RISET Geologi dan Pertambangan
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AQUIFER MAPPING BASED ON STRATIGRAPHIC AND GEOELECTRICAL DATA ANALYSIS IN BEDOYO REGION, GUNUNG KIDUL REGENCY, YOGYAKARTA SPECIAL REGION
Full text linkThe research area is a high karst area composed of carbonate rocks, where drought occurred almost every year. This study aimed to determine water availability in aquifer rocks by analyzing the surface geology and subsurface layers (geophysical method). We used the Schlumberger configuration in the geoelectric method at four locations in the study area to determine the resistivity value of the rocks beneath the surface. The results revealed that the site comprises tuff, crystalline limestone, layered limestone /chalky, and gravel to clay deposits. An aquifer was only found in Asem Lulang Village, which has a wedge layer with a thickness of 59 meters and thinned down to 5 meters towards the village of Ngalas Ombo. The value of the resistivity aquifer ranges from 0.18 to 9.18 ohm.m. The existence of that aquifer could indicate that the aquifer is only found in weakly undulated
Depositional Environmental Evolution of Nyalindung Formation based on Paleontology Molluscan Study, Ciodeng Area, Sukabumi, West Java, Indonesia
Full text linkThis research was conducted to understand and identify changes in the depositional environment of the Nyalindung Formation in the Ciodeng area, Sukabumi, based on mollusk paleontology and lithological associations. Paleontological study of mollusks to interpret the depositional environment becomes important due to the scarcity or absence of foraminifera in mollusk bearing rocks. Foraminifera is commonly used to determine depositional environment. The results of this mollusk paleontology study and lithological associations show at least sixteen changes in depositional environment. The environmental changes range from rivers, mangroves, beaches, tidal areas, shallow marine nearby the coast, shallow open marine, and relatively deeper marine. The depositional environmental changes shown by thissection might be related to tectonic activity and global sea level fall that occurred since the early Late Mioceneto Pleistocene. The sedimentary units in this section are also characterized by predominant fluvial, mangroves, beach, and tidal in the upper layers.
Wireline logs vs drilling events: Which one to believe in implying subsurface pressure?
Full text linkAs generally known, subsurface pressure can be implied using both wireline logs and drilling events. However, there may be a case where wireline logs and drilling events do not indicate the same subsurface pressure. Data from four vertical wells located in the South Sumatra Basin, Indonesia, were analyzed as a case study. Two wells, Wells A and D, encountered high overpressured zones, confirmed by drilling events and wireline logs data. The two others, Wells B and C, only encountered low overpressured zones, inferred by the relatively low mudweight used during the drilling. However, the wireline logs of Wells B and C show a reversal as Wells A and D. There are two hypotheses to explain the condition in Wells B and C. First, the wireline logs reversal is due to shallow carbonate cementation. Second, Wells B and C were drilled in an unintentional underbalanced condition. The method used includes XRD, SEM, and titration analysis. The results show that the first hypothesis is false, while the second is true. It may be due to some missing information related to drilling events in the final well report of Wells B and C
IDENTIFIKASI AKUIFER AIR ASIN DAN AIR TAWAR BERDASARKAN MODEL TAHANANJENIS DAN DATA BOR DI SIDOARJO, JAWA TIMUR
Full text linkPenelitian ini bertujuan untuk melakukan identifikasi akuifer air asin dan air tawar di daerah Sidoarjo. Penentuan akuifer dilakukan berdasarkan model tahananjenis 2D dan data bor. Interpretasi data tahananjenis menunjukkan bahwa prospek akuifer air tawar ada pada kedalaman 10 hingga 50 meter yang menyebar luas, dan akuifer air tawar dalam berada pada kedalaman 140 sampai 170 meter. Akuifer- akuifer tersebut memiliki nilai tahananjenis antara 14,7 sampai 46,2 ohm-meter. Akuifer air asin teridentifikasi pada kedalaman 51 sampai 110 meter dengan nilai tahananjenis 0,48 sampai 3,1 ohm-meter, dan air payau pada kedalaman 20 hingga 40 meter. ABSTRACT Identification of Salt and Fresh Water Aquifers Based on Resistivity Model and Logging Data in Sidoarjo, East Java. The purpose of this study is to identify saltwater and freshwater aquifers in Sidoarjo region. The aquifers were identified based on a 2D resistivity modeling from geoelectrical survey and data interpretation from resistivity logging. Analyzes from both data show that the freshwater aquifer is at the depth of 10 to 50 meters, which are widespread. A deep freshwater aquifer is in a depth of 140 to 170 meters. These aquifers has a resistivity value between 14.7 to 46.2 ohm-meters. Saltwater aquifers are identified in the depth of 51 to 110 meters with resistivity values of 0.48 to 3.1 ohms-meters. And brackish water is at the depth of 20 to 40 meters
