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.

<p class="western" lang="en-GB">Mass-transport deposits often show a low-amplitude, “acoustically transparent” seismic response compared to unfailed sediments. This amplitude signature is often interpreted as a lack of coherent internal reflectivity caused by a loss of internal structure during transport and emplacement, and is widely used to delineate mass-transport deposits in sub-bottom profiler data. An apparent contradiction is that cores penetrating such “acoustically transparent” deposits can sometimes retrieve well-stratified sediments that show little evidence of deformation.</p> <p class="western" lang="en-GB">In this study we examine the variation in the single-channel seismic amplitude response with changing heterogeneity using synthetic seismic modelling. We model the internal structure of mass-transport deposits as a two-component binarised random medium, where the lateral correlation length is used to artificially control the degree of internal deformation/scale of internal structure, while maintaining the magnitude of the internal reflectivity constant. We construct two synthetic models: i) a simplified single-source marine example and ii) a multi-source example based on a real world “acoustically transparent” mass-transport deposit imaged by a dense network of AUV sub-bottom profiles in the Black Sea. We use 2-D elastic finite-difference modelling to model the seismic response (at sub-bottom profiler bandwidths) of an ensemble of both synthetic models with varying geostatistical parameters and random seeds for the mass-transport deposit zones. For the single-source synthetic model a reduction in observed amplitude with reduced lateral scale length is consistently observed across a range of vertical correlation lengths. For the real world Black Sea example, with realistic elastic and geostatistical parameters based on cone-penetration tests and physical property measurements from sediment cores, we find that when the lateral scale length of the random medium is around 1 m, recorded seismic amplitudes are, on average, reduced by ∼15% relative to unfailed sediments.</p> <p class="western" lang="en-GB">We conclude that relatively small amounts of deformation at scales larger than the dominant seismic wavelength are, in general, able to a generate significant decrease in seismic amplitude, without requiring a reduction in the average reflectivity. Our 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 reduction in seismic amplitudes is likely much larger than the diameter of sediment cores, meaning that such mass-transport deposits may still appear well-stratified and undeformed when cored.</p>