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Geothermal study of reunion island: Audiomagnetotelluric survey
Journal o f Volcanology and Geothermal Research, 20 (1984) 311--332 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
311
GEOTHERMAL STUDY OF REUNION ISLAND: AUDIOMAGNETOTELLURIC SURVEY
Y. BENDERITTER ~,* and A. GERARD 2 1Centre National de la Recherche Scientifique, Centre de Recherches Gdophysiques, Garchy, 58150 PouiUy sur Loire (France) 2Institut Mixte de Recherche Gdothermique, Bureau de Recherches Gdologiques et Minidre, B.P. 6009, 45018 Orleans Cedex (France) (Received April 6, 1982; revised and accepted June 16, 1983)
ABSTRACT Benderitter, Y. and G~rard, A., 1984. Geothermal study of R~union Island: audiomagnetotelluric survey. J. Volcanol. Geotherm. Res., 20: 311--332. In 1979 and 1980, 535 magnetotelluric soundings using a frequency range of 1700 Hz--8 Hz were performed on the island of R~union for geothermal exploration. Few direct hot-water discharges were observed. Consequently, geophysical methods, particularly the audiomagnetotelluric method, were used extensively. Favourable geological conditions for this method were encountered and the results, which were controlled using classical electrical methods on test areas, suggest an unusual distribution of resistivities for lava flows situated above suspected geothermal areas. These layers progressively decrease in resistivity down to a very conductive layer. In areas where these conductive layers were nearest the surface, detailed studies were carried out showing a close correlation between decreasing resistivity and increasing hydrothermal alteration. In addition, gradient wells reveal high geothermal gradients in such areas. The conductive layers revealed by audiomagnetotelluric soundings seem to correlate with thermal effects creating progressive hydrothermal alteration from an inferred hot-water reservoir up toward the surface. INTRODUCTION S e v e r a l f a c t o r s m a k e t h e i s l a n d o f R d u n i o n a f a v o u r a b l e site f o r g e o t h e r mal exploration: the high temperature at depth inferred from zeolite zones i n r e c e n t lava a n d a c t i v e v o l c a n i c e r u p t i o n s , t h e p e r m e a b i l i t y e x p e c t e d f r o m b a s a l t i c lava u n d e r l y i n g t h e w h o l e i s l a n d , a n d t h e h e a v y r a i n f a l l d u e t o its t r o p i c a l l o c a t i o n . T h i s e x p l o r a t i o n , a p a r t f r o m its p u r e l y s c i e n t i f i c a s p e c t , was i n s t i g a t e d b e c a u s e o f t h e i s l a n d ' s i n c r e a s i n g n e e d f o r e l e c t r i c a l p o w e r a n d its l i m i t e d h y d r o e l e c t r i c a l r e s o u r c e s . R d u n i o n lies i n t h e I n d i a n O c e a n 8 0 0 k m e a s t o f M a d a g a s c a r a n d has a n *Present address: Institut Mixte de Recherche G~othermique, Bureau de Recherches G~ologiques et Mini~re, B.P. 6009, 45018 Orleans Cedex, France.
0377-0273/84/$03.00
© 1984 Elsevier Science Publishers B.V.
312
area of a b o u t 2500 km 2. It is made up entirely of volcanic rocks and has recently been carefully explored and surveyed geologically (13illard and Vincent, 1974). This work formed a useful basis for geothermal studies. Geological. and hydrogeochemical surveys were carried out during the first phase of the reconnaissance study (Benderitter et al., 1981). The limited number of springs and particularly of h o t springs, made the utilisation of normal geochemical methods of investigation impossible. Geophysical methods were widely used despite considerable difficulties caused by the relief and vegetation. Data obtained by audiomagnetotelluric surveying are presented here along with their interpretation, separately or in conjunction with information gained from other methods. GEOLOGICAL SETTING
Geographical description (after Vare t e t al., 19 79) R~union is made up of two basaltic shield volcanoes. The older of the two, the Piton des Neiges, rises up from ocean depths of a b o u t 4000 m to a height of 3069 m above sea-level. The part above sea-level started developing a b o u t 2 m.y. ago. It consists of a shield volcano formed by a series of olivine basalt flows. Its slopes were covered over by several hundred meters of differentiated series (basalts with feldspar phenocrysts, hawaftes, mugearites). Three main streams flow from the volcano's central part. Each of these flows across one of the three united cirques (Salazie, Cilaos, Mafate), roughly 6 km wide and 2 km deep, before escaping towards the ocean through deep gashes or gullies (Fig. 1). Zeolitized basalts crop o u t at the b o t t o m of these cirques as haloes, concentric to the volcanic massif. The corresponding isotherms suggest signs of a hydrothermal convection paleosystem which affected a region at least 10 km wide. The Piton de la Fournaise began to develop on the eastern slope of the Piton des Neiges a b o u t 0.35 m.y. ago and it is still active. Nearly all the historical eruptions t o o k place within the summit caldera inside which a shield volcano rises to 2630 m. At least five fissure eruptions have occurred on its sides during the last three centuries. The last of these destroyed part of a coastal village in 1977. To the east there is a graben-like structure 7 km wide linking the caldera to the coast and even extending b e y o n d it, according to bathymetric measurements. This supports the theory that the volcanism moved toward the east. On the northeastern and southeastern slopes historically active fissure zones radiate o u t from the caldera's summit toward the sea. An injection zone (dykes) forming a ridge oriented 140°E links up the summits of the two volcanic massifs. It is covered b y scoria, cinder cones and lava flows which, although prehistoric, are obviously recent. Even
313
© @eologic(]l
cross section of Ciloos cirque
Cirque .01 N
I2"N
23oom
deCILAOS •
Zeolitizotion limit
IN
StDENIS
® cross section of Piton de Io Fournolse
6eologiccl
Piton .
Riviere L A N G E V I N 2gg
~
dele FOURNAS IE 263om
~ I
I
Indion
I
St L
O
~
Zeolitization limit
REUNION ISLAND ~owm 0
5Km
Fig. 1. Simplified geological cross-section showing the ancient and recent volcanism of R~union Island. The different phases are marked I, II, III, IV (see text). The Piton des Neiges formations (old volcanism) are marked N. The Piton de la Fournaise formations (recent volcanism) are marked F. From 1/50,000 scale geological maps. if among t h e surface lavas there are olivine basalts similar to those of the Fournaise volcanism, it is reasonable to assume t hat this ridge could at least in part be d ue t o t h e presence of a rift zone which is part of t he Piton des Neiges system.
Chronological outline Chronologically, it was only some 2 million years ago that this island rose o u t o f the ocean. T he oldest rocks, basaltic agglomerates and breccias (phase I, mo r e than 2.1 m.y. ago), are to be f o u n d at the b o t t o m of the Cilaos, Mafate and Salazie cirques, giant excavations occupying the central part of the Piton des Neiges massif. N e x t came the olivine basalt and oceanite flows which piled up t o form the f r a m e w o r k of this ancient massif (phase II, 2.1--0.43 m.y. ago). These t h e n u n d e r w e n t an intense zeolitization. After a quiet period, activity began again (phase III, 0.35--0.25 m.y. ago). Th e Piton des Neiges was taken up to its m a x i m u m elevation by flows o f feldspathic p o r phyr i t i c basalt and of alkaline andesite. T ow ard the southeast, basalt and oceanite flows started to build up the Piton de la Fournaise massif. Eruptive p h e n o m e n a and considerable subsidence also marked the end o f this period in bot h massifs.
314 Another quiet period was followed by an active one (phase IV, less than 0.23 m.y. ago) characterized, especially in the Piton de la Fournaise massif, by basalt and oceanite flows, still occurring at the present time. Last known activity in the central part of the Piton des Neiges massif was 10,000 years B.P. OBJECTIVES OF THE GEOPHYSICAL INVESTIGATIONS An exploitable geothermal reservoir implies the existence of a very large heat source and of geothermal fluids to transport this heat up to the surface. The main physical characteristics making it possible, using geophysical methods, to detect and delimit geological structures likely to possess a geothermal reservoir are derived from this simple model. In a volcanic site the heat comes from magmatic intrusions whose density is relatively high compared to the surrounding rock, thus creating positive anomalies of the gravity field. This heat is transmitted to a geothermal fluid circulating in porous rock of relatively lower density. Furthermore, this geothermal fluid, having both a high temperature and a high dissolved salt content, corresponds to a zone of low electric resistivity (Keller and Rapolla, 1974). Inversely, a conductive zone does not always indicate the presence of a geothermal fluid. The presence of mineralized water at normal temperature is capable of lowering the resistivity of the rock containing this water considerably. It is therefore n o t surprising to find low resistivities in the superficial part of volcanic formations which can play an important role in the feeding of the geothermal reservoir. The interpretation of the resistivity anomalies of Rdunion is especially difficult because the water can be very mineralized, in particular close to the shore. One can thus expect very low resistivities, 1 ~ m or lower, which are usually considered to be indicative of the presence of hot mineralized water. This configuration of highly conductive formations in the vicinity of positive gravity anomalies is generally considered to be a necessary though not always sufficient indication of a geothermal reservoir (Gupta, 1980). Although other physical parameters may be sought, density and resistivity are usually studied first due to the ease with which the corresponding geophysical methods can be carried out during explorative prospecting. For this reason, the audiomagnetotelluric m e t h o d along with the gravimetric m e t h o d were used in R~union. The initial aim was to outline the zones over a surface area of about 1000 km 2 revealed as suitable for later investigation using more elaborate geophysical methods such as the dipole-dipole electric m e t h o d or the magnetotelluric m e t h o d at a greater investigation depth.
315 THE AUDIOMAGNETOTELLURIC METHOD
For more than 25 years n o w (Cagniard, 1953) one of the main advantages of the magnetotelluric prospection method has been recognized -- a very deep investigation using relatively light field equipment. The depth of penetration of electromagnetic waves is expressed by the following equation: 1
p =-~ x / - ~ /N p(km), p(~2m), N(Hz) being the penetration depth, the resistivity in the case of a homogeneous substratum and the frequency of the studied phenomenon, respectively, The Centre de Recherches G~ophysiques made use of this property in developing a very light apparatus (Jolivet, 1969) which could cover vast surfaces even in zones of difficult access and destined above all for the mining field (Benderitter, 1973). Since 1971 this measuring apparatus has been continually improved by the ECA Company 1. It consists of a magnetic induction receiver, a telluric line and a resistivimeter enabling the calculation in the field of the apparent resistivity according to the classic equation:
Pa - 5N
/-/y
Ex(mV/km )
and Hy(~/) being the amplitude of the horizontal c o m p o n e n t of the telluric field and the amplitude of the horizontal c o m p o n e n t perpendicular to the magnetic field. The total weight of the equipment is a b o u t 20 kg (magnetometer: 10 kg; resistivimeter: 8 kg; accessories: 1 kg). Installation at the site takes only a few minutes, the longest manoeuver being the installation of the telluric line whose maximum length is 50 m. It takes around 20 minutes to measure the resistivities for different frequencies. The surveying can be done easily by 2 people, one operator and one porter, even under the most difficult conditions. The model used in R~union (ECA 541-0) allowed measurements between 1700 and 8 Hz. Contrary to work carried out in other regions (Ballestraeci and Benderitter, 1980; Dupis et al., 1980), the first measurements showed the existence of resistivities that were exeeptionally high for volcanic sites of around several hundred to several thousand ohm-meters (Fig. 2). Identical resistivities have since been observed in the Kerguelen Islands (Ballestracci et al., 1983). An explorative survey cannot give quantitative results as precise as those ECA, 17 A v e n u e du Chateau, 9 2 1 9 0 M e u d o n Bellevue, France.
