JURNAL GEOCELEBES

Investigation of Sediment Layer Thickness Estimation at Bengkulu University Hospital Based on Microtremor Data

Andre Rahmat Al Ansory — 2024-04-01

Bengkulu is the capital of Bengkulu Province and is located in the subduction zone between the Eurasian and Indo-Australian plates. Bengkulu is also close to the major faults of Sumatra, namely the Musi fault, the Ketahun fault, and the Manna fault. Therefore, regional planning and infrastructure development for earthquake mitigation is necessary. The purpose of this study is to estimate the thickness of the sedimentary layers in the construction zone of Bengkulu Hospital during the earthquake attenuation phase. This study is conducted by surface geological analysis using the HVSR method; data analysis using microtremor data. The microtremor data were analyzed using the HVSR method to obtain the amplification value (A₀) and the dominant frequency (f₀). HVSR analysis gave dominant frequency (f₀) values from 5.1 to 5.8 Hz, amplification (A₀) from 1.64 to 5.91 times, and dominant period (T₀) from 1.49 to 2.81 seconds. The values of A₀, f₀, T₀ are interpreted with reference to the literature and surface geology as a moderate risk of seismic vulnerability, characterized by moderate to weak sedimentation and moderate amplification values.

2D Marine Seismic data Analysis Using Comparison of Kirchhoff’s Migration Method and Finite Difference Method (Case Study: Nias Basin, North Sumatera)

Cindy Fatika Nur Annisa — 2024-04-01

Seismic migration is one of the important stages in seismic data processing which aims to map seismic events to their actual positions. The migration process used in this study is post-stack time migration in the time domain using the Kirchhoff migration technique and the finite difference method to determine the results of subsurface imaging from the two migration techniques and then compare them to determine the accuracy of selecting the appropriate migration for the L08 basin trajectory research area. Nias basin, North Sumatra. The processing steps are carried out according to the preprocessing to processing stages in the Promax 5000 software. Based on the results of the study, the optimum use of aperture migration in Kirchhoff migration will produce good subsurface cross-sectional imaging. The aperture value used is 3000 ms. In the finite difference migration, subsurface imaging is much more focused with a time step variation of 10 ms, whose function is to focus the hyperbolic diffraction energy on the migration data.

Genesis of Bauxite Ore in Toba Area Sanggau District, West Kalimantan Province

Ricka Aprillia — 2024-04-01

Indonesia's largest bauxite reserves are in the province of West Kalimantan, which is 703 million tons Bauxite is formed from rocks with a high relative aluminum (Al) content, low iron (Fe) content, and small amount of quartz. The mineralogy and characteristics of lateritic bauxite deposits are closely related to several factors, one of which is the texture and composition of the bedrock such as color, mineral composition, and shape of the ore. This study discusses the genetic type of bauxite deposits based on mineralogy and geochemistry using mineragraphic, XRD, and XRF methods. The primary data from bauxite ore samples were collected from the stockpile of PT. Dinamika Sejahtera located in Toba area. The quantitative result of the geochemical analysis indicates a higher amount of alumina observed using the XRF method. Granodiorite bauxite, which is bauxite coming from granodiorite bedrock, generally has abundant geochemical elements, especially SiO2 and Al2O3. The lateritic bauxite type in the Toba area is a product of granodiorite weathering from the Sepauk Tonalite formation is embedded within a clay matrix which exhibits a brownish to red color with coarse to boulder-size of concretion texture without relict. Some important elements in bauxite laterite deposits are Al, Fe, Si (Silicon), and Ti (Titanium). The comparison between Al and Si values is a benchmark for the economics of bauxite mines. Gibbsite is the major mineral in the bauxite ore, while hematite, goethite, kaolinite, and quartz are the accessory minerals. The deposit is recognized as Low-Fe bauxite due to comparing Al2O3, Fe2O3, and Si2O3 concentrations. The weathering process has altered the primary texture, remaining resistant and secondary minerals. The petrographic analysis shows the replacement of Gibbsite as bauxite ore which presents as kaolinite replacement and fills the mineral cracks. The result of this study is expected to be useful in determining the exploration method for the bauxite deposits.

