Bull Volcanol (2000) 62 :75–89 Q Springer-Verlag 2000

RESEARCH ARTICLE

J.-F. Lénat 7 D. Fitterman 7 D. B. JacksonP. Labazuy

Geoelectrical structure of the central zone

of Piton de la Fournaise volcano (Réunion)

Received: 3 November 1999 / Accepted: 15 September 1999

Editorial responsibility: D.A. Swanson

Jean-François Lénat (Y) 7 Philippe LabazuyUniversité Blaise Pascal, OPGC-CNRS,Centre de Recherches Volcanologiques, 5 rue Kessler,F-63038 Clermont-Ferrand, Francee-mail: lenat6opgc.univ-bpclermont.fr

David FittermanU.S. Geological Survey, Box 25046, M.S. 964, Denver,Colorado 80225, USA

Dallas B. JacksonU.S. Geological Survey, Hawaiian Volcano Observatory,Hawaii National Park, Hawaii 96718, USA

Abstract A study of the geoelectrical structure of thecentral part of Piton de la Fournaise volcano (Réunion,Indian Ocean) was made using direct current electrical(DC) and transient electromagnetic soundings (TEM).Piton de la Fournaise is a highly active oceanic basalticshield and has been active for more than half a millionyears. Joint interpretation of the DC and TEM data al-lows us to obtain reliable 1D models of the resistivitydistribution. The depth of investigation is of the orderof 1.5 km but varies with the resistivity pattern encoun-tered at each sounding. Two-dimensional resistivitycross sections were constructed by interpolation be-tween the soundings of the 1D interpreted models.Conductors with resistivities less than 100 ohm-m arepresent at depth beneath all of the soundings and arelocated high in the volcanic edifice at elevations be-tween 2000 and 1200 m. The deepest conductor has aresistivity less than 20 ohm-m for soundings located in-side the Enclos and less than 60–100 ohm-m for sound-ings outside the Enclos. From the resistivity distribu-tions, two zones are distinguished: (a) the central zoneof the Enclos; and (b) the outer zone beyond the En-clos. Beneath the highly active summit area, the con-ductor rises to within a few hundred meters of the sur-face. This bulge coincides with a 2000-mV self-potentialanomaly. Low-resistivity zones are inferred to show the

presence of a hydrothermal system where alteration bysteam and hot water has lowered the resistivity of therocks. Farther from the summit, but inside the Enclos,the depth to the conductive layers increases to approxi-mately 1 km and is inferred to be a deepening of thehydrothermally altered zone. Outside of the Enclos, thenature of the deep, conductive layers is not established.The observed resistivities suggest the presence of hy-drated minerals, which could be found in landslidebreccias, in hydrothermally altered zones, or in thickpyroclastic layers. Such formations often createperched water tables. The known occurrence of largeeastward-moving landslides in the evolution of Piton dela Fournaise strongly suggests that large volumes ofbreccias should exist in the interior of the volcano;however, extensive breccia deposits are not observed atthe bottom of the deep valleys that incise the volcanoto elevations lower than those determined for the topof the conductors. The presence of the center of Pitonde la Fournaise beneath the Plaine des Sables area dur-ing earlier volcanic stages (ca. 0.5 to 0.150 Ma) mayhave resulted in broad hydrothermal alteration of thiszone. However, this interpretation cannot account forthe low resistivities in peripheral zones. It is not pres-ently possible to discriminate between these general in-terpretations. In addition, the nature of the deep con-ductors may be different in each zone. Whatever thegeologic nature of these conductive layers, their pres-ence indicates a major change of lithology at depth, un-expected for a shield volcano such as Piton de la Four-naise.

Key words Piton de la Fournaise volcano 7 Réunion 7Electrical soundings 7 Electromagnetic soundings 7Resistivity 7 Hydrothermal 7 Basaltic shield

Introduction

Réunion Island is located in the southernmost part ofthe Mascarene Basin (Indian Ocean), 800 km east of

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Fig. 1 Topographic and structural map of Piton de la Fournaisevolcano. Inset: topographic map of Réunion Island with a boxshowing the location of the Piton de la Fournaise map. The loca-tion of transient electromagnetic soundings (TEM) sounding 20 isshown

Madagascar. It is related to the hot spot that generatedthe Deccan Traps during the Cretaceous and to a vol-canic chain whose younger elements, Mauritius andRéunion Island, are at its southwestern terminus (Dun-can et al. 1989; Courtillot et al. 1986).Piton de la Fournaise (Fig. 1), on the southeast part

of Reunion Island, is one of the world’s most activebasaltic shield volcanoes. Despite numerous geologicand geophysical studies made during the past two de-cades, the internal structure of Piton de la Fournaiseremains insufficiently well known to understand someaspects of its evolution. Two issues important to ad-dress are the long-term evolution of the volcano andthe structure of its active zone. The first question iswhether the internal part of the edifice can be equated

with that of a simple basaltic shield, or if it exhibits amore complex stratigraphy and structure. For example,are there unexpected units, such as large volumes ofbreccias derived from flank landslides or unusuallythick pyroclastic layers, hidden within the edifice? Thesecond issue is the nature of the active zone. A shallowmagma reservoir underlying an active hydrothermalsystem in the summit area has been proposed by Lénatand Bachèlery (1990), but the depth and extent of thesestructures are not well constrained.In this study we address the questions of stratigra-

phy and structure using geoelectrical techniques, whichprovide an image of the internal structure as a distribu-tion of resistivities and can be interpreted in terms ofvolcanic layers. Two methods were used, the direct cur-rent (DC) Schlumberger and the transient electromag-netic (TEM) techniques. They provide complementaryinformation, and their combined interpretation allowsus to obtain more reliable 1D models of the resistivitydistribution within Piton de la Fournaise.

