3D Subsurface Research Articles

Development and Evaluation of an ODE Representation of 3D Subsurface Tile Drainage Flow Using the HLM Flood Forecasting System

AbstractWe developed a new ordinary differential equation (ODE) to represent subsurface flows from a hillslope with artificial subsurface drainage (tiling) into an adjacent channel. Our ODE is based on a derived storage‐discharge relationship from high‐resolutions HYDRUS3D simulations that solve 3D partial differential Richards' equations for a hillslope (plot scale) with internal drainage surfaces, variable soil depth, hydraulic conductivity, and varying slope in the direction of flow. The ODE does not capture the hysteresis in the storage‐drainage relation that can only be captured using partial differential equations (PDEs), however the simplification does not have major impacts on the modeled flows. The nondimensional representation of the equation is consistent with, and provides an explanation for, the empirical equation used in classical hydrological models such as the ARNO (Arno River) and Variable Infiltration Capacity (VIC) model to simulate subsurface flows, which removes the need for parameter calibration. Our equation captures the features not captured by other ODE simplifications such as DRAINMOD. The ODE can be coupled to the Hillslope Link Model (HLM) flood forecasting system allowing real‐time regional simulation of streamflow fluctuations and floods over large domains. We show that (i) the new equation improves the capability to model watershed scale hydrograph recessions and to better capture the timing above flood thresholds for multiple spatial scales from 0.1 to 18,000 km2; (ii)using realistic values of slope combined with typical values of tile separation and soil depth provides good predictive power at gauged basin outlets; and (iii) parameters can be derived for particular situations using the 3D PDE framework.

A database of Aeolian Sedimentary Architecture for the characterization of modern and ancient sedimentary systems

The Database of Aeolian Sedimentary Architecture (DASA) records the architecture and spatio-temporal evolution of a broad range of modern and recently active aeolian systems, and of their preserved deposits in ancient successions. DASA currently stores data on >14,000 geologic and geomorphic entities (including bounding surfaces and transition relationships) extracted from >60 case-study examples documented in the published literature. DASA stores data on a variety of aeolian and associated non-aeolian entities of multiple scales, including attributes that characterize their type, geometry, spatial relations, hierarchical relations, temporal significance, and textural and petrophysical properties; associated metadata are also stored.Database output describes (1) stratigraphic relationships between aeolian and associated fluvial, lacustrine and paralic depositional systems; (2) the geometry of aeolian architectural elements, and hierarchical and spatial relationships between them; (3) the probabilities of vertical and lateral transition from one type of deposit or landform to another; (4) the presence and nature of aeolian bounding surfaces at different scales, and their nested, hierarchical relationships; (5) aeolian lithofacies types, proportions and distributions, and (6) grain-scale textural parameters.DASA is applied to quantitatively characterize and compare modern and ancient aeolian sedimentary systems. Examples of database outputs demonstrate how DASA outputs can be tailored for numerous applications, including: (1) the development of bespoke quantitative facies models, specifically tailored for particular sets of boundary conditions; (2) the empirical assessment of how aeolian systems, and associated preserved sedimentary architectures, represent a response to allogenic and autogenic forcings; and (3) the instruction of forward stratigraphic models and 3D geocellular subsurface models. DASA is a valuable tool for the characterization of subsurface aeolian successions, such that output can help to (1) predict three-dimensional lithological heterogeneity in subsurface successions that are resource targets, (2) constrain geocellular stochastic models, and (3) facilitate borehole correlations of aeolian dune sets or associated non-aeolian elements.