Subsurface Interpretation of the Panjang Fault Area, Lampung, Based on Geomagnetic Method

Syamsurijal Rasimeng — 2024-04-01

Research on the area along the Panjang Fault - Lampung, the area from the Teluk Betung to Tanjung Karang Barat area using the 19-T GSM PPM tool with base rover acquisition on 2 tracks 2 kilometers apart obtained 40 acquisition points with a spacing of 0.3 kilometers. This study aims to determine the type of lithology and subsurface rock structure by utilizing the susceptibility value of rocks from magnetic anomalies. In the process of processing magnetic anomaly data, upward continuation is carried out as high as 350 m which is intended to reduce the total anomaly with the upward anomaly results so that a residual anomaly is obtained. Next, make a 2D subsurface model on the incision A - B in the residual anomaly map. Based on the results of qualitative interpretation, the total magnetic anomaly of the research area illustrates positive to negative anomaly values with a tighter contour pattern that indicates the presence of a fault structure. While based on quantitative interpretation, the 2D modeling in incision A - B shows a susceptibility value of 0.100 cgs which can be identified as breccia tuff rock, a susceptibility value of 0.0391 cgs is thought to be rhyolitic tuff rock, pumice tuff rock, and sandstone tuff, and a susceptibility value of 0.150 cgs is a rock from the intrusion of Mount Betung in the form of andesite-basalt lava. In addition, rocks with a susceptibility value of 0.0024 cgs are metamorphic rocks. The correlation between 2D modeling and regional geology is seen in the research area, which is in the Tarahan Formation (Tpot), which is suspected to be a fault structure in the Bumi Waras area with a strike direction of NW - SE which is the course of geothermal manifestations or minerals.

Analysis of Ground Vibration Levels Due to the Blasting Process at PT. Bumi Suksesindo

Nofry Hence Tarumasely — 2024-04-03

Ground vibration is one of the effects of the blasting process; when the ground vibration reaches the highest level, it will disturb comfort and even cause damage to the surrounding building structure. This research aims to determine the magnitude of ground vibrations in Pit A and Pit C, as well as determine the relationship between Peak Particle Velocity (PPV) and scaled Distance, and determine the maximum explosive charge weight per delay based on the SNI 7571: 2010 reference. Actual ground vibration measurement data during research based on PPV theory and the actual PPV power regression relationship with scaled distance was used to obtain a ground vibration prediction formula to be a reference for determining the amount of explosive filling per delay. The ground vibration produced in the blasting process is hoped not to exceed the safe threshold. Prediction of the ground vibration formula at 100 m to 1500 m according to the US Bureau of Mines where the Mean Squared Error (MSE) value is 0.54, the MSE value from the Langefors-Kihlstrom equation is 1.85 while the MSE value from the Ambersays-Hendorn equation is 0.31 with the slightest deviation is very good to use as a reference for predicting ground vibrations with the predicted PPV formula. Hence, the maximum explosive charge with a PPV limit of 2 mm/s is 2.452 kg, a PPV limit of 3 mm/s is 11.332 kg, and a PPV limit of 5 mm/s is 23.040 kg. The factors that influence ground vibration are the Distance from the blasting location to the measurement location and the maximum number of explosives per delay, so the results taken from this research are that blasting in Pit A and Pit C is still categorized as safe for infrastructure and community housing.

Application of Seismic Refraction Tomography in Determining the Soil Hardness Level in IKN Nusantara Area

Andi Alamsyah — 2024-04-29

Numerous studies supporting infrastructure construction are currently underway in the New Capital Territory of Nusantara (IKN Nusantara). A geophysical method known as Seismic Refraction Tomography (SRT) has been employed within the IKN Nusantara to identify hard and soft layers based on the P-wave velocity (Vp). The data acquisition involved 24 channels of geophone spaced at intervals 3 and 4 meters. Measurements were conducted alongfour trajectories of 69 and 92 meters, reaching penetration depths of 12 – 20 meters. P-wave velocity values ranging between 200 – 3500 m/s were recorded. Additionally, the Unconfined Compressive Strength (UCS) value was determined using an empirical equation tailored for mudrock-shale lithology, establishing the correlation between Vp and UCS. In the shallow depths of 0 – 3 meters, UCS values indicated levels below 20 MPa, classifying the materials as having low to medium hardness. However, at depths greater than 3 meters, this layer transitioned to material with high hardness levels, as evidenced by UCS rate exceeding 20 MPa across all trajectories. This suggest that the IKN Nusantara is conducive to infrastructure development.