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Geologic setting

More than half a million years of the history of Piton dela Fournaise can be observed in the walls of the deepvalleys on the flanks of the volcano (Gillot and Nativel1989). Although the structural evolution of shield vol-canoes is sometimes regarded as simple compared withthat of stratovolcanoes, studies of Piton de la Fournaisehave yielded contrasting and, sometimes, contradictoryhypotheses. In particular, some authors (Duffield et al.1982; Gillot et al. 1994) identify a series of curved rims(eastern rim of Rivière des Remparts and Plaine desSables, and the Enclos eastern fault scarps; Fig. 1) withthe headwalls of successively smaller eastward-movinglandslides, whereas others (Chevallier and Bachèlery1981; Bachèlery and Mairine 1990; Bachèlery and Lén-at 1993) consider that parts of these rims are really an-cient caldera fault scarps. All the authors, however,agree that the Grand Brûlé depression, which links thedepression named the Enclos Fouqué (the Enclos) tothe seashore, is the scar of large eastward-moving land-slides. Off-shore surveys (Lénat et al. 1990; Labazuy1991, 1996) show the presence of more than 500 km3 ofmaterial derived from a series of landslides on the eastflank of Piton de la Fournaise. A volume of 500 km3 isfar greater than that of the present-day Enclos-GrandBrûlé depression, and a succession of cycles of volcanicconstruction and landslide must have occurred duringthe evolution of the eastern flank of Piton de la Four-naise. Therefore, one suspects that brecciated depositsgenerated by landslides may be present within the vol-canic edifice, possibly in large volumes.The major features of the active zone are (Fig. 1):

(a) a 400-m-high central cone, located inside the En-clos; (b) two diffuse rift zones; and (c) two coalescentsummit craters, Bory to the west and Dolomieu to theeast. Historical and prehistoric activity has been mostlyrestricted to effusive eruptions on the central cone andalong the rift zones. On the basis of geologic and geo-physical data, Lénat and Bachèlery (1990) inferred thepresence of a shallow magma reservoir beneath thecentral cone. This reservoir is thought to be composedof a plexus of small magma bodies lying between ap-proximately 1 and 2.5 km beneath the surface. The lat-eral extent of this storage zone is inferred by Lénat andBachèlery (1990) to virtually correspond to that of thetwo summit craters. Self-potential (SP) data (Malen-greau et al. 1994) suggest the presence of an active hy-drothermal system above the postulated reservoir.

Previous geoelectrical studies

Previous geoelectrical studies on Piton de la Fournaisewere either limited reconnaissance surveys of the zonecovered by this study or more extensive surveys ofzones at its periphery. Pham Van Ngoc and Boyer(1980) made five scalar audio-magnetotelluric (AMT)

soundings in Plaine des Sables. Their interpretationshows highly resistive terrains (ca. 104 ohm-m) overly-ing a conductor (150–350 ohm-m) that they place at adepth of approximately 500 m. Benderitter and Gérard(1984) and Benderitter (1990) published a synthesis ofnumerous geoelectrical surveys acquired for a geother-mal project on Réunion Island. From scalar AMT pro-filing carried out over the flanks of Piton de la Four-naise, they defined a regional deep conductor whosetop lies between 500 and 1500 m in elevation in an areathat overlaps with our study. Courteaud (1996) madescalar AMT and TEM soundings on the flanks of Pitonde la Fournaise (a study area partially overlappingours); like Benderitter and Gérard (1984) and Bende-ritter (1990), he shows the presence of a conductivebasement whose top lies at high elevation(1000–2000 m) in the center of the volcanic edifice.Similarly, Schnegg (1997) evidenced high-level conduc-tors in the older parts of the volcano (from Plaine desCafres to Plaine des Sables) using AMT soundings. Be-cause the elevations of those conductors are considera-bly higher than that of the basal water table (Ghyben-Herzberg lens), the geologic nature of this conductivebasement is an enigma. Courteaud (1996) proposedthat the conductors correspond to hydrated minerals inbreccias of two origins: (a) breccias associated with thecollapse of calderas; and (b) breccias resulting fromflank landslides. M. Halbwachs (pers. commun.) madescalar AMT soundings in the summit craters. His mod-els suggest the presence of a 10-ohm-m conductor atapproximately 700 m depth.

