Imaging wooden artefacts using Chirp sources

This paper investigates the potential for imaging submerged and buried wooden artefacts in the marine environment using high-resolution seismic reflection techniques. Models to calculate theoretical reflection coefficients of 11 wood species buried in shallow marine sediments are developed. The models use estimates of acoustic impedance contrasts to predict the strength of reflections resulting from buried wooden artefacts. Traditionally, wooden wrecks are composed of oak, with lesser components of mahogany, pine and elm. Experimental results on wood samples from a sixteenth century oak wreck (the Mary Rose) are presented as a control on theory. Results indicate that wooden wrecks may be readily imaged by a suitable sub-bottom profiler operating under appropriate survey conditions. Particular reference is made to Chirp sonar, which transmits a frequency modulated pulse, providing high-resolution digital seismic data. Chirp data acquired over an eighteenth century buried oak wreck (the Invincible, Horsetail, East Solent) is presented. Calculations of reflection coefficients from reflection data acquired over the site support both the theoretically and experimentally derived data.