In the last 30 days

• Open/close Statistical correlations of shear wave velocity and penetration resistance for soils

  Ünal Dikmen 2009 J. Geophys. Eng. 6 61 Tag this article

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  In this paper, the correlation between shear wave velocity and standard penetration test blow counts (SPT- N) is investigated. The study focused primarily on the correlation of SPT- N and shear wave velocity ( V _s) for several soil categories: all soils, sand, silt and clay-type soils. New empirical formulae are suggested to correlate SPT- N and V _s, based on a dataset collected in a part of Eskişehir settlement in the western central Anatolia region of Turkey. The formulae are based on geotechnical soundings and active and passive seismic experiments. The new and previously suggested formulae showing correlations between uncorrected SPT- N and V _s have been compared and evaluated by using the same dataset. The results suggest that better correlations in estimation of V _s are acquired when the uncorrected blow counts are used. The blow count is a major parameter and the soil type has no significant influence on the results. In cohesive soils, the plasticity contents and, in non-cohesive soils except for gravels, the graded contents have no significant effect on the estimation of V _s. The results support most of the conclusions of earlier studies. These practical relationships developed between SPT- N and V _s should be used with caution in geotechnical engineering and should be checked against measured V _s.

• Open/close Carbonate reservoir characterization with lithofacies clustering and porosity prediction

  Abdulrahman Al Moqbel and Yanghua Wang 2011 J. Geophys. Eng. 8 592 Tag this article

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  One of the objectives in reservoir characterization is to quantitatively or semi-quantitatively map the spatial distribution of its heterogeneity and related properties. With the availability of 3D seismic data, artificial neural networks are capable of discovering the nonlinear relationship between seismic attributes and reservoir parameters. For a target carbonate reservoir, we adopt a two-stage approach to conduct characterization. First, we use an unsupervised neural network, the self-organizing map method, to classify the reservoir lithofacies. Then we apply a supervised neural network, the back-propagation algorithm, to quantitatively predict the porosity of the carbonate reservoir. Based on porosity maps at different time levels, we interpret the target reservoir vertically related to three depositional phases corresponding to, respectively, a lowstand system tract before sea water immersion, a highstand system tract when water covers organic deposits and a transition zone for the sea level falling. The highstand system is the most prospective zone, given the organic content deposited during this stage. The transition zone is also another prospective feature in the carbonate depositional system due to local build-ups.

• Open/close Extracting near-borehole P and S reflections from array sonic logging data

  Wang Bing et al 2011 J. Geophys. Eng. 8 308 Tag this article

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  There has been widespread interest in acoustic reflection logging recently due to its higher resolution compared to seismic exploration and larger lateral extension compared to logging techniques. Acoustic reflection logging is particularly useful for delineating near-borehole subtle structures and fractured reservoirs. The primary data processing of acoustic reflection logging is to extract the reflection signals from the full waveform records which are overwhelmingly dominated by the borehole direct modes. We present in this paper a method combining a frequency–wavenumber ( f–k) filter with a modified parametric prediction to separate the direct and reflection waves. Afterwards, we apply dip stacking to enhance and to further separate P–P and S–S reflection signals, and suppress the residual direct waves and other noises. Processing examples obtained with synthetic and real data show the capabilities of the proposed workflow and algorithms.

• Open/close Discovery, mapping and interpretation of buried cultural resources non-invasively with ground-penetrating radar

  Lawrence B Conyers 2011 J. Geophys. Eng. 8 S13 Tag this article

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  Ground-penetrating radar is an extremely useful tool for the mapping and interpretation of buried cultural remains within 2–3 metres of the surface, especially when the stratigraphy is complex. Standard reflection profiles can be processed to correct for depth and distance, and also filtered and processed to make cultural features visible. When many profiles are collected in closely spaced transects in a grid, reflections can be re-sampled and displayed in amplitude slice-maps, and isosurface renderings to make buried features visible. Sometimes, however, the abundance and complexity of subsurface reflections is so complex that each individual profile must be interpreted manually, which necessitates an understanding of radar wave propagation, reflection, refraction and attenuation in the ground. In order to differentiate reflections from cultural features this understanding of radar energy must be merged with an understanding of the chemistry of the ground, soil and geological stratigraphy, and how those variables affect radar reflections. When taken as a package of visualization tools, GPR can be used as an effective tool for interpreting aspects of history and culture at buried sites in ways not possible using traditional archaeological methods.