Data acquisition and processing

DC soundings

Ten vertical-electrical resistivity soundings (Fig. 2)were made with the DC Schlumberger technique in1987 (Jackson and Lénat 1989). Standard methods wereused to reduce the data (Keller and Frischknecht 1966;Koefoed 1979). Maximum AB/2 distances (half the dis-tance between the current electrodes) of up to 1600 mwere used on Piton de la Fournaise (larger-currentelectrode spacings allow deeper vertical penetration).The array expansions were limited by topography(deep valleys) and in many cases by high contact resist-ances (often ca. 20 kohms), which severely limited theamount of current that could be injected into theground.Two soundings, DC2 and DC7 (Fig. 2), had array ex-

pansion paths so curved that the resulting soundingcurves had to be corrected by finding the straight-linedistance from the sounding center to the electrode posi-tions and then recomputing their apparent resistivities.The corrected apparent resistivities were then plottedat new AB/2 positions equal to the geometric mean ofthe current electrode distances (Zohdy and Bisdorf1990).

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Fig. 2 Location map of geoel-ectrical soundings and pro-files. Coordinates in kilomet-ers (Gauss-Laborde Réunionprojection). Lines superim-posed on dots showing directcurrent electrical (DC)Schlumberger soundings loca-tion indicate the direction inwhich the DC soundings wereexpanded. Elevation contour1200 m is shown as a bold linein Rivière Langevin and Riv-ière des Remparts to shownthe approximate altitudewhere the top of the deepconductor is found at nearbysoundings (see text). The as-terisks in Rivière Langevinshow the sites where Cour-teaud (1996) described out-crops of breccias

The effect of deep valley topography on the soundings

There are deep stream valleys near some soundings(Fig. 2). The effect of a large topographic depression ona sounding curve is to remove some portion of thegeoelectric section and replace it with a nearly infiniteelectrically resistant body (air). One can estimate whateffect such a structure would have on a sounding curve.DC2 is the sounding nearest a major topographic de-pression on Piton de la Fournaise and thus should bemost affected by topography. DC2 was expandedroughly parallel to and approximately 0.5 km from theedge of the valley of Rivière des Remparts (RampartsRiver). One can approximate the effect of the valleywith a theoretical curve (Alpin et al. 1966) for an infin-itely resistive wedge (extending to infinity) whosestrike is parallel to the sounding and which dips awayfrom it at 207. The theoretical curve is subtracted logar-ithmically from the field curve to remove approximate-ly the effect of the lateral structure provided the resis-tivity of the lateral structure is either zero or infinity(Fomina 1958). For DC2 the changes produced by sub-tracting the theoretical curve representing the river val-ley from the sounding curve shows that, even for thisworst case, the effect of the valley does not significantlyalter the shape of the sounding curve (ca. 5% at the endof the sounding curve). We assume that the othersoundings near topographic depressions will be evenless affected.

Interpretation technique

Both forward and inverse 1D modeling was used to de-termine the best-fitting horizontally layered model pa-rameters for the resistivity soundings, i.e., the resistivi-ties and thicknesses of each of the layers in the 1Dearth model (Anderson 1979).

TEM soundings

The TEM method is a controlled-source electromag-netic sounding technique. A description of the physicalbasis of this method is given by Kaufman and Keller(1983), Fitterman and Stewart (1986) and Fitterman(1987).

Field procedures

Twenty TEM soundings (Fig. 2) were made in 1992 us-ing a Geonics EM-37 system in conjunction with a datalogger fabricated by the USGS. Transmitter loop sizesof 304.8 m (1000 ft) and 152.4 m (500 ft) on a side wereused. The smaller loops, which generate smaller signals,were used inside the Enclos because of the very roughterrain. Transmitter frequencies of 25 and 2.5 Hz wereused to obtain information in overlapping time rangesof 0.089–7.12 and 0.89–71.2 ms, respectively (longertimes allow deeper penetration). For the 25-Hz repeti-tion frequency, a rigid coil provided by Geonics, with

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an effective area (loop area times number of turns) of100 m2, was used. Because of the highly resistive near-surface rocks, the transient signals decayed very quick-ly. In order that information about the deeper structureof the volcano could be obtained, the 2.5-Hz data werecollected with the rigid coil as well as with a flexibleloop measuring 20!20 m with an effective area of104 m2. The first four channels of the 2.5-Hz flexibleloop data were often discarded due to inadequate loopdamping. In addition, because of very high near-surfaceresistivity (1104 ohm-m), the first three to four chan-nels of the 25-Hz data displayed sign reversals and werenot used.

Data processing

Tens of transients were stacked and averaged for eachmeasurement. To assess and improve the quality of thedata, five separate measurements were made and aver-aged together for each repetition frequency and receiv-er-coil combination. The resulting data sets had aver-age voltage errors of 1–5% for most channels, with amaximum of approximately 15%. When converted toapparent resistivity, the relative error is reduced by twothirds. The transient voltage data were converted to ap-parent resistivity using the formula:

rapm0

4p t 12L2Im0Sr5Vt 2

2/3

where L is the transmitter loop side length, I is thetransmitter current, Sr is the effective receiver coil area,V is the transient voltage measured at time t after cur-rent turnoff, and m0 is the magnetic permeability of freespace. The apparent resistivity (ra) is plotted as a func-tion of time to assess data quality. Apparent resistivitydata with more than 10% error, or those which departfrom a smooth apparent resistivity-time plot, are re-moved from the data set before interpretation.

Interpretation technique

The 1D modeling of TEM data was done in much thesame way as the DC data interpretation; a commercialinterpretation package, TEMIXGL, by Interprex and aprogram by Anderson (1993) were used.

