Multiphase Echosounder to Improve Shallow-Water Surveys

November 23, 2017

I found this article very useful to understand the capability of interferometric systems compared to the "classic" beamformer multibeam. I'd like to see your point of view about this, post your comments.

 

Hybrid Approach to Produce Bathymetry and Side Scan Data

A detailed knowledge of shallow-water bathymetry is vital for a wide range of marine activities. Surveys are required for navigation channels, dredge works, lake and dam management, environmental mapping, marine archaeology, pipeline and cable routes and offshore construction. These surveys are typically carried out by small vessels of opportunity, often operating in less than 20 m of water. The current technology utilized for this type of work is typically inefficient and costly due to the design of these legacy instruments. This inefficiency has manufacturers exploring
new innovations in swath bathymetry systems and has led EdgeTech to the development of a novel swath sonar technology, the multiphase echosounder (MPES). This article describes the technology behind the MPES, with examples from a recent survey trial to demonstrate the advantages
over traditional systems and show the new technique’s bathymetric accuracy.

A wide-swath sonar mounted on a small survey vessel is an accepted solution for high-resolution full coverage surveys in the nearshore environment, where the shallow draft and maneuverability of
the vessel enables safe operations around shorelines and hazards. This type of survey vessel
requires a compact sonar head that can be pole mounted and interfaced with a variety of ancillary position, heading and motion sensors. The single-beam echosounder was the survey tool of choice
once acoustic technology was accepted into the field, and is still in use today. Following that, and more common today, the multibeam echosounder (MBES, or beamformer), originally developed for deeper-water surveys, has been used in this role. While MBES systems have a larger coverage than the very narrow single beam, the MBES systems still have a limited beam spread, which is narrowed as water becomes shallower. This limits the survey efficiency and requires the vessel to spend extended time in shallower areas running very tight survey lines to ensure coverage. The restricted swath width also struggles to offer confidence of full coverage in complex nearshore environments,
where shoals can rise rapidly.

A Hybrid Approach

The MPES technology uses a pair of transducers, one facing port and one starboard, to produce the bathymetry and side scan data. Each transducer has 10 bathymetry receive elements to derive up to nine phase difference measurements per side. These multiple phase measurements provide
several benefits when resolving for the seafloor soundings:

  • First, the high number of receive elements improves the accuracy of each bathymetric point [...].
  • Second, the increased channel count also allows beamforming and beamsteering processing methods to focus energy at nadir, creating a denser data set in this region. This closes the nadir gap, and achieves complete bathymetric coverage of the seabed across the swath, including under the transducers.

In addition to the 10 bathymetry receive elements, