Hydrogeology of Stromboli volcano, Aeolian Islands (Italy) from the interpretation of resistivity tomograms, self-potential, soil temperature and soil CO2 concentration measurements

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Hydrogeology of Stromboli volcano, Aeolian Islands (Italy) from the interpretation of resistivity tomograms,

self-potential, soil temperature and soil CO2 concentration measurements

André Revil, Anthony Finizola, T. Ricci, Eric Delcher, Aline Peltier, Stéphanie Barde-Cabusson, G. Avard, T. Bailly, L. Bennati, Svetlana Byrdina,

et al.

To cite this version:

André Revil, Anthony Finizola, T. Ricci, Eric Delcher, Aline Peltier, et al.. Hydrogeology of Stromboli

volcano, Aeolian Islands (Italy) from the interpretation of resistivity tomograms, self-potential, soil

temperature and soil CO2 concentration measurements. Geophysical Journal International, Oxford

University Press (OUP), 2011, 186 (3), pp.1078-1094. �10.1111/j.1365-246X.2011.05112.x�. �insu-

00681400�

Hydrogeology of Stromboli volcano, Aeolian Islands (Italy) from the interpretation of resistivity tomograms, self-potential, soil temperature and soil CO ₂ concentration measurements

A. Revil, 1•

A. Finizola, ³ T. Ricci, ⁴ E. Delcher, ³ A. Peltier, ⁵ S. Barde-Cabusson, ⁶

G. Avard,7 T. Bailly, ⁸ L. Bennati, ⁹ S. Byrdina,

J. Colonge, ⁸ F. Di Gangi, ¹⁰ G. Douillet, ¹¹ M. Lupi, ¹² J. Letort ⁸ and E. Tsang Hin Sun ⁸

1

Colorado School of Mines, Department of Geophysics, Golden, CO 80401, USA. E-mail: arevil@mines.edu 2 ISTerre, CNRS, UMR 5559, Universite de Savoie, Equipe Vo/can, Le Bourget du Lac, France

3 Labora.toire GeoSciences Reunion, Universite de la Reunion, Jnstitut de Physique du Globe de Paris, Sorbonne Paris-Cite, CNRS UMR 7154, 15 rue Rene Cassin, 97715 Saint-Denis cedex 9, La Reunion, Indian Ocean, France

4

Jstituto Nazionale di Geojisica e Vulcanologia, Roma, Italy

5 Jnstitut de Physique du Globe de Paris et Universite Paris Diderot, Sorbonne Paris-Cite, CNRS UMR 7154, Paris, France

6

Institute of Ea,th Sciences Jaume Almera., Consejo Superior de Investigaciones Cientificas, Spain 1 Observatorio Vulcanol6gico y Sismol6gico de Costa Rica, Costa Rica

8

Ecole et Observatoire des Sciences de la Terre, Universite de Strasbourg, France

9

Earth and Atmospheric Sciences Department, Purdue University, West Lafayette, IN, USA

IO

Istituto Nazionale di Geojisica e Vulcanol

ia, Palermo, Italy

11

Ludwig Maximilians Universitaet, Munich, Germany

12

University of Bonn, Steinmann Institute, Geodynamics I Geophysics, Germany

1 INTRODUCTION

SUMMARY

To gain a better insight of the hydrogeology and the location of the main tectonic faults of Stromboli volcano in Italy, we collected electrical resistivity measurements, soil CO2 concentrations, temperature and self- potential measurements along two profiles. These two profiles started at the village of Ginostra in the southwest part of the island. The first profile ( 4.8 km in length) ended up at the village of Scari in the north east part of the volcano and the second one (3.5 km in length) at Forgia Vecchia beach, in the eastern part of the island. These data were used to provide insights regarding the position of shallow aquifers and the extension of the hydrothermal system. This large-scale study is complemented by two high-resolution studies, one at the Pizzo area (near the active vents) and one at Rina Grande where flank collapse areas can be observed. The Pizzo corresponds to one of the main degassing structure of the hydrothermal system. The main degassing area is localized along a higher permeability area corresponding to the head of the gliding plane of the Rina Grande sector collapse. We found that the self-potential data reveal the position of an aquifer above the villages of Scari and San Vincenzo. We provide an estimate of the depth of this aquifer from these data. The lateral extension of the hydrothermal system (resistivity ,...., 15-60 ohm m) is broader than anticipated extending in the direction of the villages ofScari and San Vincenzo (in agreement with temperature data recorded in shallow wells). The lateral extension of the hydrothermal system reaches the lower third of the Rina Grande sector collapse area in the eastern part of the island. The hydrothermal body in this area is blocked by an old collapse boundary. This position of the hydrothermal body is consistent with low values of the magnetization ( <2.5 A m-1) from previously published work. The presence of the hydrothermal body below Rina Grande raises questions about the mechanical stability of this flank of the edifice.

The localization of hydrothermal systems and aquifers in active volcanoes is a fundamental step in assessing several geological

hazards like phreatic explosions, phreato-magmatic eruptions, and

flank collapses (Petrinovic & Piiiol 2006; Lorenz & Kurszlaukis

2007; Weinstein 2007). Phreatic explosions are bursts of confined

pockets of steam and gas with no direct involvement of magma,

apar t from the source of the s team and of the invo lvemen t or no t of juven i le flu ids (Barber ie t a l. 1993) . Phrea tomagma t ic erup t ions can occur when wa ter encoun ters a magma t ic body (Barber ie t a l .1992) . Hydro therma l sys tems can a lso produce , by long- l ived a l tera t ion , mechan ica l ly weakened rocks and be respons ib le for the co l lapse of the flanks of vo lcan ic ed ifices (L´ opez & W i l l iams 1993 ; A izawa e t a l. 2009) . In the case of vo lcan ic is lands , such g ian t lands l ides can in turn genera te tsunam is . S trombo l i is a pr ime examp le (T in t i e t a l. 1999 , 2000 , 2003 , 2006 , 2008 ; T iba ld i 2001 ; Bonaccorso e t a l. 2003 ; Apuan ie t a l. 2005 ; Romagno l ie t a l. 2009a , b) . The loca l iza t ion of aqu ifers is a lso impor tan t because wa ter is a scarce resource in some vo lcan ic areas and the supp ly of fresh wa ter is increas ing ly becom ing a prob lem as the popu la t ion and tour ism increase (Cruz & F ranc ¸a 2006) .

In hydrogeo logy , the presence of aqu ifers is usua l ly de tec ted by dr i l l ing . An es t ima te of hydrau l ic conduc t iv i ty can be ob ta ined by pump ing or s lug tes ts and hydrau l ic tomography (Carrerae t a l.

2005 ; Card if fe t a l. 2009) . However , dr i l l ing a se t of boreho les in vo lcan ic forma t ions can be very expens ive and a d ifficu l t task due to s teep topography and the mechan ica l res is tance of some vo lcan ic rocks . Geophys ica l me thods represen t a non- in trus ive approach to hand l ing th is prob lem . T rad i t iona l ly , e lec tromagne t ic me thods (es- pec ia l ly the aud iomagne to te l lur ic and t ime-doma in e lec tromagne t ic me thods) have been used to look for aqu ifers in ac t ive vo lcanoes (e .g . F i t termane t a l. 1988 ; Kr ivoch ieva & Chou teau 2003 ; A izawa e t a l. 2005 ; A izawae t a l. 2009) . Th is is because e lec tr ica l res is t iv i ty is sens i t ive to the wa ter con ten t of rocks . Unfor tuna te ly e lec tr ica l res is t iv i ty is a lso known to be af fec ted by the presence of c lays and zeo l i tes through the ir ca t ion exchange capac i ty (Waxman & Sm i ts 1968 ; Rev i le t a l. 2002 ; Coppoe t a l. 2008) . Sa l in i ty of the pore wa ter and tempera ture are two o ther parame ters influenc ing the re- s is t iv i ty of porous rocks (Waxman & Sm i ts 1968 ; Rev i le t a l. 1998 ; Rev i l 1999) . Therefore , e lec tromagne t ic me thods canno t be used as s tand-a lone me thods to de term ine the presence of aqu ifers and the ex tens ion of the hydro therma l sys tem in vo lcan ic areas . A lso e lec tromagne t ic me thods have a l im i ted spa t ia l reso lu t ion , a t leas t w i th the number of da ta curren t ly ob ta ined over vo lcan ic areas and ex is t ing a lgor i thms .

