Near-surface Velocity Research Articles

Wave Front Steepness and Influence on Horizontal Deck Impact Loads

In design storm sea states, wave-in-deck forces need to be analysed for fixed and floating offshore platforms. Due to the complex physics of wave impact phenomena, numerical analyses should be complemented by model test data. With a large statistical variability, such experiments usually involve running many 3-h storm realisations. Efforts are being done to establish efficient procedures and still obtain improved statistical accuracy, by means of an initial simplified screening based on parameters derived from the incident wave record only. Here, we investigate the vertical rise velocity of the incident wave elevation at a fixed point in space, which indirectly measures both the local slope and the near-surface orbital velocity. A derived simple deck slamming model is also suggested, to support the check of the physical basis of the approach. Correlation against data from a GBS wave-in-deck model test is used for checking this model. The results show that, although there is a significant random scatter in the measured impact forces, especially in the local slamming forces but also in the global forces, there is a correlation to the rise velocity. Comparisons to the simple load model also show promising results when seen on background of the complex physics and random scatter of the impact problem.

Evaluating Implementations of the Immersed Boundary Method in the Weather Research and Forecasting Model

AbstractThe terrain-following coordinate system used by many atmospheric models can cause numerical instabilities due to discretization errors as resolved terrain slopes increase and the grid becomes highly skewed. The immersed boundary (IB) method, which does not require the grid to conform to the terrain, has been shown to alleviate these errors, and has been used successfully for high-resolution atmospheric simulations over steep terrain, including vertical building surfaces. Since many previous applications of IB methods to atmospheric models have used very fine grid resolution (5 m or less), the present study seeks to evaluate IB method performance over a range of grid resolutions and aspect ratios. Two classes of IB algorithms, velocity reconstruction and shear stress reconstruction, are tested within the common framework of the Weather Research and Forecasting (WRF) Model. Performance is evaluated in two test cases, one with flat terrain and the other with the topography of Askervein Hill, both under neutrally stratified conditions. WRF-IB results are compared to similarity theory, observations, and native WRF results. Despite sensitivity to the location at which the IB intersects the model grid, the velocity reconstruction IB method shows consistent performance when used with a hybrid RANS/LES surface scheme. The shear stress reconstruction IB method is not sensitive to the grid intersection, but is less consistent and near-surface velocity errors can occur at coarse resolutions. This study represents an initial investigation of IB method variability across grid resolutions in WRF. Future work will focus on improving IB method performance at intermediate to coarse resolutions.

A Layer-cell Tomography Method for Near-surface Velocity Model Building Using First Arrivals

Variations in the topography and thickness of the near-surface low-velocity weathering zone negatively impact the quality of onshore seismic data and images. Such impact can be mitigated using accurate near-surface velocity models. We propose to estimate near-surface velocities using a layer-cell tomography method which sequentially combines the model parameterizations of multiscale deformable-layer tomography (DLT) and cell tomography. We first estimate the long-wavelength velocity variations using multiscale DLT, which maps the undulating geometry of the weathering base where the angular coverage of first-arrival raypaths is limited. Then we convert the solution model of multiscale DLT into a cell model, and further determine the short-wavelength velocity variations using cell tomography. We test the proposed layer-cell tomography method using synthetic datasets and a field dataset. The results indicate that the new method improves the resolution of the velocity model from the multiscale DLT, and reduces the artifacts in the velocity solution compared to those observed in the cell tomography method. Reverse time migration of the synthetic data shows that using the velocity model from the layer-cell tomography method gives more accurate images of reflectors. For the field dataset test, the layer-cell tomography method yields a high-resolution near-surface velocity model that could be validated by the improvements in the tomostatic correction results.

