1,721,010 research outputs found
Real–Time Imaging of Decimetre–Resolution 3D Seismic Volumes
Full text linkThe 3D Chirp sub–bottom profiler acquires a true 3D seismic volume with decimetric horizontal and centimetric vertical resolutions, providing an ideal platform for shallow–water engineering, archaeology, military, and geological studies. In this thesis, I show how simple processing flows built around a combination of standard Chirp/Vibroseis techniques and well known industry methods produce effective and impressive results by considering an object identification case study in a shallow–water, harbour setting (Vardy et al., 2008). Both stacked and migrated volumes are used to identify 89 individual buried targets that are correlated with coincident objects. Through subsequent dredging, a 100 % detection success is demonstrated, along with the strong similarity between the migrated reflector morphology and co–incident object shape. However, this processing approach requires extensive manual input and very long processing times (? 1 month).For this reason, a new method for pre–stack 3D Kirchhoff imaging is developed. Correlation with a series of bandwidth limited theoretical source sweeps is used to frequency decompose the raw traces for pre–stack time migration using a constant velocity. By accommodating dispersion through imaging a series of band limited traces, rather than through Fourier Transform, processing times are reduced from ? 1 month to c. two days for the object detection volume (i.e., approaching real–time application). The effectiveness of this new algorithm is examined using several synthetic volumes, allowing the degenerative effects of gaps in the fold to be explored. Finally, the application of the 3D Chirp system to geological cases is demonstrated through the geomorphological mapping of a sequence of mass movement events in Windermere, UK Lake District. Three mass movement deposits are identified in a 100 m by 400 m survey area. Through mapping of the package distributions and their interaction with the pre–existing sediments stratigraphy, they are identified as Younger Dryas climate amelioration deposits, resulting from the rapid deposition of gravitationally unstable, unconsolidated sediments. A metre–scale structural interpretation allows the depositional regimes (two being debris flow and the third a mass flow deposit) and dominant transport directions to be inferred
Deriving shallow-water sediment properties using acoustic impedance inversion
No full textIn contrast to the application of marine seismic reflection techniques to reservoir-scale applications, where seismic inversion for quantitative sediment analysis is common, shallow-water, very-high-resolution seismic reflection data are seldom used for more than qualitatively reflection interpretation. Here, a quantitative analysis of very-high-resolution marine seismic reflection profiles from a shallow-water (<50 m water depth) fjord in northern Norway is presented. Acquired using Sparker, Boomer, and Chirp sources, the failure plane of multiple local landslides correlates with a composite reflection that reverses polarity to the south. Using a genetic algorithm, a 1D post-stack acoustic impedance inversion of all three profiles is performed, calibrating against multi-sensor core logger (MSCL) data from cores. Using empirical relationships the resulting impedance profiles are related to remote sediment properties, including: P-wave velocity; density; mean grain size; and porosity. The composite reflector is consistently identified by all three data sources as a finer-grained (by one phi), lower density (c. 0.2 g/cm3 less than background) thin bed, with an anomalous low velocity zone (at least 100 m/s lower than background) associated with the polarity reversal to the south. Such a velocity contrast is consistent with an accumulation of shallow free gas trapped within the finer-grain, less permeable layer. This study represents the first application of acoustic impedance inversion to very-high-resolution seismic reflection data and demonstrates the potential for directly relating seismic reflection data with sediment properties using a variety of commonly used shallow seismic profiling sources
Dataset in support of the Southampton doctoral thesis 'The optimisation of near surface seismic reflection data'
No full textSupporting Data for PhD Thesis.
