| We propose a new method named “hybrid surface wave survey” which is characterized as the combination of active and passive surface wave method by using same geophone array set along a linear survey line. A number of geophones, set along a line at 1 or 2 m intervals, are used to record active surface hitting waveforms along with microtremors. Two dispersion curves, one is for a higher frequency part calculated from CMP-CC gather data derived from active survey records, and the other is for a lower frequency part calculated from passive data applying CMP-SPAC analytical method, are combined to form a single dispersion curve for a specific CMP in a survey line. The method enables us to reconstruct a dense 2D S-wave velocity profile along a line up to 50 m in depth or more using conventional 4.5 Hz geophones. One of the advantages of hybrid surface wave survey is its robustness to the traffic noises or active utilization of them vice versa. It means it is more suitable for the method to set the line along trunk, heavy traffic highways. We conducted hybrid surface wave surveys along operational highways which had traffic flows of 300 to 1,500 vehicles per hour. The field tests demonstrated the capability of the hybrid surface wave method not only for the delineation of heterogeneous structure in embankments of highways even under heavy traffic condition but also for reconstructing deeper structures in the substrata beneath embankments. |
