Mohd Nawawi

An evaluation of optimization strategies to automatically select the optimal set of array configurations for 2D electrical imaging surveys

This paper presents results of tests involving four different strategies used to automatically select the optimal set of array configurations that generate the maximum amount of information regarding a subsurface with 2D electrical imaging surveys invoking the same number of measurements. The first strategy (Compare R) calculates the improvement in the model resolution for each array configuration added to the set of measurements. This strategy generates the best results but requires the greatest amount of time to complete. The second (BGS) and third (ETH) strategies use linear approximations to calculate a goodness function. Both strategies are less time consuming than the Compare R strategy; however, the BGS strategy produces array configurations with model resolution values that are slightly less accurate than the Compare R strategy, while the ETH strategy produces results of the poorest quality. The fourth strategy (Combined BGS–CR) uses a combination of the BGS and Compare R strategies and produces results that are almost as accurate as the Compare R strategy but requires significantly less computing time. The four optimization strategies were tested using model resolution calculations, synthetic models and field surveys. The results obtained using the different optimization strategies were also compared with conventional arrays. The various tests show that arrays selected via the optimization strategies result in subsurface inversion models that have better resolution and identify more accurate depths and shapes compared to conventional arrays.

Volumetric Assessment of Leachate from Solid Waste Using 2D and 3D Electrical Resistivity Imaging

This paper presents an approach to estimate volume of leachate from municipal solid waste using 2D and 3D electrical resistivity imaging. This process is a deviation from Water Balance Method (WBM) that is commonly used for leachate quantification at landfill sites. While the WBM emphasis is on generating rate, this geophysical approach estimates the in-situ total quantity of leachate. Five 100m profiles were used to generate five 2D inverted pseudo-sections and a 3D inverted section using RES2DINV and RES3DINV programs respectively. The resistivity of inverted pseudo-sections clearly delineated the contaminant leachate plume (ρ < 5 Ωm), which facilitated its volume estimation. The effective porosity values of clay (0.15) and sand/gravel (0.28) deduced from lithology logs were used in calculating the imaged volume of the leachate. To confirm the measured resistivity variation in the saturated subsurface around the dumpsite, in-situ electrical conductivity values of 1782 μS/cm and 4521 μS/cm were determined for uncontaminated and contaminated zones respectively. A total leachate volume of 2.21 x 103 was estimated for the 1600 survey area.

Utilization of multimode Love wave dispersion curve inversion for geotechnical site investigation

Inversion codes based on a modified genetic algorithm (GA) have been developed to invert multimode Love wave dispersion curves. The multimode Love wave dispersion curves were synthesized from the profile representing shear-wave velocity reversal using a full SH (shear horizontal) waveform. In this study, we used a frequency–slowness transform to extract the dispersion curve from the full SH waveform. Dispersion curves overlain in dispersion images were picked manually. These curves were then inverted using the modified GA. To assess the accuracy of the inversion results, differences between the true and inverted shear-wave velocity profile were quantified in terms of shear-wave velocity and thickness errors, ES and EH. Our numerical modeling showed that the inversion of multimode dispersion curves can significantly provide the better assessment of a shear-wave velocity structure, especially with a velocity reversal profile at typical geotechnical site investigations. This approach has been applied on field data acquired at a site in Niigata prefecture, Japan. In these field data, our inversion results show good agreement between the calculated and experimental dispersion curves and accurately detect low velocity layer targets.

Groundwater Exploration Using 2‐D Resistivity Imaging Technique in Marang, Terengganu, Malaysia

Surface water is critically important in supplying water to streams and wetlands, and in providing water for irrigation, manufacturing, electricity power and other uses and it is an important source of water supply especially in various regions in Malaysia and it become ever more important with an increasing population. However groundwater can be an alternative source of water to the ever increasing population. Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic water formations. This would provide alternative freshwater source. In order to determine the existence of usable groundwater for agriculture purposes in Marang Terengganu, 2‐D resistivity imaging technique was utilized. Three lines were surveyed at the site. The 2‐D resistivity imaging technique utilized the Pole ‐dipole array because of relatively good horizontal coverage but it has significantly higher signal strength. The total length of the survey lines is 400 meters. Three lines were...

Regional travel-time residual studies and station correction from 1-D velocity models for some stations around Peninsular Malaysia and Singapore

Abstract We have investigated the average P-wave travel-time residuals for some stations around Southern Thailand, Peninsular Malaysia and Singapore at regional distances. Six years (January, 2010–December, 2015) record of events from central and northern Sumatra was obtained from the digital seismic archives of Integrated Research Institute for Seismology (IRIS). The criteria used for the data selection are designed to be above the magnitude of mb 4.5, depth less than 200 km and an epicentral distance shorter than 1000 km. Within this window a total number of 152 earthquakes were obtained. Furthermore, data were filtered based on the clarity of the seismic phases that are manually picked. A total of 1088 P-wave arrivals and 962 S-wave arrivals were hand-picked from 10 seismic stations around the Peninsula. Three stations IPM, KUM, and KOM from Peninsular Malaysia, four stations BTDF, NTU, BESC and KAPK from Singapore and three stations SURA, SRIT and SKLT located in the southern part of Thailand are used. Station NTU was chosen as the Ref. station because it recorded the large number of events. Travel-times were calculated using three 1-D models (Preliminary Ref. Earth Model PREM (Dziewonski and Anderson, 1981, IASP91, and Lienert et al., 1986) and an adopted two-point ray tracing algorithm. For the three models, we corroborate our calculated travel-times with the results from the use of TAUP travel-time calculation software. Relative to station NTU, our results show that the average P wave travel-time residual for PREM model ranges from −0.16 to 0.45 s for BESC and IPM respectively. For IASP91 model, the average residual ranges from −0.25 to 0.24 s for SRIT and SKLT respectively, and ranges from −0.22 to 0.30 s for KAPK and IPM respectively for Lienert et al. (1986) model. Generally, most stations have slightly positive residuals relative to station NTU. These corrections reflect the difference between actual and estimated model velocities along ray paths to stations and can compensate for heterogeneous velocity structure near individual stations. The computed average travel-time residuals can reduce errors attributable to station correction in the inversion of hypocentral parameters around the Peninsula. Due to the heterogeneity occasioned by the numerous fault systems, a better 1-D velocity model for the Peninsula is desired for more reliable hypocentral inversion and other seismic investigations.

