Feasibility of detecting near-surface feature with Rayleigh-wave diffraction Jianghai Xia a, ⁎ , Jonathan E. Nyquist b , Yixian Xu c , Mary J.S. Roth d , Richard D. Miller a a Kansas Geological Survey, The University of Kansas, 1930 Constant Ave., Lawrence, KS 66047, United States b Department of Geology, Beury Hall, 1901 N 13th St, Temple University, Philadelphia, PA 19122, United States c The State Key Laboratory of Mineral Resources and Geological Processes, China University of Geosciences, Wuhan, Hubei, 430074, PR China d Department of Civil and Environmental Engineering, Lafayette College, Easton, PA 18042, United States Received 10 July 2006; accepted 13 December 2006 Abstract Detection of near-surfaces features such as voids and faults is challenging due to the complexity of near-surface materials and the limited resolution of geophysical methods. Although multichannel, high-frequency, surface-wave techniques can provide reliable shear (S)-wave velocities in different geological settings, they are not suitable for detecting voids directly based on anomalies of the S-wave velocity because of limitations on the resolution of S-wave velocity profiles inverted from surface-wave phase velocities. Therefore, we studied the feasibility of directly detecting near-surfaces features with surface-wave diffractions. Based on the properties of surface waves, we have derived a Rayleigh-wave diffraction traveltime equation. We also have solved the equation for the depth to the top of a void and an average velocity of Rayleigh waves. Using these equations, the depth to the top of a void/fault can be determined based on traveltime data from a diffraction curve. In practice, only two diffraction times are necessary to define the depth to the top of a void/fault and the average Rayleigh-wave velocity that generates the diffraction curve. We used four two-dimensional square voids to demonstrate the feasibility of detecting a void with Rayleigh-wave diffractions: a 2 m by 2 m with a depth to the top of the void of 2 m, 4 m by 4 m with a depth to the top of the void of 7 m, and 6 m by 6 m with depths to the top of the void 12 m and 17 m. We also modeled surface waves due to a vertical fault. Rayleigh-wave diffractions were recognizable for all these models after FK filtering was applied to the synthetic data. The Rayleigh-wave diffraction traveltime equation was verified by the modeled data. Modeling results suggested that FK filtering is critical to enhance diffracted surface waves. A real-world example is presented to show how to utilize the derived equation of surface-wave diffractions. © 2006 Elsevier B.V. All rights reserved. Keywords: Rayleigh-wave diffraction; Traveltime equation; Void/fault detection 1. Introduction Elastic properties of near-surface materials and their effects on seismic wave propagation are of fundamental interest in ground-water, engineering, and environmen- tal studies. Shear (S)-wave velocities can be derived from inverting dispersive phase velocities of the surface (Rayleigh and/or Love) waves (e.g., Dorman and Ewing, 1962; Aki and Richards, 1980, p. 664). Spectral Analysis of Surface Waves (SASW) (Stokoe and Nazarian, 1983; Stokoe et al., 1989) analyzes the Journal of Applied Geophysics 62 (2007) 244 – 253 www.elsevier.com/locate/jappgeo ⁎ Corresponding author. Tel.: +1 785 864 2057; fax: +1 785 864 5317. E-mail address: jxia@kgs.ku.edu (J. Xia). 0926-9851/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2006.12.002