3D Chirp: Underwater Sub-Bottom Profiling at the University of Southampton
The University of Southampton’s 3D Chirp sub-bottom profiler is used for underwater 3D seismic imaging. The scans it produces are of 3D data volumes, rather than the traditional 2D sections—meaning that the results can be used to reconstruct rich, three-dimensional models of the scanned areas. These models can be viewed in horizontal, vertical and arbitrary slices, offering a much more robust means to analyse the data.
Sub-bottom profiling is a standard tool in industrial hydrocarbon exploration. In these settings, the system is used to penetrate kilometers deep into the earth, and achieve a resolution of 10s of meters. However, 3D Chirp aims to achieve this same approach but with decametres (tens of metres) of penetration and decimetre (tenths of metres) resolution.
Underwater small-object detection
Detailed imaging of the seabed and subseabed morphology is crucial in the marine construction industry and in homeland defense. Preliminary surveys to identify underwater hazards traditionally combine acoustic seabed mapping and the use of divers with sounding poles. Divers are constrained greatly by environmental conditions such as water temperature, site depth, currents, tides, and visibility. Even in perfect conditions, they can cover only 10 m /day and can sample only the top 1 m of subsurface.
The 3D chirp sub-bottom profiler, however, produces a high-resolution 3D seismic volume of completely buried structures. The system was tested at an atidal basin on the southern coast of England, to map bedrock protrusions and the size and distribution of buried objects. Later dredging of the entire 23,000m basin area produced an exhaustive comparison between the shapes identified by 3D chirp, and the actual sunken objects. All objects recovered during the dredging correlated to an acoustic target identified by 3D chirp, and within the predicted dimensions.
Except for one acoustic anomaly, dredging confirmed a 100% success rate in identifying buried objects and showed a strong correlation between observed acoustic signature and object size. With those results, we successfully demonstrated that through acquisition of a high-resolution 3D seismic volume, it is possible to locate buried objects larger than 0.30×0.30m. Indeed, through fuller interpretation of scan data it is possible to estimate the approximate size and shape of objects and in some cases to determine the nature of the buried material.
Henry V’s The Grace Dieu
The remains of Henry V’s flagship, the Grace Dieu, currently lie buried within the inter-tidal sediments of the River Hamble (S. England). Previous archaeological investigations have been hindered by difficult excavation conditions resulting in a poor understanding of the dimensions, shape and degradation state of the hull’s deeper structure. This study therefore aimed to image, characterize and reconstruct the buried remains of this vessel using a high-resolution 3D acoustic sub-bottom Chirp system with RTK-GPS positioning capability. The accurate navigation and high-resolution data that were acquired enabled the construction of a full 3D image of the site that not only identified the remains of the wooden hull, but also features buried within it.
In addition, the degradation state of these buried wooden remains were investigated by calculating reflection coefficients while a hypothetical larger reconstruction of the Grace Dieu’s hull was achieved, through the use of the ShipShape ship design software package. The results of this project demonstrate that (i) acoustic data can be used to successfully image buried wooden shipwrecks, (ii) artefacts are buried within the hull of the Grace Dieu, (iii) there is variation in the degradation state of the buried timbers, as calculated from the acoustic data, with the shell of the vessel being moderately well preserved, and (iv) the Grace Dieu was a very large ship for its time (possibly over 60 m long and 16 m wide).
The outcomes of this research not only have considerable implications for the management and monitoring of submerged and buried archaeological sites but also for planning intrusive surveys, should they be required.
3D imaging of Lake Windermere, UK
Windermere is a glacially overdeepened lake located in the southeastern Lake District, UK. Using the three dimensional (3D) Chirp sub-bottom profiler, mass movement deposits related to the Younger Dryas period (approximately 12,800 to 11,500 years before present) were imaged here in unprecedented detail. Three distinct flow events were identified and scanned throughout the 3D survey area. Mapping of the debris flows and their interactions with preexisting sediments has permitted the reconstruction of a depositional history of the area.
This project has demonstrated the effectiveness of using high resolution 3D scanning for the geomorphological interpretation of small-scale, shallow water mass movement deposits. Future scanning over similarly easily accessible features can therefore improve our understanding of the formation of such structures on both small- and large-scales.
People
Underwater small-object detection
Mark E. Vardy, University of Southampton
Justin K. Dix, University of Southampton
Timothy J. Henstock, University of Southampton
Jonathan M. Bull, University of Southampton
Martin Gutowski, GeoAcoustics Ltd
Henry V’s The Grace Dieu
Ruth M.K. Plets, Memorial University, Canada
Justin K. Dix, University of Southampton
Jon R. Adams, University of Southampton
Jonathan M. Bull, University of Southampton
Timothy J. Henstock, University of Southampton
Martin Gutowski, GeoAcoustics Ltd
Angus I. Best, University of Southampton
3D imaging of Lake Windermere, UK
Mark E. Vardy, University of Southampton
Luke J.W. Pinson, University of Southampton
Jonathan M. Bull, University of Southampton
Justin K. Dix, University of Southampton
Timothy J. Henstock, University of Southampton
John W. Davis, University of Southampton
Martin Gutowski, GeoAcoustics Ltd