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Perhitungan Respons Elektromagnetik Lapisan Tipis Heterogen Menggunakan Metoda Persamaan Integral Hendra Grandis; Nanang T. Puspito
Jurnal Matematika & Sains Vol 3, No 1 (1998)
Publisher : Institut Teknologi Bandung

Show Abstract | Download Original | Original Source | Check in Google Scholar

The computation of electromagnetic response of a 3-D model which represents subsurface electric conductivity distribution is done by resolving the Maxwells equations using the integral equation method. In order to simplify the problem, it is assumed that the 3-D inhomogeneity is confined in a thin layer embedded in a horizontally stratified (1-D) medium. The algorithm is applied to compute the response of two simple models containing either resistive and conductive inhomogeneity. The apparent resistivity, phase and magnetic transfer function give general features of the models.

Multi-dimensional Inversion Modeling of Surface Nuclear Magnetic Resonance (SNMR) Data for Groundwater Exploration Warsa, W.; Grandis, Hendra; Parnadi, Wahyudi W.; Santoso, Djoko
Journal of Engineering and Technological Sciences Vol 46, No 2 (2014)
Publisher : ITB Journal Publisher, LPPM ITB

Groundwater is an important economic source of water supply for drinking water and irrigation water for agriculture. Surface nuclear magnetic resonance (SNMR) sounding is a relatively new geophysical method that can be used to determine the presence of culturally and economically important substances, such as subsurface water or hydrocarbon distribution. SNMR sounding allows the determination of water content and pore size distribution directly from the surface. The SNMR method is performed by stimulating an alternating current pulse through an antenna at the surface in order to confirm the existence of water in the subsurface. This paper reports the development of a 3-D forward modeling code for SNMR amplitudes and decay times, after which an improved 2-D and 3-D inversion algorithm is investigated, consisting of schemes for regularizing model parameterization. After briefly reviewing inversion schemes generally used in geophysics, the special properties of SNMR or magnetic resonance sounding (MRS) inversion are evaluated. We present an extension of MRS to magnetic resonance tomography (MRT), i.e. an extension for 2-D and 3-D investigation, and the appropriate inversions.

Three-Dimensional Gravity Inverse Modeling for Basement Depth Estimation Integrating Maximum Difference Reduction (MDR), Trend Surface Analysis (TSA) and Total Variation Regularization Handyarso, Accep; Grandis, Hendra
Journal of Engineering and Technological Sciences Vol 49, No 3 (2017)
Publisher : ITB Journal Publisher, LPPM ITB

In sedimentary basin studies, gravity data are typically used to estimate the basement topography. Gravity inversion methods are expected to be able to discriminate between continuous and discontinuous sedimentary basins. Most 3D gravity inversion methods require intensive computational resources (computer memory and processing time). MDR3D, a variant of the well-known Bott method, was transformed into the Gauss-Newton inversion approach for extension flexibility. Integration of trend surface analysis (TSA) into the inversion scheme for regional anomaly estimation allows basement depth estimation from the Bouguer anomaly data. The aim of the additional total variation regulariÂzation is to stabilize the inversion algorithm and to achieve a geologically feasible model, especially for discontinuous basin types. Evaluation of the proposed method led to satisfactory results both for the synthetic and the field data set. It was found that the regularization parameter can improve the stability of the algorithm and also the depth estimation from noisy data up to Â±0.5 mGal.

A Note on the Use of the Second Vertical Derivative (SVD) of Gravity Data with Reference to Indonesian Cases Sumintadireja, Prihadi; Dahrin, Darharta; Grandis, Hendra
Journal of Engineering and Technological Sciences Vol 50, No 1 (2018)
Publisher : ITB Journal Publisher, LPPM ITB

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 Grandis, Hendra; Warsa, W; Sumintadireja, Prihadi
Journal of Engineering and Technological Sciences Vol 52, No 2 (2020)
Publisher : Institute for Research and Community Services, Institut Teknologi Bandung

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.5614/j.eng.technol.sci.2020.52.2.9

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.

Aplikasi Metode Inversi Damped Least-square untuk Pemodelan 1-D Data TEM Willy Hermawan; Hendra Grandis; Widodo Widodo
Jurnal Lingkungan dan Bencana Geologi Vol 6, No 2 (2015)
Publisher : Badan Geologi

