Theoretical Transfer Function Research Articles

Vibration isolation characteristics of magnetorheological damper for the large wind turbine gearbox

ABSTRACT The gearbox vibration isolation theory in the large wind turbine transmission system is analyzed. The host frame is flexible, and the vibration isolation system is approximately equivalent to a two-degree-of-freedom system. According to large wind turbine gearbox isolation requirement, and a vibration isolation device of magnetorheological damper of the large wind turbine gearbox is designed, the magnetic field characteristics inside the magnetorheological damper are simulated and calculated by using COMSOL, when 0.5, 1, 1.5and 2 A currents are passed through the magnetorheological damper coil, the magnetic induction intensity of the damping gap and the output force of the magnetorheological damper are calculated, it can reach 1300N-5400N, which meets the gearbox vibration isolation requirements. It designed the on-off current control method of the magnetorheological damper. The prototype is tested on a certain type of offshore wind turbine, and the acceleration spectrum signals of the gearbox in the Z direction are collected under the working conditions of 990, 1200 and 2000 r/min. According to the collected data values, the vibration isolation characteristics of the designed magnetorheological damper are analyzed; the data show that the vibration level drop is about 20 dB, and the vibration isolation effect is good. By using the coherence coefficient method to process the measured data, the modified transfer function is obtained, which is consistent with the theoretical transfer function of the vibration model of two-degree-of-freedom system, so it is proved to be reasonable. The vibration isolation characteristics of rubber and the magnetorheological damper are compared at the rated speed, and it shows that the maximum vibration acceleration of the gearbox under the magnetorheological damper reduces from 0.375 m/s2 to 0.175 m/s2, which verify the damping effect of the magnetorheological damper.

The spatial properties of the site amplifications of S-waves by generalized spectral inversion technique and the correction method of the site amplifications considering the contribution of later arrivals after major S-waves

Site amplification is an important component of strong ground motion prediction as it differs among sites, reflecting its specific local subsurface geology. Here, we confirm that site amplifications are similar in a neighborhood area over a long period. However, few studies have investigated the spatial properties in a wide region (i.e., the whole of Japan). In this study, we explored the spatial properties of site amplifications based on the generalized inversion technique (GIT) using Fourier amplitude spectra (FAS) as well as pseudo-velocity response spectra (pSv) as the latter is an important index for engineering purposes and the most similar type of response spectra to FAS. The spatial distributions of S-wave site amplifications (SA-S), especially within large sediment basins (e.g., the Kanto and Osaka Basins in Japan), were found to be relatively similar in proximate areas for a long period ranging from 2 to 8 s. This suggests that we could easily predict the site amplifications using an empirical approach through spatial interpolation based on the properties obtained by the GIT. Furthermore, we propose a prediction procedure for site amplification for the whole duration from the SA-S at an arbitrary site. We used the correction function, which converts the SA-S to the site amplification for the whole duration (SA-W), including an S-wave portion and a subsequent portion. This function is called the whole-duration to S-wave spectral ratio (WSR) and is stable in terms of spatial properties. As we could estimate the SA-S either by theoretical transfer functions or observed microtremors, we can easily predict the SA-W based on the proposed WSR concept. We found that SA-S in pSv is more or less similar to SA-S in FAS, however, SA-W in pSv fails to capture the effects of the long duration of ground motions inside a large basin so that we cannot recommend to use pSv for the prediction of whole duration of ground motion.Graphical

Forward and inverse analysis of transient responses for a cantilevered rectangular plate under normal and oblique impact loadings

This study investigates the transient responses of a cantilevered rectangular aluminum plate under normal and oblique impact loadings. The transient displacement and strain histories of the aluminum plate are forward calculated as a superposition of the normal modes using first-order shear deformation plate theory and the spectral collocation method considering both in-plane and out-of-plane dominant vibrations. A laser displacement meter and strain gauges are used to measure the displacement and strain histories of the same aluminum plate when subjected to normal and oblique loadings. Subsequently, frequency-dependent damping ratios are obtained from the associated wavelet spectrograms. The measured histories correlate well with those estimated using the theoretical analysis considering the damping ratios. Additionally, the inverse problem of determining the impact force history from the transient response is investigated. First, the transfer function between the impact force and measured response is derived using a theoretical analysis developed in the time domain. Subsequently, a wavelet deconvolution method is applied to reconstruct the impact force using a discrete wavelet transform with different scale and shift components. Thus, the impact force is obtained by reconstructing the wavelet expansion coefficients using these components, thereby demonstrating the validity of the wavelet deconvolution technique utilizing the theoretical transfer function.

