Chartered Institute for Archaeologists 2020

22 |  VALUES, BENEF ITS AND LEGACY Adding value to marine geophysics with visual interpretation ALISON JAMES MCIfA and MARK JAMES MCIfA, MSDS MARINE This article explores ways in which archaeologists can add value to their marine geophysical surveys by ensuring surveys are adapted to enable new ways of visual interpretation. It builds on the experiences of MSDS Marine, which is well known for its marine geophysics capability. The world of maritime archaeology is by its very nature under water, out of sight and perhaps out of mind for the majority of the population. For this reason, MSDS Marine has been working with its geophysical data to find new methods for visual interpretation and public presentation. Primarily, remote sensing within marine archaeology consists of four sensors: Sidescan Sonar (SSS), Multibeam Echo-Sounder (MBES), Magnetometer (MAG) and Sub-Bottom Profiler (SBP). The aim for all marine geophysical surveys is that during the collection, processing and interpretation stages the data and accuracy are of the highest standard possible, that surveys are repeatable and that the outputs are suitable for archaeological assessment, analysis and presentation. Remote sensing surveys can be specified and undertaken for a number of reasons, including: prospection, either over a wide area or localised to a feature such as a wreck looking for anomalies such as debris; the establishment of an accurate position of a site; condition assessment and monitoring; and to support the creation of public engagement resources. This latter point is considered in greater detail in the next article. Each sensor collects and presents data in different ways, so not every sensor is suitable for every job. Contractors should work with their clients during the planning phase to establish the most appropriate sensor (or combination of sensors) for the task. In this article we focus on multibeam bathymetry over other geophysical techniques. Its use as a tool to identify wrecks and their extent on the seabed is well established. It offers a highly engaging image that can be readily understood by many people in a way that other geophysical techniques such as sub-bottom profiling and sidescan sonar survey can’t. The following two case studies look at ways it can be used outside of the normal hydrographic survey. CASE STUDY: MULTIBEAM AS A TOOL FOR COMPARATIVE ANALYSIS MSDS Marine has undertaken repeat geophysical and hydrographic survey over the site of the Rooswijk during 2015 (multibeam), 2016 (multibeam, sidescan, magnetometer and sub-bottom), 2017 (multibeam) and 2018 (multibeam). The works were undertaken in advance of, during, and after the fieldwork phase of the #Rooswijk1740 project running between 2016 and 2018. The Goodwin Sands, where the Rooswijk lies, is a highly dynamic environment with rapidly shifting mobile sands. In order to monitor the sand levels over and around the main site of the Rooswijk, a high resolution multibeam survey was planned that would be repeatable with equipment, methods, datums, and processing so that the excavation could be planned when the sand overburden was lowest, future sand movements predicted, and the level of environmental risk to the site monitored. The surveys also allowed the project team to prioritise the areas to be excavated and the data provided base maps to be used as an underlay for the diver acoustic tracking. The data in Figure 1 is presented to the same datum and colour scale and clearly shows the changes to the site over the four-year period. CASE STUDY: MULTIBEAM PROCESSING FOR A PUBLIC AUDIENCE The standard approach to processing multibeam data is to average the data points out into a uniform grid, typically ranging from 30cm to 50cm dependent on the specification of the survey and the data density. This grid of data points is then used to create a three-dimensional surface that is coloured by depth. The images in Figure 1: Top left clockwise, 2015– 2018 multibeam bathymetry of the Rooswijk protected wreck site clearly showing the rapidly shifting, mobile sands