Rainfall-Induced Variation of Seismic Waves Velocity in Soil and Implications for Soil Response: What the ARGONET (Cephalonia, Greece) Vertical Array Data Reveal

ABSTRACTWe apply interferometry by deconvolution to compute the shear-wave velocity in shallow sediments (0–83.4 m) based on earthquake records from a vertical accelerometric array (ARGOstoli Network [ARGONET]) on Cephalonia Island, Greece. Analysis of the time variation of measured values reveals a cyclical pattern, which correlates negatively to rainfall and a soil moisture proxy. The pattern includes a sharp reduction in velocity at the beginning of rainy seasons and a gradual rise toward dry periods, the overall variation being around 20%–25% within the shallowest depth interval examined (0–5.6 m) and estimated to reach 40% within the top 2 m. The variation itself and its amplitude are verified by surface-wave dispersion analysis, using ambient vibration data. Synthetic standard spectral ratios suggest that this seasonal effect leaves an imprint on soil response, causing differences in the level of high-frequency ground motion between dry and rainy seasons, and this is verified by earthquake records. Furthermore, the near-surface velocity decrease due to soil saturation can be of the same order of magnitude as the nonlinear coseismic variation, masking the physical process of the nonlinear response of the site due to weak-to-strong-motion shaking. Thus, seasonal variations of seismic-wave velocities in shallow sediments may be important for a number of site-effect related topics, such as high-frequency ground-motion variability, soil anisotropy, kappa measurements, nonlinear site response, and so on.

Combined use of refraction seismic, MASW, and ambient noise array measurements to determine the near-surface velocity structure in the Selinunte Archaeological Park, SW Sicily

Seismic refraction, multichannel analysis of surface waves (MASW) and ambient noise array measurements using the wireless array analysis (WARAN) system were applied to acquire near-surface profiles of seismic velocities in the Selinunte Archaeological Park. This ancient city is famous for numerous temples, which according to the literature, were destroyed by at least two earthquakes in antiquity. The morphology of the archeological park is affected by two rivers which in combination with the temple remains suggests three study sites. We determined the subsurface velocity at these three locations as essential information for further studies of the response of the temple structures to earthquake ground motions. The stratigraphy of the site indicates that low-velocity layers might exist. Seismic refraction profiles with 69 m spread and 24 geophones were employed during the active seismic experiments. The measured P-wave velocities of the top two layers were used as a constraint during the inversion of dispersion relations from the MASW and WARAN data. The reliability of the velocity profiles was tested by forward calculation of synthetic seismograms. P-wave velocities which were not well constraint throughout the dispersion curve inversions were adjusted through suitable Poisson’s ratios based on the well constraint S-velocities. The combined use of the three different kinds of measurements and multi-mode interpretation of the dispersion curves revealed velocity profiles including low-velocity layers which are supported by de-amplification observed in ratios of horizontal and vertical components of noise spectra.

Joint inversion of Rayleigh and Love wave dispersion curves for improving the accuracy of near-surface S-wave velocities

High-frequency surface-wave techniques are extensively employed for the determination of S-wave velocities of near-surface materials. The absence of the measured phase velocity dispersion curves at specific frequency band, however, is common in the real world, which may cause inaccurate S-wave velocity by inversion. Additionally, surface-wave phase velocities are relatively insensitive to S-wave velocities under the abnormal high or low velocity layer, so it is difficult for surface-wave techniques to obtain S-wave velocities in irregular models which contain an (or several) abnormal layer(s). Based on significantly different sensitivity frequency bands of Rayleigh and Love wave phase velocities, a joint inversion method of Rayleigh and Love wave dispersion curves was proposed by the conjugate gradient algorithm. We used synthetic models to test the effectiveness of the developed method. Numerical modeling results revealed that: (1) S-wave velocities acquired by joint inversion of Rayleigh and Love wave phase velocities were more accurate than that by individual inversion in the case of lacking surface-wave dispersion data; (2) joint inversion efficiently improved the accuracy of S-wave velocities of irregular models. A real-world example verified the validity in application of the proposed approach.

Observations of mean and wave orbital flows in the ocean’s upper centimetres

Sophisticated measurements of fluid velocity near to an undulating air–water boundary have traditionally been confined to the laboratory setting. Developments in camera technology and the opening of novel modes of analysis have allowed for sensitive measurements of the current profile in the ocean’s uppermost layer. Taking advantage of the Research Platform R/P FLIP as a ‘laboratory at sea’, here we present first-of-their-kind thermal and polarimetric camera-based observations of wave orbital velocities and mean shear flows in the upper centimetres of the ocean surface layer. Measurements reveal a well-defined logarithmic layer as seen in laboratory measurements and described by classical surface layer theory; however, substantial spread of observations is found at low levels of wind forcing, where the Stokes drift of swell may have a substantial impact on the near-surface current profile. A novel application of short time window Fourier transforms allows for the estimation of near-surface wave orbital velocity magnitudes. These are found to be in general agreement with the prescriptions of linear wave theory, although observations diverge from theory at high levels of wind forcing where the interface is subject to surface wave breaking. Finally, the surface gravity wave phase-coherent short wave growth is presented and discussed in the context of hydrodynamic wave and airflow modulation.

