ANALISIS BAHAYA, LINTASAN, DAN SISTEM PROTEKSI TERHADAP POTENSI LONGSORAN TIPE JATUHAN BATU PADA LERENG BANGUNAN PELIMPAH BENDUNGAN TUGU, JAWA TIMUR

The construction of the Tugu Dam spillway does not escape the problem of slope instability, especially the rock fall type landslide as a result of the rock slope cutting work at STA+80. The purpose of this study was to determine the characteristics of the rock discontinuity area and the solutions needed to address the potential hazards of rock fall on the slopes of spillway structure. In this study, a semi-quantitative method conducted based on the Rockfall Hazard Rating System (RHRS) which is carried out by identifying outcrops on rock slopes. Determination of the rock fall trajectory, was conducted by statistical methods on rock mass based on changes in velocity when rocks roll, slide, and bounce. Geologically, the research area belongs to the Mandalika Formation. Based on the RHRS weighting, the total score on the STA+80 slope is 399, which means that the slope needs to be repaired or given safely with a moderate level of urgency. The rock fall trajectory modeling at the measurement location X = 121,875 has a kinetic energy of 973.14 kJ andesite and 72.59 kJ of volcanic breccia, for high results of 0.43 meters of andesite reflection and 2.04 meters of volcanic breccia, and velocity results translational velocity obtained at 33.8 m/s andesite and 8.67 m/s volcanic breccia. The potential for rock fall requires a safety system with a type of retained flexible barriers with a height of 5 meters that can be applied to the toe of the slope.Keywords: rock fall, discontinuity, trajectory, protection system, Tugu Dam

Bouguer Anomaly of Geothermal Reservoir at Tiris Area, Probolinggo, East Java, Indonesia

Research related to the geothermal system in the Tiris geothermal area (TGA) Probolinggo Regency has been conducted using the gravity method. This study aims to investigate the subsurface structure, with a target on estimating geothermal reservoir rocks from the study area. This study utilized the Gravity meter La Coste & Romberg type G-1503 on 116 acquisition points in an area of 2.16 km2, covering all geothermal manifestation points in TGA. The gravity measurement data obtained is then processed through gravity corrections, which include: conversion into milli-Gals (mGal) units, tidal correction, drift correction, latitude correction, free air correction, Bouguer correction, and terrain correction. These corrections to obtain a complete Bouguer anomaly (CBA) value. The study area shows the CBA value on a horizontal plane which ranges from 0.1 mGal to 4.2 mGal. The separation of the regional and residual Bouguer anomaly from the CBA on a horizontal plane employed the Moving Average method through spectrum analysis. The value of residual Bouguer anomaly ranges from -0.7 mGal to 2.7 mGal. The low anomalies are scattered in the northwest, and a small number are spread in the northeast and southeast, while the high anomalies are in the middle of the study area. The result of 3D inversion modeling finds that the study area's subsurface structure consists of four rock layers, namely lapilli tuff, tuffaceous breccia, volcanic breccia, and basalt. Volcanic breccia is approximated as geothermal reservoir rocks at a depth of 700 to 1000 meters below the acquisition point. In contrast, basalt is supposed to be intrusive igneous rock because it tends to break through the surface at a depth of 348 to 350 meters below the acquisition point. The presence of these intrusive rocks can be predicted through spectrum analysis result, which shows a regional anomaly source at a depth of 348 meters below the acquisition point. This intrusion rock is suspected to be a heat source rock in the geothermal system in the study area.