• Open/close Elastic impedance variation with angle inversion for elastic parameters

  Zhaoyun Zong et al 2012 J. Geophys. Eng. 9 247 Tag this article

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  Elastic impedance (EI) and amplitude variation with offset or angle (AVO/AVA) inversion are two cardinal methods to estimate elastic parameters underground with reflection seismic data. Conventional EI inversion as a kind of pre-stack and post-stack joint inversion method has been widely applied in the industry because of its high efficiency and high stability of wavelet extraction; however, the robustness of extracting elastic parameters in conventional EI inversion is still controversial. The robustness of three-term AVO inversion has improved a lot; however, it is still challenging to extract reasonable space variant wavelets for each offset or incident angle. In this paper, a robust three-parameter estimation method, named elastic impedance variation with angle (EVA) inversion, is proposed in the Bayesian framework, which can estimate elastic parameters directly from EI. This method supposes that the parameters to be inverted are Cauchy distributed and it is implemented based on a normalized EI equation in a logarithmic domain which can reduce the nonlinearity of inversion. Application of a covariance matrix to decorrelate the parameters and constraint of well log curves introduced in an objective function enhances the robustness of EVA inversion. A model test shows that the proposed EVA inversion method enables one to estimate reasonable elastic parameters with extremely smooth initial models and moderate Gaussian noise. A real data example shows that the inverted P-wave velocity, S-wave velocity and density are identical to well log interpretation results, which shows the validity of the proposed method.

• Open/close Spatial spectral variations of microtremors and electrical resistivity tomography surveys for fault determination in southwestern Crete, Greece

  M Moisidi et al 2012 J. Geophys. Eng. 9 261 Tag this article

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  The horizontal to vertical spectral ratio (HVSR) technique using microtremors and electrical resistivity tomography (ERT) surveys reveal a potentially seismic active source in southwestern Crete located within the outer forearc of the Hellenic subduction zone in one of the most seismically active deformed regions in Europe. The combined approach is applied on the Pahia Ammos coast southwest of the Paleohora peninsula and reveals an almost E–W-striking fault crosscutting the dense populated area. Spatial HVSR variations in the fundamental frequencies and HVSR shapes using microtremors pattern the effects of surface and subsurface structure on seismic ground motion and are capable of delineating fault zones. One clear HVSR peak in the low frequencies is related to the thickness of the alluvial deposits. Two amplified frequencies are attribute to lateral heterogeneities/irregularities induced by the fault zone and thickness variations of the geological column overlying the lateral irregularities of near-subsurface structure. Dipole–dipole and Wenner–Schlumberger configuration arrays are conducted to model the surface and subsurface structure variations. The identified fault zone striking E–W inland is capable of enhancing ground seismic motion and significantly contributes to the seismic hazard assessment of the studied area. Geophysical results are cross-correlated, verifying the validity of the research outcome.

• Open/close P-wave attenuation and dispersion in a porous medium permeated by aligned fractures—a new poroelastic approach

  Jixin Deng et al 2012 J. Geophys. Eng. 9 115 Tag this article

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  An alternative method is advocated to obtain the analytical expressions of pore pressure, relative flux distribution and frame displacement in a periodic layered porous medium (White model) based on quasi-static poroelastic theory, from which the expression of the effective medium model for a P-wave propagating vertical to the layer can be obtained. Then a dispersion equation for a P-wave propagating in the porous medium permeated by aligned fractures is also presented by considering fractures as thin and highly compliant layers, with which the influence of mesoscopic fluid flow on phase velocity dispersion and attenuation is discussed under the condition of varying fracture weakness and the length scale ratio of the background layer to the fracture layer. For both cases, fluid flow induced by the Biot slow wave can result in apparent attenuation and dispersion in the usual seismic frequency band (5–500 Hz). The magnitude of velocity dispersion and attenuation of the P-wave will increase with increasing fracture weakness or with decreasing length scale ratio, and also relaxation peak and maximum attenuation move towards low frequencies. In particular, the evolution of pore pressure and relative fluxes as a function of frequency in the medium for those two cases is present, which gives us a more physical understanding of the behaviours of P-wave velocity dispersion and attenuation due to wave-induced mesoscopic flow.

