Seismic amplitude response to internal heterogeneity of mass-transport deposits

Abstract

Abstract. Compared to unfailed sediments, mass-transport deposits are often characterised by a low-amplitude response in single-channel seismic reflection images. This ‘acoustic transparency’ amplitude signature is widely used to delineate mass-transport deposits and is conventionally interpreted as a lack of coherent internal reflectivity due to a loss of preserved internal structure caused by mass-transport processes. In this study we examine the variation in the single-channel seismic response with changing heterogeneity using synthetic 2-D elastic seismic modelling. We model the internal structure of mass-transport deposits as a two-component random medium, using the lateral correlation length (ax) as a proxy for the degree of internal deformation, whilst maintaining approximately constant internal reflectivity with increasing deformation. For a controlled single-source synthetic model a reduction in observed amplitude with reduced ax is consistently observed across a range of vertical correlation lengths (az). For typical AUV sub-bottom profiler acquisition parameters, in a simulated mass-transport deposit with realistic elastic and geostatistical properties, we find that when ax ≈ 1 m, recorded seismic amplitudes are, on average, reduced by ∼ 15 % relative to unfailed sediments (ax ≫ 103 m). We also observe that deformation significantly larger than core-scale (ax > 0.1 m) can generate a significant amplitude decrease. These synthetic modelling results should discourage interpretation of the internal structure of mass-transport deposits based on seismic amplitudes alone, as ‘acoustically transparent’ mass-transport deposits may still preserve coherent, metre-scale internal structure. In addition, the minimum scale of heterogeneity required to produce a significant reduction in seismic amplitudes is likely much larger than the diameter of sediment cores, meaning that ‘acoustically transparent’ mass-transport deposits may still appear well-stratified and undeformed at core-scale.