Construction Of 2-D Vertical Shear-Wave Velocity Field By The Multichannel Analysis Of Surface Wave Technique

Abstract

We present a method that utilizes the Multichannel Analysis of Surface Waves (MASW) technique and a standard common depth point (CDP) roll-along acquisition format similar to conventional petroleum exploration seismic data acquisition to construct a vertical section of the near-surface shear (S)-wave velocity field. A one-dimensional (1-D) S-wave velocity vs. depth plot is obtained by inverting phase velocities using the MASW technique. This 1-D profile appears to be most representative of the materials directly below the middle of a geophone spread. Multiple 1-D plots of S-wave velocity vs. depth are generated as the source and receivers roll along a survey line. A two-dimensional (2-D) vertical cross-section of S-wave velocity can be constructed by contouring grids produced by combining all the 1-D S-wave velocity profiles that are a function of the middle point of geophone spread (x) and depth (z). The combination of inverting the phase velocity for S-wave velocity and the standard CDP roll-along acquisition format makes this a very effective and time-efficient method of imaging two-dimensional Swave velocity along a survey line. There are several advantages that make this method attractive in real world applications. 1. The method focuses on high-frequency ( ≥ 2 Hz) ground roll to provide a 2-D near-surface S-wave velocity map and to detect targets significantly shallower than feasible with other acoustic techniques. 2. That ground roll, which is acquired by the multichannel acquisition method, has a high signal-to-noise ratio, allowing 2-D images to be obtained in extremely noisy environments. 3. The method uses the standard CDP roll-along acquisition format, which provides an efficient way to acquire large quantities of broadband surface wave data along a line. 4. The method utilizes the redundancy of the standard CDP roll-along acquisition format so that it not only provides a reliable way to verify inverted S-wave velocities, it also reduces the ambiguity of inverted S-wave velocities. 5. A 2-D display of S-wave velocity can be produced easily and quickly by contouring the inverted S-wave velocity to provide a map of the S-wave velocity field. 6. 2-D data processing techniques, such as regression analysis, could easily be applied to a vertical S-wave velocity section to enhance local anomalies (gas or oil fields, voids, tunnels, etc.). More than five thousand shots of MASW data have been acquired and processed producing more than forty vertical near-surface S-wave velocity sections since 1997. Four real world examples demonstrate the usage of the method.