316 Apparent
resistivity Po (~m) 400O
F
J
'%x
i
I000
\ I00
~ 0 0
~
0 ~
0 I~
0 ~
I~ i~,
I
Frequency N(Hz)
I~. --
Fig. 2. Magnetotelluric sounding obtained in the Piton de la Fournaise zone. This sounding shows the high resistivities near the surface as well as the large contrast, over 100, between the resistivities of the surface and those of underlying rocks. o b t a i n e d f r o m a m o r e detailed investigation. H o w e v e r , t h e p r e s e n t meas u r e m e n t s also b e n e f i t f r o m a n o t h e r f a v o u r a b l e c i r c u m s t a n c e . Most o f the diagrams o f a p p a r e n t resistivity as a f u n c t i o n o f f r e q u e n c y c o u l d be considered t y p i c a l o f a s i t u a t i o n w h e r e resistant rocks overlie c o n d u c t i v e r o c k s with a very high resistivity c o n t r a s t o f a r o u n d 100. In this case, the thickness o f t h e resistive r o c k can easily be d e t e r m i n e d f r o m the e q u a t i o n : e = 0.356
V~a/N
e ( m ) , p a ( ~ m ) and N ( H z ) being the t o t a l thickness o f the r o c k s covering t h e c o n d u c t i v e rocks, t h e a p p a r e n t resistivity at any p o i n t o f the diagram lying o n t h e straight line o f the --2 limiting slope and t h e c o r r e s p o n d i n g f r e q u e n c y , respectively. As o n l y a few m e a s u r e m e n t s are n e e d e d to o b t a i n the limiting slope satisfactorily, t h e thickness o f t h e resistive r o c k s c o u l d be o b t a i n e d v e r y precisely. H o w e v e r , this precision c o u l d be illusory. T h e above relationship is o n l y t r u e in a t a b u l a r case. Volcanic r o c k s c a n n o t easily be r e p r e s e n t e d b y this type of model.
317 It m u s t be n o t e d t h a t t h e large n u m b e r o f m e a s u r e m e n t s d i s t r i b u t e d a c c o r d i n g to t h e profiles m a d e it possible t o o u t l i n e z o n e s w h e r e t h e app a r e n t resistivities r e m a i n quite c o n s t a n t f r o m one station t o a n o t h e r and w h e r e , t h e r e f o r e , the r o c k s t r u c t u r e is n o t t o o unlike a tabular m o d e l . In this way we avoid t h e use o f m o d e l s which are c e r t a i n l y m o r e suitable, like the c y l i n d r i c a l - t y p e m o d e l s (Th~ra, 1 9 7 7 ) , b u t whose use is m o r e complex. In the p r e s e n t case w h e r e s t r u c t u r e s resembling wide c o n d u c t i v e veins in a resistant m e d i u m are sought, the e x a m p l e given in Fig. 3 shows t h a t rKm
0
IKm
2Kml
I 0 0 0 JAm _ m ~ m m
m
\
Lo
E
~
m
I 0 0 0 J3_m
2Km
o
Fig. 3. Cylindrical structure and the multi-layered earth interpretation derived from it. In each case only half a section of the structure was represented along a plane perpendicular to its elongation. Curve 1 marks the separation between the surface rocks and the underlying rocks. Curve 2 demarcates the rocks with a resistivity of over 950 a m from those of a lower resistivity. t h e use o f a t a b u l a r - t y p e m o d e l for i n t e r p r e t a t i o n gives results b e t t e r t h a n m i g h t be e x p e c t e d at first sight. This o b s e r v a t i o n was m a d e a f t e r calculating t h e t h e o r e t i c a l soundings spread along a profile c u t t i n g t h r o u g h the cylindrical m o d e l p r o p o s e d in the left half o f t h e figure and i n t e r p r e t i n g these soundings a c c o r d i n g to a t w o - l a y e r tabular h y p o t h e s i s . T h e results were t r a n s f e r r e d t o the right-hand side o f the figure and it can be seen t h a t while the vein's resistivity deviates f r o m the t r u e value, especially t o w a r d its edges, a high degree o f a c c u r a c y is o b t a i n e d for the resistivity and thickness o f the r o c k s lying above t h e c o n d u c t i v e s t r u c t u r e . T h e struct u r e itself is satisfactorily d e m a r c a t e d . These conclusions c o n c e r n the case w h e r e the telluric field is m e a s u r e d along t h e axis p e r p e n d i c u l a r t o the axis o f the s t r u c t u r e . T h e y agree with t h o s e given by some o t h e r a u t h o r s (Hjelt e t al., 1 9 7 9 ; Meheni, 1 9 8 0 ) .
318
On the other hand, the example given in Fig. 4 shows that the relief can greatly alter the results. R~union possesses extremely narrow valleys cutting deep gashes into the slopes of the volcanic massifs and spectacular escarpments several hundred meters high. The greatest effect is on frequencies where the penetration depth is similar to the magnitude of the relief. It is always advisable to measure the telluric field parallel to the valley's axis and to keep away from the immediate vicinity of the rupture zones with precipitous slopes to reduce disturbances to a minimum. In such cases, as in the example chosen for its accentuated relief, the results are acceptable if care is taken to keep a few hundred meters back from the cliff. Apparent resistivity
pa (rim) 5000.
I000
~
I~
~,~
~,~
*\ \ + \\
i
....
/ i/
IOC
50 + - -
.
IO00Hz
+------÷
IO00Hz
IOHz
IOHz
osKil 0,5
I
1,5 Km
Fig. 4. Influence of the relief at two different frequencies (1000 and 10 Hz) and along the two main measuring directions. Solid line: E measured along the valley's axis. Dashed line: E measured perpendicular to the valley's axis. LOCATION OF THE MEASUREMENTS
The first measurements, taken in January 1979 as a reconnaissance survey, encircled the Piton de la Fournaise massif except for its southern section, at a distance varying between 8 and 16 km from its center. The
319
Rose
tO 00tom
S~ Pierre
Fig. 5. Location of the audiomagnetotelluric soundings and position of the profiles in Figs. 12 and 13.
320 eastern zone, characterized by a depression linked to the recent extension of the volcanism, and the western and especially northwestern flanks (Plaine des Palmistes), both situated between the Piton des Neiges and the Piton de la Fournaise massifs, were the main objectives of the present survey. The first measurements were followed by a second set in May 1979. These were taken towards the southwest and continued in a more detailed manner the study of the link between the two massifs. This survey also included two areas of the Piton des Neiges massif (Cilaos and Salazie cirques), interesting because of their topographical location near the center of the massif and giving hope of finding intense, though quite old, hydrothermal phenomena. A third set of measurements was made in September 1980 aimed at obtaining a cross section of the Piton de la Fournaise formations along an approximately radial plane, particularly along the Langevin River. This stream runs through a spectacular gully where zeolitized layers are found locally. A total of 535 measuring stations {see Fig. 5) were set up. They were chosen partly according to hypotheses which were drawn from the results obtained during the different stages of the magnetotelluric survey itself and partly from the results of other geological and geophysical investigations (gravimetric and electric surveys) taking place at the same time. They were spread out over a surface of about 1000 km 2. The high density of soundings in some areas permitted detailed interpretation, for example in the Plaine des Palmistes and the Langevin River. In addition, the large number of measurements taken in the different geological zones statistically enabled the elaboration of a regional model. This model takes into account the average distribution of resistivities in the island's sub-soils from which comes the notion of anomaly. REGIONAL STUDY OF THE RESULTS Once it was realized t h a t nearly all the soundings presented the same kind of apparent resistivity diagrams as the one in Fig. 2, a first general approach to the interpretation was agreed upon: a tabular model made up of resistive rocks overlying a conductive substratum whose resistivity is n o t more than 1 to 2% of that of the covering layer. The thickness and resistivity of the first rock layer were obtained in this manner for each sounding. After determining the thickness of the resistive rock, it was possible to map, with a slight smoothing, the isopleth of the resistive--conductive interface (Fig. 6). This parameter rises steeply from the island's shore (--500 m) towards the Piton des Neiges and Piton de la Fournaise massifs (+1000 m and 1500 m, respectively). The high values are found along an axis which coincides almost exactly with the topographic crest line. Moreover, we notice that the thickness of the high-resistivity rock also general-
321
\
o
jj~
p
Fig. 6. Contour lines (in kilometers) of the top of the conductive layer. Note the coincidence between the topographic crest line and the high elevation of the conductive layer. ly increases f r o m the c o a s t t o w a r d s the middle o f the island r o u g h l y acc o r d i n g t o the altitude o f the m e a s u r e d p o i n t s (Fig. 7). This shows t h a t the c o n d u c t i v e layer is p r e s e n t e v e r y w h e r e and t h a t it rises t o w a r d s the island's
322 Altitude IN]
Z(m)
/
{N)
2400. •
•
.
,"
.
•
..
•
j .
2200. 2000.[
'l 1800 o
1600=i 1400o 1200=i 1000o! !
800°i 600° (M)
,:00.
