Scattered seismic noise

Abstract

Wave test seismograms from an Aramco concession area show crisscross patterns of noise which mask the reflections. The criss-cross pattern that follows the arrival of the Rayleigh wave is sometimes very strong and entirely masks any reflections that may be present. This zone is very often referred to as the 'noise cone'. These noise patterns are also seen at larger offsets at times much earlier than the arrival of the direct Rayleigh wave. These patterns are believed to be due mainly to the scattering of Rayleigh waves into Rayleigh waves in the noise cone, and due to the conversion of refractions, multiply-reflected refractions, reflections and multiples into Rayleigh (or shear) waves outside the noise cone. A computer model was constructed using a random distribution of scatterers. Each scatterer was assumed to behave like the spherical obstacle in Lamb's problem of acoustic scattering. A source was excited and the scattered impulses were recorded on an array of seismometers. The convolution of the recorded data with a typical wavelet resulted in a section that resembles the noise cone remarkably well. This suggests that the criss-cross pattern may be due to scattering by a large number of randomly distributed inhomogeneities or by a 'perfectly' random medium.