Abstract. The total solar eclipse on 9 March 2016 was a rare phenomenon that could be observed in 12 provinces in Indonesia. The decline in solar radiation to the earth during a total solar eclipse affects the amount of electron content (TEC) in the ionosphere. The ionospheric dynamics during the eclipse above Indonesia have been studied using data from 40 GPS stations distributed throughout the archipelago. It was observed that TEC decrease occurred over Indonesia during the occurrence of the total eclipse. This TEC decrease did not instigate ionoshperic scintillation. Moreover, the relationship between eclipse magnitude and TEC decrease throughout three GPS stations was analysed using PRN 24 and PRN 12 codes. Data analysis from each station reveals that the time required by the TEC to achieve maximum reduction since the initial contact of the eclipse is faster than the recovery time. The maximum TEC reduction came about several minutes after the maximum obscuration indicating that the recombination process was still ongoing even though the peak of the eclipse had happened. The magnitude of this decline is positively correlated with the geographical location of the stations and the relative satellite trajectory with respect to the total solar eclipse trajectory. The amount of TEC reduction is proportional to the magnitude of the eclipse which is directly related to the photoionization process. Because Indonesia is located in a low latitude magnetic equator region, the dynamics of the ionosphere above it is more complex due to the fountain effect. During the solar eclipse, the fountain effect declines disturbing the plasma transport from the magnetic equator to low latitude regions.
Study of basin geometry basin is important in geosciences and geophysical exploration. Gravity method can be used to address this issue by measuring gravity anomalies on the surface caused by density contrast between the bedrock and the sediment that fills the basin, geometry of the basin and surface topography. Numerically, gravity anomaly modeling can be conducted using two-point rule Gauss-Legendre Quadrature method, for a case where density contrast varies with depth exponentially. Within the scope of this study, gravity anomalies on the surface are significantly affected by the geometry of the curvature of the bedrock as well as the topographic elevation of the surface and the selected density contrast, and are not significantly affected by the undulation of the bedrock curvature.Abstrak: Kajian mengenai geometri bidang batas antara cekungan (basin) dengan batuan dasar (basement) di bawahnya merupakan hal yang penting dalam geosains serta eksplorasi geofisika. Metode gravitasi dapat digunakan dalam kajian ini dengan cara mengukur anomali gayaberat di permukaan yang disebabkan oleh kontras densitas antara batuan dasar dengan material sedimen pengisi cekungan, geometri cekungan serta topografi permukaan. Secara numerik, pemodelan anomali gayaberat oleh struktur cekungan dilakukan menggunakan metode Gauss-Legendre Quadrature aturan dua-titik, dengan kontras densitas yang bervariasi terhadap kedalaman secara eksponensial. Dalam lingkup kajian ini, anomali gayaberat di permukaan secara signifikan dipengaruhi oleh geometri kelengkungan batuan dasar serta undulasi elevasi topografi di permukaan serta bentuk kontras densitas yang dipilih, dan tidak dipengaruhi secara signifikan oleh undulasi pada kelengkungan batuan dasar. Kata kunci: cekungan, anomali gayaberat, kontras densitas, pemodelan ke depan, metode Gauss-Legendre Quadrature PENDAHULUANStudi mengenai morfologi batuan dasar suatu cekungan (basin) melalui metode gravitasi (gayaberat) pada umumnya terkait dengan informasi nilai densitas sedimen pengisi cekungan tersebut. Karakteristik densitas batuan terhadap kedalaman, terutama dalam kasus material sedimen pengisi cekungan lazimnya dicirikan dengan peningkatan nilai densitas terhadap kedalaman atau menurunnya kontras densitas sedimen terhadap batuan dasarnya. Peningkatan nilai densitas terhadap kedalaman ini disebabkan oleh proses kompaksi mekanis dan diagenesis yang pada akhirnya menurunkan nilai porositas Chappell dan Kusznir [2008]. Pada suatu lapisan sedimen, hubungan densitas-kedalaman cukup kompleks, dipengaruhi oleh litologi, sejarah tektonisme, laju pengendapan, fluida pengisi porositas dan tekanan [Chappell dan Kusznir, 2008]. Kompleksitas variasi densitas meningkat seiring terjadinya perlapisan stratigrafi yang tak-seragam, perubahan fasies, sementasi kimiawi serta disrupsi struktural [Garcia, 1996].Inkorporasi model distribusi densitas yang bervariasi terhadap kedalaman pada sedimen pengisi cekungan merupakan faktor penting dalam khazanah pemodelan dan inversi data gravitasi (terutama pada persoalan...
The inversion of time domain electromagnetic (TDEM) data is an ill-posed problem. This makes the standard inversion procedure, which is to linearize the related objective function. Then take a deterministic approach to determine a solution that can minimize the said objective function, which has the potential to be trapped in a local minimum. In this study, we solved the problem of TDEM data inversion using the Bayesian framework by generating a sample from the posterior distribution. The posterior distribution contains information related to the uncertainty of the TDEM data and the results of the forward modeling formulation, and prior information about the subsurface parameter model. In conducting the sampling process, we use the Langevin Monte Carlo (LMC) algorithm, one of many gradient-based Markov chain Monte Carlo (MCMC) sampling algorithms. Bayesian inversion was performed on synthetic data generated through the forward modeling of several test models. We used a model with varying thickness and resistivity values for inversion. We also added a prior probability distribution related to the smoothness constraint between resistivity values in adjacent layers allowing sharp and smooth transitions.
Earthquake source parameters such as slip distribution and slip rate are useful information for understanding the physical processes behind earthquakes. The isochrone back-projection method, or isochrone-BPM, is one of the methods used to obtain slip distribution in the fault plane by incorporating the isochrone concept with back-projection method. Isochrone-BPM has advantages in its ease of implementation. However, it appears that the resulted slip distribution images always contain artifacts causing the images to be smeared. The emergence of these artifacts may lead to misinterpretation of the slip distribution, thus becoming a major weakness of isochrone-BPM. In this work, an alternative approach is proposed by utilizing least-square inversion scheme with the addition of damping factor as an alternative to the isochrone-BPM, which is then referred to as damped isochrone inversion since it still utilizes the core formulation of isochrone-BPM. The application of isochrone-BPM slip inversion to synthetic data generated from a test model shows that the quality of the resulted image will highly depend on the set of receiving stations used. In addition, the isochrone-BPM image will also depend on the focal mechanism of the earthquake, which is indicated by the difference in patterns on different mechanisms. Contrary to isochrone-BPM, the damped isochrone inversion produces slip distribution images that do not depend on the set of receiving stations used and do exhibit dependence on the focal mechanism. These results may suggest that as an alternative to the isochrone-BPM, the damped isochrone inversion offers better performance in recovering the slip distribution images.
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