Seung-Sep Kim

A tetrahedral mesh generation approach for 3D marine controlled-source electromagnetic modeling

Abstract 3D finite-element (FE) mesh generation is a major hurdle for marine controlled-source electromagnetic (CSEM) modeling. In this paper, we present a FE discretization operator (FEDO) that automatically converts a 3D finite-difference (FD) model into reliable and efficient tetrahedral FE meshes for CSEM modeling. FEDO sets up wireframes of a background seabed model that precisely honors the seafloor topography. The wireframes are then partitioned into multiple regions. Outer regions of the wireframes are discretized with coarse tetrahedral elements whose maximum size is as large as a skin depth of the regions. We demonstrate that such coarse meshes can produce accurate FE solutions because numerical dispersion errors of tetrahedral meshes do not accumulate but oscillates. In contrast, central regions of the wireframes are discretized with fine tetrahedral elements to describe complex geology in detail. The conductivity distribution is mapped from FD to FE meshes in a volume-averaged sense. To avoid excessive mesh refinement around receivers, we introduce an effective receiver size. Major advantages of FEDO are summarized as follow. First, FEDO automatically generates reliable and economic tetrahedral FE meshes without adaptive meshing or interactive CAD workflows. Second, FEDO produces FE meshes that precisely honor the boundaries of the seafloor topography. Third, FEDO derives multiple sets of FE meshes from a given FD model. Each FE mesh is optimized for a different set of sources and receivers and is fed to a subgroup of processors on a parallel computer. This divide and conquer approach improves the parallel scalability of the FE solution. Both accuracy and effectiveness of FEDO are demonstrated with various CSEM examples.

New analytic solutions for modeling vertical gravity gradient anomalies

Modern processing of satellite altimetry for use in marine gravimetry involves computing the along-track slopes of observed sea-surface heights, projecting them into east-west and north-south deflection of the vertical grids, and using Laplaces equation to algebraically obtain a grid of the vertical gravity gradient (VGG). The VGG grid is then integrated via overlapping, flat Earth Fourier transforms to yield a free-air anomaly grid. Because of this integration and associated edge effects, the VGG grid retains more short-wavelength information (e.g., fracture zone and seamount signatures) that is of particular importance for plate tectonic investigations. While modeling of gravity anomalies over arbitrary bodies has long been a standard undertaking, similar modeling of VGG anomalies over oceanic features is not commonplace yet. Here we derive analytic solutions for VGG anomalies over simple bodies and arbitrary 2-D and 3-D sources. We demonstrate their usability in determining mass excess and deficiency across the Mendocino fracture zone (a 2-D feature) and find the best bulk density estimate for Jasper seamount (a 3-D feature). The methodologies used herein are implemented in the Generic Mapping Tools, available from gmt.soest.hawaii.edu.

A Case Study of Electrical Resistivity and Borehole Imaging Methods for Detecting Underground Cavities and Monitoring Ground Subsidence at Abandoned Underground Mines

We employed electrical resistivity and optical borehole imaging methods to identify underground cavities and determine ground subsidence rate at the study area affected by land subsidence due to abandoned underground mines. At the study site 1, the anomalous zones of low resistivity ranging between 100 ohm-meter and 150 ohm-meter were observed and confirmed as an abandoned underground mine by subsequent borehole drilling and optical borehole imaging. Although the electrical resistivity survey was unavailable due to the paved surface of the study site 2, we were able to locate another abandoned underground mine with the collapsed mine shaft based on the distribution of the ore veins and confirmed it with borehole drilling. In addition, we measured vertical displacements of underground features indicating underground subsidence by conducting optical borehole imaging 6 times over a period of 43 days at the study site 2. The displacement magnitude at the deep segment caused by subsidence appeared to be 3 times larger than those at the shallow segment. Similarly, the displacement duration at the deep segment was 4 times longer than those at the shallow segment. Therefore, the combination of electrical resistivity and optical borehole imaging methods can be effectively applicable to detect and monitor ground subsidence caused by underground cavities.

Finding seamounts with altimetry-derived gravity data

Seamounts are ubiquitous manifestations of underwater volcanism that rise above the surrounding ocean floor by more than a few hundred or thousand meters. Any temporal and spatial variations of the underwater volcanic and tectonic processes that formed seamounts can primarily be understood through their geometric characterization and spatial distribution. For this study, we utilize the vertical gravity gradient (VGG) version 23.1 derived from satellite altimetry, which includes new data from the CryoSat-2, Envisat, and Jason1 missions. A repeated statistical comparison for an area with no significant geologic features shows that the standard deviation of VGG 23.1 is decreased about 48% from the previous release, indicating the signal-to-noise ratio has been improved significantly from the previous version. In order to examine whether the new data give us better opportunities to find seamounts, we choose near-ridge environments constrained by good bathymetry coverage. For a given area, the nonlinear inversion method to search for seamounts is applied. We approximate VGG anomalies over seamounts as sums of individual, partially overlapping, elliptical polynomial functions, which allows us to form a non-linear inverse problem by fitting the polynomial model to the observations. Model parameters for a potential seamount include geographical location, peak VGG amplitude, major and minor axes of the elliptical base, and the azimuth of the major axis. The non-linear inversion is very sensitive to the initial values for the location and amplitude; hence, they are constrained by the center and amplitude of the uppermost contours obtained with a 1-Eotvos contour interval. With these initial conditions from contouring, we execute a stepwise and fully automated inversion and obtain optimal model estimates for potential seamounts; these are statistically evaluated for significance using the Akaike Information Criterion and F tests. Here we present a preliminary result of seamount detection using the new global data and discuss possibilities for constructing a new synthesized global dataset of seamounts.