This is split into the three scientific Chapters:
- Chapter 2 - Modelling the Acquisition Geometry of Near-Surface Marine Seismic Reflection Data
- Chapter 3 - The importance of Velocity Modelling and Uncertainties in Deriving Physical Properties for the Marine Near-Surface
- Chapter 4 - A Quantitative Geophysical Analysis of Amplitude Anomalies in Hollandse Kust West
Within each of these you will find the relevant raw & processed data (if relevant), Parameter Grids, and additional datasets that were used in the thesis and that are able to be shared. </span
Pre-stack full waveform inversion of ultra-high-frequency marine seismic reflection data
Full text linkThe full waveform inversion (FWI) of seismic reflection data aims to reconstruct a detailed physical properties model of the subsurface, fitting both the amplitude and the traveltime of the reflections generated at physical discontinuities in the propagation medium. Unlike reservoir-scale seismic exploration, where seismic inversion is a widely adopted remote characterization tool, ultrahigh-frequency (UHF, 0.2–4.0 kHz) multichannel marine reflection seismology is still most often limited to a qualitative interpretation of the reflections’ architecture. Here we propose an elastic FWI methodology, custom-tailored for pre-stack UHF marine data in vertically heterogeneous media to obtain a decimetric-scale distribution of P-impedance, density and Poisson’s ratio within the shallow subseabed sediments. We address the deterministic multiparameter inversion in a sequential fashion. The complex trace instantaneous phase is first inverted for the P-wave velocity to make up for the lack of low frequency in the data and reduce the nonlinearity of the problem. This is followed by a short-offset P-impedance optimization and a further step of full offset range Poisson’s ratio inversion. Provided that the seismogram contains wide reflection angles (>40°), we show that it is possible to invert for density and decompose a posteriori the relative contribution of P-wave velocity and density to the P-impedance. A broad range of synthetic tests is used to prove the potential of the methodology and highlights sensitivity issues specific to UHF seismic. An example application to real data is also presented. In the real case, trace normalization is applied to minimize the systematic error deriving from an inaccurate source wavelet estimation. The inverted model for the top 15 m of the subseabed agrees with the local lithological information and core-log data. Thus, we can obtain a detailed remote characterization of the shallow sediments using a multichannel sub-bottom profiler within a reasonable computing cost and with minimal pre-processing. This has the potential to reduce the need of extensive geotechnical coring campaigns
Characterization of the slope-destabilizing effects of gas-charged sediment via seismic surveys
No full textFinneidfjord, Norway has a history of submarine slope failure and hosts areas of buried, gas-charged sediment. Using shipboard seismic survey data from Finneidfjord, we illustrate how novel seismic data processing techniques can yield estimates of geotechnical properties of this gas-charged sediment. This data processing involves two steps: 1) estimate the amount of seismic attenuation that a gas-bearing layer creates, and 2) fit a wave attenuation model to the observed attenuation. We accomplish the first step using a wavelet-based spectral ratio approach, and find the seismic quality factor (inversely related to attenuation) from the change in amplitude spectra across the gas layer. The novelty of this paper really comes in the second step, where we create a Bayesian hierarchical model that combines a wave attenuation model with a spatial random (Gaussian) process model. The advantage of this combination is that our estimates of the gas properties not only must conform to the observations of attenuation (quality factor), but must also smoothly vary over space, as one would expect of any natural process. Ultimately, we end up with posterior distributions of gas properties at points throughout our gas-bearing layer. We use these posterior distributions to probabilistically evaluate the role that gas plays in slope failures in Finneidfjord. This method has other applications in natural gas resource exploration and carbon sequestration
Microscale FEA of P-wave propagation through a saturated granular medium-Data set
No full textCode and results for a study into the propagation of P-waves through a saturated granular medium. Code will generated ABAQUS .inp files, which after completion can be post processed to extract P-wave velocity. Results are provided, and code used to produce plots.</span
An emerging tool in the site investigation toolbox: seismic inversion reduces uncertainties in site characterization
No full textThe article offers information on the use of the inversion of seismic reflection data to derive quantitative information about the subsurface for reservoir characterization. It provides an overview about deriving the subsurface conditions that created the observed seismic reflection response as well as other information related to inversion methods
