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Understanding the spatial variations in soil–rock profiles is crucial for identifying weathered rock layers and mapping geological structures, particularly in the complex feldspar-rich granitic terrain of Penang Island, Malaysia. This study employs a combination of electrical resistivity and seismic refraction tomographic techniques, along with borehole lithological data, in three distinct areas on the island. Its goal is to provide insights into surficial and subsurface soil–rock characteristics and water-bearing structures within the North Penang Pluton (including a part of the Sungai Ara axis) for sustainable groundwater development, addressing the needs of the region's growing population. The research identifies three distinct geological units at the near-surface: residual soils, highly to moderately weathered/fractured granitic layers and fresh bedrock, each with its hydrogeological properties. Promisingly, the weathered layers offer significant groundwater development potential, with deep weathered and potentially fractured zones exceeding depths of 30 m in the study area. These findings have substantial implications for sustainable groundwater development, especially in tropical hard-rock terrains. In addition, the study underscores the limitations of small geophysical spacings when probing deeper groundwater resources. This knowledge is essential for informed decision-making in water resource management and infrastructure development, addressing the ongoing global challenge of ensuring access to clean water resources amid population growth. [ABSTRACT FROM AUTHOR]

The significance of velocity–resistivity relationships has been utilized in various geologic terrains and under different conditions. The approach is yet to be exploited in tropical granitic terrains, with no definitive empirical relationships being developed. The empirical relationships are critical for rapidly delineating subsurface petrophysical, geomechanical, hydrogeological, and soil–rock features. As a result, a novel approach has been used in this study to develop velocity–resistivity relationships for tropical granitic environments, combining complex collocated velocity (Vp) and resistivity (ρ) models with simple linear regression analysis. The granitic terrain of Penang Island, Malaysia, was chosen as the study area. The geotomographic results delineated three layers, which include the residual soils (topsoil and completely weathered granite), highly to relatively weathered granitic unit (including fractures), and integral/fresh granitic bedrock. Due to the complexity, ruggedness, and varying weathering and fracturing conditions of the subsurface lithologic units in tropical regions, the supervised regression modeling successfully developed a unified and other three specific velocity–resistivity empirical relations for the lithologic units. The derived velocity–resistivity empirical relations have high practical prediction accuracies to predict Vp data. The predicted Vp data and models from the velocity–resistivity relations had good lithological and structural correlations with their observed models. The overall performance of the results indicated that the velocity–resistivity empirical relations could delineate the subsurface geologic variabilities distinctively because they are resistivity-dependent. Hence, the developed comprehensive methodological and SLR workflows and the velocity–resistivity empirical relations were posited for use in granitic terrains with similar geology to the study area, especially in areas with shallow overburden. [ABSTRACT FROM AUTHOR]

The selection of a choice electrode is pertinent to attenuating noise and improving geophysical tomographic inversion results. Besides, the detailed understanding of the geodynamic condition of subsurface formation is crucial to sustainable potable groundwater abstraction. Hence, the subsurface lithostratigraphic units and groundwater potential of two sites (i.e., Site 1 and Site 2) within the Universiti Sains Malaysia were evaluated using borehole-constrained electrical resistivity tomography (ERT) and induced polarisation (IP) tomography. Both methods employed the resolution capacities of stainless-steel and copper electrodes at dual-spacing. The ERT and IP field data and inversion results for copper electrodes were generally robust due to the generated higher positive data points and lower RMS errors, percentage relative differences, and mean absolute percentage errors (MAPE) than the stainless-steel electrodes, especially at Site 1 with a profile length of 200 m and an electrode spacing of 5 m. However, both electrodes tend to produce inversion models with almost the same parameters at Site 2, using half the profile length and electrode spacing of Site 1, i.e., 100 m and 2.5 m, respectively. Thus, the sensitivities and resolution capacities of the tomographic electrodes are heavily influenced by electrode spacing, profile length, amount of injected current, and depth of investigation. The borehole lithostratigraphic units, typically sandy silt, sand, and silty sand, have good correlations with the ERT and IP inversion results. The variability in observed resistivity and chargeability values were due to heterogeneous weathered materials and saturating water fills within the fractured and deeply-weathered granitic bedrock, with <200 m and a chargeability of >1:8 msec. The models' median depth of >40 m mapped for the weathered and/or fractured sections was suggestive of high groundwateryielding capacity in boreholes to sustain a part of the university community. However, intended boreholes in the study area must be designed to prevent silt precipitation, which is considered a potential threat to potable groundwater and transmissivity since some sections are characterised by silt with little or no sand. [ABSTRACT FROM AUTHOR]

Geophysicists use electrical methods to investigate and characterise the earth's subsurface geology. This study aims to evaluate the performance of copper and conventional stainless-steel electrodes in subsurface tomographic investigations using electrical resistivity tomography (ERT) and induced polarisation (IP) at two sites in Penang, Malaysia. Site 1 and Site 2 employed profile lengths of 200 m and 100 m, with electrodes spacing of 5.0 m and 2.5 m, respectively. In the results of the final data inversion, it was observed that the ERT and IP tomographic models of Site 1 have the best convergence limits with percentage relative differences (copper as reference model) ranging from -70% to 70%, while Site 2 recorded -8% to 8%. The electrodes performance evaluation showed that population root mean square (RMS) error and population mean absolute percentage error (MAPE) of data points between copper and stainless-steel electrodes yielded large values for Site 1 with values above 28% and that of Site 2 was less than 4%. Hence, copper (good electrical conductivity and non-polarisable) electrodes have improved the quality and quantity of infield data which give low values of population RMS error and population MAPE compared to conventional stainless-steel electrodes, especially for large unit electrode spacing surveys. Most notably, this work has contributed to the understanding of the capability of copper electrodes in providing precise and reliable inversion models for subsurface tomographic investigations in pre- and post-land uses (engineering work), hydrogeology/groundwater, environmental studies, etc. [ABSTRACT FROM AUTHOR]

