Analytical Study of Surface Wave Testing Along the Seafloor
Authors: Lee B. , Rosenblad B.L. , Wright S.G. , Stokoe K.H. 
Published: October 2008

The Spectral-Analysis-of-Surface-Waves (SASW) technique is a non-intrusive seismic technique for evaluating the stiffness in shear of seafloor soils as a function of depth. Recent experimental studies in the Gulf of Mexico and analytical studies at the University of Texas have demonstrated the feasibility of using this technique to evaluate shear wave velocity profiles and to infer, through experimental and empirical correlations, other seafloor properties. TheSASW technique can be an economical supplement to soil borings as an effective means of inferring the lateral continuity of seafloor properties. Using the SASW technique, profiles of shear wave velocity are determined by matching theoretical dispersion curves to measured experimental dispersion curves through an iterative process. In this work, analytical studies were conducted to compare 2-dimensional (2-D) and 3-dimensional (3-D) solutions used to determine theoretical dispersion curves. A series of parametric studies was performed to examine the applicability of each solution and to study further the uncertainties in the 2-D solution. Conclusions are presented concerning when the simpler and computational1y efficient 2-D solution can effectively be usedto match dispersion curves measured in the field.

Introduction

Stress wave measurements have been used on land and offshore to investigate material systems for decades. Themost commonly used measurements have involved compressional or acoustical waves. In the past 20 years, significant efforts have been directed towards developing a technique involving surface waves. This technique, cal1ed the Spectral-Analysis-of Surface-Waves (SASW) technique, employs Rayleigh-type surface waves on land and Scholte type surface waves offshore (Ref. I and Ref. 2). The technique has been used to profile many geotechnical sites onland, including natural soil deposits, compacted earth materials, and solid waste landfil1s (Ref. 3 and Ref. 4); as wel1 as concrete structures and highway and airport pavements (Ref. 5 and Ref. 6). Recently, the SASW technique has been extended to underwater environments to measure shear stiffness profiles of the seafloor (Ref. 7, Ref. 8, Ref. 9, and Ref. 10). One of the attractive attributes of the testing technique is its non-intrusive nature, especially forapplications at deepwater sites.

The SASW technique consists of exciting Scholte wave energy on the seafloor and measuring the propagation of thisenergy past two or more receivers arranged in a linear array as il1ustrated in Fig. la. Scholte waves are generated by applying a dynamic vertical load to the seafloor surface. Vertical1y sensitive receivers, also located on the seafloor surface, are used to sense the passage of wave energy. The time records recorded at each receiver are transformed into the frequency domain through a Fast Fourier Transform (FFT) algorithm, and the phase difference between receivers as a function of frequency is determined. The phase velocity of the Scholte wave, VSc is found from:

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