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

Abstract

ABSTRACTIn 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.