A modified form of diffraction tomography to image boundary structures

Geophysics ◽  
1993 ◽  
Vol 58 (8) ◽  
pp. 1136-1147
Author(s):  
Thorbjørn Rekdal ◽  
Durk J. Doornbos
Keyword(s):  
Seismic Tomography ◽  
Outer Part ◽  
Inversion Method ◽  
Unknown Boundary ◽  
Diffraction Tomography ◽  
Common Procedure ◽  
Sea Bottom ◽  
Receiver Array ◽  
Wavefield Extrapolation ◽  
Rough Sea

Wavefield extrapolation downward from the surface, as applied in migration and associated inversion methods, is a common procedure to image subsurface reflectors. These methods require adequate (i.e., extensive and unaliased) sampling of the surface wavefield. Seismic tomography on the other hand, relates parameters of the upward propagated wavefield to the diffracting image, and sampling requirements are less severe; it is usually the only option to image deep structures from sparse data. The ordinary form of ray tomography, however, imposes a severe smoothness constraint on the boundary; in particular the “tops” and “valleys” of a relatively rough structure are not well‐resolved. We have implemented a generalized form of tomography, which uses both the ray term and the diffraction term linearized in the boundary perturbation. We introduce a generalized reflection coefficient that can be linearized in terms of the (unknown) boundary gradient, and we demonstrate the adequacy of this approximation with the help of synthetic seismograms. We compare the performance of the new inversion method with migration and ray tomography in a number of model experiments where a source and receiver array are used to image (1) a rough sea bottom and (2) a rough sedimentary layer boundary. In these experiments the new method is superior, especially in the outer part of the inversion region where migration and seismic tomography suffer seriously because of the lack of adequate surface information. Even for well‐controlled surveys there is the potential to successfully image a much larger area of the reflector than is possible with migration. Our experiments involved a single reflector in a known velocity‐density structure. The method’s applicability or modifications required when relaxing these assumptions, remains to be investigated.

Investigations of past climate and sea-ice variability in the fjord area by Station Nord, eastern North Greenland

1969 ◽  
Vol 35 ◽  
pp. 67-70 ◽  
Author(s):  
Niels Nørgaard-Pedersen ◽  
Sofia Ribeiro ◽  
Naja Mikkelsen ◽  
Audrey Limoges ◽  
Marit-Solveig Seidenkrantz
Keyword(s):  
Sea Ice ◽  
Outer Part ◽  
Late Summer ◽  
Field Work ◽  
The Arctic ◽  
Research Station ◽  
Fjord System ◽  
Sea Bottom ◽  
Satellite Radar ◽  
North Greenland

The marine record of the Independence–Danmark fjord system extending out to the Wandel Hav in eastern North Greenland (Fig. 1A) is little known due to the almost perennial sea-ice cover, which makes the region inaccessible for research vessels (Nørgaard-Pedersen et al. 2008), and only a few depth measurements have been conducted in the area. In 2015, the Villum Research Station, a new logistic base for scientific investigations, was opened at Station Nord. In contrast to the early exploration of the region, it is now possible to observe and track the seasonal character and changes of ice in the fjord system and the Arctic Ocean through remote sensing by satellite radar systems. Satellite data going back to the early 1980s show that the outer part of the Independence–Danmark fjord system is characterised by perennial sea ice whereas both the southern part of the fjord system and an area 20–30 km west of Station Nord are partly ice free during late summer (Fig. 1B). Hence, marine-orientated field work can be conducted from the sea ice using snow mobiles, and by drilling through the ice to reach the underlying water and sea bottom.

Model‐based removal of water‐layer multiple reflections

Geophysics ◽  
1999 ◽  
Vol 64 (6) ◽  
pp. 1816-1827 ◽  
Author(s):  
Guochun Lu ◽  
Bjørn Ursin ◽  
Jan Lutro
Keyword(s):  
Water Layer ◽  
Computationally Efficient ◽  
Multiple Reflections ◽  
Model Based ◽  
The North ◽  
Sea Bottom ◽  
Receiver Array ◽  
Sea Bed ◽  
Reflectivity Function ◽  
Calculated Amplitude

We have developed a procedure to attenuate water‐layer multiple reflections. We estimate the sea‐bottom reflectivity function and use it plus calculated amplitude functions to model all order water‐layer multiple reflections, taking into account both amplitude and waveform shape. We model the primary and multiple reflections from the sea bottom in the frequency‐slowness domain. The amplitude function in the data modeling includes the source directivity function, source ghost response, receiver array directivity function, receiver ghost response, and offset‐dependent geometrical spreading. For small offsets we can assume that the seabed reflectivity depends only on frequency, and it is estimated using a least‐squares algorithm. An unknown scaling constant in the data is estimated using the amplitude of the primary and first multiple reflection from the sea bed. The composite sea‐bottom reflectivity is estimated as a function of frequency for each common midpoint (CMP) position. We apply the algorithm to high‐resolution seismic data from the North Sea. The modelled data match the recorded data well, and the estimated primary reflectivity is more geologically meaningful than the stacked trace. By comparison with Radon transform multiple removal applied to the same data, the model‐based method was more computationally efficient and left less residual multiple energy.

