Relationship of Resistivity Value with the Ground Material Information Obtained from Borehole Data: Case study

Abstract

In typical site investigations, borehole data offers localized information at specific depths within the borehole. It offers high vertical resolution but limited spatial coverage since it represents a point measurement. Electrical Resistivity Tomography (ERT) data, on the other hand, provides a continuous profile of subsurface resistivity distribution along a survey line or grid. It offers relatively high spatial coverage, capturing information between measurement locations. The study aims to understand on reliability of resistivity value with the soil or rock material information obtained from conventional borehole data located in Melaka, Malaysia. A resistivity lines were conducted ERT method with the Gradient XL protocol. Based on the results, high resistivity values were observed at first 5 meters, indicating the presence of Gravelly SAND and Silty SAND layers ranging from 5 to 6 SPT N-value. The resistivity value then decreases below 100 Ωm starting from a depth of 5meters and continuing until 9meters which indicate highly saturated layer. Then from a depth of 9 to 15 meters, the resistivity value increases, indicating the soil layer is less saturated with water due to its high resistivity value. It can be concluded that ERT data is reliable to represent subsurface data when comparing to borehole data. However, not all data can be fully understood or interpreted based on solely resistivity value especially around weathered materials. For instance, in area of immersed soil or rock, resistivity sometimes still gives higher value that is almost the same as in an unsaturated area. It was observed that the relationship of SPT-N and resistivity value is more consistent when the materials are in range stiff to hard layer.