Upgraded GROWTH 3.0 software for structural gravity inversion and application to El Hierro (Canary Islands)

GROWTH 3.0 is a software package to determine the subsurface 3D density distribution that adjust discrete gravity anomaly values determined from gravity observations. The software uses very general assumptions and quite automatically determines the 3D density structures described by the aggregation of thousands of small rectangular prisms. A previous version of the software was published in 2011. However, methodological developments and computational technical evolution made it necessary to upgrade the software to include new advances and recent improvements in the inversion methodology (automatic determination of density contrasts, background medium with downward density increase, etc.), and a change in the computer operating system (from Windows XP, which was compatible with the 2011 version, to Windows 10 Pro for the current version). Here we describe the new upgraded version of the software package (which can be downloaded free), its use and application, using the study of El Hierro (Canary Islands) as a test case from which to obtain a 3D density model of its crustal structure. We also give a brief geological and volcanological interpretation.

Three-dimensional geological modeling and spatial analysis from geotechnical borehole data using an implicit surface and marching tetrahedra algorithm

Three-dimensional (3D) geological modeling based on borehole data usually requires many manual operations, and the modeling process remains very complicated and time consuming. This paper presents an automatic implicit 3D geological modeling and visualization method that can be applied to urban geotechnical drilling data. First, by analyzing the general characteristics of these types of data, the implicit Hermite radial basis function (HRBF) surface is used to simulate geological interfaces according to lithological layers/units in boreholes. Second, the marching tetrahedra (MT) visualization method is optimized with new data structures and split operations to extract explicit mesh models from implicit geological surfaces. Then, the optimized tetrahedron used in the field of 3D urban drilling modeling is described in detail. Finally, the local geotechnical engineering data of two Chinese cities are used to verify the modeling algorithm, and several kinds of spatial analyses are carried out, including visualization, data querying, accuracy assessment and multiple cutting methods. The results show that the proposed urban subsurface 3D geological modeling method is effective in visualizing the structural shape, topological relationship and formation properties of strata that can be used to predict underground conditions and reduce construction risks during urban geotechnical engineering projects.

Sub-surface identification of iron sand in Lamreh, Aceh Besar using very low frequency method

This work was studied about 2D Sub-Surface Identification of iron sands in Lamreh, Aceh Besar by using Very Low Frequency method based on Resistivity mode (VLF-R). The aims of the research; (1) Predicting 2D sub-surface of iron sands resistivity in Lamreh based on VLF data. (2) Conducting 2D sub-surface identification measurement data of iron sands. VLF data were measured into 3 lines, by the first line was 250 meters and the second line was 120 meters and the third lines was 100 meters. 2D modelling in VLF-R was conducted by using 2LAYINV electromagnetic inversion program. Inversion process showed 2-iron sands-conductivity-zone, where the resistive layer in depth was covered by conductive layer on it. The conductive layer was expected as clay being rich of water content by resistivity rate at 10 – 100 Ωm. Resistive layer was expected as cap rock by resistivity rate at 20000 – 40000 Ωm.

Deep-learning-based surrogate flow modeling and geological parameterization for data assimilation in 3D subsurface flow

Data assimilation in subsurface flow systems is challenging due to the large number of flow simulations often required, and by the need to preserve geological realism in the calibrated (posterior) models. In this work we present a deep-learning-based surrogate model for two-phase flow in 3D subsurface formations. This surrogate model, a 3D recurrent residual U-Net (referred to as recurrent R-U-Net), consists of 3D convolutional and recurrent (convLSTM) neural networks, designed to capture the spatial–temporal information associated with dynamic subsurface flow systems. A CNN-PCA procedure (convolutional neural network post-processing of principal component analysis) for parameterizing complex 3D geomodels is also described. This approach represents a simplified version of a recently developed supervised-learning-based CNN-PCA framework. The recurrent R-U-Net is trained on the simulated dynamic 3D saturation and pressure fields for a set of random ‘channelized’ geomodels (generated using 3D CNN-PCA). Detailed flow predictions demonstrate that the recurrent R-U-Net surrogate model provides accurate results for dynamic states and well responses for new geological realizations, along with accurate flow statistics for an ensemble of new geomodels. The 3D recurrent R-U-Net and CNN-PCA procedures are then used in combination for a challenging data assimilation problem involving a channelized system. Two different algorithms, namely rejection sampling and an ensemble-based method, are successfully applied. The overall methodology described in this paper may enable the assessment and refinement of data assimilation procedures for a range of realistic and challenging subsurface flow problems.

