A Note on the Use of the Second Vertical Derivative (SVD) of Gravity Data, with Reference to Indonesian Cases

The analysis of gravity data is based, among others, on their horizontal and vertical gradients. Horizontal gradients can be directly estimated from the spatially distributed data, while the vertical derivative can be calculated in either space or wave-number domain. Historically, the Second Vertical Derivative (SVD) of gravity data is known to be able to delineate anomalous sources\u27 boundaries. The paper discusses the incorrect use of the SVD of gravity data, with reference to current practices in Indonesia. The SVD relative magnitude along a profile is often used to define whether a density contrast and its dipping orientation correspond to a normal or reverse fault, which might be geologically incorrect. We exemplify our analysis using both synthetic and field data

Stratigraphic Model of East Biak Based on Magnetotelluric Data

Biak Island is thought to have originated from the Australian Continental passive margin in the Mesozoic. This passive margin underwent tectonic collision to form the bedrock of Biak Island during the Paleogene. East Biak is covered by limestone, which is an obstacle to knowing the deeper stratigraphy. This study's aim was to identify the subsurface geology of the East Biak area based on rock resistivity values. Magnetotelluric (MT) surveying is a passive geophysical method that is able to interpret subsurface geology based on rock resistivity values at depths of up to 5 to 10 km. The MT measurements in East Biak used 25 measurement points. Cross-sectional variations in rock resistivity values generated from the MT measurements were used to identify rock types in the East Biak subsurface. Five rock units were identified. The East Biak stratigraphy model is dominated by carbonates, comprising two sequences, i.e. an Early Neogene sequence with a thickness of 1,000 to 3,000 m and a Late Neogene to Quaternary sequence with a thickness of 180 to 2,450 m. The unconformity between these two sequences is due to tectonic activity, which separated Supiori and Biak Island

Basin Study in Atambua, West Timor, Indonesia from Gravity Data

Timor Island, Indonesia has complex geological structures related to its tectonic history. There is an existing subsurface geological model that is based on geophysical data. It is limited to the regional crustal scale and has a relatively low spatial resolution. The objective of our study was to delineate the sedimentary basin configuration of the area, both laterally and vertically, based on gravity data. Spectral analysis of the Bouguer anomaly allowed for anomaly enhancement by wavenumber domain filtering. Two main basins were identified from elongated low gravity anomalies that follow a SW-NE trend, i.e., the Central Basin and the Atambua Basin. The 2½D gravity modeling of selected profiles perpendicular to the regional structural direction revealed the sedimentary fills of the basins and the basement based on their densities. The Bobonaro mélanges and Viqueque sequences dominate and overlay the syn-rift (Kekneno sequences) and post-rift (Kolbano sequences) with varying lithology. These para-autochthon sediments are dominated by shale and carbonaceous rocks. Their respective thicknesses and depths of burial imply the possibility of hydrocarbon generation. The underlying basement may be associated with the Australian crust protruding from the south

Basin Study in Atambua, West Timor, Indonesia from Gravity Data

Timor Island, Indonesia has complex geological structures related to its tectonic history. There is an existing subsurface geological model that is based on geophysical data. It is limited to the regional crustal scale and has a relatively low spatial resolution. The objective of our study was to delineate the sedimentary basin configuration of the area, both laterally and vertically, based on gravity data. Spectral analysis of the Bouguer anomaly allowed for anomaly enhancement by wavenumber domain filtering. Two main basins were identified from elongated low gravity anomalies that follow a SW-NE trend, i.e., the Central Basin and the Atambua Basin. The 2½D gravity modeling of selected profiles perpendicular to the regional structural direction revealed the sedimentary fills of the basins and the basement based on their densities. The Bobonaro mélanges and Viqueque sequences dominate and overlay the syn-rift (Kekneno sequences) and post-rift (Kolbano sequences) with varying lithology. These para-autochthon sediments are dominated by shale and carbonaceous rocks. Their respective thicknesses and depths of burial imply the possibility of hydrocarbon generation. The underlying basement may be associated with the Australian crust protruding from the south

Geolectrical Equipment Research Activity in Laboratory of Exploration and Geothermal, at Geology Department ITB

