FLUID ACTIVITY DETECTION IN KENDALISODO GEOTHERMAL MANIFESTATION, SEMARANG, INDONESIA, USING VP/VS RATIO

Udi Harmoko

Abstract

In areas of moderate seismic activity in which there are quite a few ground motion records and in urban or industrial contexts where noise levels are high, site effects can be identified through experimental methods involving microtremor recordings. It is common practice to undertake subsurface structure surveys using microtremor analysis. This survey's main advantage is its ease of obtaining consistent results using observation data from multiple stations. The well-known phenomenon of spatial fluctuations in Vp/Vs has been applied to estimate spatial fluctuations in subsurface structures. Anticipating temporal variations Vp/Vs is plausible, particularly in regions such as geothermal fields. This paper's goal is to indicate the thermal water sources and recharge zones around Kendalisodo geothermal manifestations. a straightforward experimental method based on microtremor recordings. The exposed area containing the geothermal manifestation served as the site of the research program. Vp/Vs ratios were calculated at 24 noise measurement locations.   In the present investigation, we analyse the product Vp/Vs and its variations. These various combinations respond in different ways to various physical factors and offer distinct perspectives on the subsurface changes. The distribution of VS30, the most common single-value measurement that best captures the behaviour of seismic soil, was also examined in this study. The results of this study become quite understandable after some analysis of the Poisson ratio distribution, which shows a value of 0.45 near the manifestation, the multiplication of compressional and shear wave velocities indicates a well-porosity area, the character of the bedrock based on geological data in accordance with the shear wave velocity value around the manifestation indicates the intrusion of hard rock, namely igneous rock and finally the water flow pattern based on satellite imagery from the summit of Mount Ungaran towards the research location.

Keywords

Kendalisodo manifestation, Vp/Vs ratio, Poisson ratio, porosity, thermal fluid

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DOI: https://doi.org/10.13057/ijap.v16i1.116778

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