• Open/close An unsplit complex-frequency-shifted PML based on matched Z-transform for FDTD modelling of seismic wave equations

  Ruiqi Shi et al 2012 J. Geophys. Eng. 9 218 Tag this article

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  The perfectly matched layer (PML) is an efficient absorbing boundary condition for mitigating undesired reflections in seismic wave modelling. However, the performance of the conventional split-field PML (S-PML) is severely reduced at grazing incidence. Very low frequency waves and evanescent waves can also cause spurious reflections on the PML interface. Such problems could be circumvented by using the complex-frequency-shifted (CFS) stretching tensor in the PML. However, it is difficult to adopt the CFS stretching tensor in the S-PML. A matched Z-transform PML (MZT-PML) technique applied in electromagnetic modelling results in a complete unsplit-field form. In this paper, we show that the MZT-PML can easily implement the CFS stretching tensor and be adopted for elastic wave finite-difference time-domain modelling. The numerical tests in an elongated model illustrate that the complex-frequency-shifted matched Z-transform perfectly matched layer (CFS MZT-PML) can substantially improve performance by eliminating the spurious reflections at grazing incidence. The numerical stability of the CFS MZT-PML is verified by long time computation of the total energy. Also, we conduct a numerical test in a two-layer heterogeneous model showing that the CFS MZT-PML can also be used to simulate efficiently the wave propagation in more complex structures.

• Open/close Strategy for the detection of vertical movements in historical environments from fast high-precision GPS measurements

  Arianna Pesci et al 2012 J. Geophys. Eng. 9 230 Tag this article

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  A continuous global positioning system station (CGPS) provides accurate coordinate time series, while episodic GPS stations (EGPSs), which operate throughout short measurement sessions, are generally used to improve the monitoring spatial density. In an urban environment, EGPSs are typically equipped with removable mounts (topographical tripod or bipod). In this paper, a method is proposed to evaluate vertical surface motions by means of differential measurements of removable mount EGPSs with respect to a nearby reference CGPS. For each day, the correct position of this CGPS is used as reference for the quick differential EGPS measurements to allow the correction of their positions. The method is applied to evaluate subsidence in the centre of Bologna, which is characterized by significant vertical movements, probably related to seasonal climatic effects, and where these movements differ significantly even among closely spaced locations.

• Open/close A hybrid approach for tomographic inversion of crosshole seismic first-arrival times

  G Göktürkler 2011 J. Geophys. Eng. 8 99 Tag this article

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  A sequential hybrid approach was presented here to invert crosshole seismic first-arrival times. The proposed tomographic scheme combined a simple simulated annealing algorithm with a linearized smoothness-constrained least-squares inversion. The simulated annealing was implemented to obtain a background velocity distribution used by the linearized inversion for the initial guess. The linearized component was based on the functional description of traveltimes. This indicates a nonlinear function, the eikonal equation, providing traveltimes for a given slowness model. Thus an explicit ray tracing was not required by the linearized scheme. The velocity updates were obtained by a matrix inversion based on an iterative conjugate gradient-like LSQR algorithm. Second-difference regularization was used to stabilize the solutions. The Jacobian matrix giving the partial derivatives with respect to the model parameters was constructed by a finite-difference approximation based on the perturbation of the cell slowness. The traveltimes for both the hybrid and linearized schemes were calculated by a fast finite-difference eikonal solver. The hybrid scheme was tested by using both synthetic and field data sets based on the crosshole geometry. According to the tests studies, the tomograms resulted from the hybrid approach better imaged the subsurface velocity distribution. Also the hybrid optimization was characterized by quicker convergence rate than the conventional optimization based on only the linearized inversion. The tests with the synthetic data set also showed that the hybrid approach yielded a solution having lower rms residual, smaller Euclidean distance and lower relative errors in the cell velocities.