PEMETAAN PERMEABILITAS MAGNETIK PERMUKAAN BERBASIS CITRA SAR POLARIMETRIK DENGAN PENGUKURAN IN SITU DI LAPANGAN GUNUNG API
Full text linkPemetaan parameter fisis berupa permeabilitas magnetik permukaan pada zona lemah yang terkait dengan aktivitas tektonik maupun vulkanik perlu dilakukan seiring dengan meningkatnya aktivitas kegempaan maupun kegunungapian dalam beberapa tahun terakhir. Pendeteksian keberadaan zona lemah umumnya dilakukan berdasarkan parameter kelurusan struktur geologi, reflektansi spektra, dan temperatur permukaan dengan memanfaatkan citra satelit optis. Makalah ini mengolah parameter yang berbeda berupa permeabilitas magnetik permukaan (μr), yaitu parameter fisis yang diturunkan dari citra satelit Polarimetric Synthetic Aperture Radar (PolSAR) dengan polarisasi penuh (quad-polarization) dan diverifikasi dengan pengukuran in situ. Parameter μr permukaan merupakan salah satu parameter yang digunakan untuk mengidentifikasi keberadaan zona lemah jalur fluida termal dari bawah sampai ke permukaan gunung api aktif. Aktivitas hidrotermal mengakibatkan degradasi nilai magnetik batuan di sekitarnya, sehingga ekstraksi nilai permeabilitas magnetik permukaan μr dianggap sangat efektif untuk mengidentifikasi jalur fluida termal di permukaan. Modifikasi pada metode dielectric from Polarimetric Synthetic Aperture Radar (dPSAR) menggunakan citra PolSAR dilakukan untuk mendapatkan parameter permeabilitas magnetik permukaan μr, yaitu kemampuan permukaan tanah dalam menerima respon medan magnet, disamping permitivitas dielektrik εr. Pengukuran μr in situ di lapangan menggunakan alat ukur Ferromaster Magnetic Permeability Meter dilakukan untuk menganalisis tingkat akurasi μr dari hasil inversi dPSAR. Berdasarkan uji validasi, nilai koefisien determinasi R2 yang diperoleh cukup tinggi, yaitu sekitar 0,73 antara μr hasil dPSAR dan pengukuran lapangan. Selain itu, hasil uji normalitas menunjukkan nilai residual antara keduanya terdistribusi normal. Hal ini mengindikasikan bahwa pendeteksian μr dengan metode dPSAR memiliki potensi untuk dikembangkan dan diaplikasikan lebih lanjut dalam pemetaan fisis geologi di permukaan. ABSTRACT - Surface Magnetic Permeability Mapping Using Sar Polarimetric and In Situ Measurement at Volcanic Field. Physical parameters mapping of surface magnetic permeability in weak zones associated with tectonic and volcanic activity is necessary due to the increasing seismic and volcanic activity in recent years. Typically, weak zones are detected from lineament related geological structures, reflectance spectra, and surface temperature visually on optical satellite images. This paper processed a different physical parameter of surface magnetic permeability (μr) derived from the Polarimetric Synthetic Aperture Radar (PolSAR) quad-polarization satellite image that was verified by field measurement. The μr was used to identify the weak zones connected to the fluid path of the subsurface hydrothermal system of an active volcano. The activity of hydrothermal fluids through the weak zones is responsible for the degradation of the magnetic permeability of the host rocks so that extracting μr is effective in identifying fluid path at the surface. A modification has been applied to the dielectric from Polarimetric Synthetic Aperture Radar (dPSAR) method using the PolSAR image by inverting the μr, in addition to εr parameters, that served as capability of the ground surface responds to the magnetic field. Measurement of μr in the field using Ferromaster Magnetic Permeability Meter was carried out to analyze the accuracy of the inverted μr from the dPSAR method. According to the validation test of μr that derived from dPSAR and field measurement, a high coefficient determination R2 of 0.73 was achieved. In addition, the normality test showed that residual values were distributed normally. Therefore, the μr inversion using dPSAR method has the potential to be developed and applied for advanced physical surface geological mapping
SEISMIC HAZARD MICROZONATION BASED ON PROBABILITY SEISMIC HAZARD ANALYSIS IN BANDUNG BASIN
