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A crucial element in the project is the remote sensing process. The aim is to acquire, process, analyze and interpret the data and to let this follow by diving on targeted locations.
We have acquired the data with a seaborne magnetometer. This device measures distortions in the natural earth’s magnetic field through the presence of iron (or ferrous) material on or under the seabed. The magnetometer is towed on a cable behind the survey vessel, sending readings up the cable to a tablet, which logs the data and provides positioning through a GPS.
Photo 1: The results of magnetic testing over know iron wreck site, Eidsvold, at Christmas Island. The image on the left clearly show distortions in the earth’s magnetic field, caused by the presence of the wreck. The three lines show the track of the survey vessel. The image on the right show a profile of a transect line across the sight with the clear characteristic ‘dipole’.
The tablet also helps the skipper to navigate along per-positioned lines called transects. These lines have been carefully arranged in mapping software to be straight, and parallel. Narrow line spacing is required to allow sufficient coverage.
Once the data is brought back ashore, it is reviewed on laptops with specialized software. This process allows for the earth’s magnetic field to be displayed and any distortions detected. Interpretation happens next.
Photo 2: A magnetic anomaly discovered in the post-processing phase of the remote sensing component in the search. The anomaly is clearly seen in the paler contour lines, along with depth contour lines. This anomaly is approximately 85 by 160 meters.
A list of potential sites is produced, and the most promising analyzed more closely. Mapping software allows for the precise positioning for the deployment of divers. Circular shapes are created and placed within the mapping software, which gives a very accurate indication of depths, and can thus inform the dive plan. In this case planning for circular dive searches.
This positioning information is then fed into the tablet, enabling precise navigation to each target.
Photo 3: Mapping and other specialized software enable the blending of magnetic isometric lines (black) and magnetic anomalies (blue and yellow), with bathymetric contour lines (white) and planned circular dive searches (red).
The entire work flow, carried out by Alex Moss and James Parkinson enables the team to react to information gathered in the field, a sort of ‘reflexive methodology’.
Penultimate day of diving on Christmas Island
Waterlogged is the best way to describe the way we all feel after the last few days. Whether diving or on the boat we have been wet. Some rain showers were so strong it was a drier feeling in the water than out of it! Sadly, our time on Christmas Island is coming to a close and we have not discovered any archaeological material that would bring us closer to the Fortuyn. While we have not yet managed to dive all the identified magnetic anomalies our time is running out. But let’s keep our fingers crossed.
Photo 4: Me descending down the shotline.
Photo 5: Alex Moss doing a 30 meter circular search. The other diver holds the line at 15 meter and makes sure the line doesn’t get caught.
Today we dived sites off the West coast and because of the very calm conditions were able to swim survey from Egeria Point northwards to Winifred Beach. The difficulty of finding a site in the 30 meter depth range available to us as divers was made very evident during that transect, as several times we encountered a vertical drop off to far greater depths not more than 50 meters from the coastline. A very small shelf for a shipwreck or shipwreck debris to stay attached to!
Photo 6: Graeme and Shinatria after their circular search.
Photo 7: The MV Eisvold shipwreck is a beautiful place to dive!
Our next blog will be the last from Christmas Island, before we go to Cocos Keeling. Stay tuned!