5.4 Underwater Surface Mapping

An increasing array of techniques is available for the investigation and mapping of palaeo-landscape surfaces that are both at the present seafloor and are currently buried beneath this. The techniques are applied as part of routine surveys for industry (oil and gas site surveys, pipe line studies, geotechnical investigations, wind farm developments) and occasionally as bespoke surveys for archaeology. See also the ScARF Marine and Maritime Panel Report. 

Single Beam Echosounder

Bathymetry is typically measured using an echo sounder with either single or dual frequency transducers. These transducers measure the travel time for an acoustic pulse and with some equipment the strength of amplitude return. A bathymetric map and amplitude map is produced by extrapolating data between survey lines.

Multibeam Echosounders

Two types of technology exist for making very high resolution maps of bathymetry, namely multibeam sonar and swath-bathymetry sonar. In both cases, acoustic signals are used to obtain multiple return energy from a swath of points beneath and to the side of the survey vessel. Recent advances in multibeam technology, the deployment of the technology and the processing of the data have led to spectacular, very high resolution models of the seafloor and wreck sites. These models provide an indispensible tool not only for archaeological site survey but also for the management of archaeological sites.

Sidescan Sonar Systems

The sidescan sonar has become one of the most often used of survey tools in the marine environment as it affords the surveyor a ‘picture’ of the seafloor in terms of amplitude of acoustic return. The sidescan sonar is a side-looking sonar with acoustic energy propagated to either side of the sonar head along a swath over the seafloor. This energy is recorded as a time-series with the amplitude of returns measuring the strength of energy reflected from the bottom and objects on the bottom. Sidescan sonars operate in the range of frequencies between 100KHz and 900KHz, the latter giving very high (cm) resolution of the seafloor. The individual swaths are captured digitally so that a mosaic can be produced for a complete image of the seafloor over the survey area.

Magnetic Signature Analysis

Marine magnetic surveying has become a standard technique for mapping the location of ferromagnetic metallic objects, such as recent wrecks, on the seafloor. The magnetometer is typically towed near to the bottom along lines that are closely spaced apart. Recent advances in technology include improvement in signal to noise ration, use of multiple instruments and use with other instrument platforms. Surveying of metallic but non-ferromagnetic remains can be achieved by the use of electromagnetic-based survey equipment. This type of survey has had limited testing in the archaeological community.

Scottish Research Capacity
Marine equipment pools are held at St Andrews University, the University of the Highlands and Islands and the Scottish Association for Marine Science (SAMS). Several commercial companies also have marine survey capacity.

Underwater Sub-Surface Mapping

Sub-surface or sub-bottom mapping is necessary when a structure such as a wreck site or a feature such as a palaeo-landscape is buried. The majority of techniques used in maritime archaeology for imaging the sub-surface rely on the propagation of an acoustic pulse. For sub-bottom surveying the depth of penetration below the seafloor and the resolution with which the data can be resolved is a function of the acoustic frequency spectrum. In general, in order to look further into the seafloor the lower the frequency the lower the resolution. A number of acoustic sources are available from high frequency pinger systems with 10-20cm resolution through to low frequency boomer systems with 20-40cm resolution. Typical deployment of these techniques is along survey lines and thus 2D cross sections to the sub-surface are acquired. Techniques for full 3D imaging are being currently developed.