The role of geophysics in the construction and stability assessment of tunnels

Geophysics play a vital role in the constructions of any major manmade structures in the world. One of those being the tunnels. In depth understanding of geophysical methods and a lot of information are needed in order to design a tunnel construction project. Comprehensive investigation on the ground condition has to be done before the field preparation study that will determine the stand-up time and the groundwater condition that may disrupt the tunnel construction. For tunnel stability assessment, an integration of geophysical methods is a must in order to obtain the most accurate results. Satellite imaging interpretation emphasizes on the structural tracing of negative lineament while field mapping emphasizes on location of underground seepage and major tectonic structures such as faults, joints and shear zones. Geoelectrical resistivity tomography survey is able to identify the differences in resistivity of Earth’s materials based on the water content inside of them. The best course of remediation could only be chosen once the output from all these studies are made available.

Lenggong Valley – Revisit Our National Treasure

Lenggong Valley was incepted as UNESCO World Heritage Site in 2012 for its marvellous cultural value. However, the status will be in peril if the valley is left neglected and nothing is done. The area is renowned for the discovery of prehistoric human settlement. Numerous studies and researches have been conducted at Lenggong Valley to increase the scientific knowledge of the surrounding areas for further conservation and development. Toba ash deposit from volcanic eruption in Sumatra, Indonesia and possible meteorite impact were also discovered within the valley. Geoelectrical resistivity survey have been conducted to determine the thickness of Toba ash. The results were correlated with borehole log from the Department of Mineral and Geoscience Malaysia (JMG), that shows the Toba ash layer is around 10 m in depth and presents low resistivity range of values (<100 Ωm). Water geochemical analysis at one of the known water spring shows the surface temperature is 27.6 °C, with a pH value of 7.47. Truth be told, Lenggong Valley has a diversity of archaeological sites and geosites that can be established as a geopark and a group of committee will be materializing it in year 2021 or 2022.

INVESTIGASI SUMBER AIR TANAH MENGGUNAKAN METODE GEOLISTRIK RESISTIVITAS KONFIGURASI SCHLUMBERGER DAN PENGEBORAN

Penelitian ini bertujuan untuk mencari keberadaan sumber air tanah. Metode yang digunakan adalah geolistrik resistivitas konfigurasi schlumberger dan proses pengeboran. Pengukuran dilakukan pada 4 titik dengan bentangan 400 meter dan jarak antar elektroda 10 meter. Hasil penelitian menunjukkan indikasi keberadaan air tanah ditemukan pada titik pengukuran 3 kedalaman 36-55 meter dibawah permukaan tanah dengan nilai resistivitas sebesar 0,97 Ωm. Proses pengeboran dilakukan pada titik 3 sesuai rekomendasi, adapun hasil dari pengeboran menunjukkan informasi keberadaan air tanah ditemukan pada kedalaman 46 m dibawah permukaan tanah, hasil ini sesuai dengan informasi yang diperoleh dari hasil penelitian. Identifikasi keberadaan air tanah menggunakan metode geolistrik resistivitas terbukti cukup ramah lingkungan, karena tidak ada kerusakan lingkungan yang ditimbulkan. Kata kunci: air tanah, geolistrik resistivitas, konfigurasi schlumberger, pengeboran. This study aims to determine the existence of groundwater. The method used in this research is the geoelectrical resistivity method with schlumberger configuration and drilling process. Measurements were made at 4 points with a span of 400 meters and a distance between electrodes of 10 meters. The results showed indications of the presence of groundwater were found at measurement point 3, a depth of 36-55 meters below the ground surface with a resistivity value of 0.97 Ωm. The drilling process was carried out at point 3 according to the recommendations, while the drilling results showed information on the presence of groundwater at a depth of 46 m below the ground surface, this result is the same as the information obtained from the research results. The identification of groundwater using geoelectrical resistivity method is proven to be quite environmentally friendly because there is no environmental damage caused. Keywords: drilling, geoelectrical resistivity, groundwater, schlumberger configuration.

