Geophysical Survey Methodology

A number of different geophysical survey methods were employed on the wreck sites chosen for survey.
 
It was intended to examine the relevance of these survey methods to archaeological evaluation and to optimise system settings and survey parameters.
 
Multibeam sonar was used to gather bathymetric data from the wreck sites for the creation of 3D terrain models and georeferenced 2D images.
 
Magnetometer surveys were conducted to establish the magnetic signature of the sites.
 
Sub bottom profiler surveys served to provide information on the general geological stratigraphy on the sites and to indicate the presence of buried material.

Multibeam Sonar

MultiBeam SonarMultiBeam SonarMultibeam sonar ensonifies a swath of seabed beneath and to either side of the survey vessel, deriving continuous and well- positioned spot heights for many thousands of points on the seabed as the vessel moves forward.
 
From multibeam data, 3D terrain models can readily be created. Depressions and features projecting from the seabed can be displayed and quantified in terms of their dimensions.
 
For the "Wrecks on the seabed" project a Reson SeaBat 8125 multibeam echosounder was used.
 
The survey data was processed by Wessex Archaeology using the IVS Fledermaus visualization system.
 
The results were 3D terrain models, 3D fly through movies and 2D georeferenced images. The 2D images were used as a base for site plans. Divers used prominent features as datum points and carried out range and bearing measurements or used offset and triangulation to map other objects, as can be seen from this plan of the Portland Stone Wreck.

Magnetometer

Marine magnetometers detect variations in the Earth's total magnetic field. These variations may be caused by the presence of ferrous material on or under the seabed, geological features or diurnal variations in the Earth's magnetic field due to solar activity. Marine magnetic surveying has become a standard technique for mapping the location of ferrous material on the seabed.
 
Magnetometers are usually towed behind the survey vessel at a sufficient distance to avoid any magnetic disturbance caused by the survey vessel itself.
 
For the 2003 fieldwork, a Geometrics G-881 caesium vapour magnetometer was deployed on each wrecksite at a linespacing in the order of 10-20m.
 
In the course of post processing the magnetometer data was imported into Fledermaus and overlayed over multibeam data to show the relationship between the anomaly and the visible wreck site.

Sub bottom profiler

Sub bottom profilers work on the same principle as simple echosounders, but use much lower frequency acoustic energy. The acoustic pulses penetrate below the seabed and into the sediment. Returning echoes from sub bottom features such as geological stratas or buried material create a trace on paper and in the digital record.
 
A number of different sub bottom profiling systems are available. These are categorised by the frequency content of transmitted pulses.
 
Low frequency systems, often called boomers achieve high ground penetration at low resolution, while higher frequency systems, called pingers, achieve high resolution but only limited ground penetration.
 
For the 2003 fieldwork, Wessex Archaeology employed an Applied Acoustics boomer plate. The survey sought to determine the geological setting of the wreck sites and to detect pieces of buried wreckage that were not visible in magnetometer, sidescan or multibeam results.
 
The data was processed using Coda software and then interpreted. Typical sub bottom profiler data, acquired from wreck site 5010 can be seen here.