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Abstract: Design of a system for analysis of surface waves

Dec. 9, 2009 -- A recent State Farm Insurance grant will facilitate earthquake research at Arkansas State University. Below is an abstract of "Design of a system for analysis of surface waves," written by Dr. Shivan Haran, Mechanical Engineering, and Dr. Ashraf Elsayed, Civil and Environmental Engineering. View a PowerPoint presentation of their work, "Shear Wave Velocity Profiling Using Simplified Equipment."

Design of a system for analysis of surface waves
Dr. Shivan Haran and Dr. Ashraf Elsayed
College of Engineering, ASU

A surface wave analysis system can be used to determine shear wave velocity profiles, and hence determine soil condition at various locations.  There are numerous applications for this, such as determining soil condition around residential houses, office buildings, prospective sites, earthquake-prone areas, etc.

We are using an in-situ seismic method for determining shear wave velocity profiles.  Testing is performed on the ground surface, allowing for less costly measurements than with traditional borehole methods.  The basis of the method is the dispersive characteristic of Rayleigh waves when traveling through a layered medium.  Rayleigh wave velocity is determined by the material properties (primarily shear wave velocity, but also compression wave velocity and material density) of the subsurface to a depth of approximately 1 to 2 wavelengths.  Longer wavelengths penetrate deeper; their velocity is affected by the material properties at greater depth.

Testing consists of measuring the surface wave dispersion curve at the given site and interpreting it to obtain the corresponding shear wave velocity profile.  The system consists of an impact source (such as a hammer) which is used to generate surface waves of different frequencies, ranging from about 25Hz. to 1000 Hz.  These waves will be monitored by two or more seismic transducers placed at known distances from the point of impact.  Digitized data from the receivers are gathered through the use of a PC-based data acquisition system and analyzed using signal processing algorithms which will include standard spectrum computation.

There are several ways of interpreting dispersion curves, depending on the accuracy required in the shear wave velocity profile.  A basic empirical analysis can be done to estimate the average shear wave velocity profile.  For greater accuracy, advanced analysis techniques can be used.  With the analytical approaches, background information on the site can be incorporated into a model and the resolution of the final profile may be quantified.  Depending upon the number of receivers used, better resolution can be obtained.

The method used by us offers significant advantages:  in contrast to borehole measurements, which are point estimates, surface wave testing is a global measurement, that is, a much larger volume of the subsurface is sampled.  The resulting profile is representative of the subsurface properties averaged over distances of up to several hundred feet.   Because the method is non-invasive and non-destructive, it is relatively easy to obtain the necessary permits for testing.  At sites that are favorable for surface wave propagation, the method allows appreciable cost and time savings.  Surface wave testing can be used to obtain velocity profiles for earthquake site response, soil compaction control, pavement evaluation, mapping subsurface stratigraphy, etc.

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