Sounding curve shapes

All the soundings, whether DC or TEM, show a patternof high-resistivity near-surface layers overlying a moreconductive layer or layers at depth.The ten DC soundings can be divided into three

groups based on the shapes of the apparent resistivitycurves (Fig. 3). The three groups also correspond tothree different areas: (a) Plaine des Remparts and west-ern Plaine des Sables (Fig. 3a); (b) eastern Plaine des

Fig. 3a–c The DC soundings at Piton de la Fournaise dividedinto three groups, based on the shapes of the apparent resistivitycurves. a Soundings on the Plaine des Remparts and westernPlaine des Sables; b soundings on the eastern Plaine des Sablesand in the western Enclos; c soundings on the Central Cone

Sables and western Enclos (Fig. 3b); and (c) CentralCone areas (Fig. 3c). The westernmost soundings (DC2and DC3; Fig. 3a) show no indication of a conductor atdepth. The apparent resistivities at the largest arrayspacings suggest that the resistivity of the deepest layer(which must be very thick) sensed by the soundings is

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Fig. 4 The TEM soundings at Piton de la Fournaise divided intotwo groups based on the shapes of the apparent resistivitycurves

near 1500–2000 ohm-m. The central group of soundings(DC11, DC10, DC4, and DC5; Fig. 3b) shows a rela-tively conductive zone of much less than 1000 ohm-m ata moderate depth; the steeply descending terminalbranches, well developed by AB/2 spacings of 800 m,reach apparent resistivities of less than 1000 ohm-m forthree of the soundings. The eastern group of soundings(DC9, DC8, DC6, and DC7; Fig. 3c) also shows a con-ductive basement (much less than 1000 ohm-m) but hasits top at a shallow depth. The descending terminalbranches are well developed at AB/2 spacings of300–400 m or less.Based on apparent resistivity values, the 20 TEM

soundings can be classified in two groups, those locatedinside the Enclos and those outside (Fig. 4). One excep-tion is TEM20, located low (285 m elevation) on thesouthern flank of the massif (Fig. 1), whose apparentresistivities are more like those of the TEM soundingsinside the Enclos. This sounding was made to charac-terize the resistivities of the lava pile outside the centralzone. In this case the deep conductor is unambiguouslyrelated to seawater-saturated rocks (Ritz et al. 1997).

Respective advantages and disadvantages of DCand TEM soundings

Both DC and TEM sounding methods allow determi-nation of resistivity variation with depth, but each haslimitations and shortcomings that are significantly re-duced if the two methods are used in combination.The DC soundings define the resistive layers in this

study (upper part of the sections), but they were notable to resolve the depth and resistivity of the deeper

Fig. 5 Illustration of suppression and the sensitivity of TEMsoundings to conductive layering and insensitivity to resistivelayering. The upper diagram shows four resistivity-depth modelsfitting sounding TEM3 equally well. Lower diagram with crossesshows observed data. The problem of suppression is shown by thefact that the resistive layers can be replaced by a single layer (sol-id-line curve) without changing the shape of the sounding curve.Note that in all cases the depth and resistivity of the deep conduc-tor vary only within a limited range of values. This illustrates theability of TEM soundings to resolve a conductor but not the resis-tivity of the overlying resistive layers. DC soundings, on the otherhand, are better suited to define the upper resistive layers

conductors because the current electrode separations,and therefore the apparent resistivity curves do not ex-tend far enough (except for DC2 and DC3; Fig. 3a). Asa result, the resistivity and depth of the deep conductorbased on DC measurements is highly uncertain.In contrast, the TEM soundings are better able to

define the resistivity and depth of the deep conductivelayer because of the TEM method’s higher sensitivityto conductive layers (Kaufman and Keller 1983). How-ever, the TEM sounding does a poor job of determin-ing the resistivity of the very resistive zones above aconductor. This is illustrated in Fig. 5, where we show

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Fig. 6 Example of a combined interpretation of coincidentallysited DC and TEM soundings. The DC-sounding calculatedcurves (upper left) and TEM-sounding calculated curves (lowerleft) are from models shown in table. Observed data are indicatedby crosses. This example is for soundings DC11 and TEM5 (loca-tion in Fig. 2). See text for comments

four models that fit the data equally well. Poor resolu-tion of high-resistivity zones is a characteristic of theTEM method. Notice that, although the shallow resis-tive layers vary significantly among the models, thedepth and resistivity of the deep conductor remainspractically unchanged. This behavior is complementaryto that of the DC soundings, which give more reliableestimates of the higher, near-surface resistivities. Con-sequently, we can provide an interpretation throughcombined use of both the TDEM and DC data sets. Fit-terman et al. (1988) used a similar approach in develop-ing a geoelectrical model for Newberry volcano, Ore-gon.

Combined interpretation of DC and TEM soundings

Both DC and TEM sounding data are available for sev-eral sites (Fig. 2). Figure 6 illustrates how a combinedinterpretation was carried out to refine the 1D modelof the geoelectric sections. A coherent model shouldagree with both types of data.

