Rayleigh waves in short-period seismic noise

Abstract

Operation of short-period vertical seismometers at depths down to 3000 m in abandoned oil wells provides a new method of studying seismic surface waves. Power spectral density functions and the cross-products of simultaneous noise samples at the surface and at depth are used to obtain the change in amplitude and phase with depth. The vertical component of the noise is shown to be caused mainly by fundamental and higher mode Rayleigh waves. The fundamental, first, and third Rayleigh modes are identified in the noise. Each higher mode can be identified by its unique variation in displacement with depth and the 180-deg phase shifts that occur at the nodal points. The experimentally determined displacement of the different Rayleigh modes with depth is in good agreement with the theoretical displacement.