In th is paper , we show tha t the comb ina t ion of se lf-po ten t ia l da ta , large sca le DC-res is t iv i ty tomography , and measuremen ts of so i l tempera ture and so i l CO

concen tra t ions and fluxes can be used to assess the ex ten t of the hydro therma l sys tem and the pres- ence of aqu ifers a t the sca le of a vo lcan ic ed ifice . The res is t iv i ty measuremen ts were performed us ing an unusua l ly long res is t iv i ty cab le (16 w ires , 2 .5 km in leng th) . Such a cab le of fers a s trong advan tage regard ing the acqu is i t ion t ime w i th respec t to c lass ica l acqu is i t ion measuremen ts us ing two cab les for d ipo le–d ipo le mea- suremen ts w i th a genera tor and a vo l tme ter . S trombo l i , an ac t ive vo lcan ic loca ted in the Aeo l ian Arch ipe lago in I ta ly , is a su i tab le na tura l labora tory to tes t these me thods because of the access ib i l i ty of the vo lcano , i ts s trong vo lcan ic ac t iv i ty , and the re la t ive ly sma l l d imens ions of i ts emerged par t . We presen t new fie ld resu l ts to de term ine the ex ten t of the hydro therma l sys tem and the presence of aqu ifers a t the sca le of th is is land us ing the comb ina t ion of the me thods men t ioned above .

2 TECTONIC SETTING AND

HISTORICAL VOLCANIC ACTIVITY S trombo l i is a s tra tovo lcano correspond ing to the nor thernmos t is- land of the Aeo l ian vo lcan ic arc in the Sou thern Tyrrhen ian sea

F igure 1 .Geograph ica l loca t ion of S trombo l i Is land . (a) The inse t shows the pos i t ion of the Aeo l ian Arch ipe lago in the Tyrrhen ian Sea . (b) C lose-up of the nor thern coas t of S ic i ly show ing the seven ma in is lands (A l icud i , F i l icud i , Sa l ina , L ipar i , Panarea , S trombo l i and Vu lcano) of the Aeo l ian arch ipe lago . (c) P ic ture of S trombo l i is land (by A . F in izo la , taken in 1999) show ing the Sc iara de l Fuoco co l lapse s truc ture .

(F ig . 1) . I t r ises from a dep th of 2000 m be low sea leve l (b .s . l .) to an e leva t ion of 924 m a .s . l . (me tres above sea leve l) (see Segre 1968 ; Romagno l ie t a l. 2009a , b) . I t is one of the four ac t ive vo l- can ic is lands (w i th Vu lcano , L ipar i and Panarea) of the Aeo l ian arch ipe lago , whose ex is tence is re la ted to the subduc t ion of the Afr ican p la te under the Euras ian p la te (Barber ie t a l. 1974) . Geo- log ica l surveys (Ros i 1980 ; F ranca lanc i 1987 ; Ke l lere t a l. 1993) showed tha t the subaer ia l par t of the vo lcan ic cone was bu i l t up dur ing the las t 100 ka . The forma t ion of the emerged par t of the ed ifice can be d iv ided in to seven d iscre te erup t ive phases separa ted by eros iona l depos i ts and / or by co l lapses of ca lderas and flanks (Pasquar` ee t a l .1993) . Defin ing these phases w i l l be impor tan t to in terpre t ing the geophys ica l da ta .

The firs t phase corresponds to Pa leoS trombo l i I be tween∼85

and 64 ka ago . A t the end of Pa leoS trombo l i I , a large ca ldera de-

press ion formed a t the top of the vo lcano . The boundary of th is

ca ldera is deno ted as PST I in F ig . 2 . The second phase corre-

sponds to Pa leoS trombo l i II ended w i th an eros iona l phase . The

th ird phase corresponds to Pa leoS trombo l i III dur ing wh ich a large

summ i t ca ldera deve loped∼34 ka ago (see PST III in F ig . 2) . The

four th phase is re la ted w i th Scar i un i ts end ing w i th s trong phrea to-

magma t ic even ts and the forma t ion of the large Scar i ca ldera∼26

ka ago (Napp ie t a l. 1999 ; G i l lo t & Ke l ler 1993) . These two de-

press ions PST III and Scar i ca lderas were fi l led w i th lava flows of

lower Vancor i un i ts wh ich represen t the fif th phase ended∼13 ka

ago (G i l lo t 1984) . The forma t ion of NeoS trombo l i cra ter occurred

abou t 13 ka ago and cons t i tu tes the beg inn ing of the s ix th phase

(Horn ig-K jarsgaarde t a l .1993) . Neo-S trombo l i per iod is charac-

ter ized by a large amoun t of lava flows , espec ia l ly to the nor thwes t

of the ed ifice . Four onshore paras i t ic cen ters can be iden t ified for the

Neo-S trombo l i per iod inc lud ing ( i) T impone de l Fuoco (Nor th of

F igure 2 .Geo log ica l map of S trombo l i vo lcano (mod ified from Ke l ler e t a l .1993 ; F in izo lae t a l .2006) . The red po in ts ind ica te the pos i t ions of the measuremen ts a long the two profi les : (1) G inos tra-Scar i and (2) G inos tra-Forg ia Vecch ia .

F rom G i l lo t & Ke l ler (1993) ,

F rom G i l lo t (1984) ,

F rom Ke l ler e t a l. (1993) ,

F rom F in izo lae t a l. (2002) .

G inos tra) , ( i i) Va l lonazzo (erup t ive fissure , NE) , ( i i i) Pun ta Labronzo (eccen tr ic cen ter , Nor th) and ( iv) Ne l Cannes tr´ a (erup t ive fissure , NE) (see the ir pos i t ion in F ig . 2) .

The trans i t ion be tween the NeoS trombo l i and the recen t S trom- bo l i cyc le (seven th phase) occurred approx ima te ly 5 ka ago (G i l lo t

& Ke l ler 1993) . Dur ing th is per iod , the eas tern aer ia l flank of S trom- bo l i has been af fec ted by severa l ma jor co l lapses . The o ldes t one loca ted in the Eas tern par t of the is land , Le Sch icc io le (see pos i t ion in F ig . 2) , invo lved the Pa leoS trombo l i un i ts . These even ts have caused the horseshoe-shaped s truc tures of ( i) the Sc iara de l Fuoco (5 ka ago , Horn ig-K jarsgaarde t a l. 1993) and ( i i) the R ina Grande area (F ig . 2) . The la t ter cu t the Vancor i basa l t ic un i ts in the nor thern and sou thern par t of the co l lapse s truc ture and af fec ted par t ia l ly the boundary of NeoS trombo l i cra ter in the upper par t of the co l lapse s truc ture , reach ing w i th the top of the co l lapse s truc ture the P izzo summ i t area .

3 FIELD SURVEY

A large-sca le survey was performed in 2011 January dur ing a 2-week per iod . E lec tr ica l res is t iv i ty measuremen ts were ob ta ined us ing a se t of 64 s ta in less s tee l e lec trodes , a se t of 16 ree ls (four

take-ou t per ree l , one every 40 m) and the ABEM Terrame ter SAS- 4000 impedanceme ter . The con tac t of the e lec trodes w i th the ground was improved by add ing sa l ty wa ter and ben ton i te . For 95 per cen t of the measuremen ts , the con tac t res is tance of the e lec trodes w i th the ground was be tween 0 .5 and 3 kohm . Ge t t ing such low con- tac t res is tances were requ ired to in jec t a 200 mA curren t in to the ground , a va lue necessary to ge t measuremen ts w i th h igh s igna l- to- no ise ra t ios . The dura t ion of the curren t in jec t ion was 1 s w i th 0 .5 s be tween in jec t ions . I t is impor tan t to no te tha t a prev ious m iss ion w i th the same equ ipmen t was performed in 2009 May bu t fa i led due to the very dry so i l cond i t ions . Th is exp la ins why a second m iss ion was p lanned in w in ter (December and January are indeed the two mon ths w i th the h ighes t ra in fa l ls a t S trombo l i vo lcano) .

