Hydrothermal system mapped by CSAMT on Karthala volcano, Grande Comore Island, Indian Ocean

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Abstract

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 self-potential (SP) mapping of the Karthala summit zone. The results of the 1D CSAMT inversion indicate the following: (1) a highly resistive (500–5000 Ω m) 200- to 400-m thick surface layer, that is characteristic of dry basaltic rocks, and made up of lava flows and/or tuff breccias; (2) a 300- to 1200-m thick layer of intermediate resistivity (20–400 Ω m), thought to be representative of the groundwater body; and (3) a deep conductor with a resistivity of less than 2 Ω 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 Ω 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.

Introduction

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 source audio-magnetotelluric (CSAMT) method has been successfully used for geothermal exploration 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 latitude 11–13°S and longitude 43–46°E (Fig. 1). This active basaltic shield volcano (2361 m a.s.l.) displays a typical Hawaiian structure, with two opposing rift zones diverging from a summit caldera, and a 290-m deep pit crater named “Choungou–Chahalé” lying in the center of the caldera (Fig. 1). At least eight eruptions have occurred during the 20th century. The Karthala eruptive style is mostly effusive, however phreatic and phreato-magmatic explosions have occurred in 1918, 1948, 1952 and 1991. The last one, which took place on the July 11, 1991, was purely phreatic, and formed a new crater (280 m in diameter, 43 m deep) in the bottom of the “Choungou–Chahalé”, which was partly filled by a lake that still exists today (Bachèlery et al., 1995). The summit of Karthala volcano presents some surface evidence of hydrothermal activity: hydrothermal alteration affecting some sites located on caldera faults, active fumaroles in the “Choungou–Chagnoumeni” (Fig. 1), an active solfatare (96 °C) called “soufrière” and located 2 km north of the caldera. Changes of the summit lake water level (metric order), not correlated with precipitation and involving a rise of the water table in a geothermal system, have also been observed between 1991 and 1997.

The geophysical investigations of the Karthala volcano have been very limited. Only a seismicity study (Savin, 1995) and self-potential (SP) measurements (Lénat et al., 1998) have been performed on the summit area. In addition, a few structural aspects have been inferred from the Karthala volcanic processes and from geological studies Bachèlery and Coudray, 1990, Bachèlery and Coudray, 1993. These studies point to the presence of a very active hydrothermal system underneath the summit caldera. Consequently, a CSAMT survey was considered suitable for providing information about the electrical structure beneath the Karthala summit region, especially in regard to the location of the hydrothermal system.

Section snippets

CSAMT method

Controlled source audio-magnetotelluric method is a frequency-domain sounding technique that utilizes artificial and natural EM fields to measure resistivity variations in the ground. Goldstein and Strangway (1975) introduced the use of a controlled source to overcome the weak signals associated with the conventional audio-magnetotelluric (AMT) method. Typical CSAMT equipment consists of a transmitting station, the source of which is a dual horizontal electrical dipole, and a receiving station

CSAMT measurements

During April 1998, CSAMT was applied to the summit region of the Karthala volcano. The 10 measurement sites are shown in Fig. 1. For each site, the controlled source transmitter was placed between 300 and 500 m from the receiving site, depending on the skin depth (close enough to obtain signal and far enough to enable plane wave interpretation). Due to the relatively rough terrain, it was difficult to perform more CSAMT measurements, resulting to little data obtained from the southern part of

Modeling

1D models (Interpex, 1993) have been adjusted to the apparent resistivity and phase curves at each site. The data were interpreted using the minimum number of concordant layers. This typically resulted in a model with three major layers. Fig. 2 depicts four examples of 1D models to data matches, as well as the inferred electrical resistivity structure beneath each site.

Generally, the results of the layered inversion indicate the following resistivity properties of the summit region of the

Discussion

The application of the CSAMT method at the Karthala volcano summit zone reveals three layers of different electrical resistivities: a resistive surface layer, an intermediate-resistivity layer and a conductor (Fig. 5).

The first resistive (500–5000 Ω m) layer obtained for all models is typical for dry basaltic rocks (Courteaud et al., 1997). For some sites (sites 4, 5 and 8), the models suggest additional thin (<4 m) conductive layers of some tens of Ω m that can be interpreted as accumulation

Conclusion

The CSAMT method, applied to the Karthala volcano, enabled us to determine the structure of the first kilometers of the summit area, which in electrical terms can be divided into three ranges of resistivity values: (1) a highly resistive (500–5000 Ω m), 200- to 400-m thick surface layer, which is characteristic of dry basaltic rocks and mainly constituted by lava flows and/or tuff breccias; (2) an intermediate-resistivity (20–400 Ω m), 300- to 1200-m thick layer, interpreted to be

Acknowledgements

Thanks to T. Mogi, an anonymous reviewer, Bernard Robineau, Sirit Coeppicus and Phil Sills for constructive criticism and help in improving the manuscript. Without Ali Youssef and the many other Comoran carriers, this CSAMT survey would not have been successful. This study was financed by the Laboratoire des Sciences de la Terre de l'Université de la Réunion, and Mission Française de Coopération on Comore Island and the CNDRS (Grande Comore Island). C. Savin is financed by a Reunion Island

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