Geological Structure Model Research Articles

Magnetic modelling as a tool in the evaluation of impact structures, with special reference to the Tvären Bay impact crater, SE Sweden

Until recently computer modelling of radial-symmetric structures, like impact craters, has presented difficulties. As a result of this, such models are scarce in the literature. In this study, the magnetic structure of the marine Tvären Bay impact crater (58°46'N, 17°25'E) south of Stockholm is investigated with a sophisticated commercially available computer program developed for gravimetric and magnetic modelling of geological structures. Aeromagnetic data along three profiles across the crater have been used. It is concluded that the method is an important supplement to other geological and geophysical investigations in estimating the shape, depth and extension of the crater and the surrounding region of basement that was fractured by the impact.

A Geodetic-Gravimetric Method for Better Modelling of Geological Structures in the Test Area of Croatia

A new approach to the modelling of the Earth's crust, especially its upper crust portion, utilising different kinds of geodetic (vertical and deflection components, geoid surface), gravimetric (Bouguer and other gravity anomalies) and geophysical (3D seismic from bore hole prospecting) data is presented in this paper. This scientific research formed part of a special contract between the Faculty of Geodesy and INA-Naftaplin, with the aim of establishing the gravity field over a test area in northern Croatia. It should improve the geophysical prospection for minerals, particularly in the field of hydrocarbon exploration.

Geospatial Modeling: A Breakthrough 3-D Technology for Understanding Complexly Faulted Geologic Structures: ABSTRACT

Today's geoscientists are required to find hydrocarbons in increasingly complicated geologic structures. These structures are usually faulted and contain complex geometric relationships. Their complexity requires modeling and mapping capabilities beyond traditional surface mapping tools. Geospatial modeling enables geoscientists to create models of complex geologic structures, calculate distributions of properties therein, visualize models in three dimensions, and analyze models through volumetric calculations. Faults and horizons are treated as individual surfaces in three-dimensional space. Fault relationships are controlled by hierarchies, and relationships between stratigraphic layers are specified by simple geologic rules. The following features can thus be accommodated: depositional layers; layers truncated by unconformities; layers cut by channels; channel in-fill; and salt diapirs. Although the resulting model is calculated from a fixed set of geological relationships, input data may change as data are added or existing interpretations altered, allowing for quick update. Property models may be calculated for each layer and/or each fault block. The three-dimensional property modeling utilizes fault and horizon surfaces as boundaries. Properties are correctly calculated near faults and show appropriate displacements across faults. Unique three-dimensionally consistent contour maps and cross sections are extracted from the models. The three-dimensional contouring process determines the spatial extent of layers, defines geometriesmore » of fault and unconformity truncation wedge zones, and accurately portrays fault gaps. Increased success is important in today's market. Geospatial modeling technology can address the sizeable problems facing geoscientists working with complexly faulted geologic structures, and can significantly increase overall understanding in a timely manner.« less

Application of unconventional geoelectrical methods to the hydrogeological examination of the Mt. S. Croce rock formations (Umbria, Italy) involved in a railway tunnel project

he project of doubling and developing of the railway line Orte-Falconara, committed by the Italian State Railway Company to the COMAVI Consortium (Rome, Italy), envisaged building the Mt. S. Croce tunnel, about 3200 m long between the stations of Narni and Nera Montoro (Umbria, ltaly). During the last phase of the feasibility project, a geophysical research based on geoelectrical prospecting methods was carried out to complement other geognostic investigations with the following goals: a) to outline the complex geotectonic model of the rock system, which will be affected by the new railway layout; b) to gain information on the hydrogeologic features of the survey area, in relation to the existing geologic situation and the consequent effects on the digging conditions of the tunnel and on the operation conditions of the railway layout. The geophysical work was thus organized according to the following scheme: a) execution of dipole electrical sounding pro-files, to depict a series of significant tomographic pseudosections, both across and along the new railway layout; b) execution of self-potential measurements, to draw an anomaly map over the whole hydrogeological network system in the survey area. The research provided information which has helped to improve the geological-structural model of the area and disclosed the hydrogelogic network, conforming to the classified field surface manifestations. At present, further detailed field investigations are being carried out, which confirm all the results obtained by the geoelectrical survey.