Curve I is the unconstrained inversion of the TEMdata for sounding TEM5, i.e., a model with the mini-mum number of layers necessary to achieve a fit com-mensurate with the data noise level. Curve II is the un-constrained inversion of sounding DC11 obtained be-fore the TEM data were available. A resistivity of200 ohm-m was assumed for the deep conductor on thebasis of the interpretation of nearby soundings withsimilar shapes within the Enclos. Curve III is a modeladjusted to fit both DC and TEM data. To obtain a co-herent model, one must rely on the DC data to definethe upper resistive layers and on the TEM data for thedepth and resistivity of deep conductors.This approach leads to far more constrained and re-

liable models than those which could be obtained fromone type of data only. One must be aware, however,that such combined interpretations cannot pretend tobe unique because of the problems known as suppres-sion (illustrated in Fig. 5; e.g., see Keller and Frisch-knecht 1966; Koefed 1979). Thus, the actual geoelectricsections can have a more complex layering than that ofthe models. Thin layers, or layers with resistivity valuesbetween those of overlying and underlying layers, canbe undetected. In addition, 1D models cannot accountfor the 3D complexity of the underground structureand for the fact that geologic noise (non-1D structure)affects the DC and TEM measurements differently.However, the major geoelectric features are assumed tobe well described by this approach.Where DC and TEM data were not collected at the

same location, the interpretation was made using the

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nearest soundings. Care was taken to obtain reasonablecorrelations between adjacent soundings for layerthickness and resistivity. Figure 7 shows the results ascross sections (located in Fig. 2) constructed from 1Dmodels obtained at the centers of the soundings. Inter-polation between the soundings was made as smooth aspossible; consequently, one cannot rule out the pres-ence of sharper lateral resistivity variations. The totaldepth of investigation of the soundings cannot be de-termined: All we know is that a conductor at least a fewhundred meters thick is present in the lower part of thevolcanic section.

Interpretation

Resistivity of volcanic rocks

In general, a basaltic shield consists of a pile of highlypermeable lava flows. This is observed at Piton de laFournaise in the stratigraphic sections of the deep val-leys and other scarps (Fig. 8). If not altered or watersaturated, such lava flows generally have resistivities ashigh as 10–100 kohm-m. [A summary of the resistivityvalues for volcanic rocks is given by Keller (1988), Pa-lacky (1988), Kauahikaua (1993), and Lénat (1995).] Alarge resistivity contrast normally occurs at the transi-tion between the vadose zone, where rocks containonly some moisture, and the zone beneath the watertable, where rocks are fully water saturated. On oceanicislands, the basal water table consists of a lens of freshwater (Ghyben-Herzberg lens; e.g., see Vacher 1988)floating over the seawater saturated zone. The top ofthe fresh-water lens generally occurs at an elevation ofa few meters to a few tens of meters above sea level.The resistivity of a basaltic pile saturated with fresh wa-ter is generally a few hundred ohm-meters but drops toa few tens of ohm-meters or less when saturated withseawater. This idealized model may be perturbed bysome types of volcanic structures such as intrusions orgeothermal activity in summit or rift zone areas aslisted below [e.g., see Kauahikaua (1993) or Lénat(1995)].1. In the zones of intrusions (central zone, rift zones),the swarms of subvertical dikes can create imperme-able barriers that impound bodies of water at highelevations (Stearns 1942; Takasaki 1981; Jacksonand Kauahikaua 1987a).

2. For a given porosity in unaltered rocks, the resistivi-ty depends primarily upon that of the pore fluid,generally water. Within the edifice of Piton de laFournaise, the water may be contaminated by theionic content of hydrothermal fluids in the activezone. This can significantly lower the rock resistivity.Raising the temperature will amplify this effect.Therefore, a hot zone with high hydrothermal fluidcontent will be characterized by low resistivities (afew ohm-meters to a few hundred ohm-meters).

PFig. 7 Geoelectric cross sections of Piton de la Fournaise ob-tained from the interpretation of DC and TEM soundings. Loca-tion of profiles in Fig. 2. The sections are constructed from 1Dmodels at the center of each sounding. Interpolation betweensoundings was made as smooth as possible. White vertical dashedlines indicate significant lateral discontinuities in resistivity, whichcould be sharper than shown by the smooth interpolation

3. Alteration, particularly hydrothermal alteration,lowers the rock resistivity through the formation ofhighly conductive, hydrated minerals such as clayminerals and zeolites.

4. Very hot, vapor-dominated zones will have resistivi-ties of thousands of ohm-meters.

5. The resistivity of basaltic magma, which can be pres-ent in intrusions at depths of a few hundred meters,is only a few ohm-meters (Rai and Manghnani1977).

6. Pyroclastic and breccia deposits, whose presencecannot be disregarded a priori within Piton de laFournaise, generally have lower resistivities thanlava flows because they typically contain hydratedminerals.

Structural interpretation

Our structural interpretation is based on the cross sec-tions presented in Figs. 7 and 9 and is summarized inFig. 10.The major feature of the geoelectrical structure of

Piton de la Fournaise is the presence of conductors (re-sistivity ~100 ohm-m) at the base of all the soundings.The conductors are located high in the edifice; theirtops are all at elevations higher than 1200–1300 m. Theexact resistivities of the conductors cannot be uniquelydetermined, but their range is well constrained. Fromthe observation of the lower apparent resistivities ofmost of the soundings inside the Enclos, and particular-ly those on the Central Cone (Fig. 4), it appears thattheir interpretation must include a deep conductorwhose resistivity is less than 20 ohm-m. For the TEMsoundings outside Enclos, the same type of observationsuggests that the resistivity of the conductor is less than100–60 ohm-m.From the distribution of resistivities, two zones can

be distinguished: (a) the central zone, nearly coincidingwith the Enclos, and (b) the outer zone. The discontin-uities shown in Fig. 7 (dashed vertical lines) are notice-able lateral variations of resistivity.

