turbidity has little quantitative relationship to flow rate. Measurement of streaming potentials could provide a practical method of measuring water velocity in the subglacial drainage system; the expressionJe = --xq implies this possibility. Unfortunately, the coupling coef-
ficient X-
p•ogCa/K involvesthe physicalproperties
a and K which are poorly determined and likely to be time
variable.
We have shown that streaming potentials are the primary source of natural potential variations in the subglacialenvironment. The streaming potential measurements have proved useful in measuringpressuregradients at the glacier bed, although more work needs to be done to discover how other subglacial processes, such as sediment deformation and varying overburden pressure,affect these potentials. The impressive correlation between subglacial water pressureand streaming
potentials(Figure5) suggests the needfor furtherwork to clarify the nature of this cross-coupledphenomenon. It would be a simple matter to place a pressure sensor in every hole that was instrumented with a voltage electrode. By using separate data loggersfor the pressure and electrical measurements,the risk of introducing unexpectedground loops could be reduced,and the spatial and temporal relations between water pressureand natural potential fields could be closely examined. Acknowledgments. We thank Tomiya Watanabe for the loan of a magnetometer and B. Barry Narod for useful discussionsand help with instrument design. We especially thank our coworkerson Trapridge Glacier, in particular, Chris Smart, Marc GSrin, and Dan Stone. Data collection was supported by grants from the Natural Sciencesand Engineering Research Council of Canada. We gratefully ac-
7494
BLAKE
AND
CLARKE:
SUBGLACIAL
knowledge Parks Canada and the Government of the Yukon for granting permission to conduct field work in Kluane National
Park.
ELECTRICAL
PHENOMENA
Fitterman, D. V., Thermoelectric self-potential anomalies and their relationship to the solid angie subtended by the
sourceregion,Geophysics, ,/9(2), 165-170, 1984. Gouy, G., Sur la constitution de la charge •lectrique & la surface d'un •lectrolyte, Ann. Phys. Paris, S•r. ,i, 9,
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Blake, E. W., The deformingbed beneath a surge-typeglacier' Measurementof mechanicaland electricalproperties, Ph.D. thesis,Univ. of B.C., Vancouver,B.C., Can., 1992. Blake, E. W., and G. K. C. Clarke, Interpretation of borehole-inclinometerdata: a general theory applied to a new
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aminingsubglacialbed deformation,J. Glacial.,$8(130), 388-396, 1992.
Bogoslovsky,V. A., and A. A. Ogilvy, Deformationsof natural electricfields near drainage structures, Geaphys.Prospect., 21, 716-723, 1973.
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Boulton, G. S., and R. C. A. Hindmarsh, Sediment deform-
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E. W. Blake, Icefield Instruments Inc., Box 5567, White-
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[email protected]) G. K. C. Clarke, Department of Earth and Ocean Sciences,129- 2219 Main Mall, University of British Columbia,
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[email protected])
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(ReceivedJanuary 29, 1998; revisedJune 18, 1998; acceptedJuly 17, 1998.)