Three-dimensional strain analysis in physical models of geological structures : J. Morgan, Tectonophysics, 230(3–4), 1994, pp 181–198

Three-dimensional strain analysis in physical models of geological structures : J. Morgan, Tectonophysics, 230(3–4), 1994, pp 181–198

Three-dimensional strain analysis in physical models of geological structures

A method is outlined for calculating three-dimensional finite strain in physical models of geological structures containing passive strain markers. This method makes it possible to determine the three-dimensional strain pattern in models of structures that lack any of the types of symmetry (such as that imparted by cylindrical folding) that simplified calculations in previous work. The strain markers in the new method are in the shape of stubby rectangular prisms or cubes. These form a three-dimensional grid or array occupying each of the active layers in a model (e.g., for a simple two-layer gravitationally unstable system, one grid for the overburden layer and one for the buoyant layer). Each of the grids can be described by positions of three families of “strain marker surfaces”, which are contacts between layers of strain markers. After deformation, the model is serial-sectioned horizontally and the traces of the strain marker surfaces on the sections are digitized. The strain state is calculated at each of several hundred points arranged in a three-dimensional “output grid” extending throughout the mechanically active part of the model. An interpolation procedure is used to estimate the spacing and orientation of the strain marker surfaces in the vicinity of each of the output grid points. The following quantities are determined for each of the three families of strain marker surfaces: 1. (1) the local horizontal orientation of the strain marker surfaces; 2. (2) the local spacing of the surfaces; and 3. (3) the local inclination of the surfaces, calculated from their change in position from the serial section above, to the serial section below, the output grid point. This information is used to generate a parallelepiped representing the strain marker geometry in the neighbourhood of the output grid point. The edges of the parallelepiped are equivalent to the coefficients of the strain matrix, from which the three principal strain magnitudes and orientations are readily derived.

Geomechanics of the South Furious field. An integrated approach towards solving complex structural geological problems, including analogue and finite-element modelling

The South Furious oil field, offshore Sabah (Malaisia), is an asymmetric anticline bounded on the steep southern flank by a series of steep E-W-striking reverse faults and affected by intense E-W crestal normal faulting immediately next to and contemporaneous with the reverse faults. In spite of the very good sand development and a high sand percentage, the reservoir performance is very poor. A study was undertaken to describe and understand the development and the mechanics of this type of structure and to explain their presence in almost undisturbed sediments around them. The regional tectonic development was studied to provide a basis for a geological model of the structure. A structural geological model was necessary to be able to predict the type of faults and fractures and their location in the structure. The proposed structural development was modelled with analogue sandbox experiments and numerically simulated with a finite-element program (GEOFEP). The results indicate that basement faulting related to the closing of the South China Sea, induced differential sediment loading, triggering asymmetric clay diapirism in the core of the anticline. The asymmetry resulted in contemporaneous formation of steep reverse faults at the leading edge of the migrating anticline and normal faults in a crestal collapse system. The reverse faults and associated shear fractures (as seen in cores) have a sealing cataclastic fabric. This explains the severe compartmentalisation of the reservoir with consequently poor production behaviour, in particular in the reverse faulted part of the field.

A computerized geologic mapping system based on logical models of geologic structures

In this article we describe the basic framework of the computerized geologic mapping system “cigma.” The system, whic is based on a mathematical formulation of geologic concepts, consists of the following six subsystems: (1) input of geologic data set; (2) inference of stratigraphic sequence; (3) construction of logical models of geologic structures; (4) determination of three-dimensional geologic boundary surfaces; (5) construction of three-dimensional solid model of geologic structures; and (6) graphical presentation. Geologic structures are summarized in several tables called “logical models of geologic structures.” Each model is constructed automatically from input data on structural relations between geologic bodies. The model interprets the data automatically to create data files necessary to determine the shapes of geologic boundaries; it also provides a threedimensional solid model of geologic structures referring to the shapes of boundaries. As a prototype, we introduce two types of contacts corresponding to conformity and unconformity into the logical model and show that it is possible to draw a geologic map automatically. More complex geologic structures can be introduced into the geologic mapping system through further formulation of geologic structures.