(N)
Linear
regression
El = 0,3Z+
485
E~=O,3Z+
85
200o
Thickness of
500
1000
1%00
the
first
layer
[I (m) 2000
Fig. 7. Variation of the thickness of the resistive covering rocks as a function of the altitude of the soundings. All the soundings to the left of the straight line (N) are considered to be located on a resistive surface layer which is abnormally thin compared to the altitude• The points are located on the Cilaos and Grand Pays cirques and in a few places on the Plaine des Palmistes.
c e n t e r b u t , in g e n e r a l , a l i t t l e m o r e s l o w l y t h a n t h e t o p o g r a p h i c a l s u r f a c e . In a d d i t i o n , a r e m a r k a b l e c o r r e l a t i o n c a n be o b s e r v e d b e t w e e n t h e resistivi t y p~ a n d t h e t h i c k n e s s E l o f t h e r e s i s t a n t r o c k s . T h i s is s h o w n b y t h e c u r v e p ~ = F ( E ~ ) p l o t t e d in Fig. 8. W h e r e t h e first r o c k l a y e r b e c o m e s v e r y t h i n its r e s i s t i v i t y falls t o a r o u n d 5 0 t o 1 0 0 ~ m . C o r r e s p o n d i n g s o u n d i n g s w e r e all s e e n t o b e o n r o c k s w h i c h w e r e v e r y rich in h y d r o t h e r m a l d e p o s i t s ( z e o l i t e s ) . T h i s is i l l u s t r a t e d by s o m e s o u n d i n g s in t h e C i l a o s , S a l a z i e a n d G r a n d P a y s c i r q u e s .
323 Resistivity of the first Ioyer
PI (rn)
?= ~ 6co~1
"
Sounding
......
• vo~)
~ooo
type I
i
,8a
i
I'~ ~ ,2o ~ /o0
I!
. "~/~-
I:
""
.[..
i
- -
-Sounding
type 5
.
(P
E
5Of} m
230m1
Thickness of the first Ioyer £4 (m) I0
ioo
~2e
,50
2~
:~
JO~
~00
~0
~
mO ~
~
o0o
,m~O
mOO
2~
Fig. 8. Variation o f the resistivity o f the first layer according to its thickness. Points were c h o s e n regularly spread o u t along the m e a n curve and equivalent multi-layered m o d e l s were calculated. The p s e u d o - c r o s s section o f Fig. 9 is the result o f this. The c o m p a r i s o n o f the interpretation s o u n d i n g w i t h s o u n d i n g is given in Fig. 10.
On t h e o t h e r h a n d , t h e first r o c k layer, over 1 0 0 0 m thick, is a s s o c i a t e d w i t h resistivity values o f over 1 0 0 0 ~2m and c o r r e s p o n d s t o s o u n d i n g s w h i c h are all l o c a t e d o n very t h i c k r e c e n t basaltic f l o w s such as t h o s e carried o u t in t h e P i t o n de la F o u r n a i s e area.
324
Preparation of a general model The conclusions of the previous discussion lead to the elaboration of a model made up of layers of decreasing resistivity down to a deep conductive layer (Fig. 9). This model seems to describe perfectly the average behavo iour of the soundings made as shown by the controls set out in Fig. 10. It is possible to recreate the observed soundings by "eroding" this model down to the lower layers. The first interpretation of the soundings using a two-layer model is therefore an approximation. It can be seen, however, that the depth of the top of the second layer corresponds quite well with the depth of the top of the deep conductive layer in the multi-layer model. Such a model implies a very conductive layer (resistivities around 1 ~2m) under all the regions studied on the island. The hydrothermal mineralizations outcropping wherever this level arrives close enough to the surface suggest that this horizon can be likened to a layer in which very mineralized waters circulate (or circulated long ago -- geophysical methods are poor at historical reconstitutions). The geothermal importance of this marker is obvious since it may be (or have been) fed by water coming from deep below the surface and therefore hot. The fact that it is covered by rocks made impermeable by hydrothermal deposits would have stopped any important escape towards the surface. The vast extent of this impermeable layer throughout the island may explain the small number of secondary volcanic features (hot springs, fumaroles, etc.). The levels with 30, 50 or even 100 ~ m can be attributed to this cover layer.
Piton de Io FOURNAISE Plolne des PALMISTES
~lome des
©
CAFRES Cirque de CILAOS ou de SALAZrE
®
®
Rivlere LANGEVIN
Cirque de GRAND PAYS
®
550
50
nm
Sounding ,yp . . . . bet (See
Fig. 9. General multi-layer model of R6union Island.
8 ond ,01
325
Pa (~,m)
S o u n d , n g type
IO000 -
Sounding type 2
I
"FOURNAISE "
I0000
" P l o , n e des
-
'r L A
Sounding type I000-
CAFRES
CRETE
" Ploine
ii
" I00 :
L" ~ O- , ~ J "x
I000"
IO=
N(Hz)
i
tO000
I0000
10(3
O" I
f ~oo o~' I00o -
500 o _-
2OO0
-
~- r oo 3 fi
--I10 _-- : : 7
.....
1500
-
55o
550
-
E
sO
4
S o u n d i n g lype LANGEVIN
Sounding
SUD "
"GRANDI~¥S" tooo-
i
I00 •
I00-
,° i
I0
"
IN n m
RESISTIVITY
4
5O 30
£0
I
~N ~'~m
50O
2ZO .0
-
150o
P=l
type
,"
i
5
Somlding
SALAZIE
type 6
II S A L A Z I E C e n t r e " , ' r C I L A O S C e n t r e "
"
" C I LAOS "
IO0~
*
IO -
i
,oo'oo....
t'4o .....
o
220 .0 50 ,
5O0-
1000
,2,20o
IO00
~ " _o
5o
I
550
t
z2o
R[SISTIVITY
" Riviere
1500
15ooe
5oo~
E
IO0
I0000
I
Or
I000
VB E E
\ IO0 ~
•
3
PALMISTES "
des
,'
200
I0000
i'~
50
E
,
I O0
•m
~:
I00
o:_ o
RESrSTIVlTy
IN
CZm
I
~
~oo:0o p= i
~0o
I
iz ,N
•
_~
7
f~ESISTIVIT ~
I0000
i
RESISTIvrTY
.
i
I
E
-
IN ~]m
Fig. 10. C o m p a r i s o n b e t w e e n the effects of the standard soundings of Fig. 8 and those of a multi-layer m o d e l w i t h progressive erosion of the upper levels. + A p p a r e n t resistivity calculated w i t h a two-layer model. - - Apparent resistivity calculated w i t h a multi-layer model. A surface layer (p = 50 ~ m , E = 4 m ) w a s added to multi-layer m o d e l s 1, 2, 3, 4.
According to the geological observations, the soundings corresponding to a surface level of 200 to 500 ~2m resistivity are mostly located either on old basalts (first series of the Piton des Neiges or Piton de la Fournaise) or in their immediate vicinity. Some of them might be typical of zones
326 s h o w i n g slight traces o f h y d r o t h e r m a l d e p o s i t s in m o r e r e c e n t f o r m a t i o n s . Finally, we m u s t c o n s i d e r t h e s o u n d i n g s d e p i c t i n g highly resistive surface layers ( 1 5 0 0 t o 1 5 , 0 0 0 g t m ) . T h e s e are f o u n d s y s t e m a t i c a l l y in t h e F o u r n a i s e massif at q u i t e high altitudes (Rivi~re de l'Est, t h e F o u r n a i s e s u m m i t , Plaine des Cafres, s o m e p a r t s o f the Plaine des Palmistes, etc.). T h e high resistivities can be e x p l a i n e d b y t h e p r e s e n c e o f r e c e n t basaltic flows. T h e s e are p r a c t i c a l l y d r y as t h e y are t o o p e r m e a b l e to h o l d w a t e r , as was p r o v e d b y a 2 0 0 - m b o r i n g d o w n t h r o u g h t h e Plaine des Palmistes.
Use o f the proposed model for definition o f the anomaly zones In spite o f t h e f a c t t h a t t h e m o d e l s h o w n gives a v e r y realistic p i c t u r e o f t h e results o f t h e 535 s o u n d i n g s carried o u t , it m u s t be n o t e d t h a t inside a n y o n e z o n e c o n s i d e r a b l e v a r i a t i o n s o f t h e p a r a m e t e r s are o f t e n o b s e r v e d (Fig. 11). Resistivity of the first layer R, (D.m)
Resistivity of the first layer
Iooo
p, (D.m)Iooo
Y
26. : '
I00
I00
.,:: :o.:, ,B
,:
SALAZIE
I00
5()0
,0~)0 *
Thickness of the first layer
CILAOS
E~ (m)
Thickness of the first layer
E~ (rn)
roo
T 500
I000
Fig. 11. Evolution of the resistive cover rocks' resistivity according to their thickness in the Cilaos and Salazie cirques..n = sounding number. Solid line = mean curve for the whole island (see Fig. 8). The places where the resistive cover rocks are thinnest (and therefore least resistive) coincides with the outcrops of rocks very laden with hydrothermal mineralizations. S o m e o f the d a t a s c a t t e r i n g is c e r t a i n l y d u e to the u n c e r t a i n t y in t h e m e a s u r e m e n t s . H o w e v e r , t h e largest d e v i a t i o n s do n o t s e e m to be e n t i r e l y d u e to r a n d o m statistical d i s t r i b u t i o n . T h e v a r i a t i o n s o b s e r v e d along t h e m e a n curve p~ = F(E~) (longitudinal
327 variations) can reflect the local changes in the depth of the most mineralized layer u n d er a cover layer described exactly by the proposed model. The deviations observed on b o t h sides o f the mean curve (transverse variations), especially when t h e y are large, indicate the differences between the true structure of the cover layer and its average structure. In particular, the points located to the left of the mean curve p~ = F(E~) correspond to soundings in areas where the average thickness of the cover rock, lower than that o f the model, suggests the existence of m ore conductive formations at an equivalent depth. Conversely, the soundings to the right are located in zones of thicker coverage. Consequently, in t he zones where the soundings give the lowest average values for p~ and E1 it is most likely t hat a convection loop, associated or n o t with erosion, could bring mineralized and probably h o t fluids up near the surface. In these conductive zones and of these soundings, it is those located furthest to the left of the line which would indicate the most interesting sites. This argument is verified in the Salazie and Cilaos cirque regions for which, in addition, several mineralized rock samples were found. Indeed, one can observe in t he Salazie cirque (Fig. 11) that it are soundings 10, 11, 12, 13, 14 and 16 which, f rom the point of view of the longitudinal position in relation to the mean curve, indicate the closest proximity to the conductive layer. The rock samples available revealed, after analysis, th at it was in this region that the h y d r o t h e r m a l mineralizations were the most abundant. A 200 m drilling near sounding 11 revealed a geothermal gradient six times larger than normal. It can also be observed in the Cilaos cirque (Fig. 11) that sounding 14 is distinct in its longitudinal position, and that sounding 13 lies noticeably t o the left with regard to its transverse position. These two soundings correspond geologically to the formation's m a x i m u m mineralization and are located n e x t to a thermal spring. A n o t h e r logical consequence of the proposed model comes from the general relationship existing between a station's altitude and the thickness o f the high-resistivity cover rock. The interesting zones are those where the cover rocks are the thinnest com pa r ed to the altitude. The cirque and Langevin River zones fulfill this condition. This could be seen from Fig. 7 where th e dots, representing the soundings made in these areas, are all located well to t he left o f t he line.