One-Dimensional Site Response Analysis: Model Error Estimation

ABSTRACT Two essential elements of partially nonergodic probabilistic seismic hazard analysis are the estimation of local site effects and the quantification of their uncertainty. Local site effects usually are evaluated using numerical site response analyses, often assuming the 1D site response. This research addresses two questions regarding the estimation of local site effects: for which stations may 1D site response analysis be beneficial? What is the model uncertainty in 1D site response analysis? In answering these two questions, we compare empirical and theoretical transfer functions (TFs) at KiK-net stations and apply state-of-the-art statistical tools. Regarding the first question, previous research associated the applicability of 1D site response to the topography of the site, geological features, and the shape and frequency of the horizontal-to-vertical spectral ratio (HVSR) peak. Among 22 candidate parameters, we find the combination of topographical position index (also known as relative elevation) and the ratio of theoretical TF predominant frequency over HVSR predominant frequency to be the most efficient predictor. However, the accuracy of predictions made by these two parameters is only 60%, highlighting the need for additional research. To address the second question, we use theoretical and empirical TFs to obtain the model error in 1D site response analyses. Model error uncertainty (in natural log units) in predicting TF ranges from 0.1 at low frequencies up to 0.4 around 10 Hz.

Seismic Characterization of Swiss Strong-Motion Borehole-Station Sites by Inversion of Full Microtremor Horizontal-to-Vertical Spectral Ratios [H/V(z,f)

ABSTRACT The assessment of the local site amplification during an earthquake requires, among other input information, a reliable estimate of the shear-wave velocity profile, including the contact with engineering and seismic bedrocks. We determine the shear-wave velocity (VS) profiles at two Swiss strong-motion borehole-station sites at Visp (Valais) and Buochs (Nidwalden) by inversion of microtremor horizontal-to-vertical spectral ratio [H/V(z,f)] curves measured at the surface and at different depths. These borehole stations were built to monitor not only the seismic activity in Switzerland and the surrounding areas but also the nonlinear site response, especially liquefaction processes during strong local and regional earthquakes. The boreholes are equipped with accelerometers at various depths, with the deepest borehole located at 102 m below the surface. In the first part, we review the forward modeling algorithm of the full-microtremor H/V(z,f), with a focus on the computational cost and accuracy. In the second part, we perform a temporal analysis of the H/V(z,f) curves obtained from the accelerometers. The results show seasonal variabilities in H/V between summer and winter. The third part presents the inversions of the H/V curves for a single day in summer and winter at both sites. From the full H/V(z,f) inversion, we obtain shear-wave velocities in the upper 30 m (VS30) of 216 and 209 m/s at Visp in winter and summer, respectively. At Buochs, the corresponding VS30 are 269 and 345 m/s. The depths of the seismic bedrock are at 219 and 210 m at Visp, and at Buochs they are at 293 and 213 m. The estimated velocity profiles compare well with independent estimates from array measurements of ambient seismic vibrations, gravimetry, and geological logging information. Finally, we use the obtained seismic velocity profile information to model the theoretical 1D shear wave transfer function. The latter result compares well with amplification function results obtained using earthquake recordings.

Ultrasonic characterization of frozen soils using a multiphase poromechanical approach

The ability to quantitatively and non-invasively characterize complex multiphase geomaterials is still a major challenge to the engineering, construction, and geophysical fields. In the context of accelerating climate change, construction on permafrost requires remedial measures and an appropriate characterization of permafrost (e.g., ice content, unfrozen water content, porosity, ice lenses, soil type, and mechanical properties). Current techniques are insufficient for efficient characterization of permafrost samples. Here, we propose an ultrasonic sensing technique and a signal interpretation method based on an inverse multiphase poromechanical approach to overcome critical gaps in permafrost characterization. This technique allows us to interpret the measured electrical signal using a theoretical transfer function obtained through an efficient multiphase poromechanical forward solver. Our study demonstrates the potential of the ultrasonic sensing technique for the rapid characterization of permafrost samples in terms of both physical and mechanical properties.

