Investigation of Soft Layer in between Hard Layer for Sedimentary Rock Formation by using Electrical Resistivity Analysis

This study aims to predict the occurrences of soft layers in between hard layers in sedimentary rock formation based on electrical resistivity analysis. ABEM Terrameter LS Box with 40 electrodes with 2.5 meters spacing in pole-dipole method was used. Standard Penetration Test (SPT) was conducted to obtain the value for the number of blows (N-value). RES2DINV was used to analyse a two-dimensional illustration of the soil layers in the pseudo section diagram. Results show that the groundwater table is located 2.0 meters below the ground surface. Based on the resistivity analysis, the subsurface profile is consists of topsoil, soft silty clay (N0-N4, 6 > Ω > 20 ohm), medium-stiff silty clay (N5-N6, 20>Ω>360 ohm), medium dense coarse sands (N13-N16, 361>Ω>550 ohm), stiff silty clay (N10-N18, 550>Ω>720 ohm), hard silty clay and gravel (N50, 720>Ω>1100 ohm), and moderate weathered and slightly fractured shale stone (N50, Ω>1100 ohm). Although the correlation between resistivity values and SPT N-values shows a very good relationship with R² ranges from 0.83 to 0.89, electrical resistivity analysis is not suitable to determine the soft layer in between hard layer in a sedimentary rock formation due to the presence of high ground water table. (paper)