Investigation of the Extent of Hydrothermal Activities of the Sikidang Crater, Dieng Volcanic Complex (Central Java), Using Geomagnetic Method

Abstract

Abstract The Sikidang Crater (± 2000 m asl) is one of the active craters in the Dieng Volcanic Complex (DVC). It is situated on the southeastern flank of Mt. Pangonan. The crater is significant for development of both tourism and energy sectors in Dieng. Thermal manifestations inside the crater which consist of boiling mud pools and pots, hot springs, fumaroles, gas vents, steaming and altered ground make the place a popular tourist attraction. They also serve as windows to the shallow subsurface processes and help better understand the dynamics of the Dieng geothermal system. However, the land use of the crater and its vicinity disregards the fact that the actual extent of the hydrothermal activity of the crater has not been figured out, and that the foci of the thermal fluid discharges in the crater often shift. Some agriculture plots, housing areas, as well as tourism and geothermal infrastructures are located within or near the morphological boundary of the crater. The inherent hydrothermal hazard of the crater therefore, must be well mitigated to minimize the negative impacts to the people, as well as their assets. Our research aims to delineate the lateral and vertical extent of the hydrothermal activity of the crater by geomagnetic method. Geomagnetic anomaly in geothermal field occurs due to the difference in the intensity of hydrothermal alteration of the rocks. The equipment used was a pair of Proton Precision Magnetometers GEOTRON G5. Our result shows that low geomagnetic anomaly (-2200 to -400 nT) correlated with demagnetized, hydrothermally altered andesitic rocks covers an area of ≥ 1.15 km2, i.e., more than seven times larger than the area outlined by morphological boundary of the crater. The anomalous zone extends N/NE - S/SW from beyond the areas of inter-volcanic plain between Pangonan and Igir Binem in the north, to the areas beyond Sikunang and Kendil cones in the south. Below the Sikidang mud pool the demagnetized rock mass reaches a maximum thickness of 2000 m. The supporting data, i.e., the fluctuations of the gas discharge, and the seismicity of the DVC indicate that the recent dynamics of the Sikidang Crater is mostly controlled by the internal processes within the crater.