Modelling Of Full Waveform Acoustic Logs In Soft Marine Sediments

Abstract

Full waveform acoustic logs obtained from the Deep Sea Drilling Project (DSDP) were modelled using synthetic full waveform acoustic logs. These synthetic logs were calculated using the discrete wavenumber method. The model is that of a fluid-filled borehole with a rigid logging tool in the center. Results from the modelling indicate that V[subscript p],V[subscript 3] and V[subscript f] along with P wave attenuation (l/Q[subscript 3]) are the primary controls on the full waveform acoustic logs of these soft sediments. S-wave attenuation (l/Q[subscript 3]) does not play a major role because the S-wave velocities (V[subscript 3]) of these fluid-saturated marine oozes are lower than the borehole fluid velocity (V[subscript f]), thus there is no refracted S-wave or pseudo-Rayleigh wave. However, the formation S-wave velocity does affect the amplitude of the observed P-wave train. Density variations by themselves have almost no discernible effect on the synthetics although in practice a change in density often is concurrent with a change in lithology and formation velocities. Matching the synthetic full waveform acoustic logs to those obtained during Leg 95 of the DSDP was formally done by a least squares linearized iteration inversion procedure. Only the P wavetrain and its associated leaky modes were taken into account. The forward model used in the inversion was a P-wave train generated by the branch cut integral method. Stable results in V[subscript 3] and Q[subscript p] were obtained. Variations in the velocity and attenuation from the inversion correlates with sedimentary units delineated from conventional logs and lithologic units identified by shipboard stratigraphers for the Baltimore Canyon Trough area. Full waveform logs, in combination with conventional logs, help to identify changes in the physical properties of these sediments as a result of the diagenesis of biogenic silica and calcium carbonate.