Transfer function model-based Bayesian estimation of dissipation and sound speed in impedance tube measurements

The air dissipation, mainly due to the boundary effect at interior walls of impedance tubes, is noteworthy for normal incidence measurements of acoustic materials using the transfer function method. This work proposes a transfer function model-based Bayesian approach to estimate the dissipation factors. Other parameters, such as the sound speed and the microphone positions, are also critical, therefore, estimated as well, along with the dissipation. The measurement of the empty impedance tube with a rigid termination applies the two-microphone transfer function method. A hypothetical air layer assumed in front of the rigid backing enables the numerical comparison between the air layer’s theoretical and experimental transfer function values. Experimental results incorporating the estimated dissipation coefficient validate the accuracy of the proposed approach. Furthermore, the estimated parameters are incorporated into further evaluations of material properties using the transfer function method, which effectively reduces the residual absorption error of the impedance tube measurement.

Quantifying the Surface Strain Field Induced by Active Sources with Distributed Acoustic Sensing: Theory and Practice.

Quantitative dynamic strain measurements of the ground would be useful for engineering scale problems such as monitoring for natural hazards, soil-structure interaction studies, and non-invasive site investigation using full waveform inversion (FWI). Distributed acoustic sensing (DAS), a promising technology for these purposes, needs to be better understood in terms of its directional sensitivity, spatial position, and amplitude for application to engineering-scale problems. This study investigates whether the physical measurements made using DAS are consistent with the theoretical transfer function, reception patterns, and experimental measurements of ground strain made by geophones. Results show that DAS and geophone measurements are consistent in both phase and amplitude for broadband (10 s of Hz), high amplitude (10 s of microstrain), and complex wavefields originating from different positions around the array when: (1) the DAS channels and geophone locations are properly aligned, (2) the DAS cable provides good deformation coupling to the internal optical fiber, (3) the cable is coupled to the ground through direct burial and compaction, and (4) laser frequency drift is mitigated in the DAS measurements. The transfer function of DAS arrays is presented considering the gauge length, pulse shape, and cable design. The theoretical relationship between DAS-measured and pointwise strain for vertical and horizontal active sources is introduced using 3D elastic finite-difference simulations. The implications of using DAS strain measurements are discussed including directionality and magnitude differences between the actual and DAS-measured strain fields. Estimating measurement quality based on the wavelength-to-gauge length ratio for field data is demonstrated. A method for spatially aligning the DAS channels with the geophone locations at tolerances less than the spatial resolution of a DAS system is proposed.

Seismo-Stratigraphic Model for the Urban Area of Milan (Italy) by Ambient-Vibration Monitoring and Implications for Seismic Site Effects Assessment

In this paper, we present the work carried out to characterize the spatial variability of seismic site response related to local soil conditions in the city of Milan and its surroundings, an area with ∼3 million inhabitants and a high density of industrial facilities. The area is located at the northwestern end of the Po Plain, a large and deep sedimentary basin in northern Italy. An urban-scale seismo-stratigraphic model is developed based on new passive and active seismic data, supported by the available geological data and stratigraphic information from shallow and deep vertical wells. In particular, 33 single-station and 4 ambient-vibration array measurements are acquired, together with 4 active multichannel analyses of surface waves (MAWS). To estimate the resonant frequencies of the sediments, the horizontal to vertical spectral ratio technique (HVSR) is applied to the ambient-vibration recordings, whereas to determine the Rayleigh-wave dispersion curves from the passive array, the data are analysed using the conventional frequency-wavenumber, the modified spatial autocorrelation and the extended spatial autocorrelation (ESAC) techniques. The array data are used to determine the local shear wave velocity profiles, VS, via joint inversion of the Rayleigh-wave dispersion and ellipticity curves deduced from the HVSR. The results from HVSR show three main bands of amplified frequencies, the first in the range 0.17–0.23 Hz, the second from 0.45 to 0.65 Hz and the third from 3 to 8 Hz. A decreasing trend of the main peaks is observed from the northern to the southern part of the city, allowing us to hypothesize a progressive deepening of the relative regional chrono-stratigraphic unconformities. The passive ambient noise array and MASW highlight the dispersion of the fundamental mode of the Rayleigh-wave in the range 0.4–30 Hz, enabling to obtain detailed Vs. profiles with depth down to about 1.8 km. The seismo-stratigraphic model is used as input for 1D numerical modelling assuming linear soil conditions. The theoretical 1D transfer functions are compared to the HVSR curves evaluated from both ambient noise signals and earthquake waveforms recorded by the IV. MILN station in the last 10 years.