The firs t profi le (5 km in leng th) s tar ts a t G inos tra , crosses the

P izzo area (a t 918 m a .s . l .) and reaches the San V incenzo v i l lage

(near the Scar i harbor) on the o ther s ide of the is land (F ig . 2) . The

second profi le (3 .7 km in leng th) s tar ts from the v i l lage of G inos tra ,

crosses the Fosse t ta from the Por te l la d i G inos tra to the Por te l la de l le

croc i , and goes through R ina Grande and Le Sch icc io le areas end ing

50 m before the Forg ia Vecch ia shore (F ig . 2) . As the to ta l leng th

of the cab le is 2 .52 km (63 spac ings be tween 64 e lec trodes w i th

40 m spac ing be tween take-ou ts) , ‘ro l l-overs’ of the e lec trodes were

requ ired to rea l ize the des ired profi le leng ths . Profi le 1 cons is ted of two ro l l-a long of e igh t ree ls and Profi le 2 cons is ted of one ro l l-a long of seven ree ls . The use of such a very long res is t iv i ty cab le is no t usua l (see G´ e l ise t a l. 2010 for a d iscuss ion) .

Acqu is i t ions were performed w i th the Wenner array because of i ts good s igna l- to-no ise ra t io . We tr ied the Wenner–Sch lumberger array as we l l and the d ipo le-d ipo le array bu t the resu l ts were less sa t isfac tory than w i th the Wenner array and a lower s igna l- to-no ise ra t io w i th these arrays were requ ir ing a h igher and proh ib i t ive num- ber of s tacks in the fie ld (acqu is i t ion t ime>10 hr) . Th is wou ld have imp l ied a longer dura t ion for the acqu is i t ion desp i te the mu l t ichan- ne l capab i l i ty of the ABEM Terrame ter SAS-4000 res is t iv ime ter . Topograph ic informa t ion was inc luded in the apparen t res is t iv i ty da ta fi les . The topography was ob ta ined from a D ig i ta l E leva t ion Map (DEM , Marse l la e t a l. 2009) w i th a prec is ion of 0 .5 m in e l- eva t ion and theXandYcoord ina tes were de term ined in the fie ld us ing a Garm in GPS w i th a prec is ion of 3 m .

In add i t ion to 2-D DC-res is t iv i ty tomography , we acqu ired se lf- po ten t ia l , so i l CO

concen tra t ion and tempera ture measuremen ts . These measuremen ts were ob ta ined w i th a spac ing of 20 m a long the two profi les .

Se lf-po ten t ia l measuremen ts were performed us ing a pa ir of non-po lar iz ing Cu /CuSO

e lec trodes . The d if ference of e lec tr ica l po ten t ia l be tween the reference e lec trode (arb i trar i ly p laced a t the beg inn ing of the profi les in G inos tra) and the mov ing e lec trode was measured w i th a h igh- impedance vo l tme ter w i th a sens i t iv i ty of 0 .1 mV and a cab le of 300 m . The impedance of the ground was a lways a t leas t 300 t imes be low the in terna l impedance of the vo l tme ter (∼60 Mohm , so<200 kohm) , an impor tan t po in t in assess ing the va l id i ty of the measuremen ts . A t each s ta t ion , a sma l l ho le (∼10 cm deep) was dug to improve the e lec tr ica l con tac t be tween the e lec- trode and the ground . The cho ice of the reference pos i t ion for the who le profi le is arb i trary bu t taken near the sea in the presen t case . The sea is indeed cons idered to be a good e lec tr ica l equ ipo ten t ia l because of the h igh conduc t iv i ty of the sea wa ter (see Corw in &

Hoover 1979) .

In the fie ld , i t is poss ib le to measure the concen tra t ion of CO

in the so i l or i ts flux from the so i l . E t iopee t a l. (1999) demons tra ted tha t a l inear re la t ionsh ip ex is ts be tween ground concen tra t ions and flux concen tra t ion va lues for concen tra t ions in the range 0 .1 to∼

12 per cen t . For h igher va lues , up to 100 per cen t ( l ike a t S trom- bo l i) , the corre la t ion is however expec ted to be poor . On S trombo l i vo lcano , the good corre la t ion be tween CO

concen tra t ion and CO

flux were shown a long the en t ire is land (see F in izo lae t a l. 2006) or in the summ i t (Fossa) area by the compar ison be tween the CO

anoma l ies ev idenced through the flux measuremen t by Carapezza

& Feder ico 2000 , and the so i l concen tra t ion techn ique by F in izo la e t a l. 2003) . A long the two profi les , we dec ided to measure on ly the CO

concen tra t ions . To ge t re l iab le da ta of CO

concen tra t ions , the so i l gas was firs t samp led through a copper tube (2 mm in d iame ter) . Th is copper tube was firs t inser ted in the so i l to a dep th of 0 .5 m . The gas was ana lysed d irec t ly in the fie ld by infrared spec trome try (Ed inburgh Ins trumen ts , mode l GasCheck) . The ana ly t ica l uncer - ta in ty was 5 per cen t of the concen tra t ion va lue .

Tempera ture measuremen ts a t 30 cm dep th were performed w i th therma l probes and a d ig i ta l thermome ter (Comark , mode l KM221) . Read ings were taken to a ten th of degree . Each tempera ture mea- suremen t was taken us ing the fo l low ing procedure : (1) a sma l l ho le was dug to a dep th of 30±1 cm w i th a s tee l rod , 2 cm in d iame ter . (2) Then , we inser ted a therma l probe in to the ho le a t the dep th of 30±1 cm by means of a wooden s t ick . (3) We compac ted the so i l around the pos i t ion of the probe . (4) F ina l ly , a tempera ture read-

ing was taken af ter 10–15 m in . Th is t ime was requ ired in order to ach ieve therma l equ i l ibr ium .

In add i t ion to the da ta acqu ired in 2011 January a long the two pro- fi les cross ing the is land , we a lso used in th is work the se lf-po ten t ia l , so i l tempera ture , CO

concen tra t ion and CO

flux measuremen ts performed in 2006 May in the summ i t area of P izzo . These da ta were co l lec ted a long e igh t para l le l l inear profi les , w i th spac ing be- tween the measuremen t po in ts a long each profi le of 2 .5 m and d is tance be tween the profi les of 20 m . On ly the CO

flux of th is da ta se t were a lready pub l ished before (Carapezzae t a l. 2009) .

4 INVERSION OF THE RESISTIVITY DATA

For each acqu is i t ion , the da ta were inver ted by means of the com- merc ia l package RES2DINV (Loke & Barker 1996) us ing a fin i te e l- emen t gr id for the forward mode l l ing of the vo l tage response to cur - ren t in jec t ion . RES2DINV is based on a fin i te e lemen t (non- l inear) forward opera tor used to compu te the pred ic ted e lec tros ta t ic po ten- t ia l d is tr ibu t iond

for a g iven res is t iv i ty mode l m : d

=K(m) . An es t ima ted mode l can be re tr ieved from the da ta us ing the inverse opera torK

:m

=K

(d

)wherem

is the es t ima ted res is t iv i ty mode l based on the observed e lec tros ta t ic po ten t ia l d is tr ibu t iond

. RES2DINV is based on a Gauss–New ton approach w i th a L

norm da ta m isfi t func t ion . For each acqu ired measuremen t , we perform s tack ing to ge t a s tandard dev ia t ion be t ter than 5 per cen t w i th a max imum of 10 s tacks ( the dura t ion of a typ ica l acqu is i t ion was 3 hr) . Da ta qua l i ty is therefore inc luded in the invers ion process .

