Horizontal-vertical Spectral Ratio Method in Microtremor to Estimate Engineering Bedrock Thickness at Sedati Mud Volcano

Based on field study, Sedati Mud Volcano located in a line with Gunung Anyar Mud Volcano and occurred by increased pressure in the compression area and rapid loss of gas. The combination of both fast-growing constructions of infrastructures and the presence of the mud volcanoes brings new challenges in Sidoarjo city. The purpose of this scientific research is to determine the sedimentary thickness around Sedati mud volcano. Only a few data show real amplitude spectrum, which represent high contrast impedance. At some point, there are several peaks indicating the presence of contrast impedance between layers. Based on 20 processed data, Sedati Mud Volcano has a 30 - 70m engineering bedrock thickness and natural frequency between 0.5 until 14.4 Hz. The enhancement of natural frequency tends to occur along decrement of layer thickness in the upper basement layer. The result shows the natural frequency parameter and its amplification is slightly variated around Sedati Mud Volcano, as caused by sedimentary lateral depth variation and/or the presence of variation on existing rock. Further analysis indicates a fault inside the area of mud volcano as possible reason behind the occurring mudflow.