Seismic tomography and modeling of complex geological structures

Several techniques have been employed in recent years for studying the velocity structure beneath seismic arrays. The most frequently used ones make use of a combination of arrival times of relevant phases from local earthquakes and explosions. The inversion of these data is performed by generalized least squares techniques to obtain a three-dimensional structure in a model of the Earth consisting of a lattice of velocity nodes (e.g. the Thurber algorithm). Ray tracing or other approximate techniques to solve wave equation in a heterogeneous medium are the most used ones to compute traveltimes. A linearized system of solutions is then solved to find the most appropriate solutions of this complex non-linear problem. The present paper discusses some results and limits of this methodology, when applied in areas of complex geology, like volcanoes. In particular studies of velocity structure beneath the volcanic areas of Campi Flegrei and Mt. Etna reveal, when properly applied, the power of seismic tomography methods, and their importance for assessing reliable models of earthquake source and velocity structure. In the Campi Flegrei region a quite extended seismological and geophysical data set exists, which allowed to infer a quite well resolved velocity image in the upper portion of the crust.

Geophysical evidence for Early Permian igneous activity in a transtensional environment, Denmark

Integrated interpretation of gravity, magnetic and seismic data in the area of the strong Silkeborg Gravity High in Central Denmark has led to a structural geological model involving a transtensional rift feature which probably formed in connection with Permo-Carboniferous wrench faulting and the development of the Oslo-Horn Graben system. Reinterpretation of the deep seismic data from profile 2 of the EUGENO-S project in the area of the Silkeborg Gravity High detected a new thin high-velocity layer (6.25 km/s) at about 7 km deep within the Palaeozoic sedimentary sequence. In the crystalline basement a separate body of high velocity (6.9–7.0 km/s) was found at 11 km deep. Beneath this body the Moho reaches its shallowest position in the area. The seismic results compare well with an interpretation based on magnetism of a shallow volcanic or subvolcanic layer at a depth of 6–8 km. This layer shows strong remanent and induced magnetization with a southward inclination of about 10°. Gravity reinterpretation of the area allows the separation of an upper body of density contrast 0.1 g/cm 3 from a lower body of density contrast 0.2 g/cm 3. The upper body roughly coincides with the magnetic body. The lower body, the major one, coincides with the deep seismic body and it is responsible for the main gravity anomaly in the area.

Reply by the authors to V. Anfiloff

Anfiloff’s discussion focused on interpretational preferences with regard to the classically nonunique problem of converting gravity data into a structural geologic model. Our interpretation was designed to answer the exploration question: Is there enough sedimentary section in the Rukwa Rift to justify further exploration? By choosing conservative and easy to understand parameters and model concepts, we were able to arrive at a clearly affirmative answer to the question posed. In contrast, Anfiloff’s proposed assumptions, while certainly valid, are complex and would have only clouded the essential elements of the exploration perspective had we used them.

A numerical study of caustics and polarizations of seismic waves interacting with three-dimensional geological structures

Ray tracing in three-dimensional, piecewise continuous models of near-surface geological structures is used to study the geometry of caustics and wave polarizations induced by refracted and multiply reflected high frequency seismic waves. The geological structures modeled are sediment-filled basins and curved reflectors. We show that multiply reflected seismic P-waves may easily induce localized intensification of both vertical and horizontal motions at the ground's surface. The results may be of importance to the prediction of earthquake hazards in urban areas. Additionally, we suggest applications to the study of the excitation of free oscillations of sedimentary basins and to the seismic detection of subsurface reflectors.

Seismic Modeling: Geologic Predictions and Pitfalls: ABSTRACT

Structural geologists have made tremendous strides in unraveling the architecture of the earth with recent studies of seismic reflection data. End_Page 1836------------------------------ This is true not only for exploration studies, but also for crustal studies. However, the seismic time section is not an undistorted cross section of the earth. Seismic sections across most geologic structures are distorted by sideswipe and/or lateral velocity changes in the subsurface. Invariably, the distortion on the 2-D migrated section hides the features that are most desired. However, through seismic models of similar geologic structures, the interpretational pitfalls caused by sideswipe and velocity are turned into practical prediction tools. Migrated seismic lines across domes and anticlines normally exaggerate the size of the anomalies. Migrated seismic lines across synclines and basins are characterized with false expressions that include grabens, contemporaneous deformation, cross-stratification, high amplitudes, and crossing reflections. Geologic areas that have large lateral velocity contrasts, such as reefs, diapirs, or fault blocks, exhibit false seismic expressions. These include relief faults, basement-controlled tectonics, facies changes, and structures that are located in geologically ambiguous positions. Even the polarity of the seismic reflection is 3-D dependent. Modeling examples show that interpretational pitfalls, such as mapping from migrated sections and interpreting from the basement upward, must be supplemented with pseudo-3-D interpretational techniques. Geologic models and their seismic analyses from salt provinces, reefs, overthrusts, etc, illustrate these pseudo-3-D interpretational tools. End_of_Article - Last_Page 1837------------

