Misinterpretation of velocity pull‐ups caused by high‐velocity infill of tunnel valleys in the southern Baltic Sea

Abstract

In this study, we undertake a renewed investigation of up-bent reflections seen in seismic time sections from the Baltic Sea, Bay of Kiel. These warped reflections stretch over the entire vertical extent of the sections, from Permian to Quaternary strata, and underlie tunnel valleys. Previous studies interpreted these structures as anticlines, explaining them together with adjacent faults and disrupted strata as the consequence of ice-load-induced salt tectonics. This conclusion would have influenced theories on how tunnel valleys formed. However, well data from tunnel valleys in other regions supported the interpretation of the up-bent reflections as imaging artefacts (pull-ups). A newly acquired long-offset, multichannel seismic data set images all strata from Base Zechstein up to the seafloor. Owing to the length of the streamer and a shallow water depth, the data display significant moveout and refracted waves, allowing the application of different quantitative methods to investigate velocities. By generating partialoffset sections, we reveal an offset dependence in the imaging of the up-bent structures caused by a local, near-surface high-velocity zone. This also explains a smoothing of the up-bending with depth in the seismic image. A velocity model gained by a traveltime tomography shows positive velocity anomalies in the upper strata correlating with tunnel valleys resolved in the reflection images. A pre-stack depth migration performed with a velocity model containing a high-velocity zone results in a seismic image almost free of the observed up-bent reflections. High-frequency reflection seismic data confirm this result as it shows a detailed image of a tunnel valley with a phase-reversed bottom reflection caused by the velocity inversion at the base of the high-velocity valley fill deposits. Hence, there is consistent evidence that all up-bent reflections in the Bay of Kiel are imaging artefacts (pull-ups) that formed beneath tunnel valleys. A salt tectonic control on tunnel valley evolution is, consequently, not likely. This study is the first purely seismic data-driven study that proves high-velocity valley fill deposits. Our findings imply that extra care must be taken when interpreting reflection undulations as tectonic features where glacial deposits are present.

Impact of Late Cretaceous inversion and Cenozoic extension on salt structure growth in the Baltic sector of the North German Basin

“…Note the small velocity pull‐ups due to high‐velocity channel infill near the seafloor (marked with grey “^” symbols along the distance axis. See Frahm et al, 2020 for further details). EHT: Eastholstein Trough; EHMB: Eastholstein‐Mecklenburg Block.…”

The Langeland Fault System unravelled: Quaternary fault reactivation along an elevated basement block between the North German and Norwegian–Danish basins

“…In the Bay of Kiel and Pomeranian Bay, Al Hseinat & Hübscher (2017) interpreted Quaternary fault reactivation of Mesozoic–Cenozoic fault systems, some of which were re‐evaluated by Frahm et al . (2020). The recent fault reactivation was related to glacially induced stress acting in combination with the present‐day stress field and thus, agree with observations along the LFS.…”

Gas - escape features along the Trzebiatów Fault offshore Poland: evidence for a leaking petroleum system

“…Problem of imaging artifacts in form of pull-downs is contrasted with the pull-ups concept caused by highvelocity localized anomaly (e.g., till within tunnel valleys; Frahm et al, 2020). Structural image below the gas charged shallow sediments could be misinterpreted as localized syncline-like features.…”