EXAMPLES OF DETAILED STUDIES T h e Plaine des Palrnistes
The m e a s u r e m e n t stations were set up along a NNE--SSW profile about 8 km long {Profile 1, Fig. 5).
328 Bouguer onomoly
(msol)
Grovimetric profile
I
250
Electricol
profile 150 1 I00
sol Interpretive audio-magnetoi'elluric Ground level
-S . S W
500
- 500
cross-section NNE Altitude (m)
.4
Jq.m
iO00
5OO Interf°ce~
/
20.Q.rn ~ 500
r
15OOm
Fig. 12. Comparison between the behaviour of the resistive-conductive interface (obtained by interpretation of the audiomagnetotelluric soundings), the variation of the apparent resistivities (obtained by resistivity mapping using direct current, electrodes 6000 m apart) and the variation of the gravity intensity in the Plaine des Palmistes (profile 1, Fig. 5). C o m p a r i s o n s w e r e m a d e with gravity a n d resistivity d a t a , t h e l a t t e r using c u r r e n t e l e c t r o d e s 6 0 0 0 m a p a r t a n d p o t e n t i a l m e a s u r i n g lines 500 m l o n g (Fig. 12). T h e a u d i o m a g n e t o t e l l u r i c results are given as a cross-section revealing in its c e n t r a l p a r t v e r y t h i c k resistive rocks. On b o t h sides t h e r e is a resistive h o r i z o n o n t h e surface laying on t o p o f a c o n d u c t i v e l a y e r which c o m e s n e a r t h e surface at s o m e spots. Since t h e i n t e r p r e t a t i o n was d o n e using a t a b u l a r m o d e l , it is advisable t o t a k e i n t o a c c o u n t w h a t was said p r e v i o u s l y if an even m o r e a c c u r a t e idea o f t h e s h a p e o f t h e c o n d u c t i v e s t r u c t u r e is sought. When these results are c o m p a r e d w i t h t h e d a t a o f the electrical survey (resistivity m a p p i n g ) we n o t i c e t h e i r p e r f e c t c o r r e l a t i o n . T h e a p p a r e n t electric resistivity decreases each t i m e t h e m a g n e t o t e l l u r i c c o n d u c t i v e l a y e r a p p r o a c h e s t h e surface. T h e r e w o u l d be n o t h i n g surprising in this if t h e c a p a c i t i e s o f t h e m a g n e t o t e l l u r i c m e t h o d did n o t still n e e d proving. T h e m e a s u r e d p a r a m e t e r is t h e s a m e a n d t h e investigation d e p t h is similar for both techniques.
329
Much more interesting is the comparison of the gravimetric data and the preceding results. Coinciding with the high-resistivity electric anomaly in the profile's center is a positive gravimetric anomaly, on a regional scale stretching with a NW--SE axis from the Cilaos and Salazie cirque zones to near the Piton de la Fournaise. Therefore, we probably have here compact formations of groups of intrusions. This can be geothermally interesting if the magmatic uprisings are recent enough to form a heat source which is still important. On both sides of this central zone, especially in the SSW part of the profile, more conductive formations can be found, coinciding with a negative gravimetric anomaly. This can be interpreted as being characteristic of more fissured zones. The SSW part of the profile gains in importance as very low frequency magnetotelluric measurements made by Dupis (Benderitter et al., 1981) revealed a conductive layer about 7 km deep. This is twice as deep as in the NNE part of the profile. The Langevin river A 15-km-long profile of 64 soundings was drawn (Profile 2, Fig. 5). • he data are presented in the form of a semi-quantitative cross-section in Fig. 13 where it can be directly compared with the data recorded along the same profile by the electric survey using a dipole-dipole system A u d i o - m o g n e t o t e l l u r i c pseudo c r o s s - s e c t i o n Soundt;~g numlN~
Dipole-dipole
pseudo cr0ss-section Geoth,,mo~ wQ,e,~o~,
AppQrenl resistivity in ~,m
Fig. 13. Comparison of the pseudo-cross section of the apparent resistivities obtained by a u d i o m a g n e t o t e l l u r i c soundings in the Plaine des Sables and along the Rivi6re Langevin (Profile 2, Fig. 5) and that of the apparent resistivities in dipole-dipole (AB = MN = 500 m) along the same profile.
-
~
~
~
/
.....
r
2Kin
....
~ ~
•',, ~ .
):"
Resistant-conductive interface
-
~
"'.
~:~:tcO:tg-~:n~i::i:~d
1
FouIf occordingto
Zeolites (Zeolitizedaphyricbasalts}
i
n
i
:
~
Ifl.m
7 E
Schlumberger sounding SE3 f " ~ / / ~ Groundlevel for -E ~ audio-magnetotellurfc /
Fig. 14. Comparison of the modeling of the audiomagnetotelluric and dipole-dipole pseudo-cross sections along the Rivi~re Langevin.
J
Altitude(m)
Electrical sounding number
5o
331 (AB = MN = 500 m). A similarity in the position of the deep conductive anomalies can be seen, confirming the validity of the audiomagnetotelluric results. A more detailed comparison reveals, in addition, the greater penetrating power of the audiomagnetotelluric method. This m e t h o d is also seen to be more reliable in a zone of accentuated relief, as is the case towards the middle of the left half of the profile. The interpretation of these two cross-sections in terms of depth gives almost identical results {Fig. 14). The geothermal significance of the conductive region near the middle of the profile was confirmed by the intense zeolitization of the recent surface formations. The region is also marked by a very large positive gravimetric anomaly which can be interpreted by an abnormal density due to intrusions of deep-seated origin. A spontaneous polarization anomaly, characteristic of a p h e n o m e n o n of thermal origin, was also revealed at this spot. CONCLUSIONS Although geochemistry and, in the last stage, drilling, are the only ways to prove the existence of a geothermal reservoir, geophysics can help determine the areas where the chances of finding the reservoir are greatest. The resistivity of rocks is an essential parameter in geothermal surveying. The electrical methods c o m m o n l y used in geophysics are poorly suited in this case because of the great depth of investigation sought to detect geothermal resources. The high resistivity of the surface volcanic rocks of R~union allows, using the audiomagnetotelluric method, a depth of investigation that is interesting for geothermal surveys. Because of the ease of operation of the equipment used, this m e t h o d allows one to carry out a large number of measurements over a large area at a reasonable cost. It thus became possible to process the data statistically and to determine the mean behaviour of the layers for the whole island. The model obtained shows a series of layers whose resistivity decreases with depth, ending with a layer whose resistivity is about 1 ~2m. Because of the scale of the phenomenon, this model is or was directly or indirectly linked to a thermal effect. This thermal effect may have induced hydrothermal circulations, which would generate conductive mineralizations, the density of which decreases gradually upwards from the paroxysmal conductive layer which forms a possible top of the geothermal prospect. The disparities between this regional model and each measurement enable the determination of anomalous areas, some of which may prove to be the main geothermal targets of R~union in agreement with the other geological and geophysical data. ACKNOWLEDGEMENTS Ph. Marie is thanked for carrying out part of the measurements and A.K. Bourg is thanked for the English editing of this paper.
332
We thank the French Ministry of Industry and FIDOM for financial support of the geothermal exploration program of R~union Island carried out by the Geothermal Energy Department of the B.R.G.M.
REFERENCES Ballestracci, R. and Benderitter, Y., 1980. Sondage magn6totellurique dans la gamme 8 Hz--170 Hz ~ proximit6 de l'Ardoukoba (Rift d'Asal, R~publique de Djibouti) R6sultats, calcul d'un module repr6sentatif et interpretation. Bull. Soc. Geol. Fr., 22(7): 873--879. Ballestracci, R., Nougier, J. and Benderitter, Y., 1983. Mise en 6vidence et interpr6tation d'une zone ~lectriquement conductrice dans des basaltes des plateaux des iles Kerguelen (T.A.A.F.). C.R. Acad. Sci., 296, serie II: 833--838. Benderitter, Y., 1973. Magneto-tellurique. Rev. Ind. Miner., Spec. Vol., 15 May, 1973: 73--81. Benderitter, Y., Dupis, A., Fitterman, D., G6rard, A., Puvilland, P., Rancon, J.Ph., Robert, D., Stieltjes, L. and Varet, J., 1981. Evaluation du potentiel g~othermique de l'ile de La R~union. Rapport du Bureau de Recherches G~ologiques et Mini~re, 81 SGN 669 GTH. Billard, G. and Vincent, P.H., 1974. Cartes g~ologiques de la France au 1/500 000. Saint Denis, Saint Benoit, Saint Pierre, Saint Joseph. Bureau de Recherches G~ologiques et Mini~re. Cagniard, L., 1953. Basic theory of the magnetotelluric method of geophysical prospecting. Geophysics, 18(3): 605--635. Dupis, A., Marie, Ph. and Petiau, G., 1980. Application des m~thodes de prospection magn6to-tellurique pour l'exploration g6othermique du Massif du Mont Dore. Contrat 582-78 EGF, G6othermie, Commissions des Communaut~s Europ~ennes, Strasbourg. Gupta, H., 1980. Geothermal Resources: an Energy Alternative. Developments in Economic Geology. Elsevier, Amsterdam, 227 pp. Hjelt, S.E., Kaikkonen, P. and Pietila, R., 1979. On the interpretation of VLF resistivity measurements. Paper presented at the 12th Meeting of the Nordic Association of Applied Geophysics, Oslo, January 1979. Jolivet, A., 1969. Etude d'un 6quipement l~ger pour la prospection magn~to-tellurique de subsurface; Th~se d'universit~, Facult6 des Sciences de Paris, 113 pp. Keller, G.V. and Rapolla, A., 1974. Electrical prospecting methods in volcanic and geothermal environments. Developments in Solid Earth Geophysics. Physical Volcanology, Elsevier, Amsterdam, pp. 133--166. Meheni, Y., 1980. Analyse de la r~sistivit6 apparente magn6totellurique sur divers modules bidimensionnels. Etude d'un sondange M.T. fi six composantes, cas particulier de la composante tellurique verticale. Th~se de Troisi~me Cycle, Universit~ Pierre et Marie Curie, Paris, 140 pp. Th6ra, A.L., 1977. Interpr6tation des mesures magn~totelluriques ~ partir d'un module deux dimensions. Th~se de Troisi~me Cycle, Universit6 Pierre et Marie Curie, Paris, 104 pp. Varet, J., Stieltjes, L. and Duffield, W., 1979. Etudes g~ologiques et g~othermiques de la R6union. R4sum~ des principaux r6sultats scientifiques et techniques du Service G~ologique National pour 1979. Rapport du Bureau de Recherches G6ologique et Mini~res. -
-
311
GEOTHERMAL STUDY OF REUNION ISLAND: AUDIOMAGNETOTELLURIC SURVEY
Y. BENDERITTER ~,* and A. GERARD 2 1Centre National de la Recherche Scientifique, Centre de Recherches Gdophysiques, Garchy, 58150 PouiUy sur Loire (France) 2Institut Mixte de Recherche Gdothermique, Bureau de Recherches Gdologiques et Minidre, B.P. 6009, 45018 Orleans Cedex (France) (Received April 6, 1982; revised and accepted June 16, 1983)
ABSTRACT Benderitter, Y. and G~rard, A., 1984. Geothermal study of R~union Island: audiomagnetotelluric survey. J. Volcanol. Geotherm. Res., 20: 311--332. In 1979 and 1980, 535 magnetotelluric soundings using a frequency range of 1700 Hz--8 Hz were performed on the island of R~union for geothermal exploration. Few direct hot-water discharges were observed. Consequently, geophysical methods, particularly the audiomagnetotelluric method, were used extensively. Favourable geological conditions for this method were encountered and the results, which were controlled using classical electrical methods on test areas, suggest an unusual distribution of resistivities for lava flows situated above suspected geothermal areas. These layers progressively decrease in resistivity down to a very conductive layer. In areas where these conductive layers were nearest the surface, detailed studies were carried out showing a close correlation between decreasing resistivity and increasing hydrothermal alteration. In addition, gradient wells reveal high geothermal gradients in such areas. The conductive layers revealed by audiomagnetotelluric soundings seem to correlate with thermal effects creating progressive hydrothermal alteration from an inferred hot-water reservoir up toward the surface. INTRODUCTION S e v e r a l f a c t o r s m a k e t h e i s l a n d o f R d u n i o n a f a v o u r a b l e site f o r g e o t h e r mal exploration: the high temperature at depth inferred from zeolite zones i n r e c e n t lava a n d a c t i v e v o l c a n i c e r u p t i o n s , t h e p e r m e a b i l i t y e x p e c t e d f r o m b a s a l t i c lava u n d e r l y i n g t h e w h o l e i s l a n d , a n d t h e h e a v y r a i n f a l l d u e t o its t r o p i c a l l o c a t i o n . T h i s e x p l o r a t i o n , a p a r t f r o m its p u r e l y s c i e n t i f i c a s p e c t , was i n s t i g a t e d b e c a u s e o f t h e i s l a n d ' s i n c r e a s i n g n e e d f o r e l e c t r i c a l p o w e r a n d its l i m i t e d h y d r o e l e c t r i c a l r e s o u r c e s . R d u n i o n lies i n t h e I n d i a n O c e a n 8 0 0 k m e a s t o f M a d a g a s c a r a n d has a n *Present address: Institut Mixte de Recherche G~othermique, Bureau de Recherches G~ologiques et Mini~re, B.P. 6009, 45018 Orleans Cedex, France.