A frequency-approximated approach to Kirchhoff migration
No full textThe integral solution of the wave equation has long been one of the most popular methods for imaging (Kirchhoff migration) and inverting (Kirchhoff inversion) seismic data. For efficiency, this process is commonly formulated as a time-domain operation on each trace, applying antialiasing through high-cut filtering of the operator or pre-/postmigration dip filtering. Migration in the time domain, however, does not allow for velocity dispersion; standard antialiasing methods assume a flat reflector and tend to overfilter the data. We have recast the Kirchhoff integral in the frequency domain, enabling robust antialias filtering through appropriate dip limiting of each frequency and implicit accommodation of true dispersion. Full frequency decomposition of the input seismogram can be approximated by band-pass filtering (or correlation with band-limited source sweeps for Chirp/Vibroseis data) into a few narrow-band traces that cumulatively retain the full source bandwidth. From prior knowledge of the source waveform, we have defined suitable bandwidths to describe broadband (3.0 octaves) data using just six frequency bands. Kirchhoff migration of these narrow-band traces using coefficients determined at their central frequencies significantly improves the preservation of higher frequencies and cancellation of steeply dipping aliased energy over traditional time-domain anti- aliasing methods. If, however, two bands per octave cease to be a robust approach, our frequency-approximated approach provides the processor with ultimate control over the frequency decimation, balancing increased resolution afforded by more bands against computing cost, whereas the number of frequency bands is few enough to permit detailed control over frequency-dependent antialias filtering parameters
Decimetric-resolution stochastic inversion of shallow marine seismic reflection data: dedicated strategy and application to a geohazard case study
Full text linkCharacterization of the top 10–50 m of the subseabed is key for landslide hazard assessment, offshore structure engineering design and underground gas-storage monitoring. In this paper, we present a methodology for the stochastic inversion of ultra-high-frequency (UHF, 0.2–4.0 kHz) pre-stack seismic reflection waveforms, designed to obtain a decimetric-resolution remote elastic characterization of the shallow sediments with minimal pre-processing and little a priori information. We use a genetic algorithm in which the space of possible solutions is sampled by explicitly decoupling the short and long wavelengths of the P-wave velocity model. This approach, combined with an objective function robust to cycle skipping, outperforms a conventional model parametrization when the ground-truth is offset from the centre of the search domain. The robust P-wave velocity model is used to precondition the width of the search range of the multiparameter elastic inversion, thereby improving the efficiency in high-dimensional parametrizations. Multiple independent runs provide a set of independent results from which the reproducibility of the solution can be estimated. In a real data set acquired in Finneidfjord, Norway, we also demonstrate the sensitivity of UHF seismic inversion to shallow subseabed anomalies that play a role in submarine slope stability. Thus, the methodology has the potential to become an important practical tool for marine ground model building in spatially heterogeneous areas, reducing the reliance on expensive and time-consuming coring campaigns for geohazard mitigation in marine areas
Recovering shear stiffness degradation curves from classification data with a neural network approach
No full textShear stiffness is critical in assessing the stress–strain response of geotechnical infrastructure, and is a complex, nonlinear parameter. Existing methods characterise stiffness degradation as a function of strain and require either bespoke laboratory element tests, or adoption of a curve fitting approach, based on an existing data set of laboratory element tests. If practitioners lack the required soil classification parameters, they are unable to use these curve fitting functions. Within this study, we examine the ability and versatility of an artificial neural network (ANN), in this case a feedforward multilayer perceptron, to predict strain-based stiffness degradation on the data set of element test results and soil classification data that underpins current curve fitting functions. It is shown that the ANN gives similar or better results to the existing curve fitting method when the same parameters are used, but also that the ANN approach enables curves to be recovered with ‘any’ subset of the considered soil classification parameters, providing practitioners with a great versatility to derive a stiffness degradation curve. A user-friendly and freely available graphical calculation app that implements the proposed methodology is also presented
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