Geometry Of Subsurface Structures Research Articles

Subsurface structure of Sumani segment in the Great Sumatran Fault inferred from magnetic and gravity modeling

The Great Sumatran Fault (GSF) 1900 km long active and segmented strike-slip faults. One of the segments is the Sumani segment in the province of West Sumatra that passes through a relatively densely populated area. Knowing the geometry of subsurface structures in this segment is one of the efforts to identify potential disasters in the future. To achieve this objective, ground magnetic measurements were carried out and the data were combined with Bouguer anomaly map. Magnetic susceptibility and density were also measured on hand samples collected during the survey so that their values could serve as initial parameters for modeling. The modeling and interpretation stages are carried out through joint forward modeling magnetics and gravity. The equivalent depth of the magnetic and gravity models built under the surface is 20 km. The magnetic and gravity modeling identify several intrusive bodies that do not reach the surface, including one beneath Lake Singkarak. Several slivers of Permian metamorphic rocks encased in the basement rocks were also identified. These unexposed intrusive bodies and slivers of encased metamorphic rocks are probably quite common in the GSF. The magnetic and gravity modeling with 2D modeling also infers an 18 km displacement in Lake Singkarak based on displacement of Quaternary colluvium volcanic deposits. Based on the presence of hot spring at the end of Sumani segment, the pull-apart pattern occurred in Lake Singkarak share similarities with that occur in another segment of GSF, namely Tarutung.

Inferring rheology and geometry of subsurface structures by adjoint-based inversion of principal stress directions

SUMMARY Imaging subsurface structures, such as salt domes, magma reservoirs or subducting plates, is a major challenge in geophysics. Seismic imaging methods are, so far, the most precise methods to open a window into the Earth. However, the methods may not yield the exact depth or size of the imaged feature and may become distorted by phenomena such as seismic anisotropy, fluid flow, or compositional variations. A useful complementary method is therefore to simulate the mechanical behaviour of rocks on large timescales, and compare model predictions with observations. Recent studies have used the (non-linear) Stokes equations and geometries from seismic studies in combination with an adjoint-based approach to invert for rheological parameters that are consistent with surface observations such as GPS velocities. Nevertheless, it would be useful to use other surface observations, such as principal stress directions, as constraints as well. Here, we derive the adjoint formulation for the case that principal stress directions are used as observables with respect to rheological parameters. Both an algebraic and a discretized derivation of the adjoint equations are described. This thus enables the usage of two data fields - surface velocities and stress directions - as a misfit for the inversion. We test the performance of the inversion for principal stress directions on simplified 3-D test cases. Finally, we demonstrate how the adjoint approach can be used to compute 3-D geodynamic sensitivity kernels, which highlight the areas in the model domain that have the largest impact on the misfit value of a particular point. This provides a simple, yet powerful, way to visualize which parts of the model domain are of key importance if changing rheological constants.

The statistical eigenvector analysis technique (SEAT) for dip data analysis

Abstract Dip data acquired from image logs (i.e. the dip/azimuth of planar features) are extensively used in the oil and gas industry to assist in interpreting subsurface geology. Gradual and/or abrupt variations in dip data with depth may indicate different structural features (e.g. faults, folds, etc.) or changes to sedimentary systems (e.g. unconformity). The statistical eigenvector analysis technique (SEAT) and accompanying computer application are useful in statistically calculating the orientation and geometry of subsurface structures from dip data. Poles to planes on a spherical projection over a given depth interval may reveal cluster or girdle distributions, which can be interactively analyzed using statistical distribution functions on the unit sphere. SEAT provides a mathematical solution for as few as two data points, and has the potential to be applied to dip data at the meter-scale. The orientation and geometry of various subsurface geological features (e.g. folds, fault drag deformation, etc.) can be interactively calculated over a chosen depth interval. SEAT is applied to a dip dataset acquired from a modern, 8-padded electronic borehole image log and a previously studied dipmeter data from the discovery well of the Railroad Gap Field to illustrate its higher accuracy in determining the orientation and geometry of subsurface structures compared to older methods. SEAT allows for a vastly improved interpretation of subsurface geological features when combined with high-resolution borehole image logs and accurate dip data. Improved understanding of subsurface geometries may allow for better business decisions in future exploration and appraisal of existing assets.

Automatic estimation of regularization parameter by active constraint balancing method for 3D inversion of gravity data

Gravity data inversion is one of the important steps in the interpretation of practical gravity data. The inversion result can be obtained by minimization of the Tikhonov objective function. The determination of an optimal regularization parameter is highly important in the gravity data inversion. In this work, an attempt was made to use the active constrain balancing (ACB) method to select the best regularization parameter for a 3D inversion of the gravity data using the Lanczos bidiagonalization (LSQR) algorithm. In order to achieve this goal, an algorithm was developed to estimate this parameter. The validity of the proposed algorithm was evaluated by the gravity data acquired from a synthetic model. The results of the synthetic data confirmed the correct performance of the proposed algorithm. The results of the 3D gravity data inversion from this chromite deposit from Cuba showed that the LSQR algorithm could provide an adequate estimate of the density and geometry of sub-surface structures of mineral deposits. A comparison of the inversion results with the geologic information clearly indicated that the proposed algorithm could be used for the 3D gravity data inversion to estimate precisely the density and geometry of ore bodies. All the programs used in this work were provided in the MATLAB software environment.

