Due to an emergency, a ship traveling within a Netherlands estuary was forced to drop anchor in a restricted area with electrical cables and pipelines transporting gasoline and/or chemicals. Because the probability of pipe or cable damage was high, the ship was not permitted to raise the anchor. As such, the chain holding the multi-ton anchor was cut and left to reside at the bottom of the seabed. Due to the sheer mass of the anchor and discarded chain, it was critical to locate and retrieve the anchor to address any pipeline damages.
Retrieval missions had been attempted by previous surveyors, but were unsuccessful. At that point, Henk de Vries of the former Metaldec Survey BV was brought in to locate the lost anchor. Mr. de Vries first task was to narrow down the area in which they would conduct their survey.
A few challenges were present from the start. The Metaldec team wanted to reduce the survey field to the smallest area possible; however, the GPS coordinates recorded when the chain was cut was within a 50 meter radius of the actual anchor location due to the length of chain. It was also unknown if the GPS coordinates were in WGS84 (World Geodetic System 1984) or ED50 (European Datum 1950) which could result in over a 100 meter discrepancy. Finally, it was unclear if the presumed GPS position incorporated the offset from the anchor chain locker location to the ship's bridge where the GPS antenna was located. After consulting with third party resources, Mr. de Vries and his team narrowed down the surveying field to a 500x300 meter area.
While this surveying area was larger than the Metaldec team would have preferred, they were able to complete the survey in a single day. The survey first began with a mounted side scan sonar (SSS) tow fish. After a few hours, they added a trailing magnetometer tow fish to gather further detail on areas of interest and to complete the remaining survey area. By combining the abilities of both the SSS and magnetometer tow fish, the chance of locating the anchor was much higher.
Upon the conclusion of the survey, the Metaldec team analyzed and post-processed the data. Eight different sonar contact points came up as possible anchor locations. The SSS data was mosaicked using SonarWiz.MAP software. Several basemaps were generated using Golden Software's Didger program and then imported into SonarWiz.MAP and Golden Software's Surfer program. All bathymetric data was then extracted for use in Surfer. A nautical chart, Google Earth base map, and SSS/magnetometer tracks and contacts were also imported into the Surfer project.
The Surfer project combining SSS mosaic data, nautical charts, Google Earth, and survey tracks and contact points.
The gold colored section of the map is the SSS mosaic, light green lines are the magnetometer survey tracks,
blue points are the SSS contacts, and red points are the magnetometer contacts.
Upon further analysis, the Metaldec team narrowed down the eight possible anchor locations to a single point. The Surfer bathymetric map was used by divers and salvage vessel crews to strategically position the salvage crane as they planned their anchor retrieval mission. This information allowed all groups to better understand what they would encounter beneath the water.
The anchor was successfully located in the area predicted by the Metaldec team.
The Surfer project combining the bathymetric and nautical map, Google Earth, and survey tracks and contact point.
The anchor was located in the red square section, as predicted by the Metaldec team.
Henk de Vries now works for RemoSens BV (short for Remote Sensing) where he and his team continue to utilize Golden Software products to create a wide variety of maps and models.
To read more about Henk de Vries and his work with Golden Software products, check out this featured article where he searches for a missing scuba diver: http://www.geoconnexion.com/uploads/publication_pdfs/int_v15i14-030-Gold16211EB.pdf