Nils Ryden

Fast simulated annealing inversion of surface waves on pavement using phase-velocity spectra

The conventional inversion of surface waves depends on modal identification of measured dispersion curves, which can be ambiguous. It is possible to avoid mode-number identification and extraction by inverting the complete phase-velocity spectrum obtained from a multichannel record. We use the fast simulated annealing (FSA) global search algorithm to minimize the difference between the measured phase-velocity spectrum and that calculated from a theoretical layer model, including the field setup geometry. Results show that this algorithm can help one avoid getting trapped in local minima while searching for the best-matching layer model. The entire procedure is demonstrated on synthetic and field data for asphalt pavement. The viscoelastic properties of the top asphalt layer are taken into account, and the inverted asphalt stiffness as a function of frequency compares well with laboratory tests on core samples. The thickness and shear-wave velocity of the deeper embedded layers are resolved within 10% deviation from those values measured separately during pavement construction. The proposed method may be equally applicable to normal soil site investigation and in the field of ultrasonic testing of materials.

Combined use of active and passive surface waves

With a surface wave method to estimate shear-wave velocity (Vs) from dispersion curve(s) of known mode(s), accurate modal identification is obviously a crucial issue. In this regard, the dispersion imaging method is an essential processing tool as it can unfold the multi-modal nature of surface waves through direct wavetield transformations. When a combined dispersion curve of an extended frequency range is prepared from analyses of both passive and active surface waves attempting to increase the maximum depth of Vs estimation, the modal nature of the passive curve (as well as the active one) must be assessed although it has usually been considered the fundamental mode. We report two experimental survey cases performed at the same soil site, but at two different times, employing the passive and active versions of the multichannel analysis of surface waves (MASW) method for an increased investigation depth. In the earlier survey, the modal nature of the imaged dispersion trends from the passive ( 20 Hz) data sets was identified as the fundamental mode, whereas it was confidently re-identified as the first higher mode from the later survey. Modal inspection with the dispersion image created by combining passive and active image data sets was the key to this confident analysis. The modal nature of the passive curve was identified from its context with active curves, whose confident analysis therefore had to come first. An active data set acquired with a small (<1.0 m) receiver spacing and an impact point located close to the receivers appears important for this purpose. (Less)

Guided wave propagation in three-layer pavement structures

A study on guided waves in a layered half-space with large velocity contrasts and a decreasing velocity with depth is presented. Multiple mode dispersion curves are calculated in the complex wave number domain, taking into consideration the attenuation caused by leakage into the underlying half-space. The excitability of the modes by a vertical point force on the surface is also calculated. Results show that the measurable wave field at the surface of a pavement structure is dominated by leaky quasi-Lamb waves in the top and second layers. The fundamental antisymmetric mode of vibration is the dominating mode generated in the stiff top layer. This mode drives the complete system and continuity across the boundaries generates higher order modes in the embedded second layer. The interaction of leaky Lamb waves in the first two layers results in large variations in the excitability and the attenuation, so that only the waves corresponding to certain portions of the dispersion curves are measurable remote fro...

Surface wave testing of pavements

Pavements are constructed using several layers of materials, and their durability depends on the quality of all of these strata. It is, therefore, valuable to be able to determine the properties of the layers nondestructively. A method is presented for evaluating the thickness and stiffness of multilayered pavement structures from guided waves measured at the surface. In this type of layered structure, the interaction of leaky Lamb waves in the embedded layers generates surface waves corresponding only to certain portions of the guided wave dispersion curves and branches measurable at the pavement surface. To resolve the different mode branches, the wavefield is measured at the surface by using a light hammer as the source and an accelerometer as receiver, generating a synthetic receiver array. The recorded data are transformed to a phase velocity spectrum, which is then inverted to give the layer properties using a global inversion algorithm. The theoretical background along with experimental results of ...

High Frequency Masw For Non-Destructive Testing Of Pavements—Accelerometer Approach

The dispersive nature of surface waves in pavement systems is imaged through a multichannel approach using one accelerometer as receiver and multiple shot points. The image obtained from a wavefield transformation method shows multimodal dispersion curves up to 10 kHz. We present results from a simplified MASW data acquisition method applied to a pavement surface. The method can simulate an arbitrary number of channels. The sensor separation can be chosen arbitrarily small. In these experiments, the upper frequency limit is 10 kHz, which can be increased by the exchange of one sensor. The method is tested by one manual and one automated procedure. Both rely on source-receiver reciprocity. The automated procedure is regarded as necessary when a large number of channels is combined with a small sensor separation. The manual method will not provide the necessary accuracy and endurance for that kind of measurement, but is promising for less complicated setups. We present recommendations for high frequency measurements on pavements. In the subsequent data processing, we follow the procedure of multichannel analysis of surface waves MASW. It has recently been developed as a geophysical method for near-surface investigation. We demonstrate that the MASW technique can identify detailed aspects of the high frequency total wave-field of both surface and body-wave events. Results of dispersion curve extraction indicate that higher modes of surface waves are dominating at depths associated with the transition between the asphalt and the base layer. However, the deviation from the fundamental mode is not large because all modes are converging in an asymptotic manner with increasing frequency. The study indicates that the MASW method is a fast and cost efficient method for measuring pavement stiffness parameters.