Res is t iv i ty tomograms are shown in F igs 3 and 4 a t i tera t ion #5 for wh ich a good convergence has been reached . Indeed , the rms error is 16 per cen t for the profi le G inos tra-Scar i and 15 per cen t for the profi le G inos tra-Forg ia Vecch ia . The h igh rms va lues are due to the no ise in the acqu ired da ta bu t as long as th is no ise is random ly d is tr ibu ted , the invers ion is very robus t to th is no ise (see d iscuss ion and numer ica l tes ts in Rev i le t a l .2008) . The co lour code used for the res is t iv i ty tomograms is the s tandard code used in DC-res is t iv i ty tomography (b lue for low res is t iv i t ies and red for h igh res is t iv i t ies) . The inver ted res is t iv i t ies typ ica l ly range from 15 to 2000–3000 ohm m . A t S trombo l i , a res is t iv i ty of 15 ohm m is typ ica l of the hydro therma l sys tem (Rev i le t a l. 2004b ; F in izo lae t a l. 2006) and va lues in the range 2000–3000 ohm m correspond to basa l t ic lava flows (espec ia l ly those assoc ia ted w i th the Vancor i or o lder Un i ts d iscussed in Sec t ion 2 and shown in F ig . 2) . The in terpre ta t ion of these tomograms is prov ided in F igs 5 and 6 . Over in terpre ta t ion of res is t iv i ty tomograms is easy . To reduce the occurrence of p i tfa l ls , the in terpre ta t ion of these tomograms has been performed carefu l ly us ing the geo log ica l map and the o ther ava i lab le CO

, se lf-po ten t ia l , and tempera ture measuremen ts . In terpre ta t ion of these da ta is d is- cussed in de ta i l in the nex t sec t ion .

5 INTERPRETATION OF THE PROFILES 5 .1 Profi le G inostra-Scar i and P izzo area

The res is t iv i ty tomogram presen ted in F ig . 3 h igh l igh ts a conduc t ive body (res is t iv i ty be low 50 ohm m) in the cen tra l par t of the vo lcano , in tercep t ing the ground surface in the P izzo area . I t is assoc ia ted w i th a therma l anoma ly ( tempera ture>80

C , F ig . 3) and w i th an e leva ted CO

concen tra t ion c lose to sa tura t ion : 100 per cen t of CO

(see ‘H’ in F ig . 3) . In th is area a pos i t ive se lf-po ten t ia l anoma ly

is a lso recorded (F ig . 3) . Such pos i t ive se lf-po ten t ia l anoma l ies are

F igure 3 .Tempera ture (

C) , se lf-po ten t ia l ( in mV) , so i l CO

concen tra t ion ( in ppm) measuremen ts and DC res is t iv i ty tomogram from RES2DINV ( in ohm m)

a long the profi le G inos tra-Scar i . Res is t iv i ty tomogram : ver t ica l sca l ing fac tor 1 .7 , rms error 16 per cen t a t i tera t ion 5 us ing a Gauss–New ton a lgor i thm . No te

the asymme try in bo th the se lf-po ten t ia l profi le and res is t iv i ty tomogram be tween the SW and NE sec t ions ; the ex tens ion of the hydro therma l sys tem is w e l l

de l im i ted to the SW and opened to the NE . The pos i t ive se lf-po ten t ia l anoma l ies a t∼2200 m a long the profi le are assoc ia ted w i th a CO

degass ing s truc ture

and tempera ture anoma ly . No te the two sca les used for the tempera ture da ta in order to show the s ign ifican t fluc tua t ions ( in the range 5–20

C) ou ts ide the area

charac ter ized by the h ighes t tempera tures . A to L represen ts tec ton ic boundar ies d iscussed in the ma in tex t .

F igure 4 .Tempera ture (

C) , se lf-po ten t ia l ( in mV) , so i l CO

concen tra t ion ( in ppm) measuremen ts and DC res is t iv i ty tomogram from RES2DINV ( in ohm

m) a long the profi le G inos tra-Forg ia Vecch ia . Ver t ica l sca l ing fac tor for the res is t iv i ty inver ted sec t ion 1 .7 . RMS error 15 per cen t a t i tera t ion 5 using a

Gauss–New ton a lgor i thm . No te the asymme try in bo th the se lf-po ten t ia l profi le and res is t iv i ty tomogram be tween the SW and E sec t ions . No te tha t the pos i t ive

se lf-po ten t ia l anoma l ies are assoc ia ted w i th CO

degass ing s truc tures . A to G represen ts tec ton ic boundar ies d iscussed in the ma in tex t .

F igure 5 .In terpre ted DC res is t iv i ty tomogram ( in ohm m) a long G inos tra-Forg ia Vecch ia profi le w i th the corre la t ion of s truc tura l boundar ies on the geo log ical

map . Ver t ica l sca l ing fac tor for the res is t iv i ty inver ted sec t ion from RES2DINV : 1 .7 . The fi l led s tar ( ) represen ts the approx ima te source loca t ion of exp los ion

quakes de term ined by Choue te t a l .(2003) . O ther symbo ls : same as F ig . 2 .

F igure 6 .Pos i t ion of the measuremen t s ta t ions for the survey of tempera ture , CO

so i l concen tra t ion and se lf-po ten t ia l measuremen ts in the P izzo area where some hydro therma l depos i ts are observed . The measuremen ts are performed a long e igh t profi les . The spac ing be tween the measuremen t po in ts a long each profi le is 2 .5 m . The d is tance be tween the profi les is 20 m . Or thopho to and D ig i ta l E leva t ion Mode l (DEM) are cour tesy of M . Marse l la (Marse l lae t a l. 2009) . usua l ly assoc ia ted w i th the upward flow of therma l flu ids (Corw in &

Hoover 1979 ; R ichardse t a l. 2010) and poss ib ly two-phase ( l iqu id wa ter and s team) flow (Byrd inae t a l. 2009) .

A h igh-reso lu t ion survey ( tempera ture , CO

concen tra t ion , CO

flux and se lf-po ten t ia l measuremen ts) of the P izzo area is shown in F igs 6–9 . Our da ta se t shows tha t degass ing and tempera ture anoma- l ies are confined a long a h igher permeab i l i ty pa thway w i th an arched shape , loca l ized be tween ‘P izzo’ and ‘he l icop ter pad’ , in the con t i- nu i ty of the nor thern s truc tura l boundary of the R ina Grande sec tor co l lapse (F igs 2 , 6–9) . Moreover , desp i te the fac t tha t tempera ture measuremen ts imp l ies co ld areas on bo th s ides of the R ina Grande s truc tura l boundary (F ig . 8) , the CO

degass ing d isp lay a c lear d if- ference be tween bo th s ides of th is s truc tura l boundary . Carapezza e t a l .(2009) defined in th is area five CO

flux popu la t ions based on probab i l i ty p lo t techn ique (S inc la ir 1974 ; see F ig . 9a) . Ins ide the co l lapse s truc ture of R ina Grande , the popu la t ion correspond ing to the lowes t CO

flux (<6gm

d) is loca ted in the same area of the CO

concen tra t ion be low a tmospher ic leve l (350 ppm , so be- low−1 .5 in logar i thm ic sca le , see dark b lue co lour area in F ig . 7) . Th is means tha t some sea ls ex is t a t dep th . These sea ls may be in- fluenced by the g l id ing p lane of R ina Grande sec tor co l lapse . Th is

impor tance of the R ina Grande sec tor co l lapse in dr iv ing ho t hy- dro therma l flu ids toward the summ i t par t of the vo lcano is shown in the h igh-reso lu t ion survey (F igs 6–8) . No te tha t the G inos tra-Scar i profi le does no t cross the R ina Grande s truc tura l boundary bu t jus t brush aga ins t the head of th is co l lapse s truc ture . I t is in teres t ing to no te tha t a long the G inos tra-Scar i profi le , the max imum peak of CO

concen tra t ion (see pos i t ion ‘H’ in F ig . 3) is no t loca ted on the P izzo cra ter boundary bu t c lear ly 40 m in the Eas t d irec t ion a long the boundary of R ina Grande sec tor co l lapse (see pos i t ion ‘H’ in F ig . 9) . These charac ter is t ics regard ing the loca t ion of the degass ing anoma l ies imp ly tha t R ina Grande has a s trong influence in bo th sea l ing and dr iv ing the magma t ic and hydro therma l flu ids toward the summ i t area (P izzo area) .