0377-0273/84/$03.00
© 1984 Elsevier Science Publishers B.V.
312
area of a b o u t 2500 km 2. It is made up entirely of volcanic rocks and has recently been carefully explored and surveyed geologically (13illard and Vincent, 1974). This work formed a useful basis for geothermal studies. Geological. and hydrogeochemical surveys were carried out during the first phase of the reconnaissance study (Benderitter et al., 1981). The limited number of springs and particularly of h o t springs, made the utilisation of normal geochemical methods of investigation impossible. Geophysical methods were widely used despite considerable difficulties caused by the relief and vegetation. Data obtained by audiomagnetotelluric surveying are presented here along with their interpretation, separately or in conjunction with information gained from other methods. GEOLOGICAL SETTING
Geographical description (after Vare t e t al., 19 79) R~union is made up of two basaltic shield volcanoes. The older of the two, the Piton des Neiges, rises up from ocean depths of a b o u t 4000 m to a height of 3069 m above sea-level. The part above sea-level started developing a b o u t 2 m.y. ago. It consists of a shield volcano formed by a series of olivine basalt flows. Its slopes were covered over by several hundred meters of differentiated series (basalts with feldspar phenocrysts, hawaftes, mugearites). Three main streams flow from the volcano's central part. Each of these flows across one of the three united cirques (Salazie, Cilaos, Mafate), roughly 6 km wide and 2 km deep, before escaping towards the ocean through deep gashes or gullies (Fig. 1). Zeolitized basalts crop o u t at the b o t t o m of these cirques as haloes, concentric to the volcanic massif. The corresponding isotherms suggest signs of a hydrothermal convection paleosystem which affected a region at least 10 km wide. The Piton de la Fournaise began to develop on the eastern slope of the Piton des Neiges a b o u t 0.35 m.y. ago and it is still active. Nearly all the historical eruptions t o o k place within the summit caldera inside which a shield volcano rises to 2630 m. At least five fissure eruptions have occurred on its sides during the last three centuries. The last of these destroyed part of a coastal village in 1977. To the east there is a graben-like structure 7 km wide linking the caldera to the coast and even extending b e y o n d it, according to bathymetric measurements. This supports the theory that the volcanism moved toward the east. On the northeastern and southeastern slopes historically active fissure zones radiate o u t from the caldera's summit toward the sea. An injection zone (dykes) forming a ridge oriented 140°E links up the summits of the two volcanic massifs. It is covered b y scoria, cinder cones and lava flows which, although prehistoric, are obviously recent. Even
313
© @eologic(]l
cross section of Ciloos cirque
Cirque .01 N
I2"N
23oom
deCILAOS •
Zeolitizotion limit
IN
StDENIS
® cross section of Piton de Io Fournolse
6eologiccl
Piton .
Riviere L A N G E V I N 2gg
~
dele FOURNAS IE 263om
~ I
I
Indion
I
St L
O
~
Zeolitization limit
REUNION ISLAND ~owm 0
5Km
Fig. 1. Simplified geological cross-section showing the ancient and recent volcanism of R~union Island. The different phases are marked I, II, III, IV (see text). The Piton des Neiges formations (old volcanism) are marked N. The Piton de la Fournaise formations (recent volcanism) are marked F. From 1/50,000 scale geological maps. if among t h e surface lavas there are olivine basalts similar to those of the Fournaise volcanism, it is reasonable to assume t hat this ridge could at least in part be d ue t o t h e presence of a rift zone which is part of t he Piton des Neiges system.
Chronological outline Chronologically, it was only some 2 million years ago that this island rose o u t o f the ocean. T he oldest rocks, basaltic agglomerates and breccias (phase I, mo r e than 2.1 m.y. ago), are to be f o u n d at the b o t t o m of the Cilaos, Mafate and Salazie cirques, giant excavations occupying the central part of the Piton des Neiges massif. N e x t came the olivine basalt and oceanite flows which piled up t o form the f r a m e w o r k of this ancient massif (phase II, 2.1--0.43 m.y. ago). These t h e n u n d e r w e n t an intense zeolitization. After a quiet period, activity began again (phase III, 0.35--0.25 m.y. ago). Th e Piton des Neiges was taken up to its m a x i m u m elevation by flows o f feldspathic p o r phyr i t i c basalt and of alkaline andesite. T ow ard the southeast, basalt and oceanite flows started to build up the Piton de la Fournaise massif. Eruptive p h e n o m e n a and considerable subsidence also marked the end o f this period in bot h massifs.
314 Another quiet period was followed by an active one (phase IV, less than 0.23 m.y. ago) characterized, especially in the Piton de la Fournaise massif, by basalt and oceanite flows, still occurring at the present time. Last known activity in the central part of the Piton des Neiges massif was 10,000 years B.P. OBJECTIVES OF THE GEOPHYSICAL INVESTIGATIONS An exploitable geothermal reservoir implies the existence of a very large heat source and of geothermal fluids to transport this heat up to the surface. The main physical characteristics making it possible, using geophysical methods, to detect and delimit geological structures likely to possess a geothermal reservoir are derived from this simple model. In a volcanic site the heat comes from magmatic intrusions whose density is relatively high compared to the surrounding rock, thus creating positive anomalies of the gravity field. This heat is transmitted to a geothermal fluid circulating in porous rock of relatively lower density. Furthermore, this geothermal fluid, having both a high temperature and a high dissolved salt content, corresponds to a zone of low electric resistivity (Keller and Rapolla, 1974). Inversely, a conductive zone does not always indicate the presence of a geothermal fluid. The presence of mineralized water at normal temperature is capable of lowering the resistivity of the rock containing this water considerably. It is therefore n o t surprising to find low resistivities in the superficial part of volcanic formations which can play an important role in the feeding of the geothermal reservoir. The interpretation of the resistivity anomalies of Rdunion is especially difficult because the water can be very mineralized, in particular close to the shore. One can thus expect very low resistivities, 1 ~ m or lower, which are usually considered to be indicative of the presence of hot mineralized water. This configuration of highly conductive formations in the vicinity of positive gravity anomalies is generally considered to be a necessary though not always sufficient indication of a geothermal reservoir (Gupta, 1980). Although other physical parameters may be sought, density and resistivity are usually studied first due to the ease with which the corresponding geophysical methods can be carried out during explorative prospecting. For this reason, the audiomagnetotelluric m e t h o d along with the gravimetric m e t h o d were used in R~union. The initial aim was to outline the zones over a surface area of about 1000 km 2 revealed as suitable for later investigation using more elaborate geophysical methods such as the dipole-dipole electric m e t h o d or the magnetotelluric m e t h o d at a greater investigation depth.
315 THE AUDIOMAGNETOTELLURIC METHOD
For more than 25 years n o w (Cagniard, 1953) one of the main advantages of the magnetotelluric prospection method has been recognized -- a very deep investigation using relatively light field equipment. The depth of penetration of electromagnetic waves is expressed by the following equation: 1
p =-~ x / - ~ /N p(km), p(~2m), N(Hz) being the penetration depth, the resistivity in the case of a homogeneous substratum and the frequency of the studied phenomenon, respectively, The Centre de Recherches G~ophysiques made use of this property in developing a very light apparatus (Jolivet, 1969) which could cover vast surfaces even in zones of difficult access and destined above all for the mining field (Benderitter, 1973). Since 1971 this measuring apparatus has been continually improved by the ECA Company 1. It consists of a magnetic induction receiver, a telluric line and a resistivimeter enabling the calculation in the field of the apparent resistivity according to the classic equation:
Pa - 5N
/-/y
Ex(mV/km )
and Hy(~/) being the amplitude of the horizontal c o m p o n e n t of the telluric field and the amplitude of the horizontal c o m p o n e n t perpendicular to the magnetic field. The total weight of the equipment is a b o u t 20 kg (magnetometer: 10 kg; resistivimeter: 8 kg; accessories: 1 kg). Installation at the site takes only a few minutes, the longest manoeuver being the installation of the telluric line whose maximum length is 50 m. It takes around 20 minutes to measure the resistivities for different frequencies. The surveying can be done easily by 2 people, one operator and one porter, even under the most difficult conditions. The model used in R~union (ECA 541-0) allowed measurements between 1700 and 8 Hz. Contrary to work carried out in other regions (Ballestraeci and Benderitter, 1980; Dupis et al., 1980), the first measurements showed the existence of resistivities that were exeeptionally high for volcanic sites of around several hundred to several thousand ohm-meters (Fig. 2). Identical resistivities have since been observed in the Kerguelen Islands (Ballestracci et al., 1983). An explorative survey cannot give quantitative results as precise as those ECA, 17 A v e n u e du Chateau, 9 2 1 9 0 M e u d o n Bellevue, France.
