Utilizing NWCR optimized arrays for 2D ERT survey to identify subsurface structures at Penang Island, Malaysia

Abstract

In this paper, the effectiveness of optimized and conventional resistivity arrays for resolving shallow geological features was investigated at Penang Island, Malaysia. The assessment was between the inversion results from Noise-Weighted Compare R (NWCR) optimized arrays, and commonly used conventional arrays (Dipole-Dipole (DD) and Wenner-Schlumberger (WS)). This is demonstrated using two field cases studies. The first case was conducted at an area characterized by fractures /cracks in the walls and ceiling of a heritage building in Georgetown city. The aim was to identify the weakness zone responsible for the observed fractures. The second case study was conducted in the main campus of Universiti Sains Malaysia (USM), to delineate the interface between the granitic bedrock and the unconsolidated sediments. In the first case, the results show a layer with a resistivity value of less than 2 Ohm-m between 2 m to 6 m depth. This layer could be the weak zone in the investigated area. The NWCR optimized arrays detected this zone better than the DD and WS. This finding correlates quite well with the shear wave velocity (Vs) model, with Vs less than 115 m/s in the same line and depth. For the second case study, the results were compared with the borehole log in the middle of the resistivity line. The ERT model obtained from the NWCR defines the granitic surface at around 32 m depth, which correlates well with the borehole log, while the model from the DD arrays underestimated depth. In summary, the NWCR optimized arrays give a more accurate image of the subsurface resistivity distribution, followed by the DD array. The WS array performed the least.