Use of Electrical Resistivity Tomography in investigating the internal structure of a landslide and its groundwater characterization (Nanka Landslide, Anambra State, Nigeria)

ABSTRACT Over two decades, this landslide in Nanka Community, Anambra State, Nigeria, in the eastern phase of Awka-Orlu upland, has been active and persistent, defying many control measures that have been put in place and causing the loss of property, lives, and assets to increase. As a result, this research work involves the study of geo-engineering and geophysical measurements of the slide site to delineate the aerial scale and direction of movement of the landslide zone, determine the lithological portion, estimate the thickness of the sliding sheet, and delineate the water-saturated areas. The data was collected using ABEM Terrameter SAS 4000 and ABEM LUND ES464 electrode selector system and processed using RES2DINV software for a 2D subsurface image. During data acquisition, a Wenner array was set-up as this array is capable of imaging deeper profile data, and apparent resistivity is easily calculated in the field. The results of the models indicate that the study area was mainly clayey and sandstone formation with mostly low resistivity values correspond to the shale layers and groundwater zones. However, gravel deposit was also present in some areas, as indicated by both the imaging and geological measurements.

Folding and faulting offshore Libya: Partly decoupled tectonics above evaporites

The Gabes-Tripoli Basin of western offshore Libya encompasses and neighbours several large hydrocarbon fields (e.g. the Bouri Field with 1000-3000 Million barrels oil), which are of major economic importance for Libya. Despite the relevance and value of the basin, there are today several unsolved geological questions concerning the basin's late Cretaceous to recent tectonic development; the role of a potential Triassic salt substratum influencing the development of hydrocarbon-bearing anticlines; and the interaction of tectonics and sedimentation controlling reservoir-rock deposition, reservoir fracturing, hydrocarbon migration and accumulation.This study presents detailed structural-stratigraphic interpretations of the Gabes-Tripoli Basin based on 3D-seismic-reflection data and borehole information. A workflow is used that is specifically designed to enhance the seismic interpretability of faults. The workflow integrates targeted 3D-seismic data pre-conditioning, volume-attribute analysis and isochore (TWT) interpretation. Detailed 3D subsurface attribute interpretations show that (i) different groups of faults with different structural orientation characterize the different stratigraphic intervals between the Late Cretaceous and today; (ii) major anticlines associated with the main hydrocarbon reservoirs trend WSW-ENE and SSE-NNW, which is oblique to the dominant NW-SE fault trend. The SSE-NNW-oriented anticlines present a newly documented fold orientation in the basin; (iii) large N-S oriented, gravity-driven, synsedimentary listric normal faults formed at the edges of major anticlines bounding kilometre-scale sedimentary depocentres; and (iv) Paleogene anticline growth and growth faulting at the anticline edges was decoupled from the dominant Late Cretaceous to recent NW-SE-oriented, deep-rooted crustal fault trend.Combined tectonic and stratigraphic interpretation documents partial decoupling of Eocene to Oligocene overburden faulting in the western and central study area from otherwise basement-rooted strike-slip tectonics. Lateral and vertical subsurface evaporite movement in the Paleogene is regarded as an important control for anticline growth and reservoir development in anticline crests, with contemporaneous and subsequent basement-involved strike-slip faulting contributing to a high variability in fault- and fracture-trends and types.