Modeling of Empirical Transfer Functions with 3D Velocity Structure

ABSTRACTEmpirical transfer functions (ETFs) between seismic records observed at the surface and depth represent a powerful tool to estimate site effects for earthquake hazard analysis. However, conventional modeling of site amplification, with assumptions of horizontally polarized shear waves propagating vertically through 1D layered homogeneous media, often poorly predicts the ETFs, particularly, in which large lateral variations of velocity are present. Here, we test whether more accurate site effects can be obtained from theoretical transfer functions (TTFs) extracted from physics-based simulations that naturally incorporate the complex material properties. We select two well-documented downhole sites (the KiK-net site TKCH05 in Japan and the Garner Valley site, Garner Valley Downhole Array, in southern California) for our study. The 3D subsurface geometry at the two sites is estimated by means of the surface topography near the sites and information from the shear-wave profiles obtained from borehole logs. By comparing the TTFs to ETFs at the selected sites, we show how simulations using the calibrated 3D models can significantly improve site amplification estimates as compared to 1D model predictions. The primary reason for this improvement in 3D models is redirection of scattering from vertically propagating to more realistic obliquely propagating waves, which alleviates artificial amplification at nodes in the vertical-incidence response of corresponding 1D approximations, resulting in improvement of site effect estimation. The results demonstrate the importance of reliable calibration of subsurface structure and material properties in site response studies.

Developing an updated set of VS profiles for the central United States seismic observatory with estimates of uncertainty between different methods

Deep dynamic site characterization was conducted at the Central United States Seismic Observatory (CUSSO), which is a 587 m deep vertical seismic array situated in the Mississippi embayment. This characterization included a combination of active and passive source surface wave methods and horizontal to vertical spectral ratio (HVSR). These measurements were used in a multimodal joint inversion of Rayleigh and Love dispersion data and the natural site frequency from HVSR (f0) to develop non-invasive VS profiles for the site. Downhole seismic measurements were also made down to 425 m to validate the surface wave method results and understand the uncertainty between the various measurements at the site. The developed VS profiles are compared to published site specific VS profiles from direct measurements, velocity models, and reference VS profile for the embayment to understand the variability and uncertainty between different methods. Major differences in the Vs profiles were noticed in the top 50 m, near the Memphis sand layer, and the Paleozoic bedrock layer. Site signatures from experimental dispersion data, HVSR f0 and empirical transfer function (ETF) were evaluated to assess the appropriateness of the candidate VS profiles. Comparison results indicate the developed VS profiles in this study performed well in capturing the site signatures. However, the theoretical dispersion fits from published VS profiles demonstrated a larger misfit with the experimental data. Theoretical transfer function and Rayleigh ellipticity from published profiles also exhibited considerable differences with the HVSR f0 and ETF amplification factor.

Evaluation of a novel application of earthquake HVSR in site-specific amplification estimation

Ground response analyses (GRA) model the vertical propagations of SH waves through flat-layered media (1DSH) and are widely carried out to evaluate local site effects in practice. Horizontal-to-vertical spectral ratio (HVSR) technique is a cost-effective approach to extract certain site-specific information, e.g., site fundamental frequency (f0), but HVSR values cannot be directly used to approximate the levels of S-wave amplifications. Motivated by the work of Kawase et al. (2019), we propose a procedure to correct earthquake HVSR amplitudes for direct amplification estimations. The empirical correction compensates HVSR by generic vertical amplification spectra categorized by the vertical fundamental frequency (f0v) via k-means clustering. In this investigation, we evaluate the effectiveness of the corrected HVSR in approximating observed linear amplifications in comparison with 1DSH modellings. We select a total of 90 KiK-net (Kiban Kyoshin network) surface-downhole sites which are found to have no velocity contrasts below their boreholes and thus of which surface-to-borehole spectral ratios (SBSRs) can be taken as their empirical transfer functions (ETFs). 1DSH-based theoretical transfer functions (TTFs) are computed in the linear domain considering uncertainties in VS profiles through randomizations. Five goodness-of-fit metrics are adopted to gauge the closeness between observed (ETF) and predicted (i.e., TTF and corrected HVSR) amplifications in both amplitude and spectral shape over frequencies from f0 to 25 Hz. We find that the empirical correction to HVSR is highly effective and achieves a “good match” in both spectral shape and amplitude at the majority of the 90 KiK-net sites, as opposed to less than one-third for the 1DSH modelling. In addition, the empirical correction does not require a velocity model, which GRAs require, and thus has great potentials in seismic hazard assessments.