CFD simulation of the wind field over a terrain with sand fences: Critical spacing for the wind shear velocity

Abstract Sand fences are often erected to reduce wind speed, prevent aeolian soil focus our investigation on the behaviour of the near-surface wind shear velocity, which is defined here using a simple model based on wind speed values at height 20 % of the fence height. Our simulations reveal novel insights about the three-dimensional structure of the turbulent wind flow between fences that are important for developing a theoretical description of this flow. We find that the area of soil associated with values of near-surface wind shear velocity that are below the minimal threshold for sand transport has two regimes, depending on the spacing L x between the fences. When L x is smaller than a critical value L xc , the wake zones associated with each fence are inter-connected (regime A), while these wake zones appear separated from each other (regime B) when L x exceeds this critical value of spacing. The system undergoes a second order phase transition at L x = L xc , with the cross-wind width of the protected zone scaling with 1 - L x / L xc β in regime A, with β ≈ 0.32 . Moreover, we also perform a comparative investigation of arrays of constant and multiple fence heights on the basis of the two-dimensional shear velocity field obtained from our simulations. Our findings have implication for a better understanding of aeolian transport in the presence of sand fences, as well as to develop optimization strategies for measures to protect soils from wind erosion.

Estimating Near-Surface Shear-Wave-Velocity Structures Via Multichannel Analysis of Rayleigh and Love Waves: An Experiment at the Boise Hydrogeophysical Research Site

Surface-wave analysis has been widely used for near-surface geophysical and geotechnical studies by using the dispersive characteristic of surface waves (Rayleigh or Love waves) to determine subsurface model parameters. Unlike Rayleigh waves, the dispersive nature of Love waves is independent of P-wave velocity in 1D models, which makes Love-wave dispersion curve interpretation simpler than Rayleigh waves. This reduces the degree of nonuniqueness leading to more stable inversion of Love-wave dispersion curves. To estimate the near-surface shear-wave velocities (Vs) using multichannel analysis of Rayleigh (MASW) and Love waves (MALW) for hydrologic characterization, we conducted an experiment at the Boise Hydrogeophysical Research Site (BHRS, an experimental well field located near Boise, Idaho, USA). We constructed the pseudo-3D velocity structures at the BHRS using both the MASW and MALW methods and compared the results to borehole measurements. We used the 3D Vs distribution to identify and resolve the extent of a relatively low-velocity anomaly caused by a sand channel. The Vs structure and anomaly boundaries were delineated at the meter scale and confirmed by the ground-penetrating radar surveys. The differences in shear-wave velocity determined by MASW, MALW and borehole measurements were discussed and interpreted to reflect the near-surface anisotropy associated with the hydrologic characteristics at the BHRS. Our results demonstrated that the combination of MALW and MASW can be a powerful tool for near-surface characterization.

Near-real-time near-surface 3D seismic velocity and uncertainty models by wavefield gradiometry and neural network inversion of ambient seismic noise

With the advent of large and dense seismic arrays, novel, cheap, and fast imaging and inversion methods are needed to exploit the information captured by stations in close proximity to each other and produce results in near real time. We have developed a sequence of fast seismic acquisition for dispersion curve extraction and inversion for 3D seismic models, based on wavefield gradiometry, wave equation inversion, and machine-learning technology. The seismic array method that we use is Helmholtz wave equation inversion using measured wavefield gradients, and the dispersion curve inversions are based on a mixture of density neural networks (NNs). For our approach, we assume that a single surface wave mode dominates the data. We derive a nonlinear relationship among the unknown true seismic wave velocities, the measured seismic wave velocities, the interstation spacing, and the noise level in the signal. First with synthetic and then with the field data, we find that this relationship can be solved for unknown true seismic wave velocities using fixed point iterations. To estimate the noise level in the data, we need to assume that the effect of noise varies weakly with the frequency and we need to be able to calibrate the retrieved average dispersion curves with an alternate method (e.g., frequency wavenumber analysis). The method is otherwise self-contained and produces phase velocity estimates with tens of minutes of noise recordings. We use NNs, specifically a mixture density network, to approximate the nonlinear mapping between dispersion curves and their underlying 1D velocity profiles. The networks turn the retrieved dispersion model into a 3D seismic velocity model in a matter of seconds. This opens the prospect of near-real-time near-surface seismic velocity estimation using dense (and potentially rolling) arrays and only ambient seismic energy.