Direct laser writing of volumetric gradient index lenses and waveguides

Direct laser writing (DLW) has been shown to render 3D polymeric optical components, including lenses, beam expanders, and mirrors, with submicrometer precision. However, these printed structures are limited to the refractive index and dispersive properties of the photopolymer. Here, we present the subsurface controllable refractive index via beam exposure (SCRIBE) method, a lithographic approach that enables the tuning of the refractive index over a range of greater than 0.3 by performing DLW inside photoresist-filled nanoporous silicon and silica scaffolds. Adjusting the laser exposure during printing enables 3D submicron control of the polymer infilling and thus the refractive index and chromatic dispersion. Combining SCRIBE’s unprecedented index range and 3D writing accuracy has realized the world’s smallest (15 µm diameter) spherical Luneburg lens operating at visible wavelengths. SCRIBE’s ability to tune the chromatic dispersion alongside the refractive index was leveraged to render achromatic doublets in a single printing step, eliminating the need for multiple photoresins and writing sequences. SCRIBE also has the potential to form multicomponent optics by cascading optical elements within a scaffold. As a demonstration, stacked focusing structures that generate photonic nanojets were fabricated inside porous silicon. Finally, an all-pass ring resonator was coupled to a subsurface 3D waveguide. The measured quality factor of 4600 at 1550 nm suggests the possibility of compact photonic systems with optical interconnects that traverse multiple planes. SCRIBE is uniquely suited for constructing such photonic integrated circuits due to its ability to integrate multiple optical components, including lenses and waveguides, without additional printed supports.

Stratigraphic analysis and geomorphological reconstruction of Grumari coastal plain, Rio de Janeiro, Brazil

Between Marambaia spit and Jacarepagua barrier island system, two major coastal plains in Rio de Janeiro State, Grumari's type of evolution remains unknown, since it was anthropologically modified from the 1960s, where all superficial sedimentary deposits were devastated, intensively changing its natural landscape, and hiding how it has been formed through the Holocene. Aiming to fill these blanks in Grumari's geological history and complementing Rio de Janeiro's coastal studies, the Ground Penetrating Radar method (GPR) was applied to enable the 2D subsurface imaging. The reflection patterns recognition and their correlation with geological features allow a radarfacies interpretation, in a radar stratigraphy context. For coastal plain studies, geological interpretation takes into consideration the sedimentary processes involved in their formation, driven by the balance between sedimentary supply and sea level variations. In the adjacent areas' researches, geological and geophysical data show, from bottom to top, continental deposits formed during the last regional marine regression, in the Late Pleistocene, covered by sand deposits and marine fossils deposited during the last marine trangression, already in the Holocene, followed by sand barriers and lagoons formed and exposed during the last sea falling until the present shoreline. For Grumari coastal plain, this work presents geophysical data interpretation of five GPR sections, collected in about 15 m thick sedimentary deposit, with a sequence of three main units, separated by regional stratigraphic surfaces: a transgressive unit followed by a regressive one, being this one covered by the most recent deposits composing a third unit. The Unit 1 is related to the Holocene sea level rise, from about 7,000 yr BP, and is characterized by truncation relationships between seawards downlapping reflectors and flat-parallel ones, showing a constant change in deposition direction, as the accommodation space was still small at the beginning of this process. At the Unit 1 top, as the accommodation space rate increased, the deposition became more regular, seen by a flat-parallel reflection pattern. While mean sea level reached its maximum range, from about 5,500 yr BP, its rising rate decreased, while the sedimentary supply rate raised due to the continental shelf and the mountain system erosion. This variation in radarfacies sequence is registered by a regional surface, marking the regressive Unit 2 beginning. The change in the rates led to the continental progradation through sand ridges formation, represented by seawards sigmoidal radarfacies. The mean sea level continued to drop gradually to the current level, exposing the sand ridges to erosion, and newer aeolian, fluvial, lagoonal and storm deposits could be formed above them, composing the Unit 3, limited at the bottom by a second regional high amplitude surface. Storm deposits from 4,450 ± 180 yr BP and 3,780 ± 200 yr BP, as washover fans, have been already identified in Jacaperagua plain, therefore, there were episodical events in the region, in the last 4,450 years, seen also in Grumari's data. This primary radar stratigraphic analysis and geomorphological reconstruction of Grumari most recent deposits show that the most recent evolution occurred through the prograding sand ridges formation, mostly attached to each other, with some inter-ridge areas development, and then is likely to be considered as a strandplain.