Resonant frequency testing of cylindrical asphalt samples

ABSTRACT There is a need to develop simple and quick non-destructive tests to measure the complex dynamic modulus of asphalt over a wide frequency and temperature range, i.e. mastercurve. In this study results from free-free resonant frequency measurements on cylindrical disk shaped asphalt samples are presented. The resulting mastercurve, obtained by resonant frequency testing at different temperatures, compares well with reference values within the high modulus range. The proposed method is simple and repeatable.

Surface waves in inversely dispersive media

The dispersion of surface waves in inversely dispersive media where the shear-wave velocity decreases with depth is studied. Theoretical dispersion curves are calculated in the complex wavenumber domain. Excitability and attenuation due to leakage are calculated for each point on the dispersion curves. These additional parameters are critical for a correct understanding of the dispersion properties of surface waves. Mode shapes are included in the study to visualize displacements inside the medium. The results of the study show that, for inversely dispersive media, the Rayleigh-wave assumption is not valid, and other types of interface waves and leaky Lamb waves contribute to the surface wavefield. They also show that, in this case, true theoretical dispersion curves can be approximated by Lamb-wave dispersion curves for a free plate in a vacuum, provided that the stiffness contrast between the top layer and the underlying half-space is large, and also that the shear-wave velocity of the stiff layer is greater than the compressional-wave velocity in the underlying media. The error in the phase velocity resulting from this approximation is investigated and it is shown that the error does not exceed 5% for the fundamental antisymmetric Lamb-wave dispersion curve. Because of the numerical simplicity of calculating its theoretical dispersion curves, the Lamb-wave approximation can provide an effective evaluation method to resolve the thickness and elastic parameters of the stiff top layer. This is exemplified using a set of field data.

Seismic Investigation of Pavements by MASW Method — Geophone Approach

A feasibility test of the multichannel approach to seismic investigation of a pavement system is described. This test followed the procedure normally taken in the multichannel analysis of surface waves (MASW) method by using geophones and a light (8-oz) hammer source. A wavefield transformation of recorded multichannel data shows a strong fundamental-mode dispersion curve image in the frequency range of 30-600 Hz with normal (30-50 Hz) and reverse (50-600 Hz) trends. However, the transformation shows that this fundamental mode disappears quite abruptly and higher modes start to dominate in the higher frequencies up to 2000 Hz. Simultaneous recording of both vertical and horizontal components of seismic wavefields facilitates identification of seismic events. In order to record the horizontally travelling direct (or possibly guided) P-wave event in the uppermost layer, it seems critical to use horizontal phones with longitudinal orientation. Results of test indicate that for an investigation focused into the uppermost layers of a pavement system it is essential to use a different acquisition system that can deal with much higher (> 2000 Hz) frequencies. In addition, complicated and unique elastic properties of pavement systems call for an inter-disciplinary study to develop an effective multichannel seismic method for this area of application.

Seismic joint analysis for non-destructive testing of asphalt and concrete slabs

A seismic approach is used to estimate the thickness and elastic stiffness constants of asphalt or concrete slabs. The overall concept of the approach utilizes the robustness of the multichannel seismic method. A multichannel-equivalent data set is compiled from multiple time series recorded from multiple hammer impacts at progressively different offsets from a fixed receiver. This multichannel simulation with one receiver (MSOR) replaces the true multichannel recording in a cost-effective and convenient manner. A recorded data set is first processed to evaluate the shear wave velocity through a wave field transformation, normally used in the multichannel analysis of surface waves (MASW) method, followed by a Lambwave inversion. Then, the same data set is used to evaluate compression wave velocity from a combined processing of the first-arrival picking and a linear regression. Finally, the amplitude spectra of the time series are used to evaluate the thickness by following the concepts utilized in the Impact Echo (IE) method. Due to the powerful signal extraction capabilities ensured by the multichannel processing schemes used, the entire procedure for all three evaluations can be fully automated and results can be obtained directly in the field. A field data set is used to demonstrate the proposed approach. (Less)

Characterizing the low strain complex modulus of asphalt concrete specimens through optimization of frequency response functions

Measured and finite element simulated frequency response functions are used to characterize the low strain (~10(-7)) complex moduli of an asphalt concrete specimen. The frequency response functions of the specimen are measured at different temperatures by using an instrumented hammer to apply a load and an accelerometer to measure the dynamic response. Theoretical frequency response functions are determined by modeling the specimen as a three-dimensional (3D) linear isotropic viscoelastic material in a finite element program. The complex moduli are characterized by optimizing the theoretical frequency response functions against the measured ones. The method is shown to provide a good fit between the frequency response functions, giving an estimation of the complex modulus between minimum 500 Hz and maximum 18|000 Hz depending on the temperature. Furthermore, the optimization method is shown to give a good estimation of the complex modulus master curve.