Identification of igneous rock distribution in Kalisonggo area using geomagnetic method

Shallow intrusions of igneous rocks are often found in the Kalisonggo area and form a columnar joint. The presence of shallow intrusions of igneous rocks in the study area is closely related to Mount Mujil as an ancient volcanic system. This study aims to prove the existence of igneous intrusion and subsurface conditions using the geomagnetic method. First Horizontal Derivative (FHD) is used to determine the edge boundaries of the anomalous source of the rock’s magnetic field. Quantitative interpretation is done by using 2.5 D modeling to determine the distribution of igneous intrusion in the study area. The results of data processing show the total anomaly value in the study area has a range of values ranging from -400 nT to 600 nT. Whereas the local anomaly value that has been reduced to the equator has a range of values between 350 nT and the lowest -400 nT with a closed contour closure pattern between the high anomaly and the low anomaly. Based on the cross-section results of 2.5 D incision 1 and 2, there are 3 layers of rock in the presence of boulder lava andesite. The andesite lava has a susceptibility value of 0.00004 SI which penetrates the bedding above it. The top layer is soil with a susceptibility value of 0.00001 SI. The next layer is the andesite breccia rocks with a susceptibility value of 0.00003 SI. The third layer is sandstone with a susceptibility value of 0.000005 SI. The model shows that the andesitic lava lumps did not break through the basement, this is following Van Bemmelen’s opinion that igneous rock in Kalisonggo is an avalanche.