Information on the sea floor and submerged deposits of archaeological interest was gathered by offshore geophysical and geotechnical surveys. Sediments retrieved during geotechnical survey were subjected to geoarchaeological and environmental analyses to provide further information on the nature of the environment that would have existed during various stages in prehistory when the study areas were dry land. This allowed archaeologists to produce a reconstruction of the prehistoric environment.

Geophysical survey

Geophysical survey methods are widely used in archaeological investigations on land and in underwater survey. The surveys undertaken during the Seabed Prehistory project used multibeam and single beam echosounders and sidescan sonar to map the bathymetry and topography of the seabed. Seismic surveys were conducted using sub-bottom profilers to penetrate the seabed and provide information on the structure of sediments beneath the seafloor.

Echosounder survey

Bathymetric dataEchosounder systems measure bathymetry using acoustic energy. Echosounders measure the bathymetry by emitting a short acoustic pulse and then accurately measuring the time it takes for the pulse to be reflected back to the transducer. Then, assuming that the speed of sound in water is accurately known, this time can be converted into water depth. Multibeam systems operate in the same way as single beam echosounders but by using up to 100 or more beams at the same time so that large swathes of the seafloor can be surveyed very rapidly. Bathymetric surveys highlight topographical features on the seabed. This is highlighted in the picture on the left which illustrates bathymetric data from the Seabed Prehistory study area in the East English Channel. The processed single beam echosounder data was input into a software program named Fledermaus from which a surface was created. A digital elevation model (DEM) of the bathymetry within the survey area was produced.

Sidescan sonar survey

Sidescan towfishSidescan uses a sonar device which may be towed from a surface vessel. The sonar device emits fan-shaped acoustic pulses down toward the seafloor across a wide angle perpendicular to the path of the sensor through the water, The intensity of the acoustic reflections from the seafloor of this fan-shaped beam is recorded in a series of cross-track slices, which when stitched together along the direction of motion, become an image of the sea bottom within the swath (coverage width) of the beam.

Seismic survey

Sub-bottom profilerSub-bottom profiling systems are used to identify and characterize layers of sediment or rock under the seafloor. The technology involved is not dissimilar to the echosounder described above. A transducer emits an acoustic pulse vertically downwards towards the seafloor, and a receiver records the return of the pulse once it has been reflected off the seafloor. Parts of the acoustic pulse will penetrate the seafloor and be reflected off of the different sub-bottom layers. The time it takes the pulse to be reflected from the seafloor and different sub-bottom layers is then accurately measured. This data is then processed and can be used to find the thickness of the layers in the seafloor and their position. The data can also provide information on the composition of the sediments, as different sediment types will reflect sound in different ways.

Geotechnical survey

Vibrocore survey

Vibrocorer being retrievedVibrocore surveys involve acquiring cores of seabed sediments using what is essentially a vibrating steel tube which penetrates the seabed to a particular depth. During this project a 6m hydraulic vibrocorer was used to acquire the vibrocores. Each core was cut into 1m lengths, capped and labelled and taken back to Wessex Archaeologyheadquarters in Salisbury for comprehensive logging. In contrast to standard vibrocore survey methodology, the cores were not opened so that visual descriptions could be made on site; instead they were packed for a subsequent, more comprehensive logging process. The second cores from some locations were recovered using black vibrocore liners and kept separately in a darkened container to prevent exposure to light, so that they could later be used for Optically Stimulated Luminescence (OSL) dating.

Grab sampling survey

Grab sample being retrievedSeabed grab-sampling surveys are used by the aggregate industry solely as part of benthic (marine life) surveys undertaken in preparation for the marine ecological assessment element of an Environmental Impact Statement. This involves using a ‘grab’ to scoop a sample of sediment from the upper strata of the seafloor. As such, artefacts found in a grab sample are unlikely to be retrieved in archaeological deposits as any material from the upper layers of the seabed is likely to have been reworked from its original context. The semi-mobile, upper strata of the seabed that will be sampled during the survey are subject to marine sediment transportation processes.

Geoarchaeological analysis

Processing the vibrocoresThe geoarchaeological analysis involves the examination and detailed description of the sediments recovered. The vibrocore log descriptions identify individual sedimentary units and record the structure, colour, texture and lithology of the sediments, describing any inclusions and the nature of the boundaries between the units. These details are used to make initial interpretations of each unit. The core log descriptions can highlight evidence of the manner in which the sediments were deposited. Features such as flood or tidal couplets, peat horizons, platy, gley clays and characteristic fluvial or tidal sequences of sedimentary units, provide evidence of the nature and relative speed of depositional processes and their environments, as well as providing material for environmental analysis such as pollen, diatoms and foraminifera.

Environmental analysis

Taking samples for environmental analysisThe sediments are sampled and analysed for pollen, diatoms, foraminifera and other organisms which can provide information on the nature of the depositional environment. A combined programme of pollen, diatom and foraminifera assessment can provide evidence of the palaeovegetation and the nature of the environmental conditions prevalent at the time. It could identify, for example, the nature of a fluvial environment, potentially suggesting details such as that it was within the tidal reach with a constant but low freshwater discharge.

Pollen

The presence, variety and quantity of pollen species can identify the vegetation and nature of the depositional environment, (i.e. saltmarsh species would suggest a different environment to a woodland assemblage), and can also be characteristic of particular prehistoric periods.

Diatoms

Diatoms are unicellular microscopic algae found in marine and fresh water environments. The presence of diatoms within the sediment relates to the salinity of the depositional environment. Analysis of diatoms can provide evidence of the size of water bodies, their salinity and the warmth of the water.

Foraminifera

Foraminifera are mainly marine benthic or planktonic organisms consisting of either a single cell or a group of cells without differentiation of function. Analysis of foraminifera can identify where the sediment was within a salt marsh zone, illustrating, for example, how far from the sea that sediment was deposited.

Dating methods

Optically stimulated luminescence

Optically stimulated luminescence (OSL) dating or optical dating, developed from thermoluminescence (TL) methods, has been used to investigate a number of terrestrial sedimentary sequences over the last ten years. It has only recently been applied to the marine environment. The technique dates the deposition of sedimentary layers through the ‘luminescence signal’ of purified quartz or feldspar-dominated fractions. This essentially means that OSL dating analyses these minerals in the sediment and determines when they were last exposed to light.

Radiocarbon dating

Radiocarbon dating measures the rate of decay of Carbon-14, a radioactive isotope of carbon, found in organic matter. Carbon-14 breaks down at a known rate, so by measuring the quantity of the isotope in an organic sample, it can be determined when the isotope began to decay thus giving us an estimate of the relative age of the sample.