THE SANGIRAN FOSSIL SHELL ASSEMBLAGES CATALOGUE

Abstrak. Katalog Himpunan Fosil Kerang Sangiran. Makalah ini merupakan pemutakhiran himpunan fosil cangkang Sangiran di Jawa Tengah. Tujuannya untuk mendokumentasikan dan menyajikan gastropoda dan kerang dari seri Kalibeng atas dan Pucangan Bawah Kubah Sangiran dari periode Plio-Pleistosen. Tercatat 61 taksa moluska hingga tingkat genus. Informasi yang dikumpulkan dari kumpulan moluska ditentukan oleh sejarah geologi dan lingkungan Sangiran. Mereka diasosiasikan dengan fasies yang berbeda: a) napal masif dan lempung biru, b) lapisan batugamping lempung, c) lempung kelabu berlumpur, d) breksi vulkanik dan lahar dan e) lempung hitam, dan terdiri dari sedimen yang merepresentasikan lingkungan laut hingga rawa yang mengarah ke perkembangan kontinental. Pola kuantitatif yang dicatat dalam kumpulan moluska menjelaskan palaeo-lingkungan dan hubungan antara kumpulan palaeodataset yang mapan dari analisis dan tingkat variabilitas dalam data paleontologi. Selain itu, cangkang yang teridentifikasi dapat digunakan sebagai referensi untuk perwakilan taksonomi Sangiran dan moluska di lapisan Kalibeng dan Pucangan Cekungan Solo di Jawa Tengah. Abstract. This paper is an update of fossil shell assemblages of Sangiran in Central Java. It is aimed to document and present the gastropods and bivalves from the Upper Kalibeng and Lower Pucangan series of the Sangiran dome from the Plio-Pleistocene period. There are 61 mollusc taxa recorded up to the genus level. Information gathered from mollusc assemblages are determined by the geological and environmental history of the Sangiran. They are associated with different facies: a) massive marls and blue clays, b) layered clayey limestone, c) silty gray clay, d) volcanic breccia and lahars and e) black clays, and composed of sediments representing marine to swampy environments leading to continental development. The quantitative patterns recorded in mollusc assemblages elucidate the palaeoenvironment and the relationship between the established palaeodatasets of analysis and the levels of variability in palaeontological data. Moreover, the identified shells may be utilized as a reference for Sangiran and molluscan taxonomic representative in Kalibeng and Pucangan layers of Solo Basin in Central Java.

Age of the basement beneath the Mesozoic Lusitanian Basin revealed by granitic xenoliths from the Papôa volcanic breccia (West Iberia)

The dyke of the Papôa volcanic breccia cross-cutting the Lower Jurassic sequence of the Lusitanian Basin (West Iberia) contains granitic xenoliths. In this study, for the first time, U-Th-Pb zircon analysis of two xenoliths yielded 298±4Ma for biotite granite and of 292±2Ma for two-mica granite, indicating that the pre-Mesozoic basement of the Lusitanian Basin includes Permian intrusions. These ages are close within the margin of error of the age of the Late Carboniferous granites of the Berlengas isle that with the Late Devonian high-grade metamorphic rocks of the Farilhões isles, located northwest of the study area, which form the pre-Mesozoic basement of the Lusitanian Basin. These new geochronological findings enable it to be established that Permo-Carboniferous magmatism lasted at least 13Ma, in this region of the Appalachian-Variscan belt. Furthermore, a comparison with available data from Paleozoic tectonic units of the Appalachian-Variscan belt located both in the Iberian Massif and outside it enables the suggestion to be made that the Lusitanian Basin (Peniche) most probably rests on the South Portuguese Zone, which may also be correlated with the Rhenohercynian Zone present in southwest England, and the Meguma terrane of Nova Scotia.

NUMERICAL EVALUATION OF TUNNEL PORTAL SLOPE STABILITY AT BAGONG DAM SITE, EAST JAVA, INDONESIA

Geometries of excavated tunnel portal slopes at Bagong Dam site was initially designed without taking into account earthquake load. The excavated slope designs also assumed the rocks consisting the slopes were homogenous. The purpose of this research was to evaluate stability of the excavated tunnel inlet and outlet slopes at the Bagong Dam site under static and earthquake loads using finite element method. Stability of the natural slopes was also analyzed for comparison. The numerical static and pseudostatic analyses of slope stability were carried out using RS2 software (Rocscience, Inc.). Input data used in the numerical analyses were obtained from engineering geological mapping, rock core analyses, and laboratory tests. Seismic coefficient applied in the pseudostatic slope stability analyses was determined following guideline described in Indonesian National Standard. The engineering geological mapping and evaluation of rock cores indicated that the inlet tunnel slope consisted of four types of materials, namely residual soil, poor quality of volcanic breccia, very poor quality of volcanic breccia, and good quality of volcanic breccia. The outlet portal slope consisted of six types of materials, namely residual soil, very poor quality of limestone, poor quality of limestone, very poor quality of volcanic breccia, poor quality breccia, and good quality breccia. Based on the secondary elastic wave velocity (Vs) values, the rock masses in the research area were classified as hard rock (SA). Seismic analyses based on the earthquake hazard source map with 10% probability of exceedance in 50 years provided by the National Earthquake Center (2017) indicated that the PGA and the corresponding amplification factor FPGA in the research area were 0.3 and 0.8, respectively. The calculated seismic coefficient for the pseudostatic slope stability analyses was 0.12. The numerical analysis results showed that, in general, earthquake load reduced critical Strength Reduction Factor (SRF) values of the slopes. However, the natural and excavated tunnel portal slopes were relatively stable under static and earthquake loads. The natural slope at the tunnel inlet with a 40° inclination had critical SRF value of 4.0, while that of at the tunnel outlet with a 51° inclination had critical SRF value of 2.6. Under static load, the excavated slopes at the tunnel inlet and outlet having a 45° inclination had critical SRF values of 2.4 and 5.0, respectively. Under earthquake load, the excavated slopes at the tunnel inlet and outlet had critical SRF values of 2.3 and 3.5, respectively.

