Estimation of Existence Geothermal Manifestation Using Very Low Frequency (VLF) Method in the PagerkandangVulcanic, Dieng, Central Java

Very Low Frequency (VLF) measurement has been done at Pagerkandang Volcanic, Dieng Volcanic Complex (DVC) to examine the possible existence of conductive zones that related with geothermal manifestation. VLF – EM survey used tilt mode with T-VLF BRGM Iris Instrument operated with two frequencies, they are 22200 Hz from Japan (JJI) and 19800 Hz from Australia (NWC). There are five lines with distance between lines is 50 m, and distance between measure points is 20 m. The parameters measured from VLF method are tilt angle (%) and elliptisity (%). Data processed by tilt angle value with fraser and Karous – Hjelt filter used WinVLF program. Karous – Hjelt filter resulted current density contour to estimate lateral location from conductive and resistive zones. The conductive zone is interpreted as the area which have high current density value. This area located at eastern dan western of Pagerkandang Volcanic. The conductive zone related to geothermal manifestation as like as fumarol that appeared because presenced of normal fault. Whereas the resistive zone is interpreted as the area which have low current density value. This area spread almost in the middle of the Pagerkandang Volcanic. The resistive zone was caused by the high weathering in claystone.


Introduction
Dieng Volcanic Complex (DVC) is one of the productive geothermal fields in Indonesia. Dieng geothermal systems have a high temperature and liquid-dominated. High temperatures in geothermal systems can be caused by the age and composition of the heat source (Wohlez and Heiken, 1992; Muffler and Duffield, 1995 in Harijoko A et al. 2016). The total potential of geothermal energy in Dieng is estimated at 355 MW but only 60 MW which has been utilized as a source of power plant. Pagerkandang volcanic is one area that has a geothermal prospects associated with Sileri Crater. Geothermal manifestations can be fumaroles, acid sulfate spring, mud pools, steaming ground and rock alteration (Harijoko A et al. 2016).
VLF method utilizing electromagnetic wave induction can be used to determine the continuity of conductive regions geothermal area based on their surface manifestation. Control of the fault structure largely determines the location of these manifestations. Open fractures formed by the structure used as the hydrothermal fluid flow path and carries with her metallic minerals. In this paper conducted qualitative interpretation of the value of the tilt angle using fraser filter and Karous -Hjelt filter. Karous -Hjelt filter will generate value for the current density equivalent (RAE) to estimate the distribution of the resistive and conductive zones.

Geological Setting
Dieng Volcanic complex had being formed by volcanic stratocones, explosion craters, parasitic vent, and also hydrothermally active which originated at the junction of two major fracture zones. The fracture is an east-west trending zone, extending due west from the Alteration process changes volcanic rocks into clay minerals which have high conductivity (low resistivity). The alterated rocks are layered by sulfur minerals and are founded surrounding the manifestation. This minerals give important information types and locations of Dieng geothermal heat source system.

Very Low Frequency (VLF) Method
VLF method uses electromagnetic waves of radio waves having a frequency ranging from 15 -30 kHz. Primary electromagnetic field of the transmitter has components vertical electric field and horizontal magnetic that perpendicular to the direction of propagation (the x axis). If a conductive medium exists in the subsurface, the magnetic component of the electromagnetic waves would induce the medium caused induced currents or Eddy currents. Eddy currents would product secondary electromagnetic field (Hs), which has horizontal and vertical components. Interference between the primary magnetic field and secondary magnetic field generates a magnetic field which is resultant polarization ellipse. The components are measured in the VLF method is a tilt angle (α) and ellipticity (ε).
Qualitative interpretation of VLF method data can be performed with fraser filter and Karous -Hjelt filter (linear filter). Data used in the process of filter is the tilt angle (%). In fraser filter large amplitude can be estimated as conductive zones (Sundararajan N et al., 2006). The highest of fraser filter graph shows the increase of the signal conductive medium, where in the conductive anomaly is considered appropriate under the peak graph fraser filter (Santos et al., 2006).The filter divides tilt angle data with 90 o to transform zero-crossing into peak, and a low pass smoothing operator to reduce noise, such as topographic effect.
2,3 is filter fraser value that plotted midway between M2and M3 tilt angle data.

Result and Discussion
Based on dissemination of tilt angle at location of VLF measured, the high tilt angles are located at the edge of Pagerkandang Volcanic, with tilt angles are 30 -80 % (red colors). Whereas in mid Pagerkandang Volcanic has tilt angles ranging from 25 -(-40) % (green colors). The tilt angle related to horizontal and vertical magnetic field, and indirectly can be used to determine conductive zone from high tilt angle value.
Tilt angle values obtained in the field are still influenced by external noise that has a high frequency.
Because of the assumption of the waves received by VLF -EM is a wave with a low frequency, the high frequency noise to be removed, one of them uses a moving average filter. Qualitative interpretation on VLF -EM method is using Fraser filter and Karous -Hjelt filter of tilt angle values. Karous -Hjelt filter will generate value for the current density to estimate the lateral location of the resistive and conductive zones. Conductive zones are estimated to be below the chart fraser filter with high amplitude (peak chart). While the linear filter (Karous-Hjelt filter) current density value is a predicted high conductive zone. On the graph of Fraser filter and Karous-Hjelt filter look anomalous responses that are interrelated. On Line A conductive zones are located at a distance of 120-240 m or on the eastern side of the Pagerkandang Vulcanic assuming depth (skin depth) up to 100 m. Conductive zones on Line A have a value of current density between 8-25%. While the resistive zones on Line A are spread between 300-600 m, with a value of current density between (-5) -(-20)%.
The conductive area on Line B is at a distance 500 m (east) and 680-1180 m or on the western side of the Pagerkandang Vulcanic has a value of current density between 10-50% if the skin depth is up to 200 m. High resistivity area in the middle of the Pagerkandang Vulcanic has a value of current density between -10 -(-30)%.
The conductive zones on Line C is between 160-220 m and 560-800 m which has a continuity with the conductive zones on Line B that located on the western side of the PagerkandangVolcanic. The value of currentdensity on Line C is between 8-40% including a conductive area. High resistivity area is located at 380-480 m which is also located in the central part of the Pagerkandang Vulcanic, with a value of current density between (-12) -(-28)%.
High conductivity zones on Line D are located at a distance of 180-360 m (east) and 600-700 m (west) with current density between 6-20%. High conductivity area in the western side predicted still associated with Line B and Line C in the south. While the resistive area is located at a distance of 120 m -180 m (east) and 380-560 m (east) with current density between (-4)% -(-20)%, and the skin depth reaches 120 m.
The measurements on Line E was began on the western side of the Sileri Crater bordering the east side of the Pagerkandang Vulcanic. High conductivity zones are located at a distance of 80-260 m or in transition from Sileri Crater and Pagerkandang Volcanic with current density between 20-50%. High