Site monitoring
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Monitoring of sites is conducted for different reasons. Pre-disturbance surveys deliver a baseline understanding of the physical, chemical and environmental information associated with the wreck site. The type of data gathered is a mixture of qualitative and quantitative data, for example, a verbal description of the sea conditions as versus a measurement of the water's pH.
Post-disturbance surveys are conducted after the excavation is completed and the site is reburied. Exactly the same data as a pre-disturbance survey is collected and this makes it possible to assess the stability of materials within the site as well as the site itself.
On behalf of the Commonwealth's Historic Shipwrecks program, MTQ is also responsible for the management of submerged cultural resources in Queensland. Monitoring for iron hulled vessels primarily includes metal corrosion surveys and residual thickness measurements. For historic shipwrecks that were primarily made of wood, surveys also include analysing pH levels, biological activity, dominant bacteria species, and other elements that reflect and effect the environment of the site.
Site stabilisation
The rationale for reburial of any archaeological excavation is that covering the site minimises the rate and extent of deterioration from physical, biological and chemical processes. In an oxygenated marine environment (normal sea conditions) materials are exposed to a high rate of deterioration. In a buried environment where there is no oxygen, (anaerobic environment) the predominant means of deterioration is through sulphate-reducing bacteria that produce pyrite and hydrogen sulfide as by products. The rate of deterioration by bacteria is significantly slower than that in an oxygenated marine environment.
Reburial practices on the Pandora were developed over a number of years. Initially, backfilling was done over a solid sheet of plastic, but over time an open weave plastic "shade cloth" was preferred. In 1999 the site was reburied as follows; a layer of shade cloth was placed down first on the exposed wreckage, followed by a layer of spoil (sediment which has already been removed from the wreck).This is then consolidated by another layer of shade cloth and polyurethane sandbags filled with new sand that has been collected beforehand from one of the sand cays. The bags are also modified so that the tied off end is frayed allowing the warp and weft threads to act like sea grass and trap more sand or coral detritus.
This reburial practice affords the excavated area with a covering of sediment of at least 50 cm thick, which is a minimum required to ensure that the timbers are sealed off from well-oxygenated water. Even after reburial is completed there is still sufficient oxygen remaining for marine organisms to survive for some time and continue to cause accelerated deterioration of the organic components of the wreck. Equally, the remaining oxygen also enables higher rates of corrosion to continue until it is depleted. Studies by Jodie Guthrie on the Pandora site for her PhD, have shown that it takes two to seven years for the pre-excavation levels of anaerobic bacteria to re establish and consequently for the area to slow down to pre-excavation rates of deterioration.
Corrosion potential measurements
The protocol for acquiring and interpreting in situ corrosion potential measurements for maritime archaeological sites was developed at the Western Australian Maritime Museum by Dr Neil North and Dr Ian MacLeod. This protocol enables the conservator to understand the mechanism and rate of deterioration of metal artefacts so as to suggest more effective methods to manage historic shipwrecks, specifically iron-hulled vessels. Understanding the mechanisms and rate of deterioration assists in the prioritisation of a site with regards to documentation, recovery, excavation or in situ stabilisation.
The protocol involves measuring and recording a number of physical and environmental parameters in situ. Quantitative and qualitative data is acquired. Measured parameters are: depth, pH, corrosion potential (Ecorr), depth of corrosion on cast iron objects, dissolved oxygen concentration of water and temperature of water. Observed parameters include: identifying the sampling point of the survey, degree of concretion cover near the sample site, whether hydrogen bubbles occur after drilling into the object, seabed make-up and conditions over the site.
Graphical relationships between Ecorr and Depth, Ecorr and pH, pH and Depth and the annual depth of graphitisation and Ecorr can be constructed. These graphs supply a semi-quantitative assessment of the shipwrecks condition. From the annual depth of graphitisation and Ecorr graph, a corrosion equation can be constructed that is site specific. This equation is dependent on the concentration of dissolved oxygen.
On the Pandora site, corrosion potential data has been collected as part of a pre-disturbance survey. The data gained is used to establish baseline thermodynamic data, to assist in the selection of an artefact for recovery and to document the change in potential for an object which has been attached to a sacrificial zinc anode (galvanic couple) to initiate conservation treatment.