Spectral Analysis of Surface Waves (SASW)

SPECTRAL ANALYSIS OF SURFACE WAVES (SASW)

Introduction

SASW tests allow the user to determine; the different profiles in a pavement system, including their depth, and also the condition of each of these layers. It is all done non-destructively from the top surface of the pavement. The test can be performed on concrete, asphalt, masonry, soil and wood and can be used to investigate profiles up to 90 meters deep.

Applications

SASW investigations can determine:

Pavement system profiles including the surface layer, base and sub grade materials.
Determination of soil velocity profiles needed for earthquake and dynamic loading analysis
Determination of abutment depths of bridges
The condition assessments of concrete liners in tunnels, and other structural concrete conditions

How it works

The

SASW method uses the dispersive characteristics of surface waves to determine the variation of the shear wave velocity (stiffness) of layered systems with depth. The SASW testing is applied from the surface which makes the method non-destructive and nonintrusive.In SASW tests, two receivers are placed on the surface, and a hammer is used to generate the wave energy. An Olson Instruments FreedomData PC or NDE360 records the hammer input and the receiver output.

Access

The SASW method requires an accessible surface for receiver attachments. The extent of the accessible surface limits the investigation depth. As a rule of thumb, if one is interested in material properties to a depth D, then the accessible surface should extend in the line of receivers direction to a distance equal to 1.5D, preferably 2D.

Setup Arrangement

This image shows the general field arrangement used in SASW testing. Receiver spacings ranging from 15cm to +90m have been used in the field to investigate depths from 5cm up to +90m.

Collection of Data

In SASW tests, two receivers are placed on the surface, and a hammer is used to generate the wave energy. Short receiver (typically accelerometers) spacings are used to sample the shallow layers while long receiver (typically velocity transducers) spacings are used in sampling the deep materials. Two profiles, a forward profile and a reverse profile, are typically obtained in SASW measurements where the accessible surface is struck by a hammer on two opposite sides of the receivers. A signal analyzer is used to collect and transform the receiver outputs to the frequency domain. Two functions in the frequency domain are of great importance in SASW tests:

the cross power spectrum between the two receivers (used in the preparation of the experimental dispersion curve)
the coherence function (used to ensure that high signalto-noise ratio data is being collected).

Accuracy

SASW measurements are accurate to within 5% for the determination of the thickness and stiffness of the top layer in a pavement system or of the concrete liner of a tunnel. Correlation between SASW and Crosshole Seismic tests on soil sites showed that the values from both tests typically compare within a 10% difference.

Case Study

FThe figure below shows dispersion curves determined from SASW measurements on asphalt pavement. Shown in this figure is the variation of the surface wave velocity (modulus) as the asphalt layer warms up. The SASW measurements were also effective in determining the thickness of the surface layer.

Platforms Available

We offer two devices available for the SASW technique. These include the NDE360 and DataPC. These offer differing levels of mobility and on-site analsysis. Please see the individual brochures for more in depth specifications for the platforms.

Additional Information

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