ABSTRAKMetode TEM (transient electromagnetic) merupakan metode alternatif yang dapat digunakan untuk mencari sumber air tanah selain metode geolistrik. Prinsip metode TEM adalah menginduksikan energi elektromagnetik ke dalam tanah dengan menggunakan transmitter loop dan receiver koil yang merekam respon bawah permukaan ketika transmitter dalam keadaan mati (tidak ada medan primer). Pemodelan data TEM pada umumnya masih menggunakan model 1-D. Penelitian ini akan membahas mengenai metode Inversi damped least-square untuk pemodelan inversi 1-D dari data TEM. Metode damped least-square membutuhkan parameter model awal untuk selanjutnya dilakukan inversi damped least-square menggunakan teknik SVD (Singular Value Decomposition). Metode ini diaplikasikan terhadap tiga data sintetik. Data sintetik I menunjukkan nilai resistivitas semakin kecil seiring bertambahnya kedalaman. Data sintetik II menunjukkan nilai resistivitas semakin besar seiring dengan bertambahnya kedalaman. Data sintetik III merupakan respon model yang mewakili daerah prospek airtanah, memperlihatkan lapisan konduktif diapit oleh lapisan resistif dan sebaliknya. Metode damped least-square juga diaplikasikan pada data lapangan daerah karst Kabupaten Tuban. Hasil pemodelan data sintetik memperlihatkan kesesuaian yang optimum antara data resistivitas semu observasi dan perhitungan. Sedangkan, hasil pemodelan data lapangan memerlihatkan kesesuaian model dengan kondisi geologi setempat.Kata kunci: metode damped least-square, singular value decomposition dan transient electromagnetic.ABSTRACTTransient electromagnetic (TEM) sounding method is an alternative method other than the geoelectrical method that can be used for groundwater exploration purposes. The principle TEM method is done by the induction of electromagnetic fields at the surface within a transmitter loop and record the subsurface response in the receiever coil when the absence of the primary field (transmitter off). TEM data are usually interpreted in term of 1-D model. The damped least-square method is presented in this paper. The damped least-square method needs an initial model parameter and employs the damped least-square solution with Singular Value Decomposition (SVD) technique. This methods have been applied to three synthetic data sets. The first synthetic data shows decreasing resistivities with depth. The second synthetic data shows increasing resistivities with depth. The third synthetic data are a response of groundwater prospect model, consisting of a conductor layer between two resistors and resistor layer between two conductors. Damped least-square methods was also applied to field data from karst area, Tuban. The results from synthetic data inversions presented good fit between the observed and the calculated apparent resistivity data. The results from field data show models subsurface resistivity consistent with the local geology.Keyword : damped least-square method, singular value decomposition and transient electromagnetic.

Multi-dimensional Inversion Modeling of Surface Nuclear Magnetic Resonance (SNMR) Data for Groundwater Exploration W. Warsa; Hendra Grandis; Wahyudi W. Parnadi; Djoko Santoso
Journal of Engineering and Technological Sciences Vol. 46 No. 2 (2014)
Publisher : Institute for Research and Community Services, Institut Teknologi Bandung

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.5614/j.eng.technol.sci.2014.46.2.1

Groundwater is an important economic source of water supply for drinking water and irrigation water for agriculture. Surface nuclear magnetic resonance (SNMR) sounding is a relatively new geophysical method that can be used to determine the presence of culturally and economically important substances, such as subsurface water or hydrocarbon distribution. SNMR sounding allows the determination of water content and pore size distribution directly from the surface. The SNMR method is performed by stimulating an alternating current pulse through an antenna at the surface in order to confirm the existence of water in the subsurface. This paper reports the development of a 3-D forward modeling code for SNMR amplitudes and decay times, after which an improved 2-D and 3-D inversion algorithm is investigated, consisting of schemes for regularizing model parameterization. After briefly reviewing inversion schemes generally used in geophysics, the special properties of SNMR or magnetic resonance sounding (MRS) inversion are evaluated. We present an extension of MRS to magnetic resonance tomography (MRT), i.e. an extension for 2-D and 3-D investigation, and the appropriate inversions.

Three-Dimensional Gravity Inverse Modeling for Basement Depth Estimation Integrating Maximum Difference Reduction (MDR), Trend Surface Analysis (TSA) and Total Variation Regularization Accep Handyarso; Hendra Grandis
Journal of Engineering and Technological Sciences Vol. 49 No. 3 (2017)
Publisher : Institute for Research and Community Services, Institut Teknologi Bandung

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.5614/j.eng.technol.sci.2017.49.3.5

In sedimentary basin studies, gravity data are typically used to estimate the basement topography. Gravity inversion methods are expected to be able to discriminate between continuous and discontinuous sedimentary basins. Most 3D gravity inversion methods require intensive computational resources (computer memory and processing time). MDR3D, a variant of the well-known Bott method, was transformed into the Gauss-Newton inversion approach for extension flexibility. Integration of trend surface analysis (TSA) into the inversion scheme for regional anomaly estimation allows basement depth estimation from the Bouguer anomaly data. The aim of the additional total variation regularization is to stabilize the inversion algorithm and to achieve a geologically feasible model, especially for discontinuous basin types. Evaluation of the proposed method led to satisfactory results both for the synthetic and the field data set. It was found that the regularization parameter can improve the stability of the algorithm and also the depth estimation from noisy data up to ±0.5 mGal.

A Note on the Use of the Second Vertical Derivative (SVD) of Gravity Data with Reference to Indonesian Cases Prihadi Sumintadireja; Darharta Dahrin; Hendra Grandis
Journal of Engineering and Technological Sciences Vol. 50 No. 1 (2018)
Publisher : Institute for Research and Community Services, Institut Teknologi Bandung

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.5614/j.eng.technol.sci.2018.50.1.9

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 Hendra Grandis; W Warsa; Prihadi Sumintadireja
Journal of Engineering and Technological Sciences Vol. 52 No. 2 (2020)
Publisher : Institute for Research and Community Services, Institut Teknologi Bandung

Show Abstract | Download Original | Original Source | Check in Google Scholar | DOI: 10.5614/j.eng.technol.sci.2020.52.2.9

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.

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