Real and synthetic data verifies the wavefield transformation method described here converts surface waves on a shot gather directly into images of multi-mode dispersion curves. Pre-existing multi-channel processing methods require preparation of a shot gather with exceptionally large number of traces that cover wide range of source-to-receiver offsets for a reliable separation of different modes. This method constructs high-resolution images of dispersion curves with relatively small number of traces. The method is best suited for near-surface engineering project where surface coverage of a shot gather is often limited to near-source locations and higher-mode surface waves can be often generated with significant amount of energy.

The conventional inversion of surface waves depends on modal identification of measured dispersion curves, which can be ambiguous. It is possible to avoid mode-number identification and extraction by inverting the complete phase-velocity spectrum obtained from a multichannel record. We use the fast simulated annealing (FSA) global search algorithm to minimize the difference between the measured phase-velocity spectrum and that calculated from a theoretical layer model, including the field setup geometry. Results show that this algorithm can help one avoid getting trapped in local minima while searching for the best-matching layer model. The entire procedure is demonstrated on synthetic and field data for asphalt pavement. The viscoelastic properties of the top asphalt layer are taken into account, and the inverted asphalt stiffness as a function of frequency compares well with laboratory tests on core samples. The thickness and shear-wave velocity of the deeper embedded layers are resolved within 10% deviation from those values measured separately during pavement construction. The proposed method may be equally applicable to normal soil site investigation and in the field of ultrasonic testing of materials.

On uncorrelated Vibroseis shot gathers, each frequency component of ground roll is represented with a unique slope as a function of arrival time and sweep function with excellent isolation from other components. The calculation of phase velocity becomes a simple matter of measuring the slope of each different frequency using an appropriate coherency measure. Possible contamination by coherent noise can readily be determined for each frequency by visual inspection. Any change in field configuration or extra efforts during data processing steps can be immediately designated. The multi-channel measurement method allows averaging and therefore effective reduction in any random noise introduced during recording. Therefore, the dispersion curve is constructed in a fast, accurate, and fully automated manner. Qualitative information about near-surface conditions can also be inferred from visual inspection, making it possible to detect near-surface anomalies. Multi-channel analysis of surface waves using Vibroseis (MASWV) has advantages over the Spectral Analysis of Surface Waves (SASW) method which employs only two receivers with an impact source. First of all, because of the high spectral integrity of acquired data, a high degree of accuracy can be placed on the results of the method. Furthermore, the method is much faster and less labor-intensive than SASW since only a single or a few recorded shot gathers are usually necessary to produce a well behaved dispersion curve.