458 Fourth /SAG. Goe ttingen (Gerntany}, 04-06// 0// 999
CONTRIBUTION OF SELF-POTENTIAL AND SOIL-TEMPERATURE SURVEYS FOR THE INVESTIGATION OF STRUCTURAL LIMITS AND HYDROTERMAL SYSTEM ON UBINAS VOLCANO (PERU)
MACEDO SANCHEZ O. ( 1). FINIZOLA A. (2.3). GONZALES K.( 1). and RAMOS D. (1 )
( 1): Instituto Geoffsico dei Per ü, IGP-Arequipa. Urb. La Marina B-19. Cayrna, Arequipa-PERU - E-mail: [email protected] - Tel/Fax: (5 1)54-251373 (2): Universite Blaise Pascal. OPGC-CNRS. UMR6524-CRV, 5 rue Kessler, 63038 Clermont-Ferrand (France) - E-mail: [email protected] - Tel: (00-33)-4-73-34-67-44 (3) IRD - (ex-ORSTOM) UR6 (France)
KEY WORDS : Self-Potential. Soil-Temperature . Ubinas volcano , South Peru
INTRODUCTION
Ubinas strato-volcano ( 16° 22' S. 700 54' W, 5672 m) is the most active voJcano of Peru: it has been reported 23 minor eruptions since 1550. This volcano has been built upon the Altiplano and the Cordillera Occidental and its truncated appearance is due to a broad summit crater or caIdera of 1,2 km wide. The south part of the caldera contains a pit funneJ-shaped crater (600 m in diarneter, 300 m deep), where a permanent fumarolic activity take place. Increasing gas emission has been reported since late in 1995. White and bluish steam commonly rose 100-500 m over the summit and sorne limes these gases reach filling cornpletely the entire caldera in a few hours. Observations and thermie measurements realized at the bottorn of the pit crater revealed the presence of six zones. located near the walls, erniting gases with strong pressure and high temperatures. until 4440 C. These fumeroles constitute the unique rising gas evidence over the entire volcanic edifice. Detailed Self-potential (SP) and soil-ternperature measurements has been carried out inside the summit caldera's floor. sr and thermie maps show no significative anomaly in the caldera. (Figs. la, lb). Fourth /SAG, Goettingen (Germanyï, 04-061/01/999 459
Thermie results seems to show an absence of comunication between hydrothermal system of ioday's active crater and the caldera's tloor. SP results show no evidence of gas rising organization inside the caldera . Four long radial SP profiles were also measured over the entire volcanic edifice (Fig. 2). The SP versus elevation profiles show a common pattern on active volcanoes: the lower part is characterised by inverse relationship between SP and elevation whereas it inverts in its higher parts (Fig.3). This lower pan is interpreted by normal hydrogeological model, whereas the upper part is a signature of an hydrotherrnal system ( Zhody et al., 1973; Corwin & Hoover, 1979; Anderson & Johnson, 1978; Fournier, 1989). On the map of the area, the limit between these two parts (hydrogeological and hydrotherrnal) shows a circular shape of approxirnatively 6 km wide. These results can be interpreted a signature of a deep structural 1imit of an older great caldera.
CONCLUSIONS
Self-potenrial and soil-temperature results results suggest that Ubinas hydrotherrnal system IS ln over-pressure state having difficulties 10 evacuate the enormous volume of superficial water infiltrated in the caldeira. Also, results of self-potential long profiles over the eruire volcanic edifice show the signature of an old great caldera of circular shape and 6 km wide.
REFERENCES
Anderson L.A. and Johnson G.R., 1978. Sorne observai ions of the self-potential effect in geotherrnal areas in Hawaii and Nevada . Geothermal Ressources Council Transactions, 2, 9- 12.
Corwin R.F. and Hoover D. B., 1979. The self-potenlial method in geotherrnal exploration. Geophysics, 44-2,226-245.
Fournier c., 1989. Spontaneous poteruials and resistivity surves applied to hydrogeology in a volcanic area: case history of the Chaine des Puys (Puy-de-Deme, France). Geophysical Prospecting 37, 647-668.
Zhody A.A.R., Anderson L.A. and Muffler L.J-P., 1973. Resistivity, self-potential and induced polarisation surveys of a vapor dominated geothermal system. Geophysics, 38-6, 1130-1/44. 460 Fourth /SAG, Goettingen (Germany). 04-061/01/999
1000 ' Caldera 's f100rlimrt
900 SP 800 , ( mV) ...... 1100 700 j Crater " . 75 2 600 :
~'77" 1 e 25 Crater 3 400 - 300 j o ·25 2 00 '
-75
'-----J--200 m. 01 1 a 100 200 300 400 500 600 700 800 900 1000
Fig. 1a.- Sc1f-potential map of Ubinas's ealdera. Crosses: location of measurement stations
10001 Caldera's f100rlimil
900 1
800 1 T ( 1IG) 700,
2 6001 400
500 1 300 Crater 3 i l ,i 400 1 Acidsourœ
•• ] "1 200 1. .• 6 1 2 ' 3 300 : " O4 10 1 l , 2001 5 . -20
200m. O' .. a 100 200 300 400 500 600 700 800 900 1000 Fig. 1b.- Infra-red thermie map of Ubinas's ea1dera. Crosses: location of measurement stations 1,2.3,4,5 and 6: permanent fumarolic zones Fourth /SAG, Goettingen (German y), 04-061/01/999 461
SP (rnV)
Fig. 2.-Self-potential map of Ubinas volcano Crosses; location of SP measurement stations
3750 4000 4250 4500 4750 5000 5250 Altitude Fig. 3. Example of Self-potential profile obtained at Ubinas volcano, (SE profile).