Analysis of a 28-nm CMOS Fast-Lock Bang-Bang Digital PLL With 220-fs RMS Jitter for Millimeter-Wave Communication

This article analyses and demonstrates a 22.5–27.7-GHz fast-lock low-phase-noise bang-bang digital phase-locked loop (PLL) for millimeter-wave (mm-wave) communication. A discrete-time PLL model, together with theoretical transfer functions, gives insight on the functionality of the automatic bandwidth control, on the effect of the gear-shift algorithm for fast lock and on the different noise contributions. The proposed gear-shift algorithm scales up the PLL bandwidth for faster acquisition and orderly reduces it for jitter performance. The PLL contains a digitally controlled oscillator (DCO) based on transformer feedback with a tunable source-bridged capacitor, which allows for a low phase noise (PN) over a wide tuning range (FoM of −184 dBc/Hz and FoM $_{T}$ of −191 dBc/Hz) and for a fine frequency resolution. The PLL occupies 0.09-mm2 core area and exhibits 220 fs rms jitter while consuming 25 mW, giving FoMRMS of −239 dB. Its frequency acquisition time improves from 780 to 45 $\mu \text{s}$ with the gear-shift algorithm. For 60-GHz communication, with a frequency multiplication factor of 2.5, this PLL covers all six channels’ frequencies of IEEE-802.11ad, allows a transmitter (TX) error vector magnitude (EVM) down to −35.9 dB assuming a TX signal to the noise-plus-distortion ratio (SNDR) of 40 dB, and, thus, is capable of supporting 256 quadrature amplitude modulation (QAM).

공기압 관로의 동적 모델링과 과도응답 특성 해석

The study deals with dynamic modeling and time domain simulation of pneumatic transmission pipes with a distributed parameter dissipative model that include frequency dependent viscous friction terms and heat transfer effects. The transient responses of pneumatic transmission pipes are considerably complex and difficult because the transcendent transfer functions consist of hyperbolic Bessel functions in the frequency domain. For the blocked pipes at the downstream, the pipes terminating into single valve and capacitance element pairs, transfer functions are derived, and their gain and phases are calculated theoretically. Based on the frequency responses of the theoretical transfer functions, we obtained approximated transfer functions with equal frequency responses using MATLAB. The approximated transfer functions with rational polynomials are accurate over the designated frequency ranges, and are used to analyze the time domain responses. The effects of varying parameters of the blocked pipe on the accuracy of the simulated responses are investigated.

The Importance of Distinguishing Pseudoresonances and Outcrop Resonances in Downhole Array Data

ABSTRACTThis short note examines the downgoing wave effect and the appearance of pseudoresonances in downhole array data. It is demonstrated that pseudoresonances, distinct from the resonances associated with outcrop conditions, occur for sites with a shallow velocity contrast (VC) or with little to no VC. An approach is outlined to distinguish pseudoresonances from outcrop resonances using the theoretical 1D transfer functions for within and outcrop boundary conditions, as well as the horizontal-to-vertical spectral ratio. This approach is applied to hypothetical shear-wave velocity profiles, as well as three downhole array sites. We establish the importance of distinguishing pseudoresonances from outcrop resonances when using downhole array data to evaluate the accuracy of the 1D site response. For the example downhole array sites shown, the pseudoresonances are not captured well by 1D analysis, whereas the outcrop resonances are captured well. We propose that when evaluating the accuracy of 1D site-response analysis using downhole array data, the comparisons of the empirical and theoretical responses only consider the frequency range associated with outcrop resonances.

Synthesis Theory of Ultra-Wideband Bandpass Transformer and its Wilkinson Power Divider Application With Perfect in-Band Reflection/Isolation

A synthesis theory of ultra-wideband (UWB) bandpass (BP) transformer with short-circuited stubs is proposed in this paper. Based on the short-circuited stub number and their positions, six combination topologies are presented when the transmission line (TL) section is fixed to 4. Although analyzing the characteristic functions of these six combination topologies, the same theoretical Chebyshev transfer function can be summarized. Therefore, by selecting different short-circuited stub number and their positions, we can make a flexible choice of the filtering transformer pattern and keep the Chebyshev response at the same time. Finally, by using the proposed transformer, a UWB BP type Wilkinson power divider can be easily designed. In order to match the numbers and frequencies of output ports reflection zeroes ( $S_{22} =0$ )/isolation zeroes ( $S_{32} = 0$ ) with those of input port reflection zeroes ( $S_{11} =0$ ), one extra TL and one isolation resistor are newly added in the power divider design. One experimental circuit is fabricated, and its measured frequency response verified well the proposed design theory and method.

