Hydrographic Surveying, Mapping

Hydrographic Surveying: Mapping the Ocean Floor

This week’s focus has been on the World’s Oceans. World Oceans Day is held every year on 8th June to raise awareness of the vital importance of our oceans and the role they play in sustaining a healthy planet.

With that in mind, in order to understand what lies beneath our oceans, hydrographic surveying is usually conducted. Hydrographic surveying involves measuring, describing and mapping physical features that can be found underwater in our oceans, seas, coastal areas, lakes and rivers .

I have been enlightened on the processes and the detailed tasks involved in conducting hydrographic surveying with some training in QPS Qinsy and CARIS (HIPS and SIPS) software programs. In a nutshell, these software programs are used to help with line planning (this helps the surveyor to collect data coherently and also helps with the navigation of the vessel), implementation of the surveys and the post-processing of collected data.

In fact, the technology for collecting underwater data has come a long way, from leadline to sonar (sound navigation and ranging) technology as depicted in the pictures below.

A nostalgic 1985 sketch of hydrographic surveying using leadline
Source: https://en.wikipedia.org/wiki/Hydrographic_survey#/media/File:Cgs01221_-_Flickr_-_NOAA_Photo_Library.jpg
Multibeam sonar used nowadays to map the ocean floor
Source: https://en.wikipedia.org/wiki/Multibeam_echosounder#/media/File:Fis01334_(27555144884).jpg

The experience I gained in hydrographic surveying involved the use of multi-beam sonar technology which was mounted to the front of the vessel. It was really engaging to be able to adjust the swath angles as information is being received from the sensor and to see first hand images of the surface below the water. Some features did amaze and startle me!

Sonar works by transmitting a wide, fan-shaped pulse from a transducer and calculating how long it takes for the echo of that sound wave to be read by the receiver on the vessel. Given that the speed of sound in water is known, using basic trigonometry, the echo can be measured to determine the depth of the area being surveyed.

Video showing the importance of line planning.

This video was captured while the hydrographic survey was being carried out. It highlights the importance of line planning.

The visual provided a clear navigation aid for the captain and also allowed for data to be collected coherently avoiding gaps in the data’s sounding grid.

To ensure that errors were minimized while scanning the ocean floor, the speed of the vessel was reduced when scanning ‘a line”. In addition, when the captain had to make a turn to get on a “new line”, the data collection process was stopped altogether and then restarted after the turn was made. The post-processing of the hydrographic data included, applying tide and sound velocity corrections and removing unwanted data called “outliers”, which can occur from incorrect sea floor detection.

Although hydrographic surveying can be seen as a very time consuming process, there is an amazing project underway, Seabed 2030, which plans to get a full comprehensive map of the entire world’s oceans by 2030 using innovative crowd sourcing methods. Learn more about it below:

Seabed 2030 film 
Source: https://www.youtube.com/watch?v=NNF9NSDKo7c

The resulting data from the post processing is called bathymetric data. Bathymetric data is used most commonly to:

  • Update nautical charts. These charts guide mariners and ensures safety of lives at sea.
  • Inform on the effects of climate change, such as beach erosion and sea-level rise.
  • Determine areas for marine protection. The depth and characteristics of the seabed define the habitat for benthic (bottom-dwelling) organisms. It can help to determine where fish and other sea life will feed, live, and breed.

One of the latest capital projects for the Island of Montserrat required hydrographic surveys to be conducted, allowing the island to benefit from subsea fibre optic cable. More information can be found at this link: https://discovermni.com/2020/01/08/marine-survey-for-montserrats-subsea-fibre-optic-cable-project-begins/.

Knowledge and understanding of our ocean floors will help us all to benefit from our blue economy.

Below I share some images showing some special views that I encountered while conducting hydrographic surveying on the west coast of Montserrat. Happy World Oceans Day 2020 to all!

Lavern Rogers-Ryan is a geospatial consultant specialising in disaster risk management and recovery. She is currently head of the GIS Centre within the Government of Montserrat. Learn more about geospatial services in disasters at www.lavernrogersryan.com.