Pemetaan Anomali Magnetik dan Log Resistivitas Batuan di Pesisir Timur Kecamatan Nusawungu Kabupaten Cilacap

Abstract

Abstrak – Pesisir Timur Kecamatan Nusawungu merupakan rangkaian kawasan di Pesisir Kabupaten Cilacap bagian timur yang disinyalir masih mengandung pasir besi. Survei magnetik dan geolistrik telah dilakukan pada bulan April hingga Oktober 2017 untuk mengetahui sebaran pasir besi di kawasan tersebut. Survei magnetik dilakukan di daerah penelitian dengan posisi 109,34619 ° – 109,37183 ° BT dan 7,69581 ° – 7,70978 ° LS. Hasil yang diperoleh dari survei magnetik adalah peta kontur anomali magnetik lokal, dengan nilai anomali berkisar -498,66 – 201,73 nT. Berdasarkan peta kontur yang diperoleh, terdapat beberapa closure anomali magnetik yang membentuk arah kelurusan 7,73 ° yang mengindikasikan adanya endapan pasir besi di bawahnya. Survei geolistrik dilakukan di kawasan yang diindikasikan mengandung pasir besi untuk memperkirakan potensinya. Berdasarkan hasil pemodelan diperoleh log resistivitas yang menunjukkan 4 – 5 lapisan batuan bawah permukaan. Pada setiap titik sounding terdapat lapisan yang diinterpretasi sebagai pasir yang mengandung butiran bijih besi yang berselingan dengan lanau dan lempung. Lapisan pasir besi ini tersebar pada titik sounding Sch-1 pada posisi 109,347722 ° BT dan 7,701611 ° LS hingga titik sounding Sch-5 pada posisi 109,362750 ° BT dan 7,703472 ° LS; dengan kedalaman berkisar 1,03 – 9,27 meter dan nilai resistivitas berkisar 2,51 – 75,55 Ωm. Berdasarkan hasil eksplorasi ini, kawasan pesisir timur Kecamatan Nusawungu Kabupaten Cilacap diperkirakan memiliki potensi pasir besi. Kata kunci: anomali magnetik, log resistivitas, pasir besi, batuan bawah permukaan, Pesisir Timur Nusawungu Abstract – The Eastern Coastal of Nusawungu District is a series of areas in the Eastern part of the Cilacap Regency Coastal which allegedly still contains iron ore. The magnetic and geoelectric surveys have been conducted in April up to October 2017 to determine the distribution of iron sand in this area. Magnetic survey has been done in the research area at position of 109.34619 ° - 109.37183 ° E and 7.69581 ° – 7.70978 ° S. The results that obtained from the magnetic survey are local magnetic anomaly contour map, with anomalies values ranging of -498.66 – 201 , 73 nT. Based on the contour map that obtained, there are several magnetic anomalous closures with 7.73 ° strike that indicate the presence of iron sand deposits in its subsurface. The geoelectric survey has been carried out in the zone which indicated to contain iron sand to estimate its potential. Based on the results of modeling, several log resistivities have been obtained which show the existence of 4 - 5 lithologic layers. At each the sounding point, there is a layer which is interpreted as sand containing iron ore grains which alternating with silt and clay. This iron sand deposits are distributed at the sounding point of Sch-1 with position of 109.347722 ° E and 7.701611 ° S to the sounding point of Sch-5 with position of 109.362750 ° E and 7.703472 ° S; a depth ranging of 1.03 – 9.27 meters and resistivity values ranging of 2.51 – 75.55 Ωm. Based on the results of this exploration, the Eastern Coastal area of Nusawungu is estimated to have the potential of iron sand. Keywords: magnetic anomaly, log resistivity, iron sand, subsurface rocks, Eastern Coastal of Nusawungu