Ambient noise analysis for characterizing sub-surface dynamic parameters

Ambient noise analysis of horizontal to vertical spectral ratio (HVSR) method has been used widely to provide reliable estimates of the site fundamental frequency and to constrain the inversion of near-surface shear wave velocity. The present paper focuses on the site measurement using the aforementioned analysis by means of the HVSR method for characterizing sub-surface dynamic parameters in the City of Banda Aceh, Indonesia. A Guralp CMG-6TD broadband seismometer was used in this study to cover a wide frequency range from 0.033 Hz to 50 Hz in standard operation. The instrument was deployed at two different sites (i.e. Location#1 of Blang Padang and Location#2 of Stadion Dirmutala) in the City of Banda Aceh for at least 2 hours for ambient noise recording. This continuous of 2 hours’ microtremor time series was separated into 30 minutes record from which the site fundamental frequency and shear wave velocity of the measured site were deduced. The later sub-surface dynamic parameter was validated using another technique of reflection seismic. This investigation suggests the fundamental frequency of 0.45 Hz at Location#1 and of 0.65 Hz at Location#2. The estimated shear wave velocity of the top 30 m, Vs,30 of this investigation is 165 m/s at Location#1 and 156 m/s at Location#2. Both the site fundamental frequency and shear wave velocity are important for infrastructure design in the high seismic region of Banda Aceh, Indonesia.

Prestack time migration from 3D irregular surfaces with near-surface-related deabsorption

We have developed a 3D prestack time migration (PSTM) approach that can directly migrate nonplanar data with near-surface-related deabsorption using three effective parameters. The proposed scheme improves the so-called topography PSTM approach by adding a near-surface effective [Formula: see text] parameter that compensates for the absorption and dispersion of waves propagating through near-surface media. The two effective velocity parameters above and below the datum can be estimated by flattening events in imaging gathers, and the additional near-surface effective [Formula: see text] parameter can be obtained using scanning technology. Hence, no knowledge with respect to near-surface media is needed in advance for implementing the proposed scheme. The proposed topography-deabsorption PSTM method can be applied to seismic data recorded on a 3D irregular surface without statics corrections. Consequently, traveltimes are obtained with improved accuracy because the raypath bends away from the vertical in the presence of high near-surface velocities, and the absorption and dispersion caused by strong intrinsic attenuation in near-surface media are correctly compensated. Moreover, we attenuated the migrated noise by smearing each time sample only along the Fresnel zone rather than along the entire migration aperture. As a result, an image with a higher resolution and superior signal-to-noise ratio is achieved. The performance of the proposed topography-deabsorption PSTM scheme has been verified using synthetic and field data sets.

Ambient Noise Tomography With Common Receiver Clusters in Distributed Sensor Networks

Near-surface imaging with distributed sensor networks (DSN) is promising for planet exploration, which affordably generates a near-surface velocity model. Recently, an Eikonal tomography-based ambient noise seismic imaging (ANSI) algorithm was implemented in a DSN to realize real-time and in-situ near-surface imaging. However, only using data from neighbors to generate a velocity map cannot have enough stacking samples to generate high-quality results. Also, the neighbor range increase will result in the exponential rise of communication costs. To overcome this problem, we propose a new decentralized Eikonal tomography algorithm in the DSN. The main idea is to change the source-based algorithm to a receiver-based one, which we call common receiver decentralized Eikonal tomography (CR-TomoEK). With CR-TomoEK, nodes fully utilize signals from neighbors to generate partial velocity maps, when combined, lead to the final output. When compared with the original Eikonal algorithm, the stacking number is significantly increased, output quality is higher than before, and there is a significant reduction in communication cost. We performed experiments on both synthetic data and real data from the USArray Transportable Array. Both imaging quality and communication cost are considered in the algorithm validation. The result shows that our algorithm significantly increases the output quality while keeping the communication cost safe to generate a real-time result.

