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. 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 ax is consistently observed across a range of vertical correlation lengths (az). 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 ax≈1 m, recorded seismic amplitudes are, on average, reduced by ∼25 % 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 typical diameter of sediment cores, meaning that acoustically transparent mass-transport deposits may still appear well stratified and undeformed at core scale.

Volcanic Flank Collapse, Secondary Sediment Failure and Flow‐Transition: Multi‐Stage Landslide Emplacement Offshore Montserrat, Lesser Antilles

“…The visually deformed and disrupted nature of the sediment comprising Deposit 2a at this site is consistent with the mostly acoustically transparent nature of the deposit in Parasound profiles. Although even small amounts of deformation may have large consequences for the seismic character of a sedimentary unit (Ford et al, 2023;Karstens et al, 2023), the seismic character suggests that the deformation observed in the cores is laterally extensive and may imply disaggregation and loss of structures within the materials mobilized in this part of Deposit 2a (Figure 6). NW/SE oriented seismic profiles show that emplacement of the blocky center and of the smooth-surfaced apron occurred broadly on the same bottom surface, although seismic amplitudes are much higher and more continuous in the apron (Figures 2b and 2c).…”