In complex tropical granitic terrains, foundation rocks' bearing strength is important in infrastructure design, while bedrock fractures that adversely impact engineering structures can boost groundwater productivity. Consequently, rock quality designation (RQD) can characterize these features effectively. This technique is difficult to determine in boreholes and relatively costly for a large area. As a result, novel approaches were employed to effectively determine RQD over a large aerial extent using statistically optimized borehole and seismic P-wave velocity (Vp) data with compensated resistivity modeling. This study was carried out in the typical granitic terrain of Penang Island, Malaysia. The first stage involves evaluating and developing borehole-based RQD models, and the results were correlated with Vp data and were regressively analyzed to develop lithology-based empirical relations. These empirical relations were used for developing 2D/3D SRT (seismic refraction tomography) based RQD models and were treated as novel models. Integrating the borehole-based and SRT-based RQD models with compensated resistivity models effectively delineate the surficial-to-subsurface soil-rock profiles based on their rock mass quality (RMQ) and conditions, as residual soil, completely weathered rock, relatively weathered rock, and integral/fresh granitic bedrock, including different directional and multiple axial fractures. Based on these results, the highlighted suitable sections for founding the foundations of infrastructure are pinned on the fresh bedrock at the depths of 8–25 m, with >2400 m/s, >2000 Ω m, and RQD >90%. The deep-weathered/fractured zones, with depth >35 m at localized sections, are water-bearing proposed for groundwater development. Overall, the methods and lithology-based empirical relationships can be adopted in granitic terrains to rapidly estimate RQD for a vast area with few and no borehole data. • Foundation rocks' bearing strength is crucial in infrastructure design in the study area. • Borehole- and SRT-based RQD with ERT models were employed for this purpose. • Infrastructure foundations are pinned on the fresh bedrock at depths of 8–25 m. • Multiple water column structures influence sustainable groundwater productivity. [ABSTRACT FROM AUTHOR]

The nature and rippability conditions (rock mass quality [RMQ]) of the near-surface lithologic units of Batu Maung, Penang Island, Malaysia, were investigated to develop sustainable groundwater and infrastructure. This study used a novel approach that combined seismic refraction tomography (SRT), electrical resistivity tomography (ERT), borehole drilling, and simple linear regression (SLR) analysis. The seismic P-wave velocity (Vp) and resistivity models revealed four distinctive units: residual soils (silty sand), very poor-to-good (weathered) granite, and the fresh granitic unit, including the fractured/faulted zones. The thicknesses of the residual and weathered materials ranged from a few centimeters to 15 m and 1.0–16 m, respectively. The developed empirical relationship effectively predicted rock quality designation (RQD) from Vp data through SLR analysis, with a prediction accuracy of 96% and a p -value<0.05. Also, the results from five key regression assumptions: linear relationship, multivariate normality, no multicollinearity, no autocorrelation, and homoscedasticity, suggested an accurate and statistically significant empirical relationship for use in granitic environments. The RMQ statistical model accurately classified the lithologic units beneath the area into Classes I–VI. Due to the low load-bearing of the rippable residual soils, very poor-to-fair weathered granitic rock masses, and fractured/faulted zones based on their Vp and resistivity values, and the steep slopes in the northern section of the study area, all intended infrastructures, particularly high-rise buildings and buildings with continuous footing foundations, should be piled to rest on the non-rippable fresh granitic units, with RQD and Vp values of >90% and >2100 m/s, respectively, in the central to the northcentral section in the study area. Conversely, the deep-weathered/fractured zones of depths >35 m, beneath Line 1, towards the central part of the area, with resistivity and Vp values of 100–900 Ω-m and <1900 m/s, respectively, were identified as potentially water-containing zones for sustainable groundwater abstraction. • This study entails strata rippabilities using SRT, ERT, and geostatistical analyses. • An empirical relationship to estimate RQD from Vp for granitic rocks was developed. • Rock masses within Classes I and II have a good-to-excellent RQD of 75% to 100%. • The excellent rocks certify the soundness criteria for founding engineering structures. • Sustainable groundwater development was envisaged at fractured/weathered zones. [ABSTRACT FROM AUTHOR]

Malaysia is one of the countries in the world that experiences landslides yearly due to natural events and human activities. Penang Island is Malaysia’s second most developed state and the largest by population. It is prone to landslides with devastating environmental impacts. Hence, the need to characterize its near-surface soil-rock conditions. This study uses remotely sensed data, with frequency ratio (FR) analysis, to identify landslide-prone areas based on different categories of landslide causative factors. To further understand the conditions and hydrodynamics of the soil-rock profiles causing landslides, electrical resistivity tomography and seismic refraction tomography were carried out at a landslide-suspected section in the study area. Also, satellite-derived Bouguer gravity anomaly modeling was performed to map the varied gravity anomalies associated with landslide-triggering factors in lithologic units. The multi-geophysical models offer strongly correlated results with the remote sensing causative factor maps and the landslide susceptibility index (LSI) map. The likelihood of landslides occurring in the area, as suggested by the area under curve modeling of LSI data, yielded a high predicted success rate of 83.47%. Hence, prospective landslides were identified in the hilly and elevated sections, while the less susceptible sections were identified on flat reliefs. Landslides may also be triggered, for instance, at steep sections with varied contractive soil bodies and shallow structures. Most importantly, leveraging the LSI map would help the necessary agencies forestall and mitigate future landslide occurrences in the area. [ABSTRACT FROM AUTHOR]