SUBSURFACE INTERPRETATION OF FAULT LINE IN BAGELEN PURWOREJO USING GEOMAGNETIC METHOD

Abstract

Abstrak Penelitian ini bertujuan untuk mengetahui pola sebaran anomali medan magnetserta mengetahui struktur bawah permukaan di kawasan jalur sesar Kecamatan Bagelen dengan menggunakan metode geomagnet. Penelitian dilakukan di 8 desa di Kecamatan Bagelen yakni Desa Kalirejo, Clapar, Hargorejo, Soko, Semagung, Semono, Sokoagung, dan Somorejo. Penelitian dilakukan menggunakan satu set alat Proton Precession Magnetometer merek Geometrics tipe G-856 di mana jarak antar titik pengukuran sejauh 500m. Tahapan pengolahan data dari lapangan berupa nilai magnet total dikoreksi menggunakan koreksivariasi harian, koreksi IGRF, koreksi topografi, reduksi ke kutub, kontinuasi ke atas yang selanjutnya digunakan untuk pemodelan 2D. Nilai anomali medan magnet pada wilayah penelitian berada pada rentang -500 nT sampai 862 nT. Nilai tersebut disumbangkan oleh formasi-formasi batuan penyusun wilayah penelitian. Berdasarkan hasil pemodelan 2D, diketahui bahwa formasi batuan penyusun wilayah penelitian terdiri dari Formasi Nanggulan, Kebobutak, Dasit, Andesit, Jonggrangan serta Aluvium, di mana pada Formasi Andesit dan Dasit diketahui adanya ubahan batuan serta mineralisasi. Kata kunci : Kecamatan Bagelen, metode geomagnet, formasi batuan. Abstract The purposes of this study were to determine magnetic field anomaly distribution and to determine subsurface structure of the fault line in Bagelen subdistrict using geomagnetic method. The study was located in 8 villages in Bagelensubdistrict there were Kalirejo, Clapar, Hargorejo, Soko, Semagung, Semono, Sokoagung, and Somorejo villages. The study was conducted using a set of G-856 Geometrics Proton Precession Magnetometer which interval was 500m between each point. Data of total magnetic field were performed using diurnal correction, IGRF correction topography correction, reduction to pole, upward continuation, and 2D modelling. The range of magnetic field anomaly values in the study area were -500 nT to 862 nT that contributed by rock formations. The results from 2D modelling, it was known that the rock formations were Nanggulan, Kebobutak, Dasit, Andesit, Jonggrangan, and Aluvium, where in Andesit and Dasit formations found that the rocks changed. Keywords: Bagelen subdistrict, geomagnetic method, rock formations.