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Applied geophysics
No full textCompletely revised and updated, this new edition of the popular and highly regarded textbook, Applied Geophysics, describes the physical methods involved in exploration for hydrocarbons and minerals. These tools include gravity, magnetic, seismic, electrical, electromagnetic, and radioactivity studies. All aspects of these methods are described, including theoretical considerations, data acquisition, and data processing and interpretation, with the objective of locating concentrations of natural resources and defining their extent. In the past fourteen years or so since the writing of Applied Geophysics, there have been many changes in the field of exploration geophysics. The authors give full treatment to changes in this field, which include improved techniques for calculating gravity fields, the use of proton-precession and optically-pumped magnetometers, improved quality of seismic data, magnetotelluric as a practical exploration method, new electromagnetic exploration methods, the use of gamma-ray spectrometers in radioactive exploration, and improved well-logging techniques
Special Issue “Remote Sensing in Applied Geophysics”
No full textThe Special Issue “Remote Sensing in Applied Geophysics” is focused on recent and upcoming advances in the combined application of remote sensing and applied geophysics techniques, sharing the advantages of being non-invasive research methods, suitable for surface and near-surface high-resolution investigations of even wide and remote areas [...
Pure and Applied Geophysics
No full textTexto completo. Acesso restrito. p. 1009-1020The presence of lateral contrasts of electrical conductivity modifies the original pattern of
electromagnetic fields radiated from remote sources. A magnetic transverse plane wave field, interacting
with a vertical conductive and outcropping dike placed between two quarter-spaces of unequal electrical
conductivities, creates an anomalous vertical component of the magnetic field. This anomalous field has
been analysed by computation, and drafting of master curves. Two case histories are presented to illustrate
the application and the effectiveness of the solution. It is concluded that: (i) the response is higher for
intermediate values of the conducting body induction number; (ii) the curves can be used for the interpretation
of magnetotelluric, AFMAG, and VLF exploration data; (iii) it is necessary to develop solutions
taking into account the vertical as well as the lateral variation of conductivity.Salvado
Remote Sensing in Applied Geophysics
Get PDFThe Special Issue is focused on recent and upcoming advances in the combined application of remote sensing and applied geophysics. Applied geophysics analyzes the distribution of physical properties in the subsurface for a wide range of geological, engineering, and environmental applications at different scales. Seismic, electrical, magnetic, and electromagnetic methods are among the most applied and well-established geophysical techniques. These methods share the advantages of being non-invasive and exploring wide areas of investigation with respect to conventional methods (e.g., drilling). Geophysical surveys are usually carried out deploying or moving the appropriate instrumentation directly on the ground surface. However, recent technological advances have resulting in the development of innovative acquisition systems becoming more typical of the remote sensing community (e.g., airborne surveys). While applied geophysics mainly focuses on the subsurface, typical remote sensing techniques have the ability to accurately image the Earth’s surface with high-resolution investigations carried out by means of terrestrial, airborne, or satellite-based platforms. The integration of surface and subsurface information is often crucial for several purposes, including the processing of geophysical data, the characterization and time-lapse monitoring of surface and near-surface targets, and the reconstruction of highly detailed and comprehensive 3D models of the investigated areas. Recent contributions showing the added value of surface reconstruction and/or monitoring in the processing, interpretation, and cross-comparison of geophysical techniques for archaeological, environmental, and engineering studies are collected in this book. Pioneering geophysical acquisitions by means of innovative remote systems are also presented
On the estimation of stochastic parameters from deep seismic reflection data and its use in delineating lower crustal structure
No full textIn this study, a new experimental method of extracting geological data from seismic reflection data was developed and tested on synthetic and real seismic data. The method uses statistics to describe the distribution of geological heterogeneity of the subsurface as well as the distribution of reflections in a resulting reflected seismic wavefield. These 2 types of statistics are then related and through this relation, the statistics of the geological heterogeneity in the subsurface can be estimated from the distribution of reflections in the resulting reflected seismic wavefield. This relation between the 2 types of statistics is found and verified in elaborate controlled synthetic experiments, involving amongst others visco-elastic Finite Difference forward modelings of wavefields in random seismic velocity fields . The estimates of geological heterogeneity are computed from seismic reflection data in the form of stochastic (von Karman) parameters correlation length and Hurst number. These parameters are properties of lateral autocorrelations, that are computed in windows of seismic data. By doing sliding-window estimations of these parameters in seismic sections, maps can be compiled of the variability of the parameters throughout the seismic section. This variability in stochastic parameters represents a variability in the distribution of underlying geological heterogeneity. A map of the latter can