In engineering application of surface waves it is critically important to accurately extract the<br>fundamental mode dispersion curve. Among several factors that may adversely affect the<br>extraction is the existence of higher modes with significant amount of energy. A calculated<br>phase velocity can be an average of the fundamental and the higher-modes phase velocities or it<br>can be the phase velocity of a specific higher mode, depending upon the specific method used for<br>the application, unless the higher modes are properly handled during the data acquisition and<br>processing steps. Therefore, it will have a practical value to observe the higher mode generation<br>through field experiments and examine for any parameter that can be controlled during data<br>acquisition.<br>A higher mode (the first overtone) of high frequency (5-30 Hz) surface waves was observed by<br>using the multi-channel analysis of surface waves (MASW) method at three boreholes located in<br>unconsolidated sediments in the Fraser River Delta, near Vancouver, British Columbia. Each<br>site has a unique near-surface shear (S)-wave velocity (Vs) structure as verified from downhole<br>Ys measurements. The relative dominance of higher mode energy is examined in association<br>with source distance as well as Ys structure. Our examination indicates that energy of higher<br>modes tends to become more significant as the source distance becomes greater. It also reveals<br>that the dominance may be related to a Vs structure: a greater dominance as fi changes little<br>with depth, or fi has an overall low value, or a combination. The dependency on the source distance<br>is observed to be stronger than that on the Vs structure. Attempts are made to explain the<br>dependency by referring to one or a combination of three factors: attenuation, the near-field<br>effects, and the intrinsic nature of surface waves. Inclusion of higher mode during a surface<br>wave measurement for near-surface (~30 m) application can be either an advantage or a-disadvantage,<br>depending on the specific type of application and the method used during the data<br>acquisition and processing steps. It is, therefore, important to recognize through field<br>observations those conditions both favorable and unfavorable to the generation of higher modes<br>of high-frequency surface waves.


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