Coastal protection in the form of dike constructions has a long history at the German North Frisian coast dating back to the High Middle Ages. As the vast majority of the dikes built prior to the devastating storm surges of the Middle Ages have been irretrievably destroyed, mostly sparse remains and only a few well preserved of these medieval dikes are found along the German North Frisian coast and within the Wadden Sea. Not all details of their construction and dimensions are yet understood. In the present case study, we investigate the historical Schardeich on the island of Pellworm in the German North Sea in a noninvasive way using shear waves (SH-waves). For the data interpretation, we applied a combination of seismic full waveform inversion and classical seismic reflection imaging to determine the interior structure of the dike and its underlying layers at the highest possible resolution. The results obtained on land are compared with dike remains found in the tidal flats. These remains show up in marine seismic sections as characteristic reflections, which probably represent a compaction layer caused by the load of the former dike. For ground truthing, we compare the seismic results with internal dike structures found in nearby excavations. The comparison highlights that FWI is a reliable tool for near-surface archaeological prospecting. We find that SH-wave FWI provides decimetre-scale velocity and density models that allow, together with the seismic reflection section, to determine distinct construction phases of the dike. The investigated dike further shows a depression at base level of about 0.75 m, which is of the same order as observed for the dike base reflections in the tidal flats. Transferring these findings to the dike remains mapped in the tidal flats, we derive a height of the former dike from 2.2 to 4.4 m.

中文翻译：

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一种用于评估中世纪海堤内部结构的新型地震全波形反演方法

以堤坝建设的形式进行海岸保护在德国北弗里斯兰海岸有着悠久的历史，其历史可以追溯到中世纪中期。由于中世纪毁灭性风暴潮之前建造的绝大多数堤坝已被不可挽回地摧毁，因此在德国北弗里斯兰海岸和瓦登海内发现的这些中世纪堤坝大多稀疏，只有少数保存完好。目前尚未了解其结构和尺寸的所有细节。在本案例研究中，我们利用剪切波（SH 波）以非侵入性方式调查了德国北海 Pellworm 岛上历史上的 Schardeich。对于数据解释，我们应用地震全波形反演和经典地震反射成像相结合，以尽可能高的分辨率确定岩脉及其下伏层的内部结构。将在陆地上获得的结果与在滩涂中发现的堤坝遗迹进行比较。这些遗迹在海洋地震剖面中显示为特征反射，可能代表了由前堤坝荷载引起的压实层。对于地面实况，我们将地震结果与附近挖掘中发现的内部堤坝结构进行比较。比较结果表明，FWI 是近地表考古勘探的可靠工具。我们发现 SH 波 FWI 提供了分米级速度和密度模型，可以与地震反射剖面一起确定堤坝的不同施工阶段。所调查的堤坝还显示出约 0.75 m 的基准面凹陷，这与在滩涂中观察到的堤坝基准反射具有相同的数量级。将这些发现转移到滩涂中绘制的堤坝遗迹中，我们得出前堤坝的高度为 2.2 至 4.4 m。