Nonlinear traveltime inversion scheme for crosshole seismic tomography in tilted transversely isotropic media

Geophysics ◽  
2008 ◽  
Vol 73 (4) ◽  
pp. D17-D33 ◽  
Author(s):  
Bing Zhou ◽  
Stewart Greenhalgh ◽  
Alan Green
Keyword(s):  
Seismic Tomography ◽  
Velocity Structure ◽  
Body Wave ◽  
Transversely Isotropic ◽  
The Body ◽  
Body Waves ◽  
Inversion Method ◽  
Perturbation Equation ◽  
Transversely Isotropic Media ◽  
Isotropic Media

Crosshole seismic tomography often is applied to image the velocity structure of an interwell medium. If the rocks are anisotropic, the tomographic technique must be adapted to the complex situation; otherwise, it leads to a false interpretation. We propose a nonlinear kinematic inversion method for crosshole seismic tomography in composite transversely isotropic media with known dipping symmetry axes. This method is based on a new version of the first-order traveltime perturbation equation. It directly uses the derivative of the phase velocity rather than the eigenvectors of the body-wave modes to overcome the singularity problem for application to the two quasi-shear waves. We applied an iterative nonlinear solver incorporating our kinematic ray-tracing scheme and directly compute the Jacobian matrix in an arbitrary reference medium. This reconstructs the five elastic moduli or Thomsen parameters from the first-arrival traveltimes of the three seismic body waves (qP, qSV, qSH) in strongly and weakly anisotropic media. We conducted three synthetic experiments that involve determining anisotropic parameters for a homogeneous rock, reconstructing a fault embedded in a strongly anisotropic background, and imaging a complicated four-layer model containing a small channel and a buried dipping interface. We compared results of our nonlinear inversion method with isotropic tomography and the traditional linear anisotropic inversion scheme, which showed the capability and superiority of the new scheme for crosshole tomographic imaging.

scholarly journals Horizontal spatial correlation of reverberation for rough sea-bottom interface

2019 ◽  
Vol 283 ◽  
pp. 07005
Author(s):  
Shanyong Yan ◽  
Changhong Wang
Keyword(s):  
Correlation Function ◽  
Spatial Correlation ◽  
Autocorrelation Function ◽  
Major Axis ◽  
Interface Roughness ◽  
Backscattering Cross Section ◽  
Sea Bottom ◽  
Bottom Interface ◽  
Covariant Field ◽  
Rough Sea

Correlation sonar, which estimates the velocity of vessel utilizing the principle of waveform invariance, can achieve the sampling of the horizontal spatial correlation of sea-bottom reverberation. The horizontal spatial correlation can be expressed as a correlation function and is affected by sea-bottom characteristics. The expression of the correlation function of the sea-bottom reverberation is derived, which is written as the convolution of the autocorrelation function of transmitted signal, the cross-correlation function of the backscattered impulse response from a plane interface, and the autocorrelation function of the probability density function of the sea-bottom roughness. The isotropic interface roughness of the sea-bottom leads to a circular planform of the correlation function whose width varies with roughness. The anisotropic interface roughness of the sea-bottom leads to an elliptical planform of the correlation function whose major axis is in the direction of weaker roughness. Simulation of submarine reverberation and correlation function verifies this conclusion. The model for the spatially covariant field is used to estimate the backscattering cross section which varies with azimuth angle under the condition of anisotropic seafloor roughness. It should be noted that the horizontal spatial correlation of reverberation is also related to sonar parameters and other sea-bottom characteristics.

scholarly journals The physical and dynamical characteristics of the asteroid 4940 Polenov

2021 ◽  
pp. 39-49
Author(s):  
E. Vchkova-Bebekovska ◽  
N. Todorovic ◽  
A. Kostov ◽  
Z. Donchev ◽  
G. Borisov ◽  
...  
Keyword(s):  
Outer Part ◽  
Astronomical Observatory ◽  
Inversion Method ◽  
Main Belt ◽  
Dynamical Characteristics ◽  
Photometric Observations ◽  
Synodic Period ◽  
Solar System Object ◽  
Stability Time ◽  
Astronomical Station

The asteroid (1986 QY4) 4940 Polenov is the first Solar system object whose 3D shape is determined using the observations from the newly built Astronomical Station Vidojevica (ASV). Here we present the results of photometric observations for Polenov, gathered from the ASV, and from the Bulgarian National Astronomical Observatory (BNAO) Rozhen, during 2014, 2019 and 2020 apparitions. Polenov is a 17.8km object located in the outer part of the main belt and belongs to the asteroid family Themis. We have determined the lightcurves, the synodic period of 4.161?0.001 h, as well as the solution for the shape and spin axis. Using the lightcurve inversion method, the combination of our lightcurves and the sparse data from ATLAS{HKO and ATLAS-MLO, we also found the sidereal period, indicating a retrograde rotation of the asteroid, with two possible mirrored pole solutions. The ratio of the largest to smallest reecting surface is about 1.4. In addition, we studied the dynamical properties of the asteroid and obtained a long stability time that exceeds 0.4 Gyrs.

