SUBSURFACE INTERPRETATION OF TINATAR-KARANGREJO GEOTHERMAL MANIFESTATION AREA IN PACITAN BASED ON 3D GEOMAGNETIC MODELLING

Abstract

Abstrak Penelitian ini bertujuan untuk mengetahui struktur bawah permukaan pada daerah manifestasi panas bumi Tinatar-Karangrejo berdasarkan pemodelan 3D metode geomagnet. Akuisisi data dilakukan menggunakan Proton Precession Magnetometer (PPM) Geometric tipe G-856 dengan 54 titik pengukuran dan jarak antar titik pengukuran ± 1 km. Pengolahan data dilakukan dengan koreksi variasi harian, koreksi IGRF, reduksi ke kutub (RTP), kontinuasi ke atas, pemodelan 2D, dan pemodelan 3D. Pemodelan 3D dilakukan dengan menggunakan software Rockwork yang didasarkan pada nilai suseptibilitas batuan di sekitar daerah penelitian dari hasil pemodelan 2D. Dari hasil analisis, diketahui bahwa pada daerah penelitian didominasi oleh batuan-batuan sedimen klastik yang terkonsentrasi pada bagian selatan hingga tengah daerah penelitian dengan nilai suseptibilitas 0 hingga 0,686 × 10-3 (dalam SI). Pada bagian tengah hingga utara daerah penelitian terdapat batuan beku dan batuan piroklastikdengan nilai suseptibilitas 0 hingga 5,578 × 10-3 (dalam SI), dimana batuan-batuan beku dan piroklastik yang letaknya dekat dengan manifestasi panas bumi Tinatar dan Karangrejotelah terdemagnetisasi. Kata kunci : metode geomagnet, pemodelan 3D, manifestasi panas bumi Tinatar-Karangrejo. Abstract This study aimed to determine subsurface structure in Tinatar-Karangrejo geothermal manifestation area based on 3D geomagnetic modelling. Magnetic data measurement had been done using Geometric G-856 Proton Precession Magnetometer (PPM) with 54 measurement points and space between each point was ± 1 km. Magnetic data then processed using diurnal correction, IGRF correction, reduction to pole (RTP), upward continuation, 2D modelling, and 3D modelling. 3D modelling process was done using Rockworksoftware also it was done based on susceptibility of rocks in the study area from 2D modelling result. The result showed that the structure in study area were dominated by clastic sedimentary rocks that are concentrated in the southern to the middle part of study area with susceptibility value ranged from 0 to 0,686 × 10-3 (SI unit). In the northen part of the study area there are igneous rocks and pyroclastic rocks with susceptibility value ranged from 0 to 5,578 × 10-3 (SI unit) where some of these igneous and pyroclastic rocks located near Tinatar and Karangrejo geothermal manifestation were demagnetized. Keywords: geomagnetic method, 3D modelling, Tinatar-Karangrejo geothermal manifestation area.