Ž . Journal of Applied Geophysics 48 2001 143–152 www.elsevier.comrlocaterjappgeo Hydrothermal system mapped by CSAMT on Karthala volcano, Grande Comore Island, Indian Ocean Cecile Savin a, ) , Michel Ritz b , Jean-Lambert Join a , Patrick Bachelery a ´ a Laboratoire des Sciences de la Terre, UniÕersite de la Reunion, BP 7151, 97715 Saint-Denis Messag. Cedex 9, France ´ ´ b IRD-UR Geophysique, BP 1386, Dakar, Senegal ´ Received 27 June 2000; accepted 18 June 2001 Abstract Ž . Controlled source audio-magnetotelluric CSAMT has been used to investigate the resistivity structure of the summit region of Karthala volcano. The major purpose of this CSAMT survey is to locate the active hydrothermal system. The presence of the hydrothermal system had already been inferred from surface evidence of hydrothermal activity and from Ž . Ž. self-potential SP mapping of the Karthala summit zone. The results of the 1D CSAMT inversion indicate the following: 1 Ž . a highly resistive 500–5000 V m 200- to 400-m thick surface layer, that is characteristic of dry basaltic rocks, and made Ž. Ž . up of lava flows andror tuff breccias; 2 a 300- to 1200-m thick layer of intermediate resistivity 20–400 V m , thought to Ž. be representative of the groundwater body; and 3 a deep conductor with a resistivity of less than 2 V m, which might be related to the active hydrothermal system. This hydrothermal system appears to be bounded by caldera edges and shows the same north–south trend. Its depth ranges from more than 1 km to less than 0.7 km in the northern part of the caldera and its resistivity ranges from 2 to 0.5 V m in the northern part of the caldera. Less resistive zones and the shallowest depths of this conductive layer are well correlated with the largest SP positive anomalies and are assumed to be generated by hot fluid circulation. The most active hydrothermal zone is situated in the northern part of the caldera. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Karthala volcano; CSAMT method; Resistivity structure; Hydrothermal system 1. Introduction Ž . In recent years, electromagnetic EM methods have become prominent tools for detailed structural resistivity studies, especially in volcanic regions where surface resistivity is high. The controlled Ž . source audio-magnetotelluric CSAMT method has ) Corresponding author. Fax: q 33-2-62-93-82-66. Ž . E-mail address: savin@univ-reunion.fr C. Savin . been successfully used for geothermal exploration Ž . Bromley, 1993; Wannamaker, 1997 to overcome a lack of telluric signal strength in the audio-frequency portion of the natural spectrum. We applied the CSAMT method to the Karthala volcano, which forms the southern two-thirds of the Grande Comore Island in the Indian Ocean between Ž . latitude 11–138S and longitude 43–468E Fig. 1 . Ž . This active basaltic shield volcano 2361 m a.s.l. displays a typical Hawaiian structure, with two op- posing rift zones diverging from a summit caldera, 0926-9851r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0926-9851 01 00078-7