Seismic amplitude response to internal heterogeneity of mass-transport deposits

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 (inline-formulaa_x) as a proxy for the degree of internal deformation. The average internal reflectivity is held approximately constant with increasing deformation by fixing the two component sediment lithologies to have realistic P-wave velocity and density based on sediment core measurements from the study area. For a controlled single-source synthetic model a reduction in observed amplitude with reduced inline-formulaa_x is consistently observed across a range of vertical correlation lengths (inline-formulaa_z). For typical autonomous underwater vehicle (AUV) sub-bottom profiler acquisition parameters, in a simulated mass-transport deposit with realistic geostatistical properties, we find that when inline-formulaa_x≈1 inline-formulam, recorded seismic amplitudes are, on average, reduced by inline-formula∼25 % relative to unfailed sediments (inline-formulaa_x≫10³ inline-formulam). We also observe that deformation significantly larger than core scale (inline-formulaa_x>0.1 inline-formulam) 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 typical diameter of sediment cores, meaning that acoustically transparent mass-transport deposits may still appear well stratified and undeformed at core scale.