Cons ider ing the G inos tra-Scar i profi le , we in terpre t the cen tra l conduc t ive body shown in F ig . 3 as the ma in hydro therma l sys tem of the vo lcan ic ed ifice . The low res is t iv i ty of the vo lcan ic rocks can be due to the a l tera t ion of these ma ter ia ls (resu l t ing in c lay m inera ls and some zeo l i tes w i th h igh surface conduc t iv i t ies , see Waxman &

Sm i ts 1968 ; Rev i l & G lover 1998 ; Rev i l & Leroy 2001) , temper -

a ture , sa l in i ty , poros i t ies or a comb ina t ion of these fac tors (Rev i l

1999 ; Rev i le t a l. 2002) . A very good examp le of the in terpre ta t ion

F igure 7 .So i l CO

concen tra t ion map ( logar i thm ic sca le in per cen t) . No te tha t the h ighes t concen tra t ions are loca l ized a long the cres t P izzo to He l icop ter pad , correspond ing to the head of the g l id ing p lane of the R ina Grande sec tor co l lapse . The va lue−1 .5 per cen t in logar i thm ic sca le (equ iva len t to∼350 ppm) corresponds to the a tmospher ic concen tra t ion of CO

.

of res is t iv i ty measuremen ts of a hydro therma l sys tem is g iven by Komor ie t a l. (2010) who were ab le to separa te pore wa ter and sur - face conduc t iv i ty us ing downho le and core measuremen ts . In the presen t case , we do no t have access to such informa t ion . However , the use of a mu l t id isc ip l inary approach as ou t l ined above po in ts ou t c lear ly tha t the conduc t ive body is re la ted to an impor tan t source of ho t flu ids r is ing and spread ing a long the ma jor s truc tura l bound- ar ies of the ed ifice . In some areas , l ike in the Fosse t ta area and in the upper par t of the R ina Grande sec tor co l lapse area , the m igra t ion of the hydro therma l flu ids are s topped by impermeab le layers . We w i l l d iscuss the or ig in of these layers be low .

A long the G inos tra-Scar i profi le , the res is t ive body above the v i l lage of G inos tra (sou th wes t par t of the ed ifice) is separa ted from the conduc t ive hydro therma l sys tem by the NeoS trombo l i cra ter fau l t (‘B’ in F igs 3 and 10) . Th is res is t ive body corresponds to the Vancor i and o lder (Pa leoS trombo l i) Un i ts , wh ich cons is ts of mass ive basa l t ic lava flows . I t is no t surpr is ing tha t these un i ts hav- ing very low-poros i ty (bu t heav i ly frac tured in some areas) and no t a l tered , have h igh res is t iv i t ies (>1000 ohm m) . The Vancor i and Pa- leoS trombo l i Un i ts are no t assoc ia ted w i th a se lf-po ten t ia l anoma ly

( imp ly ing e i ther no aqu ifer or a fla t , s t i l l , aqu ifer loca ted a t a dep th be low the dep th of exp lora t ion of our e lec tr ica l res is t iv i ty tomogra- phy survey) and a norma l so i l CO

concen tra t ion accoun t ing for the presence of vege ta t ion (F ig . 3 and 10) . For the case when an aqu ifer is presen t , the wa ter may be channe l led through frac tures d irec t ly from the ground surface to th is aqu ifer . In th is area , a sma l l CO

anoma ly (anoma ly ‘A’ in F igs 3 and 10) is assoc ia ted w i th the ax is of a deep conduc t ive body , wh ich is i tse lf assoc ia ted w i th a break in the topography . Th is CO

anoma ly cou ld represen t the s igna ture of a permeab le pa thway assoc ia ted w i th a ma jor s truc tura l boundary , name ly the Pa leoS trombo l i III ca ldera .

On the o ther s ide of the vo lcano (NE s lope) , we observe a sha l low res is t ive body w i th a th ickness of 50–120 m (F ig . 3) . Th is res is t ive body is assoc ia ted w i th the Vancor i forma t ion , wh ich is heav i ly frac tured on th is s ide of the vo lcano . Four so i l CO

concen tra t ion anoma l ies>10 000 ppm ( these anoma l ies are deno ted as ‘H’ , ‘I’ ,

‘J’ and ‘K’ in F ig . 3) , are poss ib ly re la ted to four ma jor s truc tura l boundar ies cross ing the Vancor i Un i t . The ‘H’ anoma ly is assoc ia ted w i th the top of the R ina Grande sec tor co l lapse . The anoma ly ‘I’

corresponds to the NeoS trombo l i cra ter . The anoma ly ‘J’ is poss ib ly

F igure 8 .Tempera ture map a t a dep th of 30 cm . No te tha t the tempera ture anoma ly is loca l ized a long the cres t P izzo to He l icop ter pad , correspond ing to the head of the g l id ing p lane of the R ina Grande sec tor co l lapse .

assoc ia ted w i th the con t inu i ty of the Pa leoS trombo l i III Ca ldera . F ina l ly , the anoma ly ‘K’ corresponds to the reg iona l N41

fau l t , assoc ia ted i tse lf w i th a sma l l , bu t s ign ifican t , increase in tempera ture (see F ig . 10) . The conduc t ive area observed in the cen tra l par t of the ed ifice (see d iscuss ion above) seems to expand be low the Vancor i forma t ion towards the sea . Th is may imp ly tha t the hydro therma l sys tem ex tends to the sea on th is s ide of the vo lcano . Such a m ix ing be tween hydro therma l flu ids , fresh , and sea wa ters is shown in the area jus t above the v i l lage of Scar i where the tempera ture of some we l ls reaches va lues be tween 40 and 44

C . The se lf-po ten t ia l s igna ls show a nega t ive trend w i th the e leva t ion . Th is type of trend is c lass ica l ly re la ted to the ex is tence of an unconfined aqu ifer (e .g . Rev i le t a l. 2004a ; R ichardse t a l. 2010 and references there in) . On the NE lower flank of the ed ifice , the se lf-po ten t ia l grad ien t versus e leva t ion rema ins cons tan t (–0 .22 mV m

, see F ig . 3) . Th is trend ex tends from a break in the se lf-po ten t ia l da ta (see anoma ly ‘L’) to the coas t . Th is break in the s lope ‘L’ is loca ted on the s truc tura l boundary of the o ld Pa leoS trombo l i I Ca ldera . Th is means tha t ou ts ide the Pa leoS trombo l i I Ca ldera , an unconfined aqu ifer ex tends from abou t 400 m a .s . l . down to the sea w i th a la tera l ex tend of abou t

2 km . An a t temp t to eva lua te the dep th of th is aqu ifer is presen ted in Sec t ion 6 .

A compar ison be tween the e lec tr ic res is t iv i ty tomography of

G inos tra-Scar i and the profi le performed in 2004 (w i th a take-ou t

every 20 m ins tead of 40 m , see F in izo lae t a l. 2006) d isp lay nu-

merous s im i lar i t ies inc lud ing , for examp le , the deep influence on

flu id flow of the SW boundary of the NeoS trombo l i Cra ter . There

are a lso severa l in tr igu ing d if ferences bo th in e lec tr ic res is t iv i ty

tomography and in the se lf-po ten t ia l and so i l CO

concen tra t ion

da ta : (1) For ins tance , the NE boundary of NeoS trombo l i cra ter ,

l im i t ing the la tera l ex tens ion of the hydro therma l sys tem (see fig . 2

in F in izo lae t a l. 2006) , does no t seem to p lay the same ro le in our

survey (F ig . 6) . (2) In the same sec tor ( the upper NE flank of the

ed ifice) , the three peaks of CO

concen tra t ion , up to 10 000 ppm

(see anoma l ies ‘I’ , ‘J’ and ‘K’ in F ig . 4) do no t have any equ iva-

len t in the profi le performed in 2004 (see fig . 2 in F in izo lae t a l .

2006) . These d if ferences can be eas i ly exp la ined due to the fac t

tha t the profi le loca t ion be tween the P izzo area and Pa leoS trombo l i

I ca ldera boundary is no t the same in these two surveys . In 2004 ,

the profi le was loca ted much c loser to the nor thern R ina Grande

F igure 9 .Compar ison be tween the CO

flux map d iscussed in Carapezzae t a l .(2009) and the se lf-po ten t ia l of the R ina Grande and P izzo area ( th is work) . The h ighes t CO

flux va lues in the R ina Grande area are loca ted a long two curv i l inear s truc tures , in the nor thern and sou thern par t of R ina Grande and correspond probab ly to a o ld g l id ing p lane . A th ird h igh degass ing area is loca ted a long the N64

fau l t (anoma ly ‘E’ descr ibed in F ig . 4) . A huge se lf-po ten t ia l m in imum (mauve co lour) is loca ted in the m idd le of R ina Grande be tween these three degass ing s truc tures and the fau l ts N41

and N64

.