Adaptively accelerating FWM2DA seismic modelling program on multi-core CPU and GPU architectures

This paper presents work done towards porting of FWM2DA, an open source program, on multi-core CPU and GPU architectures. FWM2DA is a Fortran90 sequential program which performs acoustic wave propagation of single source location for the 2D subsurface earth model using finite difference time domain modelling. We have reproduced this program using C programming language and upgraded its functionality for performing acoustic wave propagation of multiple source locations and allowing different grid spacing in x and z direction for the subsurface earth model. Performance of the upgraded version is improved by implementing inter-node and intra-node parallelization. Inter-node parallelization is implemented using MPI for efficiently utilizing the distributed memory while intra-node parallelization focuses on efficient utilization of underlying architecture’s resources. Outcome of these programs are compared with that of the original FWM2DA program for Sigsbee2a model and found similar thus establishing the correctness of implementation done. The developed programs are tested using two layer subsurface earth model of 5000 X 5000 grid dimension on PARAM Shreshta system having Intel Xeon Gold 6148F Skylake CPU and NVIDIA Tesla V100 GPU. Ported programming models are evaluated for execution time for wavefield propagation of single source location using two layer subsurface earth model. Performance gain of 8.6X for OpenMP, 83.22X for OpenACC and 107.77X for CUDA C implementation are recorded over the sequential C program. The multi-core CPU program is further optimized and overall 29.37X performance is achieved with respect to the sequential C program. Performance of CUDA C program is also improved by making use of shared memory in GPU and 1.18X speedup is recorded with respect to the baseline CUDA C program. The numerical experiments demonstrate the effectiveness and robustness of the developed programs with high scalability and efficiency on multi-core CPU and GPU based HPC system.

Nodal Seismograph Recordings of the 2019 Ridgecrest Earthquake Sequence

Abstract The 2019 Ridgecrest, California, earthquake sequence included Mw 6.4 and 7.1 earthquakes that occurred on successive days beginning on 4 July 2019. These two largest earthquakes of the sequence occurred on orthogonal faults that ruptured the Earth’s surface. To better evaluate the 3D subsurface fault structure, (P- and S-wave) velocity, 3D and temporal variations in seismicity, and other important aspects of the earthquake sequence, we recorded aftershocks and ambient noise using up to 461 three-component nodal seismographs for about two months, beginning about one day after the Mw 7.1 mainshock. The ∼30,000Mw≥1 earthquakes that were recorded on the dense arrays provide an unusually large volume of data with which to evaluate the earthquake sequence. This report describes the recording arrays and is intended to provide metadata for researchers interested in evaluating various aspects of the 2019 Ridgecrest earthquake sequence using the nodal data set.

Theory-Guided Machine Learning Finds Geometric Structure-Property Relationships for Chemisorption on Subsurface Alloys

Summary Developing physically transparent and quantitatively accurate models that relate the chemical interaction (chemisorption strength) between an adsorbate and a solid surface to the adsorption site's geometry is critical for our understanding of catalysis, corrosion, and electrochemistry. We develop a theory-guided machine-learning (ML) approach, which uses an interpretable class of ML models called generalized additive models (iGAM models), to discover predictive structure-property models that can quantify and explain the link between the geometric structure of an adsorption site and the chemisorption strength. We demonstrate the approach through several case studies, where we analyze the chemisorption strength of chemically distinct adsorbates (O, OH, S, and Cl) on subsurface metal alloy surfaces subjected to various strain- and ligand-induced changes in the local geometric structure. By comparing the ML-derived chemisorption models to previously established electronic-structure models, we clarify the critical physical parameters that control the chemisorption process on metal surfaces.

