7,612 research outputs found
Finite Difference Modeling of Attenuation and Anisotropy
A nite difference scheme which includes the effects of attenuation and anisotropy is tested for seismic reflection and borehole acoustic models. The validity of the scheme is established using a 3D homogenous isotropic model to compare results to the discrete wavenumber method. Three models are then investigated. First, reflections from a 3D at layered model are analyzed for o set and azimuthal dependence of attenuation. Second, discrete fractures are included in a 2D
at layered model to examine their effect on reservoir top and bottom reflections. Third, a 3D borehole in both hard and soft formations is modeled to test the effect of attenuation on guided waves.Massachusetts Institute of Technology. Earth Resources LaboratoryUnited States. Dept. of Energy (Grant DE-FC26-02NT15346)Eni S.p.A. (Firm
GPR clutter amplitude processing to detect shallow geological targets
The analysis of clutter in A-scans produced by energy randomly scattered in some specific geological structures, provides information about changes in the shallow sedimentary geology. The A-scans are composed by the coherent energy received from reflections on electromagnetic discontinuities and the incoherent waves from the scattering in small heterogeneities. The reflected waves are attenuated as consequence of absorption, geometrical spreading and losses due to reflections and scattering. Therefore, the amplitude of those waves diminishes and at certain two-way travel times becomes on the same magnitude as the background noise in the radargram, mainly produced by the scattering. The amplitude of the mean background noise is higher when the dispersion of the energy increases. Then, the mean amplitude measured in a properly selected time window is a measurement of the amount of the scattered energy and, therefore, a measurement of the increase of scatterers in the ground. This paper presents a simple processing that allows determining the Mean Amplitude of Incoherent Energy (MAEI) for each A-scan, which is represented in front of the position of the trace. This procedure is tested in a field study, in a city built on a sedimentary basin. The basin is crossed by a large number of hidden subterranean streams and paleochannels. The sedimentary structures due to alluvial deposits produce an amount of the random backscattering of the energy that is measured in a time window. The results are compared along the entire radar line, allowing the location of streams and paleochannels. Numerical models were also used in order to compare the synthetic traces with the field radargrams and to test the proposed processing methodology. The results underscore the amount of the MAEI over the streams and also the existence of a surrounding zone where the amplitude is increasing from the average value to the maximum obtained over the structure. Simulations show that this zone does not correspond to any particular geological change but is consequence of the path of the antenna that receives the scattered energy before arriving to the alluvial depositsPeer ReviewedPostprint (published version
The estimation of geoacoustic properties from broadband acoustic data, focusing on instantaneous frequency techniques
The compressional wave velocity and attenuation of marine sediments are fundamental to marine science. In order to obtain reliable estimates of these parameters it is necessary to examine in situ acoustic data, which is generally broadband. A variety of techniques for estimating the compressional wave velocity and attenuation from broadband acoustic data are reviewed. The application of Instantaneous Frequency (IF) techniques to data collected from a normal-incidence chirp profiler is examined. For the datasets examined the best estimates of IF are obtained by dividing the chirp profile into a series of sections, estimating the IF of each trace in the section using the first moments of the Wigner Ville distribution, and stacking the resulting IF to obtain a composite IF for the section. As the datasets examined cover both gassy and saturated sediments, this is likely to be the optimum technique for chirp datasets collected from all sediment environments
Modeling seismic wave propagation and amplification in 1D/2D/3D linear and nonlinear unbounded media
To analyze seismic wave propagation in geological structures, it is possible
to consider various numerical approaches: the finite difference method, the
spectral element method, the boundary element method, the finite element
method, the finite volume method, etc. All these methods have various
advantages and drawbacks. The amplification of seismic waves in surface soil
layers is mainly due to the velocity contrast between these layers and,
possibly, to topographic effects around crests and hills. The influence of the
geometry of alluvial basins on the amplification process is also know to be
large. Nevertheless, strong heterogeneities and complex geometries are not easy
to take into account with all numerical methods. 2D/3D models are needed in
many situations and the efficiency/accuracy of the numerical methods in such
cases is in question. Furthermore, the radiation conditions at infinity are not
easy to handle with finite differences or finite/spectral elements whereas it
is explicitely accounted in the Boundary Element Method. Various absorbing
layer methods (e.g. F-PML, M-PML) were recently proposed to attenuate the
spurious wave reflections especially in some difficult cases such as shallow
numerical models or grazing incidences. Finally, strong earthquakes involve
nonlinear effects in surficial soil layers. To model strong ground motion, it
is thus necessary to consider the nonlinear dynamic behaviour of soils and
simultaneously investigate seismic wave propagation in complex 2D/3D geological
structures! Recent advances in numerical formulations and constitutive models
in such complex situations are presented and discussed in this paper. A crucial
issue is the availability of the field/laboratory data to feed and validate
such models.Comment: of International Journal Geomechanics (2010) 1-1
A Simple Multi-Directional Absorbing Layer Method to Simulate Elastic Wave Propagation in Unbounded Domains
The numerical analysis of elastic wave propagation in unbounded media may be
difficult due to spurious waves reflected at the model artificial boundaries.
This point is critical for the analysis of wave propagation in heterogeneous or
layered solids. Various techniques such as Absorbing Boundary Conditions,
infinite elements or Absorbing Boundary Layers (e.g. Perfectly Matched Layers)
lead to an important reduction of such spurious reflections. In this paper, a
simple absorbing layer method is proposed: it is based on a Rayleigh/Caughey
damping formulation which is often already available in existing Finite Element
softwares. The principle of the Caughey Absorbing Layer Method is first
presented (including a rheological interpretation). The efficiency of the
method is then shown through 1D Finite Element simulations considering
homogeneous and heterogeneous damping in the absorbing layer. 2D models are
considered afterwards to assess the efficiency of the absorbing layer method
for various wave types and incidences. A comparison with the PML method is
first performed for pure P-waves and the method is shown to be reliable in a
more complex 2D case involving various wave types and incidences. It may thus
be used for various types of problems involving elastic waves (e.g. machine
vibrations, seismic waves, etc)
Near-bottom seismic profiling: High lateral variability, anomalous amplitudes, and estimates of attenuation
For almost a decade the Marine Physical Laboratory of Scripps Institution of Oceanography has been conducting nearâbottom geophysical surveys involving quantitative seismic profiling. Operating initially at 4 kHz and more recently at 6 kHz, this system has provided a wealth of fine scale quantitative data on the acoustic properties of ocean sediments. Over lateral distances of a few meters, 7âdB changes in overall reflected energy as well as 10âdB changes from individual reflectors have been observed. Anomalously high amplitudes from deep reflectors have been commonly observed, suggesting that multilayer interference is prevalent in records from such pulsed cw profilers. This conclusion is supported by results from sediment core physical property work and related convolution modeling, as well as by the significant differences observed between 4â and 6âkHz profiles. In general, however, lateral consistency has been adequate in most areas surveyed to permit good estimates of acoustic attenuation from returns from dipping reflectors and sediment wedges
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