316 Apparent
resistivity Po (~m) 400O
F
J
'%x
i
I000
\ I00
~ 0 0
~
0 ~
0 I~
0 ~
I~ i~,
I
Frequency N(Hz)
I~. --
Fig. 2. Magnetotelluric sounding obtained in the Piton de la Fournaise zone. This sounding shows the high resistivities near the surface as well as the large contrast, over 100, between the resistivities of the surface and those of underlying rocks. o b t a i n e d f r o m a m o r e detailed investigation. H o w e v e r , t h e p r e s e n t meas u r e m e n t s also b e n e f i t f r o m a n o t h e r f a v o u r a b l e c i r c u m s t a n c e . Most o f the diagrams o f a p p a r e n t resistivity as a f u n c t i o n o f f r e q u e n c y c o u l d be considered t y p i c a l o f a s i t u a t i o n w h e r e resistant rocks overlie c o n d u c t i v e r o c k s with a very high resistivity c o n t r a s t o f a r o u n d 100. In this case, the thickness o f t h e resistive r o c k can easily be d e t e r m i n e d f r o m the e q u a t i o n : e = 0.356
V~a/N
e ( m ) , p a ( ~ m ) and N ( H z ) being the t o t a l thickness o f the r o c k s covering t h e c o n d u c t i v e rocks, t h e a p p a r e n t resistivity at any p o i n t o f the diagram lying o n t h e straight line o f the --2 limiting slope and t h e c o r r e s p o n d i n g f r e q u e n c y , respectively. As o n l y a few m e a s u r e m e n t s are n e e d e d to o b t a i n the limiting slope satisfactorily, t h e thickness o f t h e resistive r o c k s c o u l d be o b t a i n e d v e r y precisely. H o w e v e r , this precision c o u l d be illusory. T h e above relationship is o n l y t r u e in a t a b u l a r case. Volcanic r o c k s c a n n o t easily be r e p r e s e n t e d b y this type of model.
317 It m u s t be n o t e d t h a t t h e large n u m b e r o f m e a s u r e m e n t s d i s t r i b u t e d a c c o r d i n g to t h e profiles m a d e it possible t o o u t l i n e z o n e s w h e r e t h e app a r e n t resistivities r e m a i n quite c o n s t a n t f r o m one station t o a n o t h e r and w h e r e , t h e r e f o r e , the r o c k s t r u c t u r e is n o t t o o unlike a tabular m o d e l . In this way we avoid t h e use o f m o d e l s which are c e r t a i n l y m o r e suitable, like the c y l i n d r i c a l - t y p e m o d e l s (Th~ra, 1 9 7 7 ) , b u t whose use is m o r e complex. In the p r e s e n t case w h e r e s t r u c t u r e s resembling wide c o n d u c t i v e veins in a resistant m e d i u m are sought, the e x a m p l e given in Fig. 3 shows t h a t rKm
0
IKm
2Kml
I 0 0 0 JAm _ m ~ m m
m
\
Lo
E
~
m
I 0 0 0 J3_m
2Km
o
Fig. 3. Cylindrical structure and the multi-layered earth interpretation derived from it. In each case only half a section of the structure was represented along a plane perpendicular to its elongation. Curve 1 marks the separation between the surface rocks and the underlying rocks. Curve 2 demarcates the rocks with a resistivity of over 950 a m from those of a lower resistivity. t h e use o f a t a b u l a r - t y p e m o d e l for i n t e r p r e t a t i o n gives results b e t t e r t h a n m i g h t be e x p e c t e d at first sight. This o b s e r v a t i o n was m a d e a f t e r calculating t h e t h e o r e t i c a l soundings spread along a profile c u t t i n g t h r o u g h the cylindrical m o d e l p r o p o s e d in the left half o f t h e figure and i n t e r p r e t i n g these soundings a c c o r d i n g to a t w o - l a y e r tabular h y p o t h e s i s . T h e results were t r a n s f e r r e d t o the right-hand side o f the figure and it can be seen t h a t while the vein's resistivity deviates f r o m the t r u e value, especially t o w a r d its edges, a high degree o f a c c u r a c y is o b t a i n e d for the resistivity and thickness o f the r o c k s lying above t h e c o n d u c t i v e s t r u c t u r e . T h e struct u r e itself is satisfactorily d e m a r c a t e d . These conclusions c o n c e r n the case w h e r e the telluric field is m e a s u r e d along t h e axis p e r p e n d i c u l a r t o the axis o f the s t r u c t u r e . T h e y agree with t h o s e given by some o t h e r a u t h o r s (Hjelt e t al., 1 9 7 9 ; Meheni, 1 9 8 0 ) .
318
On the other hand, the example given in Fig. 4 shows that the relief can greatly alter the results. R~union possesses extremely narrow valleys cutting deep gashes into the slopes of the volcanic massifs and spectacular escarpments several hundred meters high. The greatest effect is on frequencies where the penetration depth is similar to the magnitude of the relief. It is always advisable to measure the telluric field parallel to the valley's axis and to keep away from the immediate vicinity of the rupture zones with precipitous slopes to reduce disturbances to a minimum. In such cases, as in the example chosen for its accentuated relief, the results are acceptable if care is taken to keep a few hundred meters back from the cliff. Apparent resistivity
pa (rim) 5000.
I000
~
I~
~,~
~,~
*\ \ + \\
i
....
/ i/
IOC
50 + - -
.
IO00Hz
+------÷
IO00Hz
IOHz
IOHz
osKil 0,5
I
1,5 Km
Fig. 4. Influence of the relief at two different frequencies (1000 and 10 Hz) and along the two main measuring directions. Solid line: E measured along the valley's axis. Dashed line: E measured perpendicular to the valley's axis. LOCATION OF THE MEASUREMENTS
The first measurements, taken in January 1979 as a reconnaissance survey, encircled the Piton de la Fournaise massif except for its southern section, at a distance varying between 8 and 16 km from its center. The
319
Rose
tO 00tom
S~ Pierre
Fig. 5. Location of the audiomagnetotelluric soundings and position of the profiles in Figs. 12 and 13.
320 eastern zone, characterized by a depression linked to the recent extension of the volcanism, and the western and especially northwestern flanks (Plaine des Palmistes), both situated between the Piton des Neiges and the Piton de la Fournaise massifs, were the main objectives of the present survey. The first measurements were followed by a second set in May 1979. These were taken towards the southwest and continued in a more detailed manner the study of the link between the two massifs. This survey also included two areas of the Piton des Neiges massif (Cilaos and Salazie cirques), interesting because of their topographical location near the center of the massif and giving hope of finding intense, though quite old, hydrothermal phenomena. A third set of measurements was made in September 1980 aimed at obtaining a cross section of the Piton de la Fournaise formations along an approximately radial plane, particularly along the Langevin River. This stream runs through a spectacular gully where zeolitized layers are found locally. A total of 535 measuring stations {see Fig. 5) were set up. They were chosen partly according to hypotheses which were drawn from the results obtained during the different stages of the magnetotelluric survey itself and partly from the results of other geological and geophysical investigations (gravimetric and electric surveys) taking place at the same time. They were spread out over a surface of about 1000 km 2. The high density of soundings in some areas permitted detailed interpretation, for example in the Plaine des Palmistes and the Langevin River. In addition, the large number of measurements taken in the different geological zones statistically enabled the elaboration of a regional model. This model takes into account the average distribution of resistivities in the island's sub-soils from which comes the notion of anomaly. REGIONAL STUDY OF THE RESULTS Once it was realized t h a t nearly all the soundings presented the same kind of apparent resistivity diagrams as the one in Fig. 2, a first general approach to the interpretation was agreed upon: a tabular model made up of resistive rocks overlying a conductive substratum whose resistivity is n o t more than 1 to 2% of that of the covering layer. The thickness and resistivity of the first rock layer were obtained in this manner for each sounding. After determining the thickness of the resistive rock, it was possible to map, with a slight smoothing, the isopleth of the resistive--conductive interface (Fig. 6). This parameter rises steeply from the island's shore (--500 m) towards the Piton des Neiges and Piton de la Fournaise massifs (+1000 m and 1500 m, respectively). The high values are found along an axis which coincides almost exactly with the topographic crest line. Moreover, we notice that the thickness of the high-resistivity rock also general-
321
\
o
jj~
p
Fig. 6. Contour lines (in kilometers) of the top of the conductive layer. Note the coincidence between the topographic crest line and the high elevation of the conductive layer. ly increases f r o m the c o a s t t o w a r d s the middle o f the island r o u g h l y acc o r d i n g t o the altitude o f the m e a s u r e d p o i n t s (Fig. 7). This shows t h a t the c o n d u c t i v e layer is p r e s e n t e v e r y w h e r e and t h a t it rises t o w a r d s the island's
322 Altitude IN]
Z(m)
/
{N)
2400. •
•
.
,"
.
•
..
•
j .
2200. 2000.[
'l 1800 o
1600=i 1400o 1200=i 1000o! !
800°i 600° (M)
,:00.
