Integrated Geophysical Characterization of Karst Hazards Using ERT, SRT, and MASW: A Case Study from Bau, Sarawak, Malaysia

Abstract

Karst regions are considered geotechnically unpredictable terrain due to their complex subsurface conditions. The formation of cavities, sinkholes, and irregular rock morphology can become apparent without warning. Hence, traditional site investigation methods become unreliable for accurate subsurface characterization. This paper presents an integrated approach that can improve subsurface characterization and geohazard mitigation in the karst environment. Borehole-derived soil profiles and corresponding laboratory results were used to calibrate and validate the geophysical surveys, namely Electrical Resistivity Tomography (ERT), Multichannel Analysis of Surface Waves (MASW), and Seismic Refraction Tomography (SRT). The efficacy of each technique was compared against the borehole data. All techniques showed individual strengths and limitations. In this circumstance, ERT successfully delineated low resistivity anomalies (< 10 Ωm), which were subsequently confirmed by boreholes as saturated clay/slump zones associated with karst-related instability. Similarly, ERT also effectively identifies high resistivity zones (> 1000 Ωm), which is consistent with a near-surface cavity above the groundwater table. On the other hand, SRT managed to differentiate residual soil (0.4–1.6 km/s) from limestone bedrock (> 2.1 km/s). The SRT revealed a highly undulating rockhead, with varying bedrock depth reaching 10 m over a 15 m lateral distance along the surveyed profile. Conversely, MASW effectively identified low shear wave velocity zones (Vs < 150 m/s) that spatially correspond to ERT low resistivity saturated clay/slump zones, which provide complementary stiffness and thickness information. However, compared with ERT, MASW has a lower resolution to characterize cavity features. The main contribution of this study lies in the integration of application, involving ERT, SRT, and MASW, geophysics survey in characterizing karst features for the Bau limestone Formation, Bau, Sarawak, Malaysia. The combined results provide a more defensible, cross-validated subsurface model than conventional borehole only exploration. It also demonstrated extreme heterogeneity where rockhead elevation and ground stiffness vary remarkably. Practically, the study proposes a more structured site investigation workflow for karst terrain, where geophysical profiling is used to delineate zones of potential instability and rockhead irregularity. Targeted borehole verification would then be carried out at selected locations of anomalies. This would reduce the likelihood of missing narrow karst features. This integrated geophysical characterization approach provides a better basis for foundation selection and pile design.