Central zone

The central zone is characterized by a relatively shallowconductor of less than 10 ohm-m. The limits of the con-ductor toward the west, south, and northwest are seen

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Fig. 8 Photo showing the flow pile of a basaltic shield (see Fig. 2for location). Owing to a recent landslide in Rivière des Remparts(1987), the typical pile of lava flows that builds most of the shieldis well exposed. Shown are compact lava flows (light) with theirthin blocky upper and lower zones (dark). The thickness of theflows varies from less than a meter to a few meters

Fig. 9 Comparison between self-potential (SP) anomalies and re-sistivity cross sections. The SP data (relative to a base station nearthe eastern shore) are from Lénat (1987) and Malengreau et al.(1994). An elevation correction estimated to be –1.9 mV/m hasbeen removed from the data (Jackson and Kauahikaua 1987b;Lénat 1987). The SP profiles show the very high amplitude ano-malies (F2000 mV) associated with the summit area and are in-ferred to reflect upward migration of fluids or steam in a shallowhydrothermal system (Malengreau et al. 1994; Zlotnicki et al.1994; Michel 1995). The vertical lines between the SP profiles andthe geoelectric cross sections emphasize the coincidence betweenthe SP anomalies and the zone where low resistivities are found atshallow depth. Both types of data support the existence of a shal-low and active hydrothermal system (see text)

in profiles 1, 2, and 4 (Fig. 7). We do not know the lim-its of the shallow conductor toward the east and north,although we observe the deepening of the conductor inthese directions on profiles 1 and 2 in Fig. 7.Beneath the summit area there is a decrease in the

resistivities of the layers overlying the conductor, com-pared with the surrounding area, as well as a rise of the

deep conductor (profiles 1 and 2 in Figs. 7 and 9).Based only on resistivity values, the geologic identifica-tion of the conductors is not possible. However, geo-logic constraints allow us to be more positive in the in-terpretation of the structure beneath the Central Cone.The occurrence of low resistivities at shallow depth inthe summit area may be due to: (a) the presence of ashallow water table; (b) the effects of a hydrothermalsystem; or (c) a combination of both. At Karthala vol-cano (a basaltic shield volcano on Grande Comore) apermanent water lake exists in the summit crater, at analtitude of 1920 m, approximately 300 m below the ele-vation of the floor of the caldera (Bachèlery et al. 1995;Lénat et al. 1998). The presence of a similarly shallowwater table cannot be ruled out at Piton de la Four-

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Fig. 10 Structural interpreta-tion of the geoelectric andelectromagnetic soundings.Top: Location of soundings,profiles, and interpreted fea-tures. Bottom: Structural inter-pretation along profile 1. Arepresentative set of earth-quake hypocenters at Piton dela Fournaise is superimposed.The earthquakes in this zoneare thought to reflect move-ment of magma and hydro-thermal fluids

naise. The only available information comes from thedescent by one of the authors (J.F.L.) into a vent, adja-cent to DC6 and TEM10, open to 213 m beneath thesummit, where no standing water was observed. In ad-dition, the resistivities encountered at a few hundredmeters depth are too low to be attributed to saturationof unaltered lava flows by fresh water. If present, thiswater table would probably be contaminated by hydro-thermal fluids. The rise of the conductor and the rela-tively low resistivity of the overlying layers is more like-ly due to the hydrothermal system believed to exist in

this highly active volcanic area (Malengreau et al. 1994;Zlotnicki et al. 1994; Michel 1995). Alteration of rocksby steam or hot water can produce low resistivity zonesand significant SP anomalies (Zablocki 1975). Figure 9shows the correspondence between the summit SP ano-malies and the zone where low resistivities are found atshallow depth. The SP and anomalous low resistivityzone extend over a surface larger than that of the sum-mit craters.An additional possible explanation for the low resis-

tivities is the presence of molten magma intrusions.

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Such intrusions would belong to the upper part of themagma reservoir complex postulated in the model ofLénat and Bachèlery (1990). The high-temperaturerind around a molten intrusion will create a vapor-dominated zone, which would have a high resistivity,provided there were no water films coating small porespaces. Surrounding the vapor zone would be a cooler,conductive zone where groundwater would remain inthe liquid state. Because of limited expansion of theDC soundings or noise at a late time in the TEM meas-urements, we would not expect to detect the vaporzone, if it is indeed resistive. Furthermore, both meth-ods would have a hard time detecting relatively smallpockets of magma embedded in the highly conductivesummit area of the volcano.