Shallow Seismic Refraction, Two-Dimensional Electrical Resistivity Imaging, and Ground Penetrating Radar for Imaging the Ancient Monuments at the Western Shore of Old Luxor City, Egypt

A combined near surface geophysical survey conducted in archaeological site at the western bank of Luxor area shows that, the geophysical methods offer the possibility to characterize and reconstruct the geometry of subsurface structures without destroying the deposits, providing a way to find solutions to the questions of archaeological or engineering significance. The two dimensional electrical imaging (R2D) and the shallow seismic refraction (SSR) techniques were carried out through twelve profiles measured across the study area in NW-SE directions with 5 m profile spacing. The interpretation of the 2-D resistivity imaging shows that there are two layers which are arranged from the top to the bottom as soil layer consists of weathered clay and its thickness varies between 3.82 m and 4 m; wetted clay and mud with depth varies from 25.3 m to 26.23 m and a massive body may composed of Alabaster, limestone or granite is present within the second layer. Following, the inverted 2-D models were merged to form a quasi-3D resistivity model which was visualized as depth slices. A high resistive anomalous feature was detected in successive depth slices. Moreover, analysis of shallow seismic data shows that, P-velocity ranges from 400 - 1350 m/sec whereas S-wave ranges from 280 - 460 m/sec. The massive body with high resistivity is detected and shows also a high velocity. Following a detailed Ground Penetrating Radar (GPR) a survey was carried out to image the subsurface. 200 MHz antenna frequency was used on the survey. Inspection of the 2-D GPR profile shows the presence of hyperbolic anomalous features is present at a depth nearly 4 m. Moreover, three dimensional depth slices shows also the presence of an anomalous feature which appears at depth slices ranging from 3.5 m to 5 m. Finally, the interpreted geophysical results have been verified through archaeological direct excavation started by the Egyptian supreme council of Antiquities in cooperation with the European Commission working in the restoration project of Memnon’s and the Temple of Amenhotep III. They have found a body of Pharaonic statue, the statue has been known by the archaeologists as a statue of Queen Tie, wife of Amenhotep III and mother of Pharaoh Akhenaton unification, carved in Alabaster stone, and this result confirmed that the integrated geophysical method can be effectively contributed in the archaeological prospection in Egypt.

Non-destructive GPR evaluation of underpass arch-shape structures

Purpose In recent decades, road inspections have used automated vehicles that integrate LiDAR, GPR, and IRI measurements. GPR-data provide valuable information–typically about the pavement thickness–for road assessment. The aim of this work is to test the GPR-system to provide geometric information about underpasses of roads, principally related to the depth and span length. Method Both high and low frequency antennas (500 and 200 MHz) were used to determine the most appropriate survey methodology for our purpose. This technique was used to detect the different layers of an arch structure. To validate the method, GPR-data were compared with the ground-truth data provided by a mobile laser scanner. Results & Discussion The GPR results showed the potential of the system to obtain information about subsurface structures, and the arch geometry (depth and span length) were defined. However, the heterogeneity of the backfill over the arch made it difficult to use the field data we obtained. Finite-difference time-domain (FDTD) modeling was used in this work to understand the response of the radarwave and to assist in the data interpretation. The synthetic models were built from the orthoimage provided by laser scanner, defining the structure in fine detail, which resulted in large scale and more realistic models. Laser scanner errors in geometric measurements were lower than the 6 mm of the laser used as the ground truth for this work. By exhaustive interpretation of the field data–once they were analyzed through modeling–we found that the GPR-data are consistent with the laser scanner data and open the possibility of using GPR-information to obtain the geometry of subsurface structures for road inspections. This information could be useful to make structural calculations and predict critical failures.

Computer program for regression model for discontinuous structural surfaces

Abstract A regression model for discontinuous structural surfaces requires nonlinear normal equations. A computer program FAULTS is presented which estimates parameters of the regression equations. The program algorithm, which simulates possible natural-fault movements, has been tested on various multifaulted structural surfaces. The prefault geometry of subsurface structures can be reconstructed using estimates of the fault displacements. A method based on the geographic distribution of large residuals is proposed for detecting unmapped subsurface faults; a mapped fault in the Portrero oilfield (California) has been predicted accurately by this method.

TWO‐DIMENSIONAL AND THREE‐DIMENSIONAL MIGRATION OF MODEL‐EXPERIMENT REFLECTION PROFILES

A reflection profile represents an unfocused picture of the subsurface. In areas of rapid structural change, this unfocused picture may not reveal directly the true geometry of subsurface structures. Computer processing techniques, collectively called migration, have been used by many companies to focus 2-D reflection data. A description of the migration process can be given which allows immediate generalization to three‐dimensions with arbitrary source and receiver positions. Reflection profiles digitally recorded in the laboratory over known acoustically semitransparent structural models establish the effectiveness of migration. Processed reflection data over 3-D models demonstrate that 3-D migration eliminates many of the lateral correlation ambiguities caused by “sideswipes” and “blind structures.” Structure maps developed from the results of 3-D migration of reflection data give a true and precise picture of 3-D models. When the same data are processed using 2-D migration, the mapped structures are distorted. In structurally complex areas it is desirable to collect 3-D reflection data. Single profiles cannot, and conventional grids may not, reveal adequate cross‐dip information.