Subsurface Shallow Modelling Based on Resistivity Data in The Hot Springs Area of Libungo Geothermal, Gorontalo

Volcano-tectonic events in Libungo can be the cause of the presence of geothermal potential. There is no detailed research on shallow subsurface conditions in Libungo that can show the distribution of subsurface fluids. This research aims to create a shallow subsurface model of the Libungo geothermal area based on resistivity data. Resistivity data collection was carried out in the Libungo hot springs area. The electrode configuration used is the Schlumberger configuration. The variation in resistivity values is calculated using current data, potential difference data and geometry factors. The results of the calculation of the resistivity values variation are plotted versus depth. Variations of resistivity value versus depth are then displayed in the form of a single log, lithology distribution and 3D lithology model. The results showed that the shallow subsurface of the Libungo geothermal area was composed of andesite, volcanic breccia, silty clay and clay. Andesite in the research area has resistivity values ranging from 320 - 349 Ωm, has slightly fracture and is andesite dry. Volcanic breccia has a resistivity value of 177-198 Ωm, has a well to slightly fracture and is a volcanic breccia moist. Silty clay has a resistivity value of 3.25-37.99 Ωm and is a wet to moist silty clay. Clay has resistivity values in the range 1.56-2.78 Ωm and is wet to moist clay. Fluid distribution in the shallow subsurface area occurs in volcanic breccia, silty clay and clay. Shallow subsurface fluids accumulate mostly in the northern part of the Libungo geothermal area.

THREE-DIMENSIONAL RESISTIVITY MODEL FOR GROUND WATER EXPLORATION IN VOLCANIC ZONE OF TIDAR PLETEAU, MALANG, EAST JAVA

Geoelectrical resistivity method is popular technique for subsurface mapping of groundwater, mining exploration, and structural geology. For case study in Tidar Plateau which characterized by hard rock of volcanic breccia, lava, tuff, and conglomerate, the groundwater system occurs only by the secondary porosity (faulting, facturing, and weathering). Therefore it becomes difficult task for determine position of good well site in Tidar Plateau. Geoelectrical technique by Schlumberger configuration was choosen to investigated the ground water system in this area. Total 5 Vertical Electrical Sounding (VES) acqusition point (400 m length measurement; spacing 10 m) for data measurement with coverage area about 400 x 400 m2. For data processing was used IP2Win, Progress3, and RockWork 16. The data processing results showed high accuration with avarage Root Mean Square (RMS) error about 0.3 – 8.3 %. According to resistivity results, the study area has resistivity between 0.3– 2.5x104 . The rocks which have resistivity between 0.3 – 12.52 interpreted as sandy-clay; 12.52 – 50 interpreted as wet sandstone (aquifer); 129 – 178 interpreted as conglomerat; 700 – 1300 interpreted as tuff, sand, and dry gravel; 2600 – 2.5x104 interpreted as volcanic breccia. This geoelectrical results has succesful finding the fresh water at VES 5 after drilled at depth 100 m at wet sandstone formation. To findout distrubution of aquifer layer, a three-dimensional (3D) resistivity model was developed. The 3D resistivity model shows the aquifer layer image look the dome-like structure and image the fracture regions which becomes the pathway of groundwater system.