(N)
Linear
regression
El = 0,3Z+
485
E~=O,3Z+
85
200o
Thickness of
500
1000
1%00
the
first
layer
[I (m) 2000
Fig. 7. Variation of the thickness of the resistive covering rocks as a function of the altitude of the soundings. All the soundings to the left of the straight line (N) are considered to be located on a resistive surface layer which is abnormally thin compared to the altitude• The points are located on the Cilaos and Grand Pays cirques and in a few places on the Plaine des Palmistes.
c e n t e r b u t , in g e n e r a l , a l i t t l e m o r e s l o w l y t h a n t h e t o p o g r a p h i c a l s u r f a c e . In a d d i t i o n , a r e m a r k a b l e c o r r e l a t i o n c a n be o b s e r v e d b e t w e e n t h e resistivi t y p~ a n d t h e t h i c k n e s s E l o f t h e r e s i s t a n t r o c k s . T h i s is s h o w n b y t h e c u r v e p ~ = F ( E ~ ) p l o t t e d in Fig. 8. W h e r e t h e first r o c k l a y e r b e c o m e s v e r y t h i n its r e s i s t i v i t y falls t o a r o u n d 5 0 t o 1 0 0 ~ m . C o r r e s p o n d i n g s o u n d i n g s w e r e all s e e n t o b e o n r o c k s w h i c h w e r e v e r y rich in h y d r o t h e r m a l d e p o s i t s ( z e o l i t e s ) . T h i s is i l l u s t r a t e d by s o m e s o u n d i n g s in t h e C i l a o s , S a l a z i e a n d G r a n d P a y s c i r q u e s .
323 Resistivity of the first Ioyer
PI (rn)
?= ~ 6co~1
"
Sounding
......
• vo~)
~ooo
type I
i
,8a
i
I'~ ~ ,2o ~ /o0
I!
. "~/~-
I:
""
.[..
i
- -
-Sounding
type 5
.
(P
E
5Of} m
230m1
Thickness of the first Ioyer £4 (m) I0
ioo
~2e
,50
2~
:~
JO~
~00
~0
~
mO ~
~
o0o
,m~O
mOO
2~
Fig. 8. Variation o f the resistivity o f the first layer according to its thickness. Points were c h o s e n regularly spread o u t along the m e a n curve and equivalent multi-layered m o d e l s were calculated. The p s e u d o - c r o s s section o f Fig. 9 is the result o f this. The c o m p a r i s o n o f the interpretation s o u n d i n g w i t h s o u n d i n g is given in Fig. 10.
On t h e o t h e r h a n d , t h e first r o c k layer, over 1 0 0 0 m thick, is a s s o c i a t e d w i t h resistivity values o f over 1 0 0 0 ~2m and c o r r e s p o n d s t o s o u n d i n g s w h i c h are all l o c a t e d o n very t h i c k r e c e n t basaltic f l o w s such as t h o s e carried o u t in t h e P i t o n de la F o u r n a i s e area.
324
Preparation of a general model The conclusions of the previous discussion lead to the elaboration of a model made up of layers of decreasing resistivity down to a deep conductive layer (Fig. 9). This model seems to describe perfectly the average behavo iour of the soundings made as shown by the controls set out in Fig. 10. It is possible to recreate the observed soundings by "eroding" this model down to the lower layers. The first interpretation of the soundings using a two-layer model is therefore an approximation. It can be seen, however, that the depth of the top of the second layer corresponds quite well with the depth of the top of the deep conductive layer in the multi-layer model. Such a model implies a very conductive layer (resistivities around 1 ~2m) under all the regions studied on the island. The hydrothermal mineralizations outcropping wherever this level arrives close enough to the surface suggest that this horizon can be likened to a layer in which very mineralized waters circulate (or circulated long ago -- geophysical methods are poor at historical reconstitutions). The geothermal importance of this marker is obvious since it may be (or have been) fed by water coming from deep below the surface and therefore hot. The fact that it is covered by rocks made impermeable by hydrothermal deposits would have stopped any important escape towards the surface. The vast extent of this impermeable layer throughout the island may explain the small number of secondary volcanic features (hot springs, fumaroles, etc.). The levels with 30, 50 or even 100 ~ m can be attributed to this cover layer.
Piton de Io FOURNAISE Plolne des PALMISTES
~lome des
©
CAFRES Cirque de CILAOS ou de SALAZrE
®
®
Rivlere LANGEVIN
Cirque de GRAND PAYS
®
550
50
nm
Sounding ,yp . . . . bet (See
Fig. 9. General multi-layer model of R6union Island.
8 ond ,01
325
Pa (~,m)
S o u n d , n g type
IO000 -
Sounding type 2
I
"FOURNAISE "
I0000
" P l o , n e des
-
'r L A
Sounding type I000-
CAFRES
CRETE
" Ploine
ii
" I00 :
L" ~ O- , ~ J "x
I000"
IO=
N(Hz)
i
tO000
I0000
10(3
O" I
f ~oo o~' I00o -
500 o _-
2OO0
-
~- r oo 3 fi
--I10 _-- : : 7
.....
1500
-
55o
550
-
E
sO
4
S o u n d i n g lype LANGEVIN
Sounding
SUD "
"GRANDI~¥S" tooo-
i
I00 •
I00-
,° i
I0
"
IN n m
RESISTIVITY
4
5O 30
£0
I
~N ~'~m
50O
2ZO .0
-
150o
P=l
type
,"
i
5
Somlding
SALAZIE
type 6
II S A L A Z I E C e n t r e " , ' r C I L A O S C e n t r e "
"
" C I LAOS "
IO0~
*
IO -
i
,oo'oo....
t'4o .....
o
220 .0 50 ,
5O0-
1000
,2,20o
IO00
~ " _o
5o
I
550
t
z2o
R[SISTIVITY
" Riviere
1500
15ooe
5oo~
E
IO0
I0000
I
Or
I000
VB E E
\ IO0 ~
•
3
PALMISTES "
des
,'
200
I0000
i'~
50
E
,
I O0
•m
~:
I00
o:_ o
RESrSTIVlTy
IN
CZm
I
~
~oo:0o p= i
~0o
I
iz ,N
•
_~
7
f~ESISTIVIT ~
I0000
i
RESISTIvrTY
.
i
I
E
-
IN ~]m
Fig. 10. C o m p a r i s o n b e t w e e n the effects of the standard soundings of Fig. 8 and those of a multi-layer m o d e l w i t h progressive erosion of the upper levels. + A p p a r e n t resistivity calculated w i t h a two-layer model. - - Apparent resistivity calculated w i t h a multi-layer model. A surface layer (p = 50 ~ m , E = 4 m ) w a s added to multi-layer m o d e l s 1, 2, 3, 4.
According to the geological observations, the soundings corresponding to a surface level of 200 to 500 ~2m resistivity are mostly located either on old basalts (first series of the Piton des Neiges or Piton de la Fournaise) or in their immediate vicinity. Some of them might be typical of zones
326 s h o w i n g slight traces o f h y d r o t h e r m a l d e p o s i t s in m o r e r e c e n t f o r m a t i o n s . Finally, we m u s t c o n s i d e r t h e s o u n d i n g s d e p i c t i n g highly resistive surface layers ( 1 5 0 0 t o 1 5 , 0 0 0 g t m ) . T h e s e are f o u n d s y s t e m a t i c a l l y in t h e F o u r n a i s e massif at q u i t e high altitudes (Rivi~re de l'Est, t h e F o u r n a i s e s u m m i t , Plaine des Cafres, s o m e p a r t s o f the Plaine des Palmistes, etc.). T h e high resistivities can be e x p l a i n e d b y t h e p r e s e n c e o f r e c e n t basaltic flows. T h e s e are p r a c t i c a l l y d r y as t h e y are t o o p e r m e a b l e to h o l d w a t e r , as was p r o v e d b y a 2 0 0 - m b o r i n g d o w n t h r o u g h t h e Plaine des Palmistes.
Use o f the proposed model for definition o f the anomaly zones In spite o f t h e f a c t t h a t t h e m o d e l s h o w n gives a v e r y realistic p i c t u r e o f t h e results o f t h e 535 s o u n d i n g s carried o u t , it m u s t be n o t e d t h a t inside a n y o n e z o n e c o n s i d e r a b l e v a r i a t i o n s o f t h e p a r a m e t e r s are o f t e n o b s e r v e d (Fig. 11). Resistivity of the first layer R, (D.m)
Resistivity of the first layer
Iooo
p, (D.m)Iooo
Y
26. : '
I00
I00
.,:: :o.:, ,B
,:
SALAZIE
I00
5()0
,0~)0 *
Thickness of the first layer
CILAOS
E~ (m)
Thickness of the first layer
E~ (rn)
roo
T 500
I000
Fig. 11. Evolution of the resistive cover rocks' resistivity according to their thickness in the Cilaos and Salazie cirques..n = sounding number. Solid line = mean curve for the whole island (see Fig. 8). The places where the resistive cover rocks are thinnest (and therefore least resistive) coincides with the outcrops of rocks very laden with hydrothermal mineralizations. S o m e o f the d a t a s c a t t e r i n g is c e r t a i n l y d u e to the u n c e r t a i n t y in t h e m e a s u r e m e n t s . H o w e v e r , t h e largest d e v i a t i o n s do n o t s e e m to be e n t i r e l y d u e to r a n d o m statistical d i s t r i b u t i o n . T h e v a r i a t i o n s o b s e r v e d along t h e m e a n curve p~ = F(E~) (longitudinal
327 variations) can reflect the local changes in the depth of the most mineralized layer u n d er a cover layer described exactly by the proposed model. The deviations observed on b o t h sides o f the mean curve (transverse variations), especially when t h e y are large, indicate the differences between the true structure of the cover layer and its average structure. In particular, the points located to the left of the mean curve p~ = F(E~) correspond to soundings in areas where the average thickness of the cover rock, lower than that o f the model, suggests the existence of m ore conductive formations at an equivalent depth. Conversely, the soundings to the right are located in zones of thicker coverage. Consequently, in t he zones where the soundings give the lowest average values for p~ and E1 it is most likely t hat a convection loop, associated or n o t with erosion, could bring mineralized and probably h o t fluids up near the surface. In these conductive zones and of these soundings, it is those located furthest to the left of the line which would indicate the most interesting sites. This argument is verified in the Salazie and Cilaos cirque regions for which, in addition, several mineralized rock samples were found. Indeed, one can observe in t he Salazie cirque (Fig. 11) that it are soundings 10, 11, 12, 13, 14 and 16 which, f rom the point of view of the longitudinal position in relation to the mean curve, indicate the closest proximity to the conductive layer. The rock samples available revealed, after analysis, th at it was in this region that the h y d r o t h e r m a l mineralizations were the most abundant. A 200 m drilling near sounding 11 revealed a geothermal gradient six times larger than normal. It can also be observed in the Cilaos cirque (Fig. 11) that sounding 14 is distinct in its longitudinal position, and that sounding 13 lies noticeably t o the left with regard to its transverse position. These two soundings correspond geologically to the formation's m a x i m u m mineralization and are located n e x t to a thermal spring. A n o t h e r logical consequence of the proposed model comes from the general relationship existing between a station's altitude and the thickness o f the high-resistivity cover rock. The interesting zones are those where the cover rocks are the thinnest com pa r ed to the altitude. The cirque and Langevin River zones fulfill this condition. This could be seen from Fig. 7 where th e dots, representing the soundings made in these areas, are all located well to t he left o f t he line.
