Some Properties of Backscattered Seismic Returns

Abstract

Seismic amplitudes occurring in typical marine records indicate that the source-generated wavefield commonly exceeds ambient noise for much, if not all, of the normal recording time. When this behaviour is observed in areas of poor specular reflections and with small source-receiver separation, a reasonable inference is that the wavefield is sustained by backscattered source energy. There is abundant evidence that sea-floor irregularities play a particularly important role in this. Even in good reflection areas, sea-floor scattering can influence the seismic section in subtle, often unsuspected ways. Out-of-plane scatterers generate artifacts in response to normal 2D migration. Velocity stratification allows multipath propagation of scattered energy, so that a single scatterer may evoke several responses at different times characterised by different velocities. Whereas common midpoint (CMP) stack selectively attenuates scattered noise propagating within some range of azimuth angles determined by the primary velocity function, an unfortunate side effect is to enhance backscattered noise over the complimentary range of angles. Moreover, the source-receiver offset distances usually employed for stacking must increase susceptibility to interference from converted modes.