Sec tor co l lapse boundary than the profi le carr ied ou t in 2011 . I t seems tha t , in the v ic in i ty of the s truc tura l boundary of R ina Grande sec tor co l lapse , the influence of frac tura t ion and fau l t ing is sma l ler than severa l hundred me ters in the nor th d irec t ion . Th is in teres t ing resu l t imp l ies an inheren t comp lex i ty in the organ iza t ion of the ge- o logy of such s tra tovo lcano ed ifices in term of permeab i l i ty change a long the same s truc tura l boundary a t the sca le of on ly severa l hundred me ters .

5 .2 Profi le G inostra-Forg ia Vecch ia and R ina Grande area The res is t iv i ty tomogram of Profi le G inos tra-Forg ia Vecch ia is shown in F ig . 4 w i th the se lf-po ten t ia l , so i l CO

concen tra t ions and tempera ture measuremen ts . Profi le G inos tra-Forg ia Vecch ia is s im i lar to the resu l ts of Profi le G inos tra-Scar i in the SW par t of the ed ifice . So i l CO

concen tra t ion anoma l ies ‘A’ and ‘B’ may be

assoc ia ted w i th the Pa leoS trombo l i III ca ldera and NeoS trombo l i cra ter fau l ts , respec t ive ly (F ig . 5) . Th is profi le shows a lso tha t the hydro therma l sys tem observed in the cen tra l par t of the ed ifice ex- tends towards the Eas tern par t of the vo lcan ic ed ifice fi l l ing more than two- th irds of the R ina Grande area . Indeed a conduc t ive body (res is t iv i ty<50 ohm m) can be c lear ly observed be low the R ina Grande area where four s trong so i l CO

concen tra t ions are observed (see anoma l ies ‘C’ , ‘D’ , ‘E’ and ‘F’ in F ig . 4) . A l l these anoma l ies are assoc ia ted w i th d is t inc t s truc tura l boundar ies w i th one excep- t ion . The anoma ly ‘C’ is assoc ia ted w i th the NeoS trombo l i cra ter boundary and the R ina Grande sec tor co l lapse . The anoma ly ‘D’

is assoc ia ted w i th the N41

reg iona l fau l t . The anoma ly ‘E’ corre-

sponds to the N64

fau l t , wh ich is a lso assoc ia ted w i th CO

con-

cen tra t ions reach ing va lues c lose to fu l l sa tura t ion (F ig . 4) . F ina l ly ,

the anoma ly ‘F’ , wh ich l im i ts sharp ly the la tera l ex tens ion of the

hydro therma l sys tem toward the Eas t , is no t assoc ia ted to a known

F igure 10 .In terpre ted DC res is t iv i ty tomogram ( in ohm m) a long G inos tra-Scar i profi le w i th the corre la t ion of s truc tura l boundar ies on the geo log ica l map .

Ver t ica l sca l ing fac tor for the res is t iv i ty inver ted sec t ion from RES2DINV : 1 .7 . The fi l led s tar ( ) represen ts the approx ima te source loca t ion of exp los ion

quakes de term ined by Choue te t a l .(2003) . O ther symbo ls : same as F ig . 2 .

F igure 11 .In terpre ta t ion of the two profi les in terms of flu id flow pa thways . The low so i l CO

concen tra t ion in the Fosse t ta imp l ies the ex is tence of a sea l confin ing the hydro therma l sys tem excep t a t the pos i t ions of ma jor fau l ts of h igh permeab i l i ty pa thway for CO

r is ing sys tems : the fau l t border ing the R ina Grande sec tor co l lapse , the fau l t of the g l id ing p lane l im i t ing the la tera l ex tens ion of the hydro therma l sys tem toward the Eas t in the lower par t of R ina Grande area , and the reg iona l fau l t N41

and N64

.(

) The fi l led-s tar represen ts the approx ima te source loca t ion of exp los ion quakes de term ined by Choue te t a l . (2003) . No te the re la t ive ly h igh ra in ra te of abou t 600 mm yr

.

s truc tura l boundary . Never the less , the e lec tr ica l res is t iv i ty tomog- raphy (F ig . 5) s trong ly sugges ts tha t the boundar ies ‘F’ and ‘G’

corresponds to two over lapped co l lapse s truc tures or ien ted towards the eas t . The anoma ly ‘G’ i tse lf is assoc ia ted w i th the ‘Le Sch icc i- o le’ co l lapse area (Pasquar` ee t a l. 1993) .

Maps of CO

flux measuremen ts (Carapezzae t a l .2009) and se lf- po ten t ia l measuremen ts ( th is work) are shown in F ig . 9 . Ins ide the R ina Grande area , the CO

flux map d isp lays three ma in degass ing areas . Two of these areas , one on the nor th and one on the sou th , show a curv i l inear or ien ta t ion para l le l to the a lready known inner co l lapse of R ina Grande . These areas are re la ted to ano ther sma l ler co l lapse s truc ture over lapped ins ide the two o ther co l lapse s truc tures of R ina Grande (see dashed l ines in F ig . 9) . The th ird degass ing area is loca ted be tween these two co l lapse boundar ies and encompasses the pos i t ive ‘E’ anoma ly of CO

concen tra t ion , tempera ture and se lf- po ten t ia l (see anoma ly ‘E’ in F igs 4 and 9) . Th is anoma ly is c lear ly e longa ted a long a NE–SW ax is correspond ing to the N64

Fau l t . The se lf-po ten t ia l map d isp lays a s trong nega t ive se lf-po ten t ia l anoma ly

in an area of approx ima te ly 200 m×200 m (see the mauve co lour in F ig . 9b) . Th is nega t ive anoma ly is loca ted be tween the three ma in degass ing s truc tures and a secondary m in imum (see the b lue co lour in F ig . 9b) cu t t ing the nor thern degass ing s truc ture . Th is se lf- po ten t ia l m in imum area is bordered by sharp se lf-po ten t ia l grad ien ts in agreemen t w i th the pos i t ion of the N41

and N64

Fau l ts . Th is area of nega t ive se lf-po ten t ia l anoma ly cou ld be in terpre ted as the resu l t of downward wa ter infi l tra t ion dragged a long the sha l lowes t co l lapse(s) g l id ing p lane(s) of the R ina Grande area . Accord ing to th is hypo thes is , the N41

fau l t cou ld have p layed a ro le in the h is tor ica l co l lapse of R ina Grande .

6 DISCUSSION OF THE GENERAL FLO W PATTERN

The genera l flow pa t tern for the two profi les is summar ized in

F ig . 11 . Accord ing to A izawae t a l. (2005) (see a lso F i t termane t a l.

F igure 12 .Cross-sec t ion of 3-D magne t ic imag ing of S trombo l i vo lcano and i ts v ic in i ty (mod ified from Okumae t a l. 2009) . Con tour in terva l is 0 .25 A m

. Ver t ica l exaggera t ion is 1 .25 . The area w i th low magne t iza t ion (<2 .50 A m

) corresponds to the hydro therma l sys tem (F igs 3 and 11a) wh i le areas w i th h igh magne t iza t ion (>2 .50 A m

) correspond to less a l tered lava flows .

1988) , the conduc t ive zone in a vo lcan ic ed ifice is cons idered to correspond to the zone of coex is t ing c lay (resu l t ing from a l tera t ion) and ho t flu ids w i th tempera tures be low 200

C , bo th assoc ia ted w i th the hydro therma l sys tem . Fo l low ing th is idea , the conduc t ive body observed in the cen tra l par t of the ed ifice is in terpre ted as the ma in hydro therma l sys tem . Th is in terpre ta t ion is cons is ten t w i th the vo lume of low magne t iza t ion observed by Okumae t a l. (2009) in the cen tra l par t of S trombo l i (F ig . 12) .

The range of res is t iv i ty va lues for the hydro therma l sys tem (15–50 ohm m) is cons is ten t w i th the res is t iv i ty cons idered by A izawae t a l. (2005) for Japanese s tra tovo lcanoes . The hea t and CO

degass ing source are assoc ia ted w i th the ex is tence of a sha l- low s torage of magma be low the ac t ive ven ts . Indeed , Choue te t a l.