EXAMPLES OF DETAILED STUDIES T h e Plaine des Palrnistes
The m e a s u r e m e n t stations were set up along a NNE--SSW profile about 8 km long {Profile 1, Fig. 5).
328 Bouguer onomoly
(msol)
Grovimetric profile
I
250
Electricol
profile 150 1 I00
sol Interpretive audio-magnetoi'elluric Ground level
-S . S W
500
- 500
cross-section NNE Altitude (m)
.4
Jq.m
iO00
5OO Interf°ce~
/
20.Q.rn ~ 500
r
15OOm
Fig. 12. Comparison between the behaviour of the resistive-conductive interface (obtained by interpretation of the audiomagnetotelluric soundings), the variation of the apparent resistivities (obtained by resistivity mapping using direct current, electrodes 6000 m apart) and the variation of the gravity intensity in the Plaine des Palmistes (profile 1, Fig. 5). C o m p a r i s o n s w e r e m a d e with gravity a n d resistivity d a t a , t h e l a t t e r using c u r r e n t e l e c t r o d e s 6 0 0 0 m a p a r t a n d p o t e n t i a l m e a s u r i n g lines 500 m l o n g (Fig. 12). T h e a u d i o m a g n e t o t e l l u r i c results are given as a cross-section revealing in its c e n t r a l p a r t v e r y t h i c k resistive rocks. On b o t h sides t h e r e is a resistive h o r i z o n o n t h e surface laying on t o p o f a c o n d u c t i v e l a y e r which c o m e s n e a r t h e surface at s o m e spots. Since t h e i n t e r p r e t a t i o n was d o n e using a t a b u l a r m o d e l , it is advisable t o t a k e i n t o a c c o u n t w h a t was said p r e v i o u s l y if an even m o r e a c c u r a t e idea o f t h e s h a p e o f t h e c o n d u c t i v e s t r u c t u r e is sought. When these results are c o m p a r e d w i t h t h e d a t a o f the electrical survey (resistivity m a p p i n g ) we n o t i c e t h e i r p e r f e c t c o r r e l a t i o n . T h e a p p a r e n t electric resistivity decreases each t i m e t h e m a g n e t o t e l l u r i c c o n d u c t i v e l a y e r a p p r o a c h e s t h e surface. T h e r e w o u l d be n o t h i n g surprising in this if t h e c a p a c i t i e s o f t h e m a g n e t o t e l l u r i c m e t h o d did n o t still n e e d proving. T h e m e a s u r e d p a r a m e t e r is t h e s a m e a n d t h e investigation d e p t h is similar for both techniques.
329
Much more interesting is the comparison of the gravimetric data and the preceding results. Coinciding with the high-resistivity electric anomaly in the profile's center is a positive gravimetric anomaly, on a regional scale stretching with a NW--SE axis from the Cilaos and Salazie cirque zones to near the Piton de la Fournaise. Therefore, we probably have here compact formations of groups of intrusions. This can be geothermally interesting if the magmatic uprisings are recent enough to form a heat source which is still important. On both sides of this central zone, especially in the SSW part of the profile, more conductive formations can be found, coinciding with a negative gravimetric anomaly. This can be interpreted as being characteristic of more fissured zones. The SSW part of the profile gains in importance as very low frequency magnetotelluric measurements made by Dupis (Benderitter et al., 1981) revealed a conductive layer about 7 km deep. This is twice as deep as in the NNE part of the profile. The Langevin river A 15-km-long profile of 64 soundings was drawn (Profile 2, Fig. 5). • he data are presented in the form of a semi-quantitative cross-section in Fig. 13 where it can be directly compared with the data recorded along the same profile by the electric survey using a dipole-dipole system A u d i o - m o g n e t o t e l l u r i c pseudo c r o s s - s e c t i o n Soundt;~g numlN~
Dipole-dipole
pseudo cr0ss-section Geoth,,mo~ wQ,e,~o~,
AppQrenl resistivity in ~,m
Fig. 13. Comparison of the pseudo-cross section of the apparent resistivities obtained by a u d i o m a g n e t o t e l l u r i c soundings in the Plaine des Sables and along the Rivi6re Langevin (Profile 2, Fig. 5) and that of the apparent resistivities in dipole-dipole (AB = MN = 500 m) along the same profile.
-
~
~
~
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....
~ ~
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):"
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-
~
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~:~:tcO:tg-~:n~i::i:~d
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i
n
i
:
~
Ifl.m
7 E
Schlumberger sounding SE3 f " ~ / / ~ Groundlevel for -E ~ audio-magnetotellurfc /
Fig. 14. Comparison of the modeling of the audiomagnetotelluric and dipole-dipole pseudo-cross sections along the Rivi~re Langevin.
J
Altitude(m)
Electrical sounding number
5o
331 (AB = MN = 500 m). A similarity in the position of the deep conductive anomalies can be seen, confirming the validity of the audiomagnetotelluric results. A more detailed comparison reveals, in addition, the greater penetrating power of the audiomagnetotelluric method. This m e t h o d is also seen to be more reliable in a zone of accentuated relief, as is the case towards the middle of the left half of the profile. The interpretation of these two cross-sections in terms of depth gives almost identical results {Fig. 14). The geothermal significance of the conductive region near the middle of the profile was confirmed by the intense zeolitization of the recent surface formations. The region is also marked by a very large positive gravimetric anomaly which can be interpreted by an abnormal density due to intrusions of deep-seated origin. A spontaneous polarization anomaly, characteristic of a p h e n o m e n o n of thermal origin, was also revealed at this spot. CONCLUSIONS Although geochemistry and, in the last stage, drilling, are the only ways to prove the existence of a geothermal reservoir, geophysics can help determine the areas where the chances of finding the reservoir are greatest. The resistivity of rocks is an essential parameter in geothermal surveying. The electrical methods c o m m o n l y used in geophysics are poorly suited in this case because of the great depth of investigation sought to detect geothermal resources. The high resistivity of the surface volcanic rocks of R~union allows, using the audiomagnetotelluric method, a depth of investigation that is interesting for geothermal surveys. Because of the ease of operation of the equipment used, this m e t h o d allows one to carry out a large number of measurements over a large area at a reasonable cost. It thus became possible to process the data statistically and to determine the mean behaviour of the layers for the whole island. The model obtained shows a series of layers whose resistivity decreases with depth, ending with a layer whose resistivity is about 1 ~2m. Because of the scale of the phenomenon, this model is or was directly or indirectly linked to a thermal effect. This thermal effect may have induced hydrothermal circulations, which would generate conductive mineralizations, the density of which decreases gradually upwards from the paroxysmal conductive layer which forms a possible top of the geothermal prospect. The disparities between this regional model and each measurement enable the determination of anomalous areas, some of which may prove to be the main geothermal targets of R~union in agreement with the other geological and geophysical data. ACKNOWLEDGEMENTS Ph. Marie is thanked for carrying out part of the measurements and A.K. Bourg is thanked for the English editing of this paper.
332
We thank the French Ministry of Industry and FIDOM for financial support of the geothermal exploration program of R~union Island carried out by the Geothermal Energy Department of the B.R.G.M.
REFERENCES Ballestracci, R. and Benderitter, Y., 1980. Sondage magn6totellurique dans la gamme 8 Hz--170 Hz ~ proximit6 de l'Ardoukoba (Rift d'Asal, R~publique de Djibouti) R6sultats, calcul d'un module repr6sentatif et interpretation. Bull. Soc. Geol. Fr., 22(7): 873--879. Ballestracci, R., Nougier, J. and Benderitter, Y., 1983. Mise en 6vidence et interpr6tation d'une zone ~lectriquement conductrice dans des basaltes des plateaux des iles Kerguelen (T.A.A.F.). C.R. Acad. Sci., 296, serie II: 833--838. Benderitter, Y., 1973. Magneto-tellurique. Rev. Ind. Miner., Spec. Vol., 15 May, 1973: 73--81. Benderitter, Y., Dupis, A., Fitterman, D., G6rard, A., Puvilland, P., Rancon, J.Ph., Robert, D., Stieltjes, L. and Varet, J., 1981. Evaluation du potentiel g~othermique de l'ile de La R~union. Rapport du Bureau de Recherches G~ologiques et Mini~re, 81 SGN 669 GTH. Billard, G. and Vincent, P.H., 1974. Cartes g~ologiques de la France au 1/500 000. Saint Denis, Saint Benoit, Saint Pierre, Saint Joseph. Bureau de Recherches G~ologiques et Mini~re. Cagniard, L., 1953. Basic theory of the magnetotelluric method of geophysical prospecting. Geophysics, 18(3): 605--635. Dupis, A., Marie, Ph. and Petiau, G., 1980. Application des m~thodes de prospection magn6to-tellurique pour l'exploration g6othermique du Massif du Mont Dore. Contrat 582-78 EGF, G6othermie, Commissions des Communaut~s Europ~ennes, Strasbourg. Gupta, H., 1980. Geothermal Resources: an Energy Alternative. Developments in Economic Geology. Elsevier, Amsterdam, 227 pp. Hjelt, S.E., Kaikkonen, P. and Pietila, R., 1979. On the interpretation of VLF resistivity measurements. Paper presented at the 12th Meeting of the Nordic Association of Applied Geophysics, Oslo, January 1979. Jolivet, A., 1969. Etude d'un 6quipement l~ger pour la prospection magn~to-tellurique de subsurface; Th~se d'universit~, Facult6 des Sciences de Paris, 113 pp. Keller, G.V. and Rapolla, A., 1974. Electrical prospecting methods in volcanic and geothermal environments. Developments in Solid Earth Geophysics. Physical Volcanology, Elsevier, Amsterdam, pp. 133--166. Meheni, Y., 1980. Analyse de la r~sistivit6 apparente magn6totellurique sur divers modules bidimensionnels. Etude d'un sondange M.T. fi six composantes, cas particulier de la composante tellurique verticale. Th~se de Troisi~me Cycle, Universit~ Pierre et Marie Curie, Paris, 140 pp. Th6ra, A.L., 1977. Interpr6tation des mesures magn~totelluriques ~ partir d'un module deux dimensions. Th~se de Troisi~me Cycle, Universit6 Pierre et Marie Curie, Paris, 104 pp. Varet, J., Stieltjes, L. and Duffield, W., 1979. Etudes g~ologiques et g~othermiques de la R6union. R4sum~ des principaux r6sultats scientifiques et techniques du Service G~ologique National pour 1979. Rapport du Bureau de Recherches G6ologique et Mini~res. -
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