High resolution seismology, archaeology and submerged landscapes - an interdisciplinary study

Abstract

Archaeological investigation of the shallow coastal environment has traditionally been conducted through exclusively marine or terrestrial methods, creating artificial division between sites located in the intertidal and subtidal zones. This thesis demonstrates a successful seamless inter-disciplinary survey methodology, utilising side scan sonar, sub-bottom profilers, aerial photography and archaeological field inspection to establish the correct spatial and contextual relationships of submerged archaeological structures. To facilitate survey of the shallow intertidal zone, a novel catamaran-based platform is developed, which carries side scan and Chirp sub-bottom sonar systems, and is proved operational in water depths greater than 1.3m and 2.4 m respectively. Digital survey technologies offer the potential for post-survey signal enhancement of high-frequency seismic data. Investigation into the characteristics of commercial Chirp and Boomer seismic indicates that Chirp systems offer superior repeatability and vertical resolution, making possible development of a rapid, robust processing flow for high-resolution investigation of sediment structure. Development of shoreface exploitation sites is strongly controlled by interaction between contemporary physical processes and the underlying geomorphology. High resolution seismic techniques are used here to study the drumlins, late-Quaternary seismic stratigraphy and relative sea level history of Strangford Lough, Northern Ireland. Unique seismic sections through submerged late-Midlandian (Devensian) drumlins, parallel to the direction of ice flow, indicate that adjacent drumlins share a common internal structure and are preferentially located over subtle topographic maxima in the underlying substrate. A high-resolution seismo-stratigraphy is developed from Chirp profiles through the northern lough. Correlation with field observations, cores and published RSL data from the Irish Sea suggests that Strangford Lough underwent isolation from the sea during the early Holocene, implying that early Mesolithic settlers were attracted to the area by an exploitable freshwater, rather than marine resource. The intertidal mudflats of the Blackwater Estuary contain an extensive, but largely inaccessible and rarely exposed archaeological resource, including wooden coastal fish weirs dated to Saxon times. Application of side scan sonar to areas previously photographed during aerial reconnaissance has facilitated accurate mapping of six large (100-2000m length) structures, increasing the site record content up to 250%, and improving understanding of the manner in which these structures operated. Structures on the southern shore of the estuary extend into the subtidal zone to c. -2m OD. It is suggested that the lower altitudes of these sites imply a significantly earlier date for development of this fishing technique than presently accepted.

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