Analisis struktur Geologi daerah Ranu Gedang berdasarkan data anomali medan magnet

INDONESIA: Telah dilakukan survei magnetik untuk menduga model anomali magnetik bawah permukaan Ranu Gedang dan mengidentifikasi litologi batuan bawah permukaannya. Pengambilan data dilakukan pada 60 titik ukur yang tersebar pada daerah penelitian yang memiliki luas 16 km2 dengan spasi pengukuran 50-500 m menggunakan base magnetometer dan field magnetometer. Pengolahan data diawali dengan koreksi IGRF dan koreksi variasi harian untuk mendapatkan anomali medan magnet total, kemudian reduksi bidang datar, reduksi ke kutub, selanjutnya kontinuasi ke atas pada ketinggian 400 meter di atas sferoida referensi untuk memisahkan anomali lokal dan regional. Hasil interpretasi kualitatif menunjukkan adanya anomali dipole magnetik di sebelah barat dan diantara lokasi Ranu Gedang. Interpretasi kuantitatif dilakukan dengan pemodelan benda anomali menggunakan program Mag2DC for Windows. Hasil dari interpretasi kuantitatif menunjukkan bahwa secara umum strukruktur bagian dalam Ranu Gedang tersusun oleh batuan vulkanik Gunung Lamongan berupa basal, lava, breksi vulkanik, piroklastik/ tufa, lahar. Ranu gedang berbentuk cekungan yang diduga terbentuk akibat letusan ekplosif. Letusan tersebut diduga telah memotong muka air tanah, dan pori- pori batuan/ rekahan yang terbentuk mengontrol pergerakan air hingga tertampung pada cekungan. ENGLISH: Magnetic survey has been carried out to estimate the subsurface magnetic anomaly model in Ranu Gedang and identify the lithological rocks in the subsurface. The data was collected on 60 measuring points which spread on the research area that has 16km2 widths in measuring space 50 m up to 500m by using base magnetometer and field magnetometer. Data processing started by correcting IGRF and diurnal variation to achieve anomaly of total magnetic area. Then, it was reduced to a horizontal plane, reduction to the pole, and than upward continuation to 400 m height above referential spheroid references to separate the local anomalies and regional anomalies. The results of qualitative interpretation indicate the presence of a dipole magnetic anomaly in the west and between location of Ranu Gedang. Quantitative interpretation is done by modelling the anomalous objects using Mag2DC for Windows program. The results of quantitative interpretation reveal that generally the inner core of Ranu Gedang composed by volcanic rocks of Lamongan volcano such as basalt, lava, volcanic breccia, pyroclastic/tuff, magma. Therefore, Ranu Gedang is a basin which formed because of the explosive eruption. Those eruptions are supposed cutting the ground water and rocks pore which is formed and aims to control the water movement so that the water is retained in the basin