Determination of the Rock Contact Lithology of the Nanggulan Formation with the Magnetic Method in the Hargorejo Area of Kulon Progo

Abstract

The Nanggulan Formation is a formation unit consisting of sandstone, clay, and other sedimentary rocks. Lithological contact between clay and geologically old andesite could indicate an avalanche. The magnetic method in collecting data uses a natural magnetic source with a proton magnetometer. Acquisition data was determined according to the survey design, namely a grid with a distance of 250 meters per point. Data processing to remove noise data, namely noise from magnetic storms, position and altitude, and other natural disturbances. Later, Processing will produce a magnetic anomaly map, which must be separated between local and regional anomalies. This local anomaly will later form the basis for 2.5-dimensional modeling. This 2.5-dimensional modeling can get an overview of lithology, relationships between lithologies, and rock contacts according to the geological model in the field. The modeling results can describe the relationship between the claystone of the Nanggulan Formation and the igneous rocks of the Old Andesite Formation. Its relationship claystone is overlapping Andesite rocks in the Horgorejo Kulon Progo area. This can cause soil movement and landslides if the Andesite rocks experience massive weathering and erosion and if rainfall is high.