Sound Velocity in a Thin Shallowly Submerged Terrestrial-Marine Quaternary Succession (Northern Adriatic Sea)

Ana Novak,Marko Vrabec,Bogomir Celarc,Andrej Šmuc,Sašo Poglajen

doi:10.3390/w12020560

Abstract

Estimating sound velocity in seabed sediment of shallow near-shore areas submerged after the Last Glacial Maximum is often difficult due to the heterogeneous sedimentary composition resulting from sea-level changes affecting the sedimentary environments. The complex sedimentary architecture and heterogeneity greatly impact lateral and horizontal velocity variations. Existing sound velocity studies are mainly focused on the surficial parts of the seabed sediments, whereas the deeper and often more heterogeneous sections are usually neglected. We present an example of a submerged alluvial plain in the northern Adriatic where we were able to investigate the entire Quaternary sedimentary succession from the seafloor down to the sediment base on the bedrock. We used an extensive dataset of vintage borehole litho-sedimentological descriptions covering the entire thickness of the Quaternary sedimentary succession. We correlated the dataset with sub-bottom sonar profiles in order to determine the average sound velocities through various sediment types. The sound velocities of clay-dominated successions average around 1530 m/s, while the values of silt-dominated successions extend between 1550 and 1590 m/s. The maximum sound velocity of approximately 1730 m/s was determined at a location containing sandy sediment, while the minimum sound velocity of approximately 1250 m/s was calculated for gas-charged sediments. We show that, in shallow areas with thin Quaternary successions, the main factor influencing average sound velocity is the predominant sediment type (i.e. grain size), whereas the overburden influence is negligible. Where present in the sedimentary column, gas substantially reduces sound velocity. Our work provides a reference for sound velocities in submerged, thin (less than 20 m thick), terrestrial-marine Quaternary successions located in shallow (a few tens of meters deep) near-shore settings, which represent a large part of the present-day coastal environments.

Highlights

In geophysical investigations of the subsurface, velocity modeling is essential for converting two-way travel time of the observed reflections into the depth domain
Estimating sound velocity in seabed sediment of shallow near-shore areas submerged after the Last Glacial Maximum is often difficult due to the heterogeneous sedimentary composition resulting from sea-level changes affecting the sedimentary environments
Our study shows that an average sound velocity through the Quaternary sedimentary column is sufficient for depth conversion of high-resolution geophysical profiles acquired in thin Quaternary successions in shallow water depths

Summary

Introduction

In geophysical (acoustic/seismic) investigations of the subsurface, velocity modeling is essential for converting two-way travel time of the observed reflections into the depth domain. Restricted navigation, legal constraints, busy marine traffic, relatively low resolution of the acquired data and the surveying cost itself often make acquisition and maneuvering with streamers and seismic sources impractical or even impossible In such settings, high-resolution single-channel seismic and acoustic surveys provide a common alternative, but the velocity data must be obtained by other means, such as in situ. When velocity data for depth conversion of single-channel seismic or acoustic data are not acquired during surveying, a velocity value corresponding to the surficial sediment grain size (e.g., [19]) or a previously published value from a nearby location is usually used Whereas this approach is sufficient for geophysical surveys of uniform sedimentary layers, it produces significant uncertainties when dealing with pronounced lateral and vertical variability in sediment composition and architecture

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