Outer zone

Only a few soundings are located outside of the Enclos,but they provide information about the rocks underly-ing Plaine des Sables and Plaine des Remparts areas.The shallow, high resistivity (11000 ohm-m) layers

thicken from the Enclos toward the Plaine des Sablesto the west (Fig. 7, profiles 1 and 4). Near the surfacethese layers are unambiguously known to be a pile oflava flows, as is observed in section in the east-facingfault scarps of the Plaine des Sables and the Enclos(Fig. 2). In general, young, largely unweathered flowshave higher resistivities than do more weathered olderflows. Thus, the increasing thickness of high resistivity(18000 ohm-m) layers toward the east suggests the fill-ing of depressions by young, relatively unaltered, lavaflows. In the western part of Enclos, it is easy to visual-ize such an accumulation of lava flows since the calderacollapse approximately 4500 years ago (Mohamed-Ab-chir 1996). There is no straightforward explanation forthe sharp thickening of high resistivity (18000 ohm-m)layers within the Plaine des Sables, between TEM2 andTEM4, other than assuming a concealed collapse block.Such a feature was also suggested by Schnegg (1997).The shallow resistivities beneath Plaine des Remparts(DC2) are lower than those beneath Plaine des Sablesand the floor of the Enclos: This can be explained bythe relatively greater age of this area (Bachèlery andLénat 1993), which implies weathering of the pile oflava flows by meteoric water.The nature of the deep conductors is more proble-

matic. Resistivities lower than 100 ohm-m cannot be at-tributed to unsaturated-unaltered lava flows. The top ofthe 50–100 ohm-m conductor is at a higher elevationthan the bottom of nearby deep valleys (Fig. 2), so thatthe geoelectric section can be checked against the geo-logic sections exposed in the nearly vertical valley walls(over 1 km high). These walls are composed of layers offlows (Fig. 8) intersected by a few nearly vertical dikes,arranged in a subradial pattern, which converge be-tween the sites of soundings TEM1 and TEM5 (Bachèl-ery and Mairine 1990). Such a scattering of dikes is in-

sufficient to impound large bodies of water, as oftenhappens in dense swarms of high-angle dikes underly-ing well-developed rift zones. Only small springs are re-cognized in the valley walls, apparently related toperched groundwater bodies supported by thin im-permeable ash beds and a few mud flows. Therefore,no evidence of widespread, very low resistivity, layershas as yet been observed in these natural sections; how-ever, Courteaud (1996) reports observations of poorlyexposed outcrops of breccias near the bottom of Riv-ière Langevin (location in Fig. 2).At high elevations in a basaltic shield, the extensive

presence of layers with resistivities less than 100 ohm-mprobably implies the presence of hydrated minerals, be-cause the alternative explanation, the presence of high-ly mineralized and possibly hot water is ruled out bythe temperature and composition of springs emanatingfrom the nearby valley walls (Coudray et al. 1990).Off-shore surveys suggest that the eastern flank of

Piton de la Fournaise has generated large landslidesduring its evolution. Rocks involved in a slide are af-fected, more or less severely, by block collapse and/orrotation, sometimes resulting in complete fragmenta-tion and crushing. Because of the greatly increased sur-face areas, the resulting breccias are typically prone toclay-forming alteration. Since the headwalls of the east-ward-moving landslides were probably subaerial (Lénatet al. 1990; Labazuy 1991, 1996), large volumes of brec-ciated rocks may constitute part of the interior of theedifice, producing the observed deep conductors. Thisinterpretation would be partly similar to that of Cour-teaud (1996).On the other hand, Bachèlery and Mairine (1990)

and Bachèlery and Lénat (1993) have proposed that thecenter of the ancient shield of Piton de la Fournaisewas beneath Plaine des Sables. This eruptive centermay have developed a zone of hydrothermal alteration,such as the one that appears to underlie the presentcenter of volcanism.Our data do not allow us to discriminate between

these two hypotheses; moreover, the genesis of thedeep conductors can differ from one location to an-other.

Discussion and conclusions

The distribution of resistivities in the interior of thecentral zone of Piton de la Fournaise provides new in-sights into the structure of the basaltic shield. Thereare, however, limitations on our understanding im-posed by the methods we used. Firstly, the depth of in-vestigation is limited to detection of a deep conductorwhose top lies a few hundred meters to approximately1 km beneath the surface. Thus, we have, at most, in-formation about the first kilometer and a half of theinternal structure of Piton de la Fournaise. Secondly,our interpreted resistivity values, or range of values,

87

cannot unambiguously be attributed to a particulargeologic unit.On the other hand, our interpretations are strength-

ened by the combined interpretation of TEM and DCsoundings and by the comparison of the geoelectricalmodels with available geologic and geophysical data.Our combined interpretation of DC and TEM sound-ings yields coherent models that describe the geoelec-trical structure of the volcano within the study areawith reasonable confidence. This could not have beenachieved using either geoelectrical data set alone. Forthe identification of geoelectrical layers and contrastswith volcanic structures, reliable constraints have beensought in the natural sections offered by the adjacentdeep valleys, in the off-shore surveys, in the generalknowledge of the evolution of the volcano and of itsrecent activity, and in SP data in the central area.The presence of conductors at elevations high within

the edifice must be considered to understand the evolu-tion of Piton de la Fournaise. In the summit zone,where layers of very low resistivity are detected todepths of only a few hundred meters from the surface,our geoelectric data reinforce the hypotheses for a well-developed hydrothermal system previously suggestedby others (Bachèlery and Lénat 1993; Malengreau et al.1994; Zlotnicki et al. 1994; Michel 1995). Farther fromthe summit, but still inside the Enclos, the deep con-ductors are assumed to belong to the outer parts of thehydrothermal system.This hypothesis can be compared with the geoelec-

