Frequency-dependent Amplitude Versus Offset analysis of a Cenozoic mass-transport deposit on the Namibian slope

Abstract

Abstract A partially tuned, ‘soft’ seismic reflection, overlying a regional limestone marker on the west African slope, is shown to represent a sequence, up to 30 m thick, within which various individual stratigraphic units are identified, some of which display characteristics of a mass-transport deposit (MTD). The seismic reflection is described in terms of its two-way time structure, reflection amplitude pattern, complex seismic attributes and AVO response. Frequency-dependent aspects of the AVO response, resulting from velocity attenuation and dispersion of seismic signals, are shown to indicate variations in pore-fluid gas concentration. These are exploited to assess lithological, petrophysical and pore-fluid characteristics of the various sub-units within the sequence. A relative acoustic impedance inversion of the seismic data is combined with a low-frequency model, constructed from acoustic velocity data, to approximate absolute acoustic impedance characteristics of the sequence and to model behaviour of the various sub-units across the area. These pseudo-absolute impedance data are combined with evidence from AVO analysis, to construct a 2D model of the MTD unit and to generate a synthetic seismic record, which confirms the interpretation of stratigraphic, petrophysical and lithological properties of the MTD complex.