1D Audio Magnetotelluric Modelling for Deep Aquifer Identification in the Lava Fan Area of Haruman Peak, Malabar Mountains

Nabilah Rahmawati — 2024-04-29

The Malabar Mountains area acts as a catchment and infiltration zone for rainwater. Haruman Peak is the location of one of these areas. Information on the well-preserved depth of the shallow aquifer at 40 meters reinforces this. This research reviews the results of subsurface 1D resistivity structures from AMT data to obtain information on the depth of deep aquifers on the western slope of the Haruman Peak Lava Fan, Haruman Mountains. 1D modelling shows an aquifer at depth of 140.56-2080.07 meters with resistivity ranging from 5.25-68.08 Ωm. At depths of 453.32 m (HR02), 530.8 m (HR03), 1464.97 m (HR01), and 2080.07 m (HR02), interbedded tuff-andesite with minor pumice identifies the deep aquifer. In addition, a depth of 140.56 m (HR02) with a resistivity value of 68.08 Ωm indicates a shallow aquifer. Looking at the elevation of the Bandung Basin, water from aquifers located at elevations > 700 meters above sea level will flow into the Bandung Basin.

Regional Lineament Pattern and Morphotectonic Analysis: The Investigation of Geological Structures and Present-Time Relative Tectonic Activity in the Tin Granite Area of Belitung Island, Indonesia

Harnanti Yogaputri Hutami — 2024-04-29

Belitung Island is located on the East Coast of Sumatra and is the southernmost extension of the Southeast Asian granite belt. Despite the flat terrain of the island, numerous granite outcrops provide insight into the past tectonic activities that caused the uplift in the region. This study analyzes the current state of Belitung's tectonic activity by examining its morphotectonic index and lineament pattern. A National Digital Elevation Model (DEMNAS) dataset with a resolution of up to 8.1 m will be used to assess the geological patterns and relative tectonic activity from the surface. The relationship between the regional lineament system and morphotectonic quantification throughout the landforms of Belitung Island will also be considered. The modified Segmented Tracing Algorithm (m-STA) technique extracted the lineament features. The Index of Relative Active Tectonic (IATR) was calculated by averaging several morphotectonic indices, such as asymmetry factors (AF), stream-length index (SL), mountain-front sinuosity (Smf), and valley floor width-height ratio (VF) factors, to quantify the relative tectonic activity of the area. The combination of the two methods shows that Belitung is currently experiencing relatively weak tectonic activity compared to the past. This is supported by the surface appearance, which is mainly composed of lowlands. Several granite outcrops and highlands are aligned along the NW-SE and NE-SW directions, corresponding to the main geological structures in the area.

Andesite Quality based on Compressive Strength Tests in the Ulujadi area, Palu City and the Banawa Area, Donggala Regency

Fauziah Alimuddin — 2024-04-30

The research location is located in the Ulujadi area, Palu City, Central Sulawesi Province, which is one of the areas that has abundant andesite mineral content. This research aims to optimizing use of andesite rocks in the Ulujadi area of Palu City. The research was carried out using qualitative and quantitative methods in the form of rock sampling and laboratory analysis, namely compressive strength test analysis to determine the technical properties of rocks and petrographic analysis to determine the quality of andesite based on the minerals that make up it. The results of petrographic analysis show that the andesite rocks in the study area are composed of minerals that have a good level of resistance. ST 07 has a mineral composition of plagioclase (38%), pyroxene (5%), biotite (5%), hornblende (7%), and soil mass (45%), and has a compressive strength test of 123 MPa, included in the classification strong, and is used as a foundation for light to medium buildings (SNI 03-0394-1989). ST 08 has a mineral composition of plagioclase (32%), ground mass (50%), opaque minerals (5%), orthoclase (5%), quartz (4%), and hornblende (4%), with a compressive strength test value of 97MPa, including in the strong classification, and can be used as a foundation for light to medium buildings. Meanwhile, ST 09 has a mineral composition of plagioclase (30%), hornblende (12%), quartz (3%), biotite (3%), orthoclase (7%), pyroxene (5%), and soil mass (40%). ), it is included in the weak category due to the relatively high degree of weathering, and can only be used as an ornamental stone.

Stochastic Inversion in Determining the Distribution of Petroleum Carrying Sandstones in the "JS" Field of the South Sumatra Basin

Johannes Kurni Bintang Awan Situmorang — 2024-04-30

The "JS" field is a field located in the South Sumatra Basin, where the field has good prospects for the distribution of petroleum-bearing sandstone. The target of this research is the Air Benakat formation. This research uses the stochastic seismic inversion method to determine the probability of finding petroleum in sandstone. Stochastic seismic inversion has the advantage that it can overcome thin layers and can reduce existing data misalignments. So stochastic inversion can overcome the shortcomings of other seismic inversions, especially model-based seismic inversion which is the initial model for stochastic seismic inversion. Stochastic seismic inversion produces several realizations by showing uncertainty so as to get results that are close to the actual situation. Probability map of oil-bearing sandstones located in the north and east of the study area. with the slice results obtained for the acoustic impedance range of 8517-9051(m/s)*(g/cc) and oil sand probability with a value range of 0.61-0.78%.