Full text linkThe geological and seismic-tectonic setting in the Bandung Basin area proliferates the seismicity risk. Thus, it is necessary to investigate the seismic hazards caused by the foremost seismic source that affects the ground motions in the bedrock. This research employed Probability Seismic Hazard Analysis (PSHA) method to determine the peak ground acceleration value. It considers the source of the earthquakes in the radius of 500 km with a return period of 2500 years. The analysis results showed that the Peak Ground Acceleration (PGA) in this region varies from 0.46 g to 0.70 g. It correlates with the magnitude and hypocentre of the dominant earthquake source of the study locations. The PGA value on the bedrock was used as an input to develop the seismic hazard microzonation map. It was composed using the Geographic Information System (GIS) to visualise the result. This research provides a scientific foundation for constructing residential buildings and infrastructure, particularly as earthquake loads in the building structure design calculations. ABSTRACT - Mikrozonasi Bahaya Seismik Berdasarkan Probability Seismic Hazard Analysis di Cekungan Bandung. Kondisi geologi dan seismik-tektonik di Cekungan Bandung meningkatkan risiko kegempaan di wilayah tersebut. Oleh karena itu, perlu dilakukan penelitian tentang bahaya seismik yang disebabkan oleh sumber-sumber gempa di sekitarnya yang mempengaruhi gelombang gempa di batuan dasar. Penelitian ini menggunakan metode Probability Seismic Hazard Analysis (PSHA) untuk menentukan nilai percepatan gelombang gempa di batuan dasar. Lebih lanjut penelitian ini menggunakan sumber gempa dalam radius 500 km dengan periode perulangan 2500 tahun. Hasil analisis menunjukkan bahwa Peak Ground Acceleration (PGA) di wilayah ini bervariasi dari 0,46 g hingga 0,70 g. Hal ini berkorelasi dengan magnitudo dan jarak hiposenter sumber gempa dominan terhadap lokasi penelitian. Nilai PGA di batuan dasar digunakan sebagai input data dalam pembuatan peta mikrozonasi bahaya seismik. Peta mikrozonasi bahaya seismik disusun dan divisualisasikan menggunakan Sistem Informasi Geografis (SIG). Luaran penelitian ini menghasilkan landasan ilmiah pada konstruksi bangunan tempat tinggal dan infrastruktur, khususnya sebagai pembebanan gempa dalam perhitungan desain struktur bangunan
GEOLOGI DAN PETROKIMIA ENDAPAN ZEOLIT DAERAH BAYAH DAN SUKABUMI
Full text linkZeolit dijumpai pada batuan piroklastik di daerah Cikembar (Sukabumi) dan Bayah (Banten) dari zona fisiografi pegunungan Bogor dan Bayah. Karakteristik, genesa batuan, dan tipe zeolit di kedua lokasi tersebut belum dijelaskan, terutama kaitannya dengan potensi pengkayaan unsur tanah jarang (UTJ) pada batupasir tufan di daerah Cikembar dan tuf teralterasi di daerah Swakan. Metode yang digunakan pada studi ini terdiri dari pengamatan geologi lapangan, petrografi, difraksi sinar-X (XRD), dan kimia batuan (whole rock) menggunakan XRF dan ICP-MS. Tujuannya adalah untuk mengungkap genesa pembentukkan zeolit dan UTJ di Swakan dan Cikembar. Batupasir tufan dari Cikembar dan tuf dari Swakan menunjukkan kehadiran zeolit bertipe mordenit dan klinoptilolit. Zeolit di daerah Cikembar terdapat sejajar perlapisan, sedangkan di daerah Swakan berasosiasi dengan kumpulan mineral alterasi. Pengendapan zeolit Cikembar dipengaruhi oleh air meteorik yang dicirikan oleh anomali negatif Ce pada pola diagram laba-laba yang di normalisasi terhadap kondrit. Total (∑UTJ) pada batupasir tufan dari Cikembar 82– 94 ppm dan pada tuf Swakan 71–83 ppm. Perbedaan kandungan ∑UTJ tersebut lebih mencerminkan komposisi UTJ pada batuan asal.ABSTRACT – Geology and petrochemistry of zeolite deposits of Bayah and Sukabumi areas. Zeolites are found in pyroclastic rocks in Cikembar (Sukabumi) and Bayah (Banten) from the Bogor and Bayah mountainous physiographic zones. The characteristics, rock genesis, and types of zeolites from these two locations have not been explained, especially in relation to the potential of REEs (Rare Earth Elements) enrichment in Cikembar tuffaceous sandstone and Swakan altered tuff. The method used in this study consists of field geological observations, petrography, X-ray diffraction (XRD), and whole rock geochemistry using XRF and ICP-MS. The aim is to uncover the formation of zeolites and REEs in Swakan and Cikembar. The Cikembar tuffaceous sandstones and the Swakan tuffs show the presence of mordenite and clinoptilolite types of zeolites minerals. The zeolites in Cikembar area are parallel to the bedding plane, while in Swakan area are associated with alteration mineral assemblages. The deposition of Cikembar zeolite was influenced by meteoric water which is characterized by the negative Ce anomalies in chondrite-normalized spider diagram pattern. The total (∑REEs) concentration in Cikembar tuffaceous sandstones are 82–94 ppm and Swakan tuffs are 71–83 ppm. The difference in the ∑REE content more likely reflects the parent’s rock REEs composition