1BRANCHING OF DISPERSION CURVES IN SURFACE WAVE TESTING OF PAVEMENTS

When a set of simulated multichannel seismic records acquired over pavement is pro-cessed by a 2-D wavefield transformation technique normally used in the Multichannel Analysis of Surface Waves (MASW) method, several branches are observed in the dispersion-curve image constructed from the transformation. Some investigators theoretically anticipated this branching phenomenon a few decades ago in connection with discontinuities in a layer model that has de-creasing stiffness with depth. Although this phenomenon has long been speculated about during measurements with a conventional two-receiver approach, sometimes by attributing the results to several other possible causes like higher modes, an objective observation confirming its link to the predicted theory was never made. The dispersion curve image shows several frequency-phase velocity branches that match fairly well with the discontinuities in the dispersion curve predicted by theory. With a case study and numerical modeling we discuss a new approach that can yield thickness and stiffness of layers in a pavement system simply from the characteristics of this branching phenomenon. These determinations can be made without going through the normal procedure of dispersion curve analysis followed by inversion for the shear wave velocity (Vs) profile