(2003) showed tha t the source of exp los ion quakes and tremor are assoc ia ted w i th the summ i t par t of the sha l lower feed ing sys tem and are loca ted a t dep ths of approx ima te ly 220 m benea th and 160 m nor thwes t of the ac t ive ven ts (see the pos i t ion of the s tar in F ig . 11) . On the Wes tern s ide of the ed ifice , the hydro therma l sys tem is confined by the fau l t border ing the NeoS trombo l i cra ter . The ev i- dence of a tmospher ic leve ls of so i l CO

concen tra t ions (360 ppm) in the Fosse t ta imp l ies the ex is tence of a sea l capp ing the hydro ther - ma l sys tem . S im i lar conc lus ions have been reached by A izawae t a l.

(2009 , the ir fig . 5) for some Japanese s tra tovo lcanoes . Accord ing to F in izo lae t a l .(2002) , Rev i le t a l. (2004b) and A izawae t a l. (2009) , the upper layer of the hydro therma l sys tem is large ly sea led by c lay-r ich ma ter ia ls . A s tr ik ing examp le of such a sea l is d iscussed by Rev i le t a l. (2004b) for the summ i t cra ter of S trombo l i . The permeab i l i ty of these sea ls is so low tha t CO

is preven ted to cross such a boundary . The occurrence of sea ls in hydro therma l sys tems is common (e .g . , Facca & Tonan i 1967 ; Ingebr i tsen & Sorey 1988 ; Lowe l le t a l. 1993 and references there in) . The infi l tra t ion of me- teor ic wa ter can flow on the upper s ide of such a sea l to form a perched aqu ifer (F ig . 11 , F in izo lae t a l .2002 and Rev i le t a l. 2004b , for an examp le) .

The sea l ing of hydro therma l sys tems is a lso suppor ted by the fac t tha t surface geo therma l ac t iv i ty and gas d ischarge is genera l ly

h igh ly loca l ized where fau l ts are loca ted (see the examp les g iven by F in izo lae t a l .2002 , 2006 ; Rev i le t a l. 2004b) . In our case , on ly the fau l t border ing the head of the R ina Grande sec tor co l lapse near the P izzo area exh ib i ts a s trong near surface (30 cm deep) tempera ture anoma ly (>80

C , see F igs 3 and 8) . H igh so i l CO

concen tra t ions are usua l ly ind ica t ive of the presence of open fau l ts , frac tures and l im i ts of co l lapse zones .

The adven t ive hydro therma l c ircu la t ion on the NE s ide of the vo lcan ic cone is cons is ten t w i th the observa t ions of F in izo lae t a l . (2010) and we l ls a t Scar i and San V incenzo record ing a ground wa ter tempera ture of abou t 40

C . Regard ing the downward ex- tens ion of the hydro therma l sys tem , we can use the pre l im inary invers ion of aeromagne t ic da ta by Okumae t a l .(2009) (F ig . 12) . The profi le shown in F ig . 12 is charac ter ized by the lowes t mag- ne t iza t ion in the cen tra l par t of the ed ifice correspond ing to the hydro therma l sys tem and h igher magne t iza t ion a long the flanks correspond ing to lava flows . Th is imp l ies a much deeper roo t for the ma in hydro therma l sys tem .

Adven t ive hydro therma l c ircu la t ion seems a lso to ex is t be low the R ina Grande area . Because of the mechan ica l weaken ing of the rocks by long-s tand ing therma l a l tera t ion , such an observa t ion may exp la in the var ious co l lapses observed in th is area (F igs 2 and 9) . Th is a l tera t ion cou ld represen t a r isk of fu ture co l lapses . Th is area shou ld be therefore be t ter imaged in 3-D and mon i tored con t inuous ly in terms of deforma t ion .

F ina l ly , we inves t iga te the poss ib le pos i t ion of the aqu ifer above the v i l lages of Scar i and San V incenzo by in terpre t ing sem i- quan t i ta t ive ly the se lf-po ten t ia l in the NE par t of the vo lcano (end par t of Profi le G inos tra-Scar i) . We firs t need to de term ine a reason- ab le va lue for the s tream ing po ten t ia l coup l ing coeffic ien t , wh ich represen ts the sens i t iv i ty be tween the e lec tr ica l po ten t ia l d if ference produced in response to a pore flu id pressure grad ien t . We measure th is coup l ing coeffic ien t for a se t of seven samp les taken from the R ina Grande area because th is area is l ike ly to represen t an ou t- crop of the sec t ion above Scar i a long wh ich we expec t to find the aqu ifer . These samp les were firs t crushed , washed to remove organ ic ma t ter , and s ieved to ob ta in a gra in s ize compr ises be tween 100 and 200μm . The samp les were sa tura ted w i th NaC l e lec tro ly tes a t d if- feren t ion ic s treng ths dur ing severa l days . In order to be cer ta in tha t equ i l ibr ium is reached , the conduc t iv i ty and the pH of the so- lu t ion were measured over t ime . The me thodo logy used to perform these measuremen ts is descr ibed in Rev i le t a l. (2004b) w i th two non-po lar iz ing Ag /AgC l e lec trodes loca ted a t the end faces of the samp le . The samp le is enc losed in a g lass ho lder a l low ing flu id flow on ly through the two end faces . We measure the e lec tr ica l po ten t ia l d if ference be tween the two end-faces of the samp le subm i t ted to a known pore flu id pressure d if ference in dra ined cond i t ions . The resu l ts (F ig . 13) show tha t the coup l ing coeffic ien t depends s trong ly on the conduc t iv i ty of the pore wa ter as pred ic ted by the e lec trok i- ne t ic theory . The ground wa ter a t Le Sch icc io le seasona l spr ing , wh ich is loca ted in be tween the R ina Grande and Le Sch icc io le ar - eas , was samp led . I ts e lec tr ica l conduc t iv i ty is 0 .128±0 .001 S m

(a t 25

C) . In teres t ing ly , Rev i le t a l. (2004b) p laced dem inera l ized pore wa ter in con tac t w i th vo lcan ic ashes co l lec ted ins ide the P izzo cra ter area . The e lec tr ica l conduc t iv i ty was mon i tored over severa l days ind ica t ing tha t i t reached equ i l ibr ium in less than 2 d . They ob ta ined a wa ter conduc t iv i ty va lue in equ i l ibr ium w i th the vo lcan ic ash of 0 .1 S m

(a t 25

C) in agreemen t w i th the prev ious va lue .

The s tream ing po ten t ia l coup l ing coeffic ien t is be tween

−0 .6 mV m

and−2 .5± 1 .0 mV m

w i th a l ike ly va lue of

−1 .2 mV m

. If the se lf-po ten t ia l trend observed in the NE par t

of the vo lcano is re la ted to the presence of an unconfined aqu ifer ,

F igure 13 .Measuremen ts of the s tream ing po ten t ia l coup l ing coeffic ien t . (a) . Labora tory measuremen ts of the s tream ing po ten t ia l coup l ing coeffi- c ien t . The s tream ing po ten t ia l coup l ing coeffic ien t is g iven as a s lope of the recorded e lec tr ic po ten t ia l versus the flu id pressure d if feren t ia ls im- posed be tween the two end-faces of a cy l indr ica l core samp le . (b) Measured s tream ing po ten t ia l coup l ing coeffic ien tCversus the flu id conduc t iv i ty . The pH va lues represen t the pH of so lu t ion in equ i l ibr ium w i th the core samp le and the a tmosphere . The spr ing wa ter conduc t iv i ty has been measured a t the (seasona l) spr ing of Le Sch icc io le . Compar ison w i th the da ta from Rev i l e t a l .(2004b) us ing scor ia from S trombo l i . The grey area corresponds to the expec ted l inear trend in a log– log p lo t .

we can rough ly es t ima te the e leva t ion of the wa ter tab le as fo l low . We cons ider the reference a t the sea leve l (0 mV) . A t the boundary of the aqu ifer defined w i th a cons tan t se lf-po ten t ia l /e leva t ion ra t io (see anoma ly ‘L’ in F ig . 3) , the se lf-po ten t ia l s igna l is−110 mV in the case of profi le G inos tra-Scar i . There are severa l mode ls tha t l ink se lf-po ten t ia l s igna ls to the dep th (or the e leva t ion) of the wa ter tab le (see Jackson & Kauah ikaua 1987 ; Auber t & A tangana 1996 for ear ly inves t iga t ions and A izawae t a l. 2009 ; On izawae t a l. 2009 ; Ba l le t a l. 2010 , for more recen t s tud ies) . Tak ing the above va lue for the s tream ing po ten t ia l coup l ing coeffic ien t and assum ing tha t the se lf-po ten t ia l response is con tro l led by the hydrau l ic head y ie lds a hydrau l ic head of 100±60 m a t the hydro therma l /aqu ifer con tac t . Th is es t ima te and the fac t tha t the e leva t ion of the aqu ifer is nu l l a t the shore l ine are used to ske tch the pos i t ion of the aqu ifer in F ig . 11 .