scholarly journals NEW ITERATIVE AND MULTIFREQUENCY APPROACHES IN GEOPHYSICAL DIFFRACTION TOMOGRAPHY

2019 ◽  
Vol 37 (2) ◽  
Author(s):  
Danilo Sande ◽  
Amin Bassrei ◽  
Jerry Harris
Keyword(s):  
Seismic Tomography ◽  
Seismic Inversion ◽  
Single Frequency ◽  
Diffraction Tomography ◽  
Matrix Formalism ◽  
De Se ◽  
Weak Scattering ◽  
Resolution Imaging ◽  
Multiple Frequencies ◽  
Working Frequency

ABSTRACT. Seismic tomography is used in reservoir geophysics as an important method for high-resolution imaging. The classical Born approach, which is used in single-frequency diffraction tomography under the condition of weak scattering, is limited by the requirement to know the background velocity in advance. We propose tomographic inversion approaches within matrix formalism and the Born approximation conditions. These approaches are iterative (in the sense that the background velocity field is updated at each iteration) and do not require knowledge of the true background velocity. In the first approach, a single-frequency that is kept constant is used. In the second approach, several frequencies are also kept constant and are used simultaneously. In the third approach, in addition to the background velocity, the working frequency is also updated. Finally, in the last approach, the multiple frequencies used simultaneously are updated throughout the iteration. The proposed approaches were tested on a synthetic model containing a dipping layer and a paleochannel, with cross-well acquisition geometry, and the data were contaminated with Gaussian noise. When compared to the standard, single-frequency non-iterative approach, the iterative process with the use of multiple frequencies generated results with smaller RMS errors for model parameter, velocity and data.Keywords: seismic inversion, seismic tomography, wave numerical modeling, reservoir characterization.RESUMO. A tomografia sísmica é usada na geofísica de reservatórios como um método importante para obtenção de imagens de alta resolução. A abordagem clássica de Born, usada na tomografia de difração de única frequência sob a condição de espalhamento fraco, é limitada pela necessidade de se conhecer antecipadamente a velocidade do fundo homogêneo. Propomos abordagens iterativas de inversão tomográfica dentro do formalismo matricial e sob a condição da aproximação de Born. Essas abordagens têm uma natureza iterativa, onde o campo de velocidade de fundo é atualizado em cada iteração, sendo que o conhecimento da velocidade verdadeira do fundo homogêneo não é necessário. Na primeira abordagem é usada uma única frequência mantida constante. Na segunda abordagem são usadas simultaneamente várias frequências também mantidas constantes. Na terceira abordagem, além da velocidade do fundo homogêneo, a frequência de trabalho também é atualizada. Finalmente, na última abordagem, as múltiplas frequências usadas simultaneamente são atualizadas durante a iteração. As abordagens propostas foram testadas em um modelo sintético contendo uma camada inclinada e um paleocanal, com geometria de aquisição poço a poço, sendo os dados contaminados com ruído gaussiano. Quando comparado com a abordagem padrão não-iterativa de única frequência, o processo iterativo com o uso de múltiplas frequências gerou resultados com erros menores de RMS para o parâmetro de modelo, a velocidade e o vetor de dados.Palavras-chave: inversão sísmica, tomografia sísmica, modelagem numérica de ondas, caracterização de reservatórios.  

Anisotropic 3D inversion of towed-streamer electromagnetic data: Case study from the Troll West Oil Province

2014 ◽  
Vol 2 (3) ◽  
pp. SH97-SH113 ◽  
Author(s):  
Michael S. Zhdanov ◽  
Masashi Endo ◽  
Daeung Yoon ◽  
Martin Čuma ◽  
Johan Mattsson ◽  
...  
Keyword(s):  
Integral Equation Method ◽  
Inversion Method ◽  
Huge Number ◽  
3D Inversion ◽  
Actual Case ◽  
The North ◽  
Rock Formations ◽  
Sea Bottom ◽  
High Production Rate

One of the critical problems in the interpretation of marine controlled-source electromagnetic geophysical data is taking into account the anisotropy of the rock formations. We evaluated a 3D anisotropic inversion method based on the integral equation method. We applied this method to the full 3D anisotropic inversion of towed-streamer electromagnetic (EM) data. The towed-streamer EM system makes it possible to collect EM data with a high production rate and over very large survey areas. At the same time, 3D inversion of towed-streamer EM data has become a very challenging problem because of the huge number of transmitter positions of the moving towed-streamer EM system, and, correspondingly, the huge number of forward and inverse problems needed to be solved for every transmitter position over the large areas of the survey. We overcame this problem by exploiting the fact that a towed-streamer EM system’s sensitivity domain is significantly smaller than the area of the towed-streamer EM survey. This approach makes it possible to invert entire towed-streamer EM surveys with no approximations into high-resolution 3D geoelectrical sea-bottom models. We present an actual case study for the 3D anisotropic inversion of towed-streamer EM data from the Troll field in the North Sea.

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