Application of GSTAR(1,1) model for layer peat soil predicted based on resistivity log data

In this study, GSTAR modeling was carried out with the inverse of distance weight matrix obtained from Geoelectrical Resistivity data at several peatland locations around the Universitas Tanjungpura, Pontianak. This data can identify the subsurface layer of the soil through the electric current that binds into the soil. However, due to the limitation of the tool to measure the resistivity value, it can only measure 1/5 of the depth of the observation length. To overcome this problem, predictions are made at the next depth using the GSTAR model. The study began by measuring the resistivity value of the land using the geoelectric method and mapping it. Through this GSTAR modeling, predictions are made for the unobserved subsurface to determine the type of soil layer. Knowing the type of deeper soil layer can help contractors build plant concrete stakes to keep buildings safe on peatland. The results of the GSTAR(1.1) model are not accurate enough to estimate the resistivity value data. This is possible because the correlation between rock ages is not the same, so further analysis is required.

An investigation of groundwater condition by geoelectrical resistivity method: A case study in Bengalon, East Kutai, East Kalimantan Province Indonesia

Bengalon is located in East Kutai Regency, East Kalimantan Province. Bengalon is included in the Samarinda-Bontang groundwater basin. The development of agricultural and industrial areas, followed by an increase in residential areas, will cause an increase in the water demand. This study aims to determine the groundwater potential of alluvial areas using the resistivity geoelectric method. The resistivity survey resulted in subsurface lithology, shallow groundwater conditions in the form of depth, thickness, and location of the aquifer as well as the type of groundwater quality. There are 10 locations for geoelectric data collection using the 2D geoelectric method. The results showed that the majority of the aquifers were alluvial deposits consisting of sand and sandy clay. The resistance indicating groundwater is 1 - 4 Ω m, at a depth of 10 – 50 m with a thickness varying from 10 – 20 m which has been dominated by sand. The presence of sea water intrusion in aquifers causes the quality of groundwater to tend to be brackish because the research location is in a swamp area bordering the coast.

Investigation of Aquifer Zone in Tambakromo Village, Ponjong Subdistrict, Gunung Kidul Regency, Yogyakarta, Indonesia

The presence of surface water is always being a primary issue in Gunung Kidul regency. An aquifer investigation would support groundwater exploration. The research area is located in Tambakromo village, Ponjong subdistrict, Gunung Kidul Regency, Daerah Istimewa Yogyakarta Province. The aim of this research is about geological and subsurface conditions based on geological data and geoelectrical resistivity methods. By resistivity value interpretation, the position, depth, and thickness of aquifer could be determined. The purpose of this research is to identification of geological conditions that supported being aquifer rock. The method in this research is used geological mapping combined with petrographic analysis. The geophysics method is used to identification of aquifer in research area. Three locations were selected to acquisition geoelectrical resistivity survey. In Grogol village, there is no aquifer based on resistivity values of more than 10000 ohmmeters (andesitic breccia). An akuifuge is interpreted there in andesitic breccia instead of the presence of aquifer. The tuffaceous is interpreted as lapillistone that contained the glass with resistivity value of 43.63 and 340.11 ohmmeters. The Garon and Sumberejo village have aquifer at depth of 50 to 80 meters with a thickness of 13 to 70 meters. The type of aquifer in both area is included an unconfined aquifer, specifically located in between the limestone rock layer and andesitic breccia. By regional stratigraphic correlation, tuffaceous lapillistone is interpreted as derived from Semilir Formation. In addition, the presence of aquifer in research area could recommend for local people and government to reduce the drought disaster.

