Identifying the Distribution of Alteration Zone Using Very Low Frequency Method in Candi Gedong Songo, Ungaran, Semarang, Central Java

Abstract

The alteration zone could be a key link and a proof to know where the paleo heat source was previously located. An electromagnetic survey to identify and to map the lateral and vertical distribution of alteration zone had been done at geothermal area Candi Gedong Songo, Ungaran, Central Java, from 9th - 19th of June 2014, using VLF method. The survey consisted of 6 profiles, with NW - SE direction, which were located nearby the fumaroles spots and then went down to the observed alteration zone. Each profile was 600 m long and the distance between each profile was 20 m. The space between each measurement point of a profile was 20 m. In this study, tilt and ellipticity data with frequency of 19.8 kHz (Japan) and 24 kHz (Panama) were used. First, the data was processed to get the cross features anomaly between tilt and ellipticity data on the chart. Then, the derivative fraser and the relative current density pseudosection were also made to support the cross features anomaly. The interpretation of this data was done qualitatively using fraser and relative current density pseudosection. The result shows that the alteration zone gives high response of conductivity compared to its surrounding area. This is supported by the anomaly cross features between tilt and ellipticity data on the chart, also by high value of fraser and relative current density. Thus, the alteration zone are located in meter 150 - 250 in V1 and V2 profiles, also in meter 180 - 250 in V5 and V6 profiles. This result indicates that the ancient heat source was previously located nearby the fumaroles area and it is physically shown by the presence of sulphuric clay mineral content at the alteration surface area.