Improving the Spectral Analysis of Hydrological Signals to Efficiently Constrain Watershed Properties

AbstractThe footprint of catchment properties on water flow is reflected into hydrological signals, such as stream discharge. Here we demonstrate that it is possible to constrain catchment properties from the spectral analysis of hydrological signals but only when an appropriate transfer function (TF) is used for interpretation. We show that the appropriate theoretical TF, newly derived, is the only one to robustly describe a large diversity of experimental TFs that could be encountered in nature, because it entails the role of diffuse overflow and flow through the vadose zone, which have never been considered in spectral approaches before. The properties that may be estimated are the characteristic time scales of transfer in each compartment (surface, vadose zone, and aquifer) and the flow partitioning between surface and subsurface. We validate our approach by comparing the new and previous theoretical TFs to experimental TFs generated by a physically based distributed hydrological model, for a wide range of properties. The results confirm that without the use of the new TF, the interpretation of observed spectra may often lead to severe misestimations of catchment properties. The potential of the new TF to constrain catchment characteristics is exemplified by analyzing real hydrological signals from two watersheds with distinct behaviors. We finally discuss the broad implications of our findings and how they may contribute to a variety of topics in hydrology, thereby opening the way to a more widespread and robust use of spectral analysis to describe hydrosystems from effective rainfall, river discharge, and piezometric data.

Horizontal-to-vertical spectral ratio of S-waves and SH-wave transfer functions at the vertical seismic and strong-motion arrays in the Central United States

Spectral analyses were performed on S-waves of earthquake recordings from two vertical seismic and strong-motion arrays in the central United States to obtain horizontal-to-vertical spectral ratios (HVSR) and transfer functions. Spectral analyses were also performed on ambient noises to obtain HVSRs at both sites. Theoretical SH-wave transfer functions were obtained using Thomson-Haskell propagation matrices and soil profiles at both sites. The results demonstrate that the S-wave HVSR is similar to the observed and theoretical SH-wave transfer functions in terms of peak frequencies up to the fifth mode. In other words, the S-wave HVSR could be used as an empirical SH-wave transfer function and provide in situ constraints on the shear-wave velocity structure of soils at these sites. The results also demonstrate that the ambient-noise HVSR is different from the S-wave HVSR and SH-wave transfer functions of the soils. Thus, the ambient-noise HVSR is not applicable for characterizing earthquake ground-motion site-effect, as well as determining the shear-wave velocity structure.

A frequency-domain method for solving linear time delay systems with constant coefficients

In an active control system, time delay will occur due to processes such as signal acquisition and transmission, calculation, and actuation. Time delay systems are usually described by delay differential equations (DDEs). Since it is hard to obtain an analytical solution to a DDE, numerical solution is of necessity. This paper presents a frequency-domain method that uses a truncated transfer function to solve a class of DDEs. The theoretical transfer function is the sum of infinite items expressed in terms of poles and residues. The basic idea is to select the dominant poles and residues to truncate the transfer function, thus ensuring the validity of the solution while improving the efficiency of calculation. Meanwhile, the guideline of selecting these poles and residues is provided. Numerical simulations of both stable and unstable delayed systems are given to verify the proposed method, and the results are presented and analysed in detail.

Characterisation of ground motion recording stations in the Groningen gas field

The seismic hazard and risk analysis for the onshore Groningen gas field requires information about local soil properties, in particular shear-wave velocity (VS). A fieldwork campaign was conducted at 18 surface accelerograph stations of the monitoring network. The subsurface in the region consists of unconsolidated sediments and is heterogeneous in composition and properties. A range of different methods was applied to acquire in situ VS values to a target depth of at least 30 m. The techniques include seismic cone penetration tests (SCPT) with varying source offsets, multichannel analysis of surface waves (MASW) on Rayleigh waves with different processing approaches, microtremor array, cross-hole tomography and suspension P-S logging. The offset SCPT, cross-hole tomography and common midpoint cross-correlation (CMPcc) processing of MASW data all revealed lateral variations on length scales of several to tens of metres in this geological setting. SCPTs resulted in very detailed VS profiles with depth, but represent point measurements in a heterogeneous environment. The MASW results represent VS information on a larger spatial scale and smooth some of the heterogeneity encountered at the sites. The combination of MASW and SCPT proved to be a powerful and cost-effective approach in determining representative VS profiles at the accelerograph station sites. The measured VS profiles correspond well with the modelled profiles and they significantly enhance the ground motion model derivation. The similarity between the theoretical transfer function from the VS profile and the observed amplification from vertical array stations is also excellent.