Keep ing a d is tance of abou t 1 .5 km be tween the sea shore and the boundary be tween the aqu ifer and the hydro therma l sys tem (cor - respond ing to the pos i t ion ‘L’ in fig . 3) , th is y ie lds a hydrau l ic head grad ien t of 0 .05 for the unconfined aqu ifer . Accord ing to the Ghyben-Herzberg formu la (based on isos ta t ic equ i l ibr ium , see Domen ico & Schwar tz 1990 ; M ichae le t a l. 2005) in an homoge- neous is land , the in terface be tween the fresh wa ter and the sea wa ter

occurs a t a dep th be low sea leve l tha t is 40 t imes the he igh t of the wa ter tab le (above sea leve l) . The dep th of the sea wa ter in trus ion cou ld be as low as 3 km b .s . l . be low the boundary be tween th is aqu ifer and the hydro therma l sys tem , so very deep ins ide the vo l- cano . We p lan to check th is assump t ion by us ing MT measuremen ts in a fu ture work .

On the SW par t of the is land , the−50 mV anoma ly cou ld im- p ly a re la t ive ly fla t aqu ifer w i th a wa ter tab le e leva t ion of 40 m a .s . l . assum ing tha t the s tream ing coup l ing coeffic ien t is equa l to

−1 .2 mV m

. Th is assump t ion is usua l ly va l id as long as the ef fec t of surface conduc t iv i ty assoc ia ted w i th c lays and zeo l i tes is neg- l ig ib le (Rev i le t a l. 2003 , the ir fig . 3) . In turn , th is wou ld imp ly a dep th of the sea wa ter in trus ion deeper than 1 .5 km accord ing to the Ghyben-Herzberg re la t ionsh ip .

7 CONCLUDING STATE MENTS

Th is s tudy a l lowed us to cons tra in the hydrogeo logy of the S trombo l i vo lcan ic is land in the Aeo l ian Arch ipe lago in I ta ly . The fo l low ing conc lus ions have been reached :

1 . A hydro therma l sys tem (res is t iv i ty in the range 15–50 ohm m) is loca ted in the cen tra l par t of the vo lcan ic ed ifice and ex tend ing in bo th the NE and E d irec t ions . In the NE d irec t ion i t exp la ins why warm wa ter is found a t the v i l lages of Scar i and San V incenzo ( tempera ture in the range 40–44

C) . In the R ina Grande co l lapse area (E d irec t ion) , the ex is tence of the hydro therma l sys tem exp la ins the h igh degass ing ra te of th is zone a lso crossed by two ma jor fau l ts (N41

and N64

) . The ex is tence of a hydro therma l body is cons is ten t w i th low magne t iza t ions (<2 .5 A m

) in these areas . Our survey po in ts ou t tha t the R ina Grande sec tor co l lapse is one of the mos t impor tan t s truc tura l con tro l for magma t ic and hydro therma l flu ids in the upper par t of S trombo l i vo lcan ic edfice . Th is area , formed of three horse shoe-shape over lapp ing s truc tures opened toward the eas t , is respons ib le of the two ma in d if fuse degass ing areas in the upper par t of the ed ifice . (a) The firs t degass ing area is assoc ia ted w i th the top of the larges t horse shoe-shape s truc ture dragg ing magma t ic flu ids toward the summ i t area (more prec ise ly toward the P izzo area ; see anoma ly ‘H’ in bo th F igs 3 and 9) . The second degass ing area is assoc ia ted w i th the sma l les t horse shoe-shape s truc ture , wh ich drags a lso hydro therma l and magma t ic flu ids a long i ts sou thern border . In add i t ion , the R ina Grande sec tor co l lapse is charac ter ized by the sha l low dep th of the hydro therma l sys tem of S trombo l i . In the lower Eas tern par t of the R ina Grande area , the la tera l ex tens ion of the hydro therma l sys tem is cons tra ined by ano ther boundary of sec tor co l lapse , loca ted less than 200 m in d is tance above the Le Sch icc io le sec tor co l lapse de l im i ta t ing the Forg ia Vecch ia area .

2 . There is no ev idence for a sha l low hydro therma l sys tem in the SW par t of the ed ifice above the v i l lage of G inos tra . The res is t iv i ty mode ls show a res is t ive body ( in the range 1000–3000 ohm m) tha t cou ld be assoc ia ted w i th the Vancor i and Pa leoS trombo l i un i ts . Th is assump t ion is confirmed w i th a h igher magne t iza t ion of th is area (>2 .5 A m

) .

3 . The se lf-po ten t ia l da ta show the presence of an unconfined aqu ifer above the v i l lage of Scar i . A s imp le order of magn i tude es t ima te from the se lf-po ten t ia l da ta leads to a s lope of 0 .05 for th is aqu ifer (50 m of head per k i lome tre) .

The informa t ion d iscussed above shou ld be comb ined w i th ad-

d i t iona l informa t ion (espec ia l ly magne t ic and grav i ty) to perform

a 3-D jo in t invers ion of these geophys ica l da ta . The use of the re-

s is t iv i ty informa t ion wou ld a l low reduc ing the non-un iqueness of

the inverse prob lem in inver t ing the magne t ic da ta a lone . 3-D re- s is t iv i ty tomography of Vu lcano has been performed recen t ly by Rev i le t a l .(2010) . Such type of work cou ld be performed a t S trom- bo l i as we l l . Numer ica l mode ls s imu la t ing vo lcan ic hydro therma l sys tems (see Ingebr i tsene t a l. 2010) requ ire accura te and de ta i led geophys ica l inves t iga t ions . We env is ion tha t our s tudy can be the bas is for such a work in tegra t ing geo log ica l , geophys ica l , and hy- drogeo log ica l informa t ion in an accura te numer ica l inves t iga t ion of the hydro therma l sys tem of S trombo l i . Such mode l l ing cou ld be usefu l to mon i tor th is vo lcano .

ACKNO WLEDG MENTS

The INSU-CNRS , the Labora to ire G´ eoSc iences R´ eun ion-IPGP , the Is t i tu to Naz iona le d i Geofis ica e Vu lcano log ia (INGV) and the DOE (Energy Effic iency and Renewab le Energy , Geo therma l Techno lo- g ies Program , award DE-FG36–08GO018195) are thanked for fund- ing . We thank the personne l of the Na t iona l C iv i l Pro tec t ion and Mar io Za ia (Zaz` a) for the ir con t inuous suppor t dur ing the fie ld survey . Th is is the IPGP con tr ibu t ion number 3148 . We thank Jean- F ranc ¸o is L´ ena t for the use of the res is t iv i ty equ ipmen t (co-funded by the Labora to ire G´ eoSc iences R´ eun ion of Reun ion Is land , the Lab- ora to ire Magmas e t Vo lcans of C lermon t-Ferrand , and the INSU- CNRS) , G ladys Le Masson , Cas i Lock , Xav ier Rass ion , Barbara Susk i , Agn` es Crespy and N ico le Wo lyn iec for the ir he lp dur ing the 2011 January fie ld campa ign for the two geoe lec tr ica l pro- fi les cross ing the is land . Em i l ie Rou l leau and Ju l ie Mor in are a lso thanks for the ir he lp in 2006 May for the fie ldwork performed in the summ i t area of P izzo . AF thanks Lauren t M ichon for fru i tfu l d iscuss ions . We thank the Ed i tor , Joach im Wassermann , Theodore H . Asch and one anonymous referee for the cons truc t ive rev iews of our manuscr ip t .

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