Slope failures evaluation and landslides investigation using 2-D resistivity method

Abstract Slope failure is a complex phenomenon that may caused to landslides. Buildings and infrastructure such as transportation facilities and pipelines located within the boundaries of a landslide can be damaged or destroyed. Slope failure classification and various factors contributing to the instability using 2-D resistivity survey conducted in Selangor, Malaysia are described. Six 2-D resistivity survey lines with 5 m minimum electrode spacing using Pole-dipole array were performed. The data were processed using Res2Dinv and surfer10 software to evaluate the subsurface characteristics. The 2-D resistivity results show that the subsurface consist of two main zones. The first zone was alluvium or highly weathered with resistivity value of 100–1000 Ω m and depth of >30 m. This zone consists of saturated area with resistivity value of 1–100 Ω m and boulders with resistivity value of 1200–7000 Ω m. The second zone with resistivity value of >7000 Ω m was interpreted as granitic bedrock. The study area was characterized by saturated zones, highly weathered zone, highly contain of sand and boulders that will trigger slope failure in the survey area. This will cause to low strength of soil, debris flow and movement of earth. On the basis of the case examples described, 2-D resistivity method is categorized into desirable and useful method in determination of slope failure and future assessments.

Seismic reflection survey at Ayer Hangat site to investigate shallow subsurface structures

Ayer Hangat site is located in the island of Langkawi, northwest Malaysia. The site is characterized by the presence of hot spring. This hot spring is believed to be related to granitic intrusion nearby. Hence the present work is focusing on defining the shallow subsurface structures that control the migration of hot water to the surface. Seismic reflection method is used to achieve the goal of the present study. Forty three shot points were used with an offset of 5m of the nearest geophone. The shot-points interval is set to 1m. Seismograms were recorded on 24 channel TERRALOC instrument. The Geophone interval used was 1m. Conventional seismic data processing scheme was adopted. However, due to the fact that TERRALOC produce SEG2 data files, a script based on Obspy was written and used to convert to SEG-Y format. Afterwards, analyses were carried out using SU Package. The processed data is used to develop a model for the subsurface controlling structures. Such model will help in the understanding of the geothermal hot spring system in the area.

A preliminary mineralogical evaluation study of natural asphalt rock characterization, southeast Sulawesi, Indonesia

The mineral content of asphalt rocks located in Buton Island (southeast Sulawesi, Indonesia) had remained unknown/unexplored despite the economic importance that may be attached to it. In order to characterize it, evaluation of the natural asphalt rock using x-ray fluorescence analysis (XRF) and resistivity survey have been carried out. For the XRF analysis, two rock samples were collected from the study area and analyzed at different temperatures in order to observe the variations in concentration and composition of rock samples. Vertical stacking technique (VerStack) was used for the characterization of rock sample into elements with either similar or different patterns at varying depths. The results obtained show 15 elements which are previously unexplored, and some of these elements constitute the innate elements in the parent rock minerals. The VerStack technique indicated that the characteristics of upper rock minerals (topsoil) are similar in patterns with bottom rock minerals. The radial plotting technique was also carried out and it shows good correlation with VerStack patterns. More detail geophysical investigation is recommended to ascertain the quantity of these elements identified by this study.

Studying Effect of Riverbank Filtration (RBF) on Ground Water Using ‘Time-Lapse Electrical Resistivity Imaging’ at Kuala Kangsar, Perak

Riverbank filtration (RBF) is a natural water pre – treatment, safer and cost – effective that uses geology instead of chemicals to pre – treat river water. The process occur during the infiltration process of river water passing through riverbank material towards a production well during a pumping test, where the removal of biological and colloids take place. Abstraction of ground water are required for RBF to occur, hence pumping test were done. As pumping test took place, ground water was in continuous discharging and recharging process throughout time. Objective of this research is to study the dynamic changes of ground water during discharging and recharging process for 72 hours by using time – lapse electrical resistivity imaging (TLERI). Resistivity, which uses current to measure the resistivity of subsurface was used during the pumping test to monitor ground water changes and infiltration of river water during RBF process. Results show significant changes on TLERI data from the first session until the last session. Dynamic changes occurred around the riverbank and production well section while others show less significant changes.

Improved Subsurface Mapping by Enhancing Resistivity Horizontal Resolution

Geotechnical studies are usually used for subsurface, engineering and environmental works. Geophysical study provides supported data in order to save cost and time. The survey was executed to solve engineering problems and environment. Geophysical methods can be used to determine depth of bedrock, thickness of soils, near surface structure such as fault and cavity. A resistivity study is another method of mapping shallow subsurface and become increasingly more widespread in their use. In this paper, resistivity technique will be developing in order to get detail and deeper penetration in shallow subsurface study. By enhancing resistivity horizontal resolution to establish targets, most of the required information can be mapped and the results verified, thus improving the subsurface mapping of the engineering and environmental works. This technique is called as EHR (Enhancing Horizontal Resolution). EHR technique is applied at two sites, miniature model and real site. From the results, EHR technique improves the horizontal changes in subsurface resistivity.