give valuable information about the scale lengths of heterogeneity in the subsurface and how they came to be as a product of regional rock-mechanical deformation history. A pleasant by-product of the statistical nature of the method is the assessment of uncertainties in the results. By looking at the uncertainties in the estimated parameters, their validity can be discussed. A synthetic test was done to ascertain the ability of the method to differentiate between two tectonic regions with clearly different distributions of geological heterogeneity, through estimations in the (highly complex) reflected wavefield. This test was succesful, the difference in heterogeneity was picked up by the method in the right proportion, also in the case of noisy seismic data and migrated seismic data. Given the good perspective from the synthetic test-case, application to 2 real deep seismic datasets was done. 1 dataset was the AG48 seismic line in the Abitibi-Grenville transect across Quebec, Canada, part of the LITHOPROBE project. The other dataset was the DOBRE 2000/2001 seismic line, a transect of the Donbas Basin in southeastern Ukraine. Both datasets contain reflections from as deep as upper mantle depth. With some enhancements to the method to deal with real-data effects, maps were made of the estimated stochastic parameters. The maps reveal patterns in the distribution of geological heterogeneity that point to clusters of equal scale lengths. These clusters coincide grosso modo with previous tectonic line-drawing interpretations, but also deliver additional patterns that were previously undetected by the line-drawing interpretations. A modified view on regional rock-mechanical deformation history is the most positive outcome of this real-data application
The internal structure of modern barchan dunes of the Ebro River Delta (Spain) from ground penetrating radar
No full textGround penetrating radar is a non-invasive technique that allows the study of the structure of dune Systems when outcrops are limited or protected. GPR response of sand dunes of the Holocene aeolian dunes of the Ebro River Delta (Spain) has been analyzed in this study in order to: characterise their internal architecture, determine their development and recent evolution, and calculate electromagnetic (EM) waves mean velocities in fine-grained sedimentary deposits. Several GPR profiles carried out in different representative areas have revealed the existence of different reflector packages that are related to differences in barchantype dune activity. The area with a highest sand movement activity is characterized by small dunes, with overlapping reflector packages exhibiting reflections which dip up to 25°. When dune activity is moderate, dunes are higher (up to 5 m height) and their internal structure shows low-angle dip reflections except for the avalanche face, where dips up to 22° are identified. The area with the lowest sand movement, nearest to the coast line, is represented by small dunes with internal geometry consisting of partially overlapping elongated reflector packages defined by subhorizontal reflections. In all cases, a reflection associated to the location of the water table has been recognized at about 0.7 m depth. The results obtained from the GPR survey have allowed us to improve our knowledge about the dynamics of the coastal dune field and its relative evolution. They have shown that the morphology and geometry of the dune bodies adapt themselves to wind conditions, which permits the construction of coastal dune development models in order to establish the evolution of dunes.Biología y Geologí
The role of layer-induced anisotropy in seismic exploration
No full text184In
this thesis we focus on anisotropy caused by fine layering. We analyse
the conditions that must be satisfied so that fine layering is equivalent to
anisotropy. In the long-wavelength (or quasi-static) approximation an interval of
thickness H, consisting of a sequence of layers, is effectively homogeneous and
anisotropic to seismic wave propagation. This approximation implies that H is
much smaller than the seismic wavelength A. Closer inspection of this
approximation shows that the degree of equivalence depends on several
parameters. The equivalence is exact for infinitely long wavelengths. It is also
exact (for all wavelengths) for those waves which are not back-scattered at the
interfaces between the layers. This is the case when reflection coefficients are
zero. We use a simple model where the interval of thickness H consists of a N
times repeated set of two layers. These two layers form one period, the whole
interval consists of N periods. The wave field that propagates through this
sequence oflayers, consists of the sum of the primary wave and all the multiples.
The effective medium for a given wave number depends on the interference of
primary and multiples. For infinitely thin layers this results in an effectively
homogeneous medium, with a pseudo-primary that is delayed compared to the
primary wave
Inversion of potential field data : theory and applications in gravimetry and magnetometry
No full textThis thesis deals with the inversion of potential field data. Theoretical aspects and
applications in gravimetry and magnetometry are treated. Inverse theory provides mathematical
techniques to obtain useful infonnation about the earth based on measurements
(data). These techniques estimate numerically parameters which have some properties
of the earth. These properties are called model parameters and are related to data by a
theory or model. The object of all inversion methods is to obtain a model which is consistent
with all available data and is physically plausible
Coordinate free representation of the hierarchically symmetric tensor of rank 4 in determination of symmetry
No full textGeneral theory of elasticity treats the anisotropic behaviour of media i.e. that
property-dependence on spatial direction is taken care of. Examples of elastic media
are rocks, building- and biological materials.