Calculation of finite incremental deformations in ductile geological materials and structural models

Although the total continuous deformation of rocks can be the net result of several tectonic pulses (finite increments), it will be assumed that there are only two. It is further assumed that differential displacements are unknown, but that two generations of rotation gauges are available in a rock body, together with two respective generations of strain gauges. A mathematical procedure is outlined herein for calculating the first or second deformation increment in natural structures and putty models with elliptical markers. To illustrate their practical value, one set of equations is used to determine the pattern of large incremental strain associated with the lateral spreading of a mature model diapir. This model pattern may help to interpret igneous fabric patterns of undeformed granitoid plutons.

A new method of determining finite strain in models of geological structures : 6F, 2T, 9R. Tectonophysics, V24, N1–2, 1974, P99–114 Dixon J.M.

A new method of determining finite strain in models of geological structures : 6F, 2T, 9R. Tectonophysics, V24, N1–2, 1974, P99–114 Dixon J.M.

A new method of determining finite strain in models of geological structures

A new method of constructing models of cylindrical geological structures has been developed which allows the finite strain to be determined for each of a large number of small, initially square, elements. Each material layer is made up of uniformly thick, alternately colored laminations which are oriented vertically in one half of the model and horizontally in the other half. After the model has been deformed it is cut into slices perpendicular to the axis of the cylindrical structure, and the lamination patterns from the two model halves are superimposed graphically to reveal the strained shapes of the initially square elements. The three-dimensional finite-strain tensor of each element can be easily determined algebraically from measurements of its deformed shape, provided that the deformation involved rotation about no more than one principal direction. As an example the method is applied to a silicone putty model of a diapiric ridge which was formed by spinning the model in a centrifuge.

Boundaries and fractures in finite element models of geological structures : Sevice, KG Douglas, A Geophys. J. Roy. Astron. Soc. V32, N1, 1973, P1–14

Boundaries and fractures in finite element models of geological structures : Sevice, KG Douglas, A Geophys. J. Roy. Astron. Soc. V32, N1, 1973, P1–14

A Three-Dimensional Seismic Model for Laboratory Use

A three-dimensional seismic model has been developed for laboratory instruction. To simulate seismic energy sources, a transducer sends a train of modulated ultrasonic waves of frequency 60 kHz into a model ground. Various informative signals, such as the direct, refracted, reflected and diffracted waves from models of geological structures in the subsurface are displayed on the screen of an oscilloscope. The travel time of each signal is read directly from the oscilloscope, or from photos taken with an oscilloscope camera. Although the instrumentation involved is simple and easily assembled, nevertheless the model can simulate real field conditions with uneven typography, weathered layer and various subsurface geological structures. Several seismograms obtained with the equipment are illustrated in this article.

Boundaries and Fractures in Finite Element Models of Geological Structures

Summary The finite element method provides a useful tool for setting limits to the forces which form crustal structures. In this paper, we present methods of obtaining realistic boundary forces and of simulating and predicting fractures within a model. The use of edge forces, representing the horizontal, gravitational and residual tectonic forces, is found to be a satisfactory boundary condition for the ends of models as they simulate the model extending to infinity. Airy isostasy is used to constrain the base of crustal models but no universal boundary conditions exist for intracrustal situations. The Murrell-Griffith fracture criteria are shown to provide a method of the predicting fracture initiation and the subsequent development into a major discontinuity. Three models containing discontinuities are described to illustrate the application of the techniques described above.

An experimental study of potential distribution in different geological structures

The principle of analogy between the flow of electricity and the flow of heat has been used to study the thermal distribution in models of geological structures by obtaining the pattern of equipotential lines. This principle of analogy has been further extended to the study of the distribution of seismic energy in these models. Seventeen different models of geological structures have been investigated. The physical parameters of the media and the structures have a definite effect on the distribution of energy and the shape of the equipotential lines.