trical structure of Kilauea volcano, which is regarded tobe, in many respects, an analog to Piton de la Four-naise. The geoelectrical structure of the first kilometeror so of the central zone of Kilauea is described byelectromagnetic soundings (Jackson and Keller 1972),DC Schlumberger soundings (Jackson and Kauahikaua1987a) and electromagnetic soundings by Kauahikauaet al. (1986), which provide information to a depth ofapproximately 5 km. Beneath the central zone of Ki-lauea, a similar succession of highly resistive layers (ca.1000 to several thousand ohm-meters) overlying a con-ductor (3–30 ohm-meters) is observed. Here, the depthof the conductor, approximately 500 m beneath the sur-face, can be identified with that of a high-level watertable observed in a research drill hole (Keller et al.1979). Electrical logs confirm that a sharp decrease ofresistivity in the volcanic pile coincides with the top ofthe water table. The resistivity of the water is lowerthan that of fresh water, and contamination by magmat-ic fluids is inferred. The rise of steam across the shallowresistive layers is attested to by fumaroles and positiveSP anomalies (Zablocki 1976; Jackson and Kauahikaua1987b). D. Jackson and J. Kauahikaua (unpublished re-port from Hawaiian Volcano Observatory) noted thatthe resistivity of the shallow layers was significantlylowered (down to several hundred ohm-meters) inzones probably related to hydrothermal alteration.Thus, there are similarities in the geoelectrical struc-

ture of the central zones of Kilauea and Piton de la

Fournaise. There may be, however, differences in thestructure of their central geothermal systems. Both vol-canoes are assumed to have shallow magma chambers,but Kilauea’s is inferred to be deeper than Piton de laFournaise’s, 3–4 km beneath the 1100-m-high summitof Kilauea (Fiske and Kinoshita 1969; Koyanagi et al.1976) vs 1–2 km beneath the 2630-m-high summit of Pi-ton de la Fournaise (Lénat 1988; Lénat and Bachèlery1990). Thus, it is possible that the conductors beneaththe summit of Piton de la Fournaise do not necessarilycoincide with a water table, although only drilling couldprovide the answer.In the area outside of the Enclos, the interpretation

of the deep, highly conductive layers (~60–250 ohm-m) has not been established. The observed resistivitiesimply the presence of hydrated minerals, which are inbreccias generated by landslides, in hydrothermally al-tered zones, or in fine-grained pyroclastic deposits. Theknown occurrence of large, eastward-directed land-slides in the evolution of Piton de la Fournaise stronglysuggests that large volumes of breccias should exist inthe interior of Piton de la Fournaise. However, exten-sive breccia has not been observed at the bottom of thedeep valleys that incise the flanks of Piton de la Four-naise, down to an elevation lower than that determinedfor the top of the conductors outside the Enclos(Fig. 2). This may be due to extensive colluvial, alluvial,and vegetation cover, which precludes the observationof the volcanic rocks in most places, or to the fact thatbreccias, if present, are within zones not yet exposed byerosion.The center of an ancient (ca. 0.5–0.150 Ma) volcanic

shield beneath the Plaine des Sables may have resultedin broad hydrothermal alteration of the overlying zone.This possibility, however, cannot explain the presenceof low resistivities in peripheral zones, beneath TEM3and TEM19 (Fig. 2). Similarly, thick and extensivelayers of pyroclastic rocks have never been observed atPiton de la Fournaise. In fact, the problem of explain-ing the origin of the conductors at depth, within theedifice, is even more far reaching if we consider the re-gional geoelectric surveys of Benderitter and Gérard(1984), Benderitter (1990), and Courteaud (1996),which suggest that similar conductive horizons are alsopresent beneath the flanks of Piton de la Fournaise.Thus, except in the summit area, the interpretation

of the deep conductors remains a problem whose geo-logic nature has not been determined. Nevertheless,three main conclusions are drawn for the structure ofthe edifice in the zones outside the Enclos: (a) Piton dela Fournaise is not simply constructed of a homogene-ous pile of lava flows; (b) the conductors at depth markone or more major lithologic discontinuities that mustreflect notable events in the evolution of the volcano;and (c) the storage and circulation of water in the edi-fice must be influenced at depth by extensive, highlyconductive, probably clay-rich layers.Refining the geoelectric structure of the interior of

Piton de la Fournaise will require more soundings to

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extend both the density and the spatial coverage of thedata. Exploration drill holes would be necessary to ver-ify the geologic nature of the deep conductors discov-ered in this study.

Acknowledgements The acquisition of the DC data were sup-ported by CNRS-INSU (PIRPSEV) in 1987. The TEM soundingswere made with support from the University of Réunion, the“Conseil Général de la Réunion,” and the USGS Volcano Haz-ards Program and the former Branch of Geophysics. A. Hos-kuldsson provided very efficient help during the TEM fieldwork.We also thank the staff of Piton de la Fournaise VolcanologicalObservatory for its frequent logistic help during fieldwork. Final-ly, the paper benefited from accurate and constructive remarksand suggestions from J. Kauahikaua and Y. Sasai.

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