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 partial‐offset 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 travel‐time 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.

中文翻译：

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波罗的海南部隧道河谷高速填埋引起的速度上拉的误解

摘要 在这项研究中，我们对基尔湾波罗的海地震时间剖面中的向上弯曲反射进行了重新调查。这些扭曲的反射波延伸到剖面的整个垂直范围，从二叠纪到第四纪地层，并位于隧道山谷的下方。先前的研究将这些结构解释为背斜，将它们与相邻断层和地层破坏一起解释为冰荷载引起的盐构造的结果。这一结论将影响有关隧道山谷如何形成的理论。然而，来自其他地区隧道谷的井数据支持将向上弯曲的反射解释为成像伪影（上拉）。新获得的长偏移距、多道地震数据集对从泽希斯坦基地到海底的所有地层进行成像。由于拖缆的长度和浅水深，数据显示显着的时差和折射波，允许应用不同的定量方法来研究速度。通过生成部分偏移剖面，我们揭示了由局部近地表高速带引起的上弯结构成像中的偏移依赖性。这也解释了地震图像中随着深度的向上弯曲的平滑。通过走时层析成像获得的速度模型显示，上层地层的正速度异常与反射图像中解析的隧道谷相关。使用包含高速带的速度模型进行的叠前深度偏移会产生几乎没有观察到的向上弯曲反射的地震图像。高频反射地震数据证实了这一结果，因为它显示了隧道谷底反射的详细图像，这是由高速谷填充沉积物底部的速度反演引起的。因此，有一致的证据表明，基尔湾的所有向上弯曲反射都是在隧道山谷下方形成的成像人工制品（上拉）。因此，对隧道谷演化的盐构造控制是不可能的。这项研究是第一个纯地震数据驱动的研究，证明了高速填埋沉积物。我们的发现意味着在将反射波动解释为存在冰川沉积物的构造特征时必须格外小心。