Sea Water Intrusion Monitoring on the Sendangbiru of the Southern Malang Coastal Area based on Geoelectrical Resistivity Data

The study was conducted with the objective to distinguish the presence of seawater intrusion layer or salt-water aquifer distribution along the data acquisition line at the locations. Data acquisition was conducted by using the Wenner-Schumberger configuration of geoelectrical resistivity. From this research, 4 lines and 4 points of vertical electrical sounding (VES) data for every line were obtained with the distance between electrode a as 10m. Based on the data processing, obtained depth up to 120m with the smallest resistivity value is 0.02Ωm and the largest is 6764.52Ωm. To make the distribution of resistivity values along the path line of the study, cross sections were made until a depth of 120m. Based on the cross-section, the low resistivity value (less than 1.5 Ωm) that interpreted as a seawater intrusion layer or salt water aquifer distribution is located at varying depths. There are intrusions for the SB1 cross section, there is an intrusion at a depth of 6m-7m as far as 10m, at a depth of 6m-8m as far as 10m for the SB2 cross section and at a depth of 22m - 26m as far as 25m for the SB3 cross section.

Geoelectrical Exploration of Groundwater by Using Vertical Electrical Sounding Technique at Wadi Morra, South Sinai, Egypt

Wadi Morra area is one of the most important areas targeted by the government for sustainable development. This development depends mainly on groundwater occurrences. This research aims to explore groundwater in this region, which depends on groundwater for its needs. To achieve this aim, the geoelectrical resistivity method was used, represented by Schlumberger Vertical Electrical Soundings (VESs). Seventeen VESs have been carried out with a maximum half current electrode spacing (AB/2) of 700m. The measured VESs are interpreted qualitatively and quantitatively to construct a geoelectrical model. To detect the water-bearing layers and their extensions, three geoelectric cross-sections in addition to a one-panel diagram were constructed using all the available data. Also, both true resistivity and isopach contour maps of waterbearing layers were created based on these data. The interpretation results indicate that there are five geoelectric layers (A, B, C, D, and E), two of them (C and D) are water-bearing layers. The first water-bearing layer is layer C, which represents the Nubian sandstone aquifer, and the second layer is layer D, which represents the fractured basement aquifer. These two aquifers were observed along the northern part of the Saint Catherine - Nuweiba road that dissects the Wadi Morra basin, while it is absent at the southern part of this road. It is due to the presence of a few faults. According to groundwater occurrences, the study area was divided into two zones. The first zone is zone (A) where the groundwater is present, either in the Nubian sandstone aquifer or in the fractured basement aquifer and the second zone is zone (B) where there is no groundwater at all due to the presence of basement rocks at a few depths. This research also recommends that the best places for drilling productive wells be at zone A.

Geoelectrical and Geotechnical Characterization of Different Types of Soil in Ede, Osun State, Nigeria

Geoelectrical and geotechnical investigations were carried out to characterize soils from different locations in Ede, Osun State, Nigeria for engineering purposes. Wenner electrode configuration technique was deployed in carrying out the geoelectrical survey of about 40m for each profile. The data analysis was approached by plotting the apparent conductivity against electrode spacing (s) and the result was interpreted. The geoelectrical resistivity survey revealed locations 1 and 2 with resistivity values of 25.01-419.22ohm-m (conductivity 0.002 - 0.194 (ohm-m)-1and 5.5-1246.57 ohm-m (conductivity = 0.0002 - 0.001 (ohm-m)-1) respectively could be classified as clay. Sample 3 recorded a resistivity value of 1.00- 22,787.39 ohm-m (conductivity= 0.00004- 1.00 (ohm-m)-1) thus, was classified as silt/sand respectively. Soil from the said different locations in Ede, Nigeria, were tested in the Laboratory for certain properties like Atterberg limits, specific gravities, sieve analysis, compaction test, etc and the results showed that samples 1, 2 and 3 have specific gravity values of 2.50, 2.13 and 2.40 respectively and could therefore be referred to as organic soil. Samples 1, 2 and 3 have maximum dry density (MDD) of 1.45g/cm3­, 1.92g/cm3, 1.95g/cm3 and optimum moisture content (OMC) of 15.40%, 13.36% and 9.61% respectively. The analysis conducted in this study revealed that the soil type found in Ede, Nigeria could be classified as silt-clay, sandy clay, clay and sand.Keywords- Electrical conductivity, Characterization, Compaction, Plasticity, Wenner array.