The tensor concept is the most fundamental concept in the description of elastic
anisotropy. Although a tensor describes a physical property and as such is independent
of coordinate systems, the tensor can be represented by components referred to
a coordinate system. A vector - which is a first rank tensor - is the most familiar
quantity where the components are dependent on the coordinate system. The set of
components is a representation of the vector. A scalar quantity like the length of a
vector is independent of the coordinate system to which the vector is referred to.
A main subject in my thesis, is a representation of the anisotropic elastic tensor by
means of coordinate-free - or invariant - quantities to describe symmetry properties
of the medium.
A question in elastic anisotropy is how material symmetry can be determined
from the components of the elastic tensor. This is of main concern in my thesis.
In an arbitrary coordinate system an elastic tensor has 21 non-zero components.
The exception is the isotropic tensor with some vanishing components in all coordinate
systems and only two independent components. For ideal media with specified
symmetry, there exist coordinate systems where some of the components are zero.
How can a coordinate system which reduces the number of constants be determined,
and what can be said about symmetry? The previous questions motivated investigating
general theories which could represent a tool in solving such and related
problems.
For experimentally observed tensors, there are no vanishing components due to
deviation from ideal symmetry and inaccuracy in measurements. Thus there are 21
components for real media in any coordinate system.
A theory for representing elastic tensors geometrically was given in Backus (1970).
The theory is based on a specified decomposition of the elastic tensor into harmonic
tensors, and Maxwell multipoles are the geometrical representation of the harmonic
tensor in 3-dimensional space. Harmonic decomposition is also done with different
approaches by e.g. Mochizuki (1988) and Cowin (1989) (see Ch. 2).
Kelvin (1856) and Sutcliffe (1992) present decompositions of the elastic tensor by
means of eigentensors and eigenstiffnesses (see Ch. 1). The concept was proposed by Kelvin (1856), but does not appear to have been accepted at that time. It was independently
discovered by Pipkin (1976), Rychlewsky (1984), Mehrabadi and Cowin
(1990), and Sutcliffe(1992). A thorough discussion is given in Helbig (1994).
My work on coordinate-free representation is mainly based on Backus's theory
including Maxwell multipoles. This thesis contains applications and further developments
of his theory.
In the literature, the notation 'elastic tensor' is normally used for tensors of
rank four and dimension three, describing real i.e., stable, media. Backus's theory,
however, is valid regardless of the stability conditions. A tensor of rank four in three
dimensions satisfying" elastic" symmetry is here defined as a hierarchically symmetric
tensor. Stability conditions need not be satisfied.
According to the introduction of the concept 'hierarchically symmetric tensor',
this concept was progressively taken into use in my work (Chapters 3, 4, 5 and 6).
However in my first publication (Ch. 2) hierarchically symmetric tensors were not
defined. I have kept it in my thesis as it was published, except for minor changes.
Theoretical analysis of elastic anisotropy has a much wider range of application
than in earth sciences, and is presented in scientific diciplines like mathematics,
physics, material sciences and biomechanics as well.
Most of my work is related to ideal media, with symmetry properties as in crystal
physics. In Ch. 6 modeling of real media are performed by perturbation of tensors of
ideal symmetry. Perturbation of a triclinic tensor models the inaccuracy of physical
measurements.
Perturbation of a tensor of ideal higher symmetry, models deviation from ideal
symmetry. Further work has to be done in applying the fundamentals to obtain tools
for determine symmetry elements for experimentally determined tensors
Multiple beam generation with a digital computer for echosounding at low and high frequencies
No full textThe fact that the velocity of sound in (sea)water does not vary much enables
us to determine through ray theory the direction from which underwater sound
is received. In the same manner the direction can be chosen in which underwater
sound is emitted. These principles are applied in echosounders for
hydrographic use (Glenn, 1970, Burke and Robson, 1975) sidelooking sonars
(Belderson, et al., 1972), sectorscanning sonar (Tucker, 1960, Ballard and
van Andel, 1977, Wolff, 1976) and imaging echosounders (Metherel et al.,
1969). The class of hydrographic echosounders can be divided into narrowbeam
systems and multi-beam systems. The former use an antenna, i.e. an array
of transducers, to generate a single beam with a small beamwidth, the latter
generate several beams with different azimuths.
Up to now the most sophisticated deep-sea multibeam echosounders (Sonarray
Subsystem, Glenn, 1970, and Seabeam, Renard et Allenou, 1979) generate simultaneously
beams with different azimuths, utilizing electronic circuits. These
beams lie in a plane perpendicular to the ship's track. In that way information
in three dimensions is gathered along one track, hence the name threee-dimensional
echosounders
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