Near the Port of Tallinn, old pieces of wood were unearthed during the construction of Reidi Road. Excavation was immediately halted and a closer investigation was launched. It turned out that this could be a very old shipwreck, the existence of which was previously unknown.
TASK
The construction project involved building large pipes from a collector to the sea.
However, pieces of a shipwreck were found between the collector and the sea. Our task was to use GPR to determine the size of the shipwreck, its exact location and depth below the ground. Based on the information obtained, it was possible to decide what to do with the shipwreck – leave it where it was or excavate it.
SOLUTION
Since the task was to determine the exact location of the shipwreck and it was known that shipwrecks in this area could be up to 10 meters deep, it was decided to use a low-frequency GPR radar, which in ideal conditions can “see” up to a few dozen meters deep. Although pieces of the wreck were found a few meters deep with a grab, this was probably the highest part of the wreck and the object extended much deeper. The most suitable ground-penetrating radar for this type of work is the CrossOver CO730. It has two frequencies, 70 MHz and 300 MHz, and is a towable radar that allows movement on uneven ground. As it was not known where the wreck was located or how much of it remained, it was decided to scan the entire survey area with a survey grid with 1 m intervals. Due to the long wavelength of the 70 MHz antenna, sufficient coverage was achieved at a depth of just a few meters with a 1 m grid. For positioning, the start and end points of all scan lines were measured with an RTK GNSS device.
RESULTS
Raw data was collected on 25 profiles in the survey area. The raw data was processed using special software called CrossPoint and GPR Slice. CrossPoint is ImpulseRadar software designed for post-processing data from CrossOver and PinPointR GPR. This software allows the collected raw data to be quickly and easily processed into a radargram. GPR Slice is professional software that allows the raw data from virtually all manufacturers’ GPR to be processed. With this software, various sections and volumes can be created and the processed data can be visualized in different forms. The survey determined the extent and depth of the wreck, and this information was used to carry out further excavation work.
A georadar survey revealed the exact location and depth of a shipwreck that was unexpectedly discovered during the construction of Reidi Road.
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