Ground-Motion Analysis of Hydraulic-Fracturing Induced Seismicity at Close Epicentral Distance

ABSTRACT The application of seismic hazard analysis methods developed for natural earthquakes can provide an effective framework for managing risks of induced seismicity (IS) sequences, particularly for assessment of potential risk to nearby infrastructure. Among various factors, the reliability of these methods depends on the accuracy of the ground-motion prediction equation (GMPE)—especially at close epicentral distances where ground motions are expected to be highest. In addition, potential impacts on local infrastructure can be assessed based on ground-motion-derived intensity values, which provide a basis for some traffic-light protocols that are used for managing fluid IS. GMPEs in many areas of the world, however, are poorly calibrated at close epicentral distances, because the availability of near-source seismograph stations is generally very sparse. This study investigates ground motions generated by an IS sequence, up to local magnitude (ML) 3.77 that occurred during a hydraulic-fracturing stimulation in the Duvernay shale play in central Alberta, western Canada. The sequence was monitored using a near-surface array that was located directly above the hydraulically fractured horizontal wells, providing accurate ground-motion measurements at hypocentral distances <10 km. The local array consisted of a combination of geophones cemented in shallow wellbores to depth of ∼27 m, shallow buried broadband seismometers, and a strong-motion accelerometer. This configuration enabled direct measurement of near-surface seismic velocities in the top 27 m, which provided robust VS30 data used to correct observed ground motions for local site-amplification effects. Our data set extends previous analyses in this region by providing new measurements very close to the epicenters of moderate earthquakes and shows that a recently developed GMPE provides accurate near-source ground-motion estimates.

Circulation in Tasman-Golden bays and Greater Cook Strait, New Zealand

ABSTRACT Analyses are presented of model simulations and Lagrangian drifter experiments designed to quantify circulation and dispersal in Tasman and Golden Bays, and their connection to Cook Strait, New Zealand. Observations made in autumn 2017 and summer 2018 are compared with numerical model results. The drifter experiments were focussed on three repeat experiments in each bay designed to measure the local horizontal eddy diffusivity. An additional experiment investigated the connection with, and flow through, Greater Cook Strait. Point by point velocities extracted from the model show low correlation with, but on average are in the same direction as observed velocities. Eddy diffusivities computed from the drifter experiments are highly variable, but the observations support the modelling. We find that the mean velocity in Tasman and Golden Bays is weak and the circulation is dominated by wind and tidal flows. The mean depth-averaged velocity shows anticyclonic flows in the south of both Tasman and Golden Bays, with cyclonic flow in the north of Golden Bay. The mean near-surface velocity is generally out of each bay and is compensated for by inward circulation at depth.

High-Frequency Time-Lapse Seismic Spatial Autocorrelation Imaging Shallow Velocity Variations

Shallow velocity variations can be caused by different reasons, which could be related to infrastructure security. Among seismic-based temporal velocity analysis methods, ambient-noise-based spatial autocorrelation (SPAC) provides the finest shallow imaging resolution. We present a continuous time-lapse SPAC (tSPAC) approach to retrieve the shallow velocity variations. In our field application, due to soil moisture changes caused by water leakage, the seismic velocity changes over the time. Based on the SPAC method, we use the extracted surface wave (Rayleigh wave) to estimate the shear wave velocity as a function of depth. Using a time-lapse manner, we demonstrate that velocity variations due to water leakage can be detected from passive ambient seismic noise. The field deployment results agree well with the ground truth of experiment setups. The success of our study demonstrates that the in situ near-surface seismic velocity can be accurately imaged by tSPAC. This technique can be used to monitor seismic velocity change and further investigate not only the fluid saturation, but also other associated changing conditions, such as stress and temperature.