Modeling Geoelectric Fields Induced by Geomagnetic Disturbances in 3D Subsurface Geology, an Example From Southeastern Australia

AbstractGeomagnetic storms can cause power grid instabilities and blackouts due to excessive geomagnetically induced currents (GICs) flowing in electric transmission systems. In this study, we assess regional vulnerability to GICs by modeling the geoelectric fields induced by significant historic geomagnetic disturbance events in the presence of 3D subsurface geology using data from the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) magnetotelluric array, Australia‐Wide Array of Geomagnetic Stations (AWAGS) magnetometer array, and Geoscience Australia geomagnetic observatory network. We analyze the vertical component of the magnetic field with respect to the horizontal magnetic‐field polarization for two magnetic storms and gain insight into the inductive effects associated with field polarization orientations in the 3D case. We also analyze the telluric field intensity and polarization for a unit geomagnetic field polarized in northerly and easterly directions at AusLAMP sites and find that in the presence of 3D geology the induced field has a very polarization‐sensitive anomaly. We model the geoelectric fields in southeastern Australia for the 1989 “Québec storm.” The induced ground electric fields are typically in the range 1,000–2,000 mV/km with a few sites within 2,000–5,000 mV/km on highly resistive regions and in coastal areas, and below 300 mV/km on inland sedimentary basins. The current study focuses on magnetic‐field variations with periods between 120 and ~20,000 s due to bandwidth limits in our magnetotelluric tensor data and the Nyquist limit for the 60 s sampling of our geomagnetic‐field data. Hence, our modeled maximum values should be considered lower estimates of potential real values.

Sinkhole detection with 3D full seismic waveform tomography

Sinkhole collapse may result in significant property damage and even loss of life. Early detection of sinkhole attributes (buried voids, raveling zones) is critical to limit the cost of remediation. One of the most promising ways to obtain subsurface imaging is 3D seismic full-waveform inversion. For demonstration, a recently developed 3D Gauss-Newton full-waveform inversion (3D GN-FWI) method is used to detect buried voids, raveling soils, and characterize variable subsurface soil/rock layering. It is based on a finite-difference solution of 3D elastic wave equations and Gauss-Newton optimization. The method is tested first on a data set constructed from the numerical simulation of a challenging synthetic model and subsequently on field data collected from two separate test sites in Florida. For the field tests, receivers and sources are placed in uniform 2D surface grids to acquire the seismic wavefields, which then are inverted to extract the 3D subsurface velocity structures. The inverted synthetic results suggest that the approach is viable for detecting voids and characterizing layering. The field seismic results reveal that the 3D waveform analysis identified a known manmade void (plastic culvert), unknown natural voids, raveling, as well as laterally variable soil/rock layering including rock pinnacles. The results are confirmed later by standard penetration tests, including depth to bedrock, two buried voids, and a raveling soil zone. Our study provides insight into the application of the 3D seismic FWI technique as a powerful tool in detecting shallow voids and other localized subsurface features.

Regional subsurface mapping and 3D flexural modeling of the obliquely converging Putumayo foreland basin, southern Colombia

The Putumayo foreland basin (PFB) is an underexplored hydrocarbon-bearing basin located in southernmost Colombia. The PFB forms a 250 km long segment of the 7000 km long corridor of Late Cretaceous-Cenozoic foreland basins produced by eastward thrusting of the Andean mountain chain over Precambrian rocks of the South American craton. We have used approximately 4000 km of 2D seismic data tied to 28 exploratory wells to describe the basin-wide structure and stratigraphy. Based on seismic interpretation and comparison with published works from the southward continuation of the PFB into Peru and Ecuador, three main across strike, structural zones include (1) the 20 km wide, western structural zone closest to the Andean mountain front characterized by inversion of older, Jurassic half-grabens during the Late Miocene, (2) the 45 km wide, central structural zone characterized by moderately inverted Jurassic half-grabens, and (3) the 120 km wide, eastern structural zone (ESZ) characterized by the 40 km wide, north–south-trending Caquetá arch. The five mainly clastic tectonosequences of the PFB include (1) Lower Cretaceous pre-foreland basin deposits, (2) upper Cretaceous-Paleocene foreland basin deposits, (3) Eocene foreland basin deposits related to the early uplift of the eastern Cordillera, (4) Oligocene-Miocene underfilled, foreland basin deposits, and (5) Plio-Pleistocene overfilled, foreland basin deposits. We used 3D flexural modeling to identify the elastic thickness ([Formula: see text]) of the lithosphere below the PFB, and to model the location of the sedimentary-related and tectonically related forebulges of Cretaceous to Oligocene age. Flexural analysis finds two pulses of rapid, foreland-related subsidence first during the Late Cretaceous-early Paleocene and later during the Oligocene-Miocene. Despite the present-day oblique thrusting of the mountain front, flexure of the PFB basement has produced a tectonic forebulge now located in ESZ and controls a basement high that forms the eastern updip limit for most hydrocarbons found in the PFB.

