Mid-ocean ridges spreading at ultraslow rates of <20 mm yr-1 can exhume serpentinized mantle to the seafloor, or they can produce magmatic crust. However, seismic imaging of ultraslow spreading centres has not been able to resolve the abundance of serpentinized mantle exhumation, and instead supports 2-5 km of crust. Most seismic crustal thickness estimates reflect the depth at which the 7.1 km s-1 P-wave velocity is exceeded. Yet, the true nature of the oceanic lithosphere is more reliably deduced using the P-to S-wave velocity (Vp/Vs) ratio. Here, we report on seismic data acquired along off-axis profiles of older oceanic lithosphere at the ultraslow spreading Mid-Cayman Spreading Centre. High Vp/Vs ratios of >1.9 and continuously increasing P-wave velocity, changing from 4 km s-1 at the seafloor to >7.4 km s-1 at 2 to 4 km depth, indicate highly serpentinized peridotite exhumed to the seafloor. Elsewhere, either magmatic crust or serpentinized mantle deformed and uplifted at oceanic core complexes underlies areas of high bathymetry. The Cayman Trough provides therefore a window into mid-ocean ridge dynamics that switch between magma-rich and magma-poor oceanic crustal accretion, including exhumation of serpentinized mantle to ~25% of the seafloor. About 60% of the Earth's surface is oceanic crust and new seafloor is continually created along the ~65,000 km long mid-ocean ridge (MOR) system 1. Most oceanic crust has a relatively uniform

We apply seismic full waveform inversion to SH‐ and Love‐wave data for investigating the near‐surface lithology at an archaeological site. We evaluate the resolution of the applied full waveform inversion algorithm through ground truthing in the form of an excavation and sediment core studies. Thereby, we investigate the benefits of full waveform inversion in comparison with other established methods of near‐surface prospecting in terms of resolution capabilities and interpretation security. The study is performed in a presumed harbour area of the ancient Thracian city of Ainos. The exemplary target is the source of a linear magnetic anomaly oriented perpendicular to the coast, which was found in a previous magnetic gradiometry survey, suggesting a mole. The SH‐wave full waveform inversion recovered a subsurface SH‐wave velocity model with submeter resolution showing lateral and vertical velocity variation between 40 and 150 m/s. To tame the non‐linearity of the full waveform inversion, a sequential inversion of frequency bands has to be combined with time‐windowing in order to separate the Love wave from the reflected SH wavefield. We compare the full waveform inversion results with multichannel analysis of surface waves, standard seismic reflection imaging, electrical resistivity tomography and electromagnetic induction. It turns out that the respective depth sections are correlated to a certain degree with the full waveform inversion results. However, the structural resolution of the other geophysical methods is significantly lower than for the full waveform inversion. An exception is the reflection seismic imaging, which shows the same resolution as full waveform inversion but can only be interpreted together with the full waveform inversion–based velocity model. An archaeological excavation as well as coring data allows ground truthing and a direct understanding of the geophysical structures. The results show that the target was a sort of near‐surface trench of about 3–4 m width and 0.8 m to 1.0 m depth, filled with silty sediment, which differs from the layered surrounding in colour and composition. The ground truthing revealed that only SH‐wave full waveform inversion and seismic reflection imaging could image the trench and sediment structure with satisfying lateral and depth resolution. We emphasize that the velocity distribution from SH‐wave full waveform inversion agrees closely with the excavated subsurface structures, and that the discovered changes in seismic velocity correlate with changes in the sand content in the respective sediment facies sequences. The study demonstrated that SH‐wave full waveform inversion is capable to image structural and lithological changes in the near subsurface at scales as low as 0.5 m, thus providing the high resolution needed for archaeological and geoarchaeological prospection.

We performed geophysical and geoarchaeological investigations in the Wadden Sea off North Frisia (Schleswig-Holstein, Germany) to map the remains and to determine the state of preservation of the medieval settlement of Rungholt, especially its southern dyke segment, called the Niedam dyke. Based on archaeological finds and historical maps, Rungholt is assumed to be located in the wadden sea area around the island Hallig Südfall. During medieval and early modern times, extreme storm events caused major land losses, turning cultivated marshland into tidal flats. Especially the 1st Grote Mandrenke (or St. Marcellus’ flood), an extreme storm surge event in 1362 AD, is addressed as the major event that flooded and destroyed most of the Rungholt cultural landscape. Cultural traces like remains of dykes, drainage ditches, tidal gates, dwelling mounds or even plough marks were randomly surveyed and mapped in the tidal flats by several authors at the beginning of the 20th century. Due to the tidal flat dynamics with frequently shifting tidal creeks and sand bars, the distribution of cultural remains visible at the surface is rapidly changing, making it hard to create a comprehensive map of the cultural landscape by surveying. Today, the Niedam dyke area is fully covered by tidal flat sediments, depriving any remains from further archaeological investigation. Since little is known about the precise location or state of preservation of these remains, our investigation aimed at the rediscovery of the medieval dyke system and associated structure with modern and accurate geophysical, geodetical and geoarchaeological methods. Magnetic gradiometry revealed a large part of the medieval dyke, confirming two tidal gates and several terps connected inland with the dyke, providing a detailed example of a Frisian medieval dyke system. Based on our results, the so far inaccurate and incomplete maps of this part of Rungholt can now be specified and completed. Beyond that, seismic reflection profiles give a first depth resolving insight in the remains of the dyke system, revealing a severe threat to the medieval remains by erosion. The site is exemplary for the entire North Frisian coast, that was influenced by multiple flood events in the middle ages to modern times.

We present a new three-dimensional (3D) marine seismic data acquisition system, named PingPong, developed for archaeological prospection in shallow water.Prospection targets for the system are ancient harbour sites and sedimented remains of shipwrecks. The prospection of such targets often means working at the transition from land to water, in areas of only a few meters of water depth and hardly accessible waters. An acquisition system for such environments needs to meet specific demands, especially low draught and marginal weight besides the requirements of archaeological prospection, meaning decimetre resolution and 3D imaging capabilities, together with fast multichannel acquisition to be able to cover large areas. We explain the properties of the PingPong system and show its imaging capabilities using the case study of a sedimented medieval shipwreck. The study area is located at the innermost part of the Baltic fjord Schlei, Germany. In 2014, divers found a wreck in this area, mostly covered by mud. Findings and two timbers, dated by dendrochronology, indicated that the wreck is a Scandinavian transport ship dating to the middle of the twelfth century and related to Schleswig, which is located 2 km northwest of the study area.We show that the PingPong system is able to image the major remains of the wooden wreck at the seafloor and underneath. The acquired seismic datacube has a resolution of 0.15 m. It shows a number of distinct reflections that can clearly be assigned to the shipwreck, helping to understand the overall condition of the wreck. The reflections originate from one half the ship's hull, which is tilted to the side. The reflections concentrate in the first metre below the seafloor and correlate well with the results from the diving prospection.

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