Impact of the Electromagnetic Brake Position on the Flow Structure in a Slab Continuous Casting Mold: An Experimental Parameter Study

Flow measurements are performed in a slab model for continuous casting of steel under the influence of a ruler type Electromagnetic Brake (EMBr). The Mini-LIMMCAST facility utilizes the low melting GaInSn alloy for flow modeling. Two-dimensional velocity distributions in the center plane of the rectangular mold with a cross section of $$300 \times 35\,\hbox {mm}^{2}$$ are determined by means of the ultrasound Doppler velocimetry. This study especially focuses on the influence of the vertical position of the EMBr and its magnetic flux density as well as the effect of different immersion depths of the submerged entry nozzle. The horizontal flow velocity just below the free surface can effectively be reduced by choosing an optimal position of the EMBr, while an improper positioning even increases the near-surface velocity compared to the case without activated brake. A general braking effect of the EMBr on the submerged jet is not observed. The decisive mechanism for controlling the near-surface flow results from a modification of the jet geometry and a reorganization of the flow field. In terms of an effective flow control an appropriate positioning of the EMBr has at least the same significance as the regulation of the magnetic field strength.

Joint refraction traveltime tomography and migration for multilayer near-surface imaging

In near-surface velocity structure estimation, first-arrival traveltime tomography tends to produce a smooth velocity model. If the shallow structures include a weathering layer over high-velocity bedrock, first-arrival traveltime tomography may fail to recover the sharp interface. However, with the same traveltime data, refraction traveltime migration proves to be an effective tool for accurately mapping the refractor. The approach downward continues the refraction traveltime curves and produces an image (position) of the refractor for a given overburden velocity model. We first assess the validity of the refraction traveltime migration method and analyze its uncertainties with a simple model. We then develop a multilayer refraction traveltime migration method and apply the migration image to constrain traveltime tomographic inversion by imposing discontinuities at the refraction interfaces in model regularization. In each subsequent iteration, the shape of the migrated refractors and the velocity model are simultaneously updated. The synthetic tests indicate that the joint inversion method performs better than the conventional first-arrival traveltime tomography method with Tikhonov regularization and the delay-time method in reconstructing near-surface models with high-velocity contrasts. In application to field data, this method produces a more accurately resolved velocity model, which improves the quality of common midpoint stacking by making long-wavelength static corrections.

FWI as an effective solution for land near-surface model building into the area with complex geological settings: Eastern Siberia case study

Seismic imaging in the Eastern Siberia region is challenging due to the presence of complex overburden anomalies such as magmatic intrusions and thrusts in the suprasalt complex leading to significant vertical and lateral property variations of high-velocity halite-carbonate geological sections with multiple velocity inversions, complex shallow stratigraphy and substantial topography. Such geological settings create amplitude dimming and wipeouts in the images and cause structural distortions at the reservoir level. We applied full-waveform inversion (FWI) technology to overcome limitations of the conventional workflow and to support the exploration programme. The primary output of FWI is a high-resolution subsurface velocity model, which can be used as an attribute to improve interpretation of the overburden sediments, to identify shallow hazards, to derisk exploration prospects and to optimize well placement. We tested FWI on a conventional 3D land data set to create a near-surface velocity model. Detailed subsurface models generated with FWI provided an accurate description of the overburden and helped us to investigate shallow geological environments. Predrill predictions using the FWI velocity volume showed excellent agreement with well measurements and made it possible to recommend the technology for inclusion in the processing flow.

Research of near-wall thermodynamic state for indoor airflow over the vertical heating unit using TIV/PIV/RTD

Up to now, few studies focus on thermodynamic state including the velocity and temperature of the air near heating unit. The thermodynamic state of the airflow over an indoor heating unit has a significant influence on indoor thermal comfort and energy consumption. This study analyzed the thermal and dynamic state of the near-wall airflow over the heating unit. The thermal state was measured using resistance thermal detectors (RTDs). The near-wall airflow fields were measured by particle image velocimetry (PIV) and thermal image velocimetry(TIV). The performance of TIV in natural and mixed convection was evaluated by comparing the TIV and PIV measurement results. Under natural convection, the velocity shows vertical variation and the spatial difference changes more pronounced with the increase of heating temperature. Under mixed convection, the near-wall temperature changes uniform and the velocity exhibits a decreasing trend with the increase of height. Through the spectrum analysis of the temperature, it is found that the velocity measured by TIV is close to the velocity near the boundary layer to some content. The positions of the near-surface velocity measured by TIV are not fixed in all cases and change with the change of the boundary layer. The findings in this study can provide a convenient and feasible flow field measurement method suitable for actual space scale. This method can predict the effect of heating terminal units on indoor airflow and thermal environment, so as to optimize the form and arrangement of the heating terminal units, and improve heating efficiency and occupants’ thermal comfort.