Multi-sensor core logging (MSCL) and X-ray computed tomography imaging of borehole core to aid 3D geological modelling of poorly exposed unconsolidated superficial sediments underlying complex industrial sites: An example from Sellafield nuclear site, UK

The 3D characterisation of geology underlying complex industrial sites such as nuclear plants is problematic due to the presence of the built infrastructure that restricts or in some cases completely prevents access for geologists to the subsurface environment. Outcrops are rare, geophysics surveys are often impossible (particularly at nuclear plants where activities such as vibroseis are frowned upon due to their effect on the infrastructure itself), and boreholes are often the only way to obtain subsurface data. Yet, with sedimentary deposits in particular, geotechnical logging undertaken to specific standards sometimes misses key information that could have been used to directly inform the creation of geological 3D models.Multi-sensor core logging (MSCL) and X-ray computed tomography (XCT) undertaken on core obtained from a borehole within the Sellafield nuclear plant, is used to illustrate the potential for the techniques to contribute significantly to the creation of 3D subsurface geological models, particularly where access is restricted, such as within nuclear industry locations. Geophysical characteristics are recorded and used to reassess and enhance geotechnical descriptions, leading to the modification of existing unit boundaries or the creation of new ones. A new sedimentary log was created and this was used in a comparison with existing logs and nearby historic exposures, and as the basis for an illustration of industrial site to regional correlation.

MODEL GAYABERAT 2D UNTUK MENGUNGKAP STRUKTUR GEOLOGI BAWAH PERMUKAAN PADA DAERAH PANAS BUMI NATAR

Daerah panas bumi Natar terletak di Kecamatan Natar yang berjarak kurang lebih 10 km utara Bandar Lampung, Provinsi Lampung. Lapangan panas bumi ini terbilang cukup unik karena manifestasi mata air panas yang muncul ke permukaan berada pada daerah yang relatif datar. Pengamatan geologi pada singkapan untuk mengetahui struktur geologi yang mengontrol keluarnya air panas tersebut sulit dilakukan karena terbatasnya singkapan batuan sehingga diperlukan metode geofisika untuk menginterpretasi kondisi bawah permukaan seperti metode gayaberat. Geologi daerah penelitian ditutupi oleh Formasi Lampung yang terdiri dari batuan piroklastik–vulkanoklastik dengan tebal mencapai 200 m. Hasil pemrosesan data gayaberat menunjukkan nilai anomali Bouguer yang berkisar antara 51,0-62,6 mGal. Hasil interpretasi peta anomali Bouguer memperlihatkan keberadaan lima struktur sesar, yang menjadi jalur keluarnya air panas, yang dominan berarah baratlaut-tenggara dan utara-selatan. Arah tersebut menandakan bahwa struktur geologi yang terdapat pada daerah penelitian masih berkaitan dengan Sesar Besar Sumatera yang juga berarah baratlaut-tenggara. Pemodelan 2D bawah permukaan yang dilakukan dengan menggunakan perangkat lunak ‘Oasis Montaj 8.4’, menghasilkan tiga lapisan densitas batuan yaitu densitas yang berasosiasi dengan Formasi Lampung dengan densitas 1,9 g/cm3, dan yang berasosiasi dengan dua struktur basement dengan densitas masing-masing 2,54 g/cm3 dan 2,76 g/cm3, yang membentuk struktur seperti horst dan graben. Pengeboran eksplorasi untuk penelitian selanjutnya disarankan dilakukan pada sebelah barat mata air panas untuk mengonfirmasi model bawah permukaan.

