Mapping and Monitoring of decimeter-level seabed change

Hughes Clarke, Parrott GSC, Danforth USGS

(Funded through CFI, Prov. NB. and Simrad)

 

With sufficient calibration and absolute control, shallow water multibeam sonars should be capable of mapping decimetre level seabed change. The sedimented seafloor can be a very dynamic interface. This is particularly true in the high energy of the coastal zone where the sediment on the seabed is continually being remobilized. This is often the natural result of winds, waves, current and tides but also increasingly it is a product of man’s interaction with that environment.

 

Typical scales of erosion and deposition range for a few centimetres to a few decimetres. Conventional depth measurements are normally only achievable with an absolute accuracy of a several decimetres. Until recently, even this has only been possible at spot locations or along two-dimensional profiles using single beam echo sounders. Multibeam echo sounders, however, for the first time, potentially allow us to view the sediment-water interface at that resolution over the whole surface. The same sources of error, however, that plague single-beam echo-sounding (and some addition new ones that affect multibeam sounders only), make it operationally extremely difficult to be able to resolve this small scale erosion and deposition using underway acoustics.

 

If a robust method to map and monitor the sediment water interface at scales of a decimetre could be developed it would allow researchers and engineers to better understand the effect of natural and man made influences on the seabed. Whilst the possible field of applications would be very broad, three applications, particularly relevant to Canada, will be specifically addressed using the funded infrastructure:

 

  1. Build-up and dispersal of organic debris under Aquaculture sites
  2. Quantification of naturally occurring sediment transport rates and volumes
  3. Scour and deposition around man made structures

 

To support this research, infrastructure funding has been successfully obtained to buy hardware including:

 

·       Simrad EM3000 multibeam sonar

·       Seatex MRU-6 motion, heave and heading sensor

·       RDI 600 kHz Monitor ADCP.

 

The intent is to use this hardware to support local field research programs into the three applications using frequent repetitive surveys (weekly, monthly) to demonstrate the feasibility of monitoring decimetre-level seabed changes