Scientists have found that the watery graves of coastal shipwrecks leave sediment plumes at the sea's surface, that can be detected using satellites to locate wreckage.
Using data from the NASA/USGS Landsat 8 satellite, researchers have detected plumes extending as far as 4 kilometres downstream from shallow shipwreck sites.
This discovery demonstrates for the first time how Landsat and Landsat-like satellites may be used to locate the watery graves of coastal shipwrecks.
While there is a romantic association of shipwrecks and buried treasure, it is desirable to know where they are located for many other practical reasons.
The ships may be of historical significance or, if the hard substrate of the ship has created a reef, of ecological significance, researchers said.
Modern-era shipwrecks are also commonly sources of pollution, leaking onboard fuel and corroded heavy metals.
While airborne lidar (which uses light pulses to measure distance) can be used to detect shipwrecks close to shore and multibeam echosounders and other sound-based methods can be used anywhere deep enough for a survey vessel to sail, the former method requires clear water and cost prohibits both methods from being used to conduct exhaustive coastal surveys.
The researchers, including those from the Royal Belgian Institute of Natural Sciences and Ulster University in Northern Ireland, have found a way to use freely available Landsat satellite data to detect shipwrecks in sediment-laden coastal waters.
Their study, conducted in a coastal area off of the Belgium port of Zeebrugge, relied on a detailed multibeam echosounder survey of wreck sites, previously conducted by the Flemish government. This part of the Belgian coast is strewn with shipwrecks, in often sediment-laden waters.
The researchers started with the known location of four fully submerged shipwrecks.
Using 21 Landsat 8 images and tidal models, the researchers mapped sediment plumes extending from the wreck locations.
They found that the two ships with substantial portions of their structure unburied created sediment plumes that could be traced downstream during ebb and flood tides.
The researchers postulate that the exposed structure of these ships created scour pits that then fill with fine sediments during slack tides (the period of relatively still currents between ebb and flood tides).
These scour pits then serve as sediment repositories from which sediments are re-suspended during flood and ebb tides. When these sediments reach the surface, they create their telltale plumes.
Uncharted shipwrecks could be located by using the researchers' methodology in reverse - ie mapping sediment plumes during various tidal stages and then following the plumes upstream to their point of origin.
The study was published in the Journal of Archaeological Science.