Automatic triangular and triangular-prism mesh generation for overland and subsurface water flow simulations

In this paper, an automatic mesh generator is developed to simulate density-dependent water flow and transport problems in watershed systems. The two-dimensional (2D) triangular mesh is generated by global Delaunay triangulation, and the three-dimensional (3D) triangular-prism mesh is generated by vertical stretch. The implementation procedures of mesh generation are adjusted according to the interaction options of one-dimensional (1D) river, 2D overland, and 3D subsurface flows. An improved boundary point generation algorithm is proposed to maintain appropriate correspondence of points and edges according to the discrete patterns of river reaches and dead ends. Overlapped nodes are generated on zero-width river reaches and zero-width junctions without storage to construct the numerical models for 2D overland simulations. An additional triangular mesh generation method is proposed to create additional triangles for filling the empty water zones of finite-width river reaches, junctions with storage, finite-width ponds, lakes, and dead ends. The boundary loops bounding each water area are identified correctly, and the additional grids are created compatibly, aiming at the finite-depth and zero-depth patterns of storage zones. The computation equations of relevant parameters used for 3D stretch from triangles to triangular-prisms are built, and the 1D/2D/3D correspondence on river reaches, junctions, ponds, lakes, dead ends, and control structures is established. Finally, practical examples for discretizing real-world water areas are provided to demonstrate the robustness and effectiveness of our developed mesh generator by using the skewness as the mesh quality metric.

Characterization of sub-seismic resolution structural diagenetic heterogeneities in porous sandstones: Combining ground-penetrating radar profiles with geomechanical and petrophysical in situ measurements (Northern Apennines, Italy)

Deformation bands and structurally-related diagenetic heterogeneities, here named Structural Diagenetic Heterogeneities (SDH), have been recognized to affect subsurface fluid flow on a range of scales and potentially promoting reservoir compartmentalization, influencing flow buffering, and sealing during production. Their impact on reservoir hydraulic properties depends on many factors, such as their permeability contrast with respect to the undeformed reservoir rock, their anisotropy, thickness, geometry as well as their physical connectivity and arrangement in the subsurface. We used the Ground Penetrating Radar (GPR) for detection and analysis of the assemblage of deformation bands – carbonate nodules, in high-porosity arkose sandstone in Northern Apennines of Italy. 2D GPR surveys allowed the description of the SDH spatial organization, their geometry, and their continuity in the subsurface. Petrophysical (air-permeability) and mechanical (uniaxial compressive strength) properties of host rock, deformation bands, and calcite-cement nodules were evaluated along a 30-m thick stratigraphic log to characterize the permeability and strength variations of those features. The assemblage “deformation bands – nodules” degrade porosity and permeability and produce a strengthening effect of the rock volume, imparting a strong mechanical and petrophysical heterogeneity to the pristine rock. Different textural, petrophysical, and geomechanical properties between deformation bands, nodules, and host rock result in different GPR response (permittivity). Thus, GPR response could be used to extend outcrop data (petrophysical and geomechanical) of SDH to 3D subsurface volumes in a way to reconstruct realistic and detailed outcrop analogs of faulted aquifers and reservoirs in porous sandstones. • GPR effectively detects deformation bands and diagenetic heterogeneities in sandstones. • Deformation bands and cement nodules show high compressive strength and low permeabilities. • GPR response correlates with petrophysical and geomechanical rock properties. • An improved reservoir analog is obtained from integrating outcrop-based and GPR data. • Deformation bands and nodules impart anisotropy to the host rock.