GRSG Conference 2022: Orbit to Outcrop
Title: Shipwrecks From Space: Satellite Imagery Oil Slick Observations for Monitoring and Rediscovery of Sunken Vessels
Author: Geoffrey Thiemann
During the Second World War more than 8000 ships were sunk over the course of the conflict, with many wreck sites still not accurately located to this day. Of these 8000 sunken vessels, over 10% were oil tankers or fleet oilers, with many lost whilst transporting significant cargoes of fuel oils, aviation fuels, diesels, etc. As much as 20 million tonnes (~140 million barrels) of various oil types is believed to still be contained within the worlds’ shipwrecks.
As these shipwrecks deteriorate over the years the potential for major oil spills is an increasingly concerning environmental hazard; especially for wrecks around the ecologically sensitive atolls and reefs of the Pacific. The potential impacts of a major spill on such an environment and their local communities were demonstrated in August 2001 when a State of Emergency was declared in Yap State, Federated States of Micronesia (FSM) following a significant oil spill from the sunken oil tanker USS Mississinewa at the remote and environmentally sensitive Ulithi Lagoo atoll.
Whilst deoiling efforts were undertaken on that vessel, most Potentially Polluting Wrecks (PPW) remain untreated and many remain undiscovered. Locating undocumented shipwreck sites and monitoring oil releases from them is key to remediation efforts to tackle this issue.
Over the past two years, CGG Satellite Mapping have adapted the methodology previously utilised for CGG’s Seep Explorer natural seepage detection work towards the detection and monitoring of temporally repeating oil slicks over known and suspected shipwreck locations as a precursor to potential remediation efforts by other agencies. With large open access datasets of both SAR (synthetic aperture radar) and optical satellite imagery now available, the presence of repeating oil slicks can be observed across timescales of decades, with potential for even infrequent/irregular oil releases to be caught on imagery.
The first of these projects was to aid NOAA and SEARCH Inc. in the discovery of the wreck site of the sunken oil tanker SS Bloody Marsh off the coast of South Carolina. The final resting place of the ship had been documented with a low degree of accuracy, leading to unsuccessful search dives in 2019 onto false positives identified by sonar bathymetry. Oil slick mapping provided by CGG Satellite Mapping indicated a repeating oil slick occurrence documented on SAR and Optical imagery between 1992 – 2019 which was 12km South-West of the historically documented location of the SS Bloody Marsh.
This information proved crucial in refining the search area for the wreck in the ‘Windows to the Deep’ survey cruise performed by the vessel Okeanos Explorer in the 2021 season. Within this new search area, a shipwreck-like target was found in the multibeam bathymetry data 4 km downstream of where the oil slicks originated at the surface. A dive on the 28th October 2021 confirmed this new target was the shipwreck of the Bloody Marsh.
The second of these projects has been a collaboration with the Australian non-profit Major Projects Foundation providing historical oil slick investigation over several sites across the Pacific (Bikini Atoll, Chuuk Lagoon, Solomon Islands, Coral Sea and individual shipwreck locations off New South Wales, Australia and North Island, New Zealand).
At Bikini Atoll optical imagery from Sentinel-2 and Landsat 8 identified five actively leaking shipwrecks between 2015 – 2021, with the Japanese Battleship Nagato noted for the scale and consistency of the oil slicks originating from it. Oil slick thickness estimates were possible on several images in the study using the BONN code methodology with volumes of at least 4 barrels of oil on the lagoons surface documented on one image.
At Iron Bottom Sound in the Solomon Islands, a mixture of SAR and optical imagery from 2000 – 2001 was utilised in the study which identified 23 sites of temporally repeating oil slick occurrences in the area. Notable oil slicks were associated with seismic activity in the area, revealing sites which did not slick under other circumstances. These locations were provided to partner organisation Sealark Exploration, who have attributed the repetition sites to potential origin vessels based on last documented locations, these attributions may help the future discovery of the exact locations of the vessels wreck sites in future.
Following this work, CGG are developing the Seascope monitoring system to support continual proactive oil pollution monitoring from fixed infrastructure, shipping, and shipwrecks. This satellite imagery based monitoring system will enable the establishment of production water baselines and provide early detection of anomalous events and third-party pollution incidents, as well as surveillance of natural seeps. This will support the creation of a growing evidence base of responsible operations for stakeholders such as operators, regulators, investors, insurers, etc.