Active and passive surface wave techniques

Active and passive surface wave techniques are relatively new in-situ seismic methods for determining shear wave velocity profiles. These techniques are basic methods used to classify site of UBC / IBC code for seismic design, seismic microzonation, liquefaction analysis, soil compaction control, mapping subsurface stratigraphy, locating potentially weak zones in earthern embankments and levees, etc. Testing is performed on the ground surface, allowing for less costly measurements than with traditional borehole method.

configuration-of-a-MASW-survey-spread

                                          The configuration of a MASW survey spread

The basis of surface wave technique is the dispersive characteristic of Rayleigh waves when traveling through a layered medium. Rayleigh-wave phase velocity is determined by the material properties (primarily shear wave velocity, but also to a lesser degree primary wave velocity and material density) of the subsurface to a depth of approximately 1 to 2 wavelenghts. Surface wave testing consists of measuring the surface wave dispersion curve at site and modeling it to obtain the corresponding shear wave velocity profile.

dispersion curve

                                                             A dispersion curve

There are two kinds of surface wave techniques, they are active and passive techniques. Active surface wave techniques (multi-analysis surface wave or MASW) measure surface waves generated by dynamic sources such as hammers, weight drops, electromechanical shakers, etc. MASW field layout is similar to that of the seismic refraction technique. Geophones are laid out in a linear array with 1 to 5 m spacing and connected to a multi-channel seismograph. Passive surface wave techniques measure noise; surface wave from ocean wave activity, traffic, factories, wind, etc. These techniques include the array microtremor and refraction microtremor (REMI) techniques. The array microtremor technique typically uses 7 or more 4.5 or 1 Hz geophones arranged in a two-dimensional array such as the triangle, circle, “L” or line (REMI) array.

MASW survey result

                                                         A result of MASW survey at a test site

Surface wave data are processed by SeisImager SW software.