Transition from buckling to subduction on strike-slip continental margins: Evidence from the East Sea (Japan Sea)

Initiation of subduction is rarely encountered in modern tectonic environments due to its ephemeral and destructive nature. We report the geological and geophysical evidence indicating a transitional phase from buckling to embryonic subduction along the eastern Korean margin. The transition appears to be caused by compressional reactivation of the strike-slip boundary between the continental (Korean Peninsula) and oceanic (Ulleung Basin) crusts since the Early Pliocene. Evidence for compressional reactivation includes (1) a west-dipping major thrust and coincident crustal buckling of the Ulleung Basin; (2) an east-west structural asymmetry inferred from the gravity anomaly and P-wave tomography; and (3) ongoing crustal uplift and high-angle faults along the eastern Korean margin. The juxtaposition of underthrusting and buckling of the crust in the Ulleung Basin, and its associated ubiquitous reverse faulting on the eastern Korean margin, imply the potential development of a new subduction system along the western margin of the East Sea (Japan Sea). We propose that the East Sea comprises two incipient subduction margins (i.e., the Korean and Japanese sides), which are now competing to reach a self-sustaining subduction stage. INTRODUCTION The opening of an oceanic basin eventually ceases, and occasionally the basin switches to the closing regime accompanied by subduction initiation (SI). SI is a transient phase, before the onset of a mature subduction system, which can develop under tectonic conditions associated with preexisting mechanical heterogeneities and/or external stress transmission (Stern and Gerya, 2017). However, evidence for SI is rarely observed in either modern tectonic environments or geological records because SI itself is a destructive process. Due to such limitations, the understanding of SI mostly relies on inferences regarding an embryonic phase preserved in some mature subduction systems (e.g., Whattam and Stern, 2015) and on numerical simulations (e.g., Gurnis, 1992). Many efforts have been made to find a link between hypothetical models of SI and known geological examples, including seismic reflection analysis of structural components (Duarte et al., 2013), focal mechanism and aftershock analysis of seismicity along a master thrust fault (No et al., 2014), and P-wave velocity tomography of deep lithospheric structures (Eakin et al., 2015). However, no consensus has been achieved on the criteria for defining ongoing SI in modern tectonic settings. Therefore, to identify SI and unravel its geodynamic mechanism, geological and geophysical data on surface-level to crustallevel deformation should be integrated. Our study reports a new example of the transitional phase from buckling to SI observed in the western margin of the East Sea (Japan Sea). Observations are based on multi-scale deformation structures constrained by seismic reflection, gravity, and P-wave velocity data. We focus on the formative processes and mechanical relationship between deep-crustal deformation features, and discuss the geodynamic driving forces for SI in this region. For the stratigraphic and structural analysis, we utilized ~10,700 km of multi-channel seismic reflection data (Fig. 1). Digital SEG-Y (Society of Exploration Geophysicists standard) data were loaded into the PETREL software system (Schlumberger, https:// www .software .slb .com /products /petrel) for stratigraphic correlation and mapping. The complete spherical isostatic gravity anomaly data of the World Gravity Map (WGM; Bonvalot et al., 2012) was then used to identify the gravitational manifestation of crustal-level deformation imaged by the seismic reflection data. NEOTECTONIC FEATURES OF THE EAST SEA MARGINS The East Sea, one of the major marginal basins in the western circum-Pacific (inset in Fig. 1), has experienced pull-apart–style backarc extension and subsequent multi-phase tectonic reactivations during the Cenozoic (Jolivet et al., 1992). Though typical oceanic crust is only evident in the northern region of the East Sea, the crust in the south is also regarded as oceanic, which was thickened (~11 km) by magmatic underplating (Sato et al., 2014). The western (i.e., Korean side) and eastern (i.e., Japanese side) boundaries of the sea, developed originally as strike-slip principal displacement zones of the pull-apart system, are currently under a transverse shortening regime (Jolivet et al., 1992). In particular, the eastern margin of the sea is regarded as an incipient subduction boundary based on compressional structures and large-magnitude earthquakes (e.g., the A.D. 1983 M7.7 Nihonkai-Chubu earthquake) hosted by an east-dipping reverse fault (No et al., 2014, and references therein). The western margin of the sea, which exhibits a drastic transverse transition from oceanic to continental crust (Cho et al., 2004), also displays evidence of ongoing compressional deformation, such as high-angle reverse faults (Yoon and Chough, 1995), marine terraces (Choi et al., 2008), and moderate-sized earthquakes (e.g., the 2004 M5.1 Uljin earthquake; Kang and Baag, 2004). *E-mail: bdso@kangwon.ac.kr GEOLOGY, July 2018; v. 46; no. 7; p. 603–606 | https://doi.org/10.1130/G40305.1 | Published online 7 June 2018

Imaging Inner Structure of Bukbawi at Mt. Palgong Provincial Park Using Ground Penetrating Radar

Hyeong-Gi Kim, Seung-Ho Baek, Seung-Sep Kim*, Na Young Lee and Jang-Soon Kwon Department of Astronomy, Space Science and Geology, Chungnam National University, Daejeon 34134, South Korea Department of Geology and Earth Environmental Sciences, Chungnam National University, Daejeon 34134, South Korea Faculty of Earth Systems and Environmental Sciences, Chonnam National University, Gwangju 61186, South Korea Korea Atomic Energy Research Institute, Daejeon 34057, South Korea (Received: 14 August 2017 / Revised: 27 December 2017 / Accepted: 28 December 2017)