Multichannel analysis of surface waves

The frequency-dependent properties of Rayleigh-type surface waves can be ut ilized for imaging and characterizing the shallow subsurface, Most surface- wave analysis relies on the accurate calculation of phase velocities for th e horizontally traveling fundamental-mode Rayleigh wave acquired by steppin g out a pair of receivers at intervals based on calculated ground roll wave lengths. Interference by coherent source-generated noise inhibits the relia bility of shear-wave velocities determined through inversion of the whole w ave field. Among these nonplanar, nonfundamental-mode Rayleigh waves (noise ) are body waves, scattered and nonsource-generated surface waves, and high er-mode surface waves. The degree to which each of these types of noise con taminates the dispersion curve and, ultimately, the inverted shear-wave vel ocity profile is dependent on frequency as well as distance from the source . Multichannel recording permits effective identification and isolation of no ise according to distinctive trace-to-trace coherency in arrival time and a mplitude. An added advantage is the speed and redundancy of the measurement process. Decomposition of a multichannel record into a time variable-frequ ency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency component in a unique and continuous format. Coherent noise contamination can then be examined and its effects appraised in both frequency and offset space. Separation of frequency components per mits real-time maximization of the SM ratio during acquisition and subseque nt processing steps. Linear separation of each ground roll frequency component allows calculatio n of phase velocities by simply measuring the linear slope of each frequenc y component. Breaks in coherent surface-wave arrivals, observable on the de composed record, can be compensated fur during acquisition and processing. Multichannel recording permits single-measurement surveying of a broad dept h range, high levels of redundancy with a single field configuration, and t he ability to adjust the offset, effectively reducing random or nonlinear n oise introduced during recording. A multichannel shot gather decomposed into a swept-frequency record allows the fast generation of an accurate dispersion curve. The accuracy of disper sion curves determined using this method is proven through field comparison s of the inverted shear-wave velocity (v(s)) profile with a downhole v(s) p rofile.