Integrating ground-penetrating radar and borehole data from a Wadden Sea barrier island

Abstract Sea level rise may have large implications for low-gradient barrier coastal systems. This problem motivated an integrated ground-penetrating radar (GPR) and sedimentological study of the Romo Wadden Sea barrier island. Crossing W–E and N–S-oriented 100 MHz GPR reflection profiles with a total length of about 30 km were acquired on the island. In this case study, we process and analyze GPR data and investigate the feasibility of using integrated GPR and sedimentological log data to constrain spatial variations in lithology, structures and changing depositional environments of the Romo barrier island. We document different standard processing steps which lead to increased signal-to-noise ratio, improved resolution and trustworthy GPR-to-borehole correlation. The GPR signals image the subsurface layering with a vertical resolution of ~ 0.2–0.3 m. The penetration depth of the GPR reflection signals varies between 7 and 15 m in the interior of the island where the shallow subsurface is not influenced by saltwater intrusion or fine-grained salt marsh sediments. Analysis of common midpoint reflection data constrains the radar wave velocity distribution in the subsurface and facilitates depth conversion of the reflection profiles. The GPR reflections are correlated with sedimentological facies logs, and we test to which extent it is possible to map the architecture of different sedimentary units of the Romo barrier island based on joint interpretation of the GPR and core data. Detailed investigation of signal waveform variations and amplitude decay analysis are used for assessing lateral variation in lithology and composition. The combined GPR and borehole investigations constrain the nature of different marine and aeolian sedimentary sequences that have formed the barrier island system. We observe two prominent reflections which, in general, exhibit good continuity. The upper prominent reflection (0–2 m above present mean sea level (pmsl)) outlines swales and beach ridge flanks. In depressions (swales), this reflection has a complex signature originating from the contrasts in electrical properties caused by the presence of a thin layer of organic mud or gyttja. On the beach ridge flanks, this reflection has a simpler appearance and most likely represents contrasting sandy beds. The lower prominent reflection (0–3 m below pmsl) is relatively strong and represents the top of a clay layer in some areas. In other parts of the data set, this lower reflection is weaker and interpreted to represent the transition between two marine sand deposits of different origin. Mounded structures with internal dipping reflections are interpreted to represent a system of 20–40 m wide and up to ~ 2 m high fossil beach ridges in the shallow subsurface of the Romo barrier island. The base of the beach ridges is found 0–1 m above pmsl. Some beach ridges show signs of erosion, and aeolian dunes have developed above them. The dunes appear with a variety of internal structures, but are often characterised by steeply dipping internal structures.

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