This article is presented in the 5th ICONTES Kuala Lumpur, Malaysia. Author: Prihadi Sumintadireja. The Exploration Geology and Geothermal Laboratory (LEGG) was established in June 2017 as a research institution under the Faculty of Earth Science and Technology (FITB), Institut Teknologi of Bandung (ITB). As indicted by the three principle of higher education (Education, Research, and Community Services), the research lab has a goal to conduct researches on applied geosciences and modeling surface and subsurface geosciences data for exploration, geological hazard mitigation, and environmental geosciences and to publish the results both in national as well as international level. The lab specializes in creating a geological model from surface and subsurface data in order to solve real world problem. LEGG’s research activities that was done in between 2005 – 2017 are funded by ITB’s Research and Community Services Institute in addition to funding from petroleum and geothermal industry. The researches done in several stages that is listed below: 1.) Lab scale geo-electric measuring device prototype development, 2.) Field scale geo-electric measuring device development, 3.) Field scale measurement simulation, 4.) Environmental geoscience implementation and 5.) Petroleum industry implementation. This paper is meant to highlight the LEGG’s success story of more than 10 years of researching geoelectric acquisition and its implementaiton in the industry. The fruit of the research is LVG GLS, newly developed geoelectric acquisition instrument, that could record vast amount of higher quality data in shorter amount of time. Latest version of the device has 120 channels which means it has almost twice wider coverage and could be used to map three dimensional subsurface configuration quicker compared to any previous version. Keywords: Current and potential electrode, mise-a-la-masse, R&D, LEG

A Note on the Use of the Second Vertical Derivative (SVD) of Gravity Data with Reference to Indonesian Cases

Gravity data analysis and interpretation are based, among others, on their spatial variation represented by horizontal and vertical gradients. The gradient or derivative of a gravity field can be calculated either in the spatial domain or the wave-number domain. Historically, the second vertical derivative (SVD) of gravity data can be used to delineate the boundaries of anomalous sources. This paper addresses inappropriate use of the SVD of gravity data, with reference to current practices in Indonesia. The SVD's relative magnitude along a profile is widely used to define whether a density contrast and its dipping orientation correspond to a normal or reverse fault, which may be geologically incorrect. Furthermore, the SVD is calculated by approximation using the horizontal derivative, which may be erroneous especially with poorly distributed data and anomalous 3D sources. We exemplify our analysis with synthetic data and propose a more appropriate spectral-based analysis using field data

Layer Stripping in Magnetotellurics (MT) for Enhancement of Resistivity Change Effect in Reservoir: Equivalence Analysis

Magnetotellurics (MT) can be applied to monitor resistivity change at depth that is for example due to fluid injection in enhanced oil recovery or CO2 storage. The observed MT data changes at the surface may be insignificant, but the effect can be enhanced using the layer stripping method, i.e. calculating MT data changes that would be observed at depth based on data from the surface. Two well-known formulas for MT 1D forward modeling were reformulated to allow for calculation of the impedance at depth based on the impedance at the surface. We applied the layer stripping technique to synthetic data associated with models that were representative of a likely CO2 storage site. We also used an equivalent model and the Monte Carlo approach to estimate the sensitivity of the method to cope with the uncertainty of the host model and the input data. The layer stripping calculation has the greatest uncertainty at short periods, where the real and imaginary parts of the complex impedance tend to be equal, i.e. an homogeneous medium response. The layer stripping technique should be used with great caution based on a relatively precise 1D host model

Development LVG GLS Geolectrical Instrument for Monitoring Near Surface Geohazard

This paper was presented in The 3rd Padjadjaran International Physics Symposium 2017, Holiday-Inn Hotel, Bandung , 14-15 November 2017, Department of Physics Universitas Padjadjaran. Authors: Prihadi Sumintadireja1, Ihsan Imaduddin2,4, Wahyu Srigutomo2, Enjang Jaenal Mustopa2, Diky Irawan3, Ihsanuddin A. Lubis3. Abstract: This paper demonstrated a new geolectrical equipment of LVG GLS multi channel , which as part of Laboratory of Application and Geosciences Modeling research, which funded by Institut Teknologi Bandung. Initially, the LVG GLS equipment is designed for delineating geothermal area by mise a la masse configuration. Furthermore, the application is plan to modify for geohazard study, since many landslide occurrence without clear explanation about the involving process due to lack data measurement. The ability of equipment is able to measure resistivity of subsurface condition in real time and continuous. Therefore, spatial and temporal data for monitoring vulnerable landslide area, which identified by geological mapping can be visualized to mitigate the geohazard area more precisely. The simultaneous 80 channel electrode measurement can reduced inhomogeneity of spontaneous potential value, which change by time interval