New results on the resistivity structure of Merapi Volcano (Indonesia), derived from three-dimensional restricted inversion of long-offset transient electromagnetic data

Abstract

S U M M A R YThree long-offset transient electromagnetic (LOTEM) surveys were carried out at the active volcano Merapi in Central Java (Indonesia) during the years 1998, 2000 and 2001. The measurements focused on the general resistivity structure of the volcanic edifice at depths of 0.5-2 km and the further investigation of a southside anomaly. The measurements were insufficient for a full 3-D inversion scheme, which could enable the imaging of finely discretized resistivity distributions. Therefore, a stable, damped least-squares joint-inversion approach is used to optimize 3-D models with a limited number of parameters. The models feature the realistic simulation of topography, a layered background structure, and additional coarse 3-D blocks representing conductivity anomalies. 28 LOTEM transients, comprising both horizontal and vertical components of the magnetic induction time derivative, were analysed. In view of the few unknowns, we were able to achieve reasonable data fits. The inversion results indicate an upwelling conductor below the summit, suggesting hydrothermal activity in the central volcanic complex. A shallow conductor due to a magma-filled chamber, at depths down to 1 km below the summit, suggested by earlier seismic studies, is not indicated by the inversion results. In conjunction with an anomalous-density model, derived from a recent gravity study, our inversion results provide information about the southern geological structure resulting from a major sector collapse during the Middle Merapi period, approximately 14 000 to 2200 yr BP. The density model allows to assess a porosity range and thus an estimated vertical salinity profile to explain the high conductivities on a larger scale, extending beyond the foothills of the volcano.

Structure and CO2 budget of Merapi volcano during inter-eruptive periods

“…Multiform fluid circulations and hydrothermal activity resulting from these features have been already outlined through conductivity and resistivity measurements (Muller and Haak 2004;Commer et al 2006), self-potential mapping (Aubert et al 2000), self-potential continuous monitoring (Friedel et al 2004) and geochemical characteristics of high-temperature fluids (Zimmer and Erzinger 2003). Commer et al (2006) suggested from resistivity modelling the existence of both secondary minerals derived from hydrothermal alteration and hypersaline (∼25 eq. wt.%) high-temperature fluids extending southerly and heated by ascending lava and/or gas-steam mixtures.…”

“…Merapi undergoes permanent volcanic activity, leading to high temperatures below the central edifice (Commer et al 2006). Tropical rainfall leads to huge meteoric inflows through the young volcanic deposits at the surface.…”

“…Large displacements are observed in the summit area before and during episodes of unrest (Beauducel et al 2006), reflecting magma dynamics within a highly fractured crater and stress acting upon the rocks of the volcano's feeder system. Merapi's main magma reservoir could be either a 2.6 km 3 magma chamber (Ratdomopurbo 1995), or a system of associated magma pockets (Commer et al 2006) at about 1.5 km depth.…”

Combined electrical and electromagnetic imaging of hot fluids within fractured rock in rugged Himalayan terrain

“…Hördt et al (1992) explained the observed sign reversal in LOTEM data by considering the laterally inhomogeneous subsurface. Hördt and Müller (2000) and Commer et al (2006) used topography to model the observed sign-reversals in the field data. Recently a sign reversal has been observed in the SHOTEM measurements by Goldman et al (2009).…”

1-D multimodel joint inversion of TEM-data over multidimensional structures

“…Caldwell & Bibby (1998) presented a scheme to visualize distortions in the LOTEM electrical components. The horizontal field components are included in calculations shown by Hördt & Müller (2000) and Commer et al (2007) for data from mountainous areas. Hördt & Scholl (2004) present calculations for a distorting body close to either the TX or RX for all LOTEM components.…”