Marine high-resolution reflection seismology : acquisition, processing and applications

Buried wooden artefacts can be readily imaged by Chirp-sourced reflection seismology. Compressional wave velocity parallel to the wood grain (V_L) is consistently faster than that across-grain (V_R and V_T). Theoretical and experimental predictions of reflection coefficients (K_R) calculated for wood buried in unconsolidated marine sediments are typically large and negative (K_R = +0.27 to -0.79; K_Roak = 0.03 to -0.64). Variations in K_R are dependent upon the burial-sediment, wood species and structural coherency of the artefact. Synthetic modelling demonstrates that small changes in the seafloor impedance profile causes random switches in the polarity of the Chirp pulse. Reflection coefficients are calculated from Chirp sub-bottom data using amplitude-time relationships and polarity information derived from trace-mixing. Calculations from Chirp data acquired over a partially buried 17^th Century oak-wreck provide an average K_R of -0.26, in agreement with the predicted range. A pseudo 3-dimensional Chirp survey of the excavated Mary Rose wreck site (East Solent, UK) identified two brightspot anomalies, buried to a depth of 4-5 m, trending east-west adjacent to the western margin of the excavation hole. These anomalous reflectors are interpreted as infilled palaeo-scour features associated with the wrecking and subsequent degradation of the Mary Rose. Fill material comprises wreck fragments and coarse sediment sourced from the ship's ballast. Longitudinal scour features were previously unrecognised on the site, and represent the first time such palaeo-scour marks have been recognised in the sedimentary record. Integrated Chirp and side-scan sonar surveys of the Invincible site (East Solent, UK) demonstrate the capability of high-resolution acoustic techniques to: identify the extent and coherency of semi-exposed wrecks, determine the wrecking history of a site and act as an effective management tool. The distribution of wreck material is primarily controlled by storm-associated wave action, depositing the bulk of fragmented wreck-structure to the north and north-east of the in situ port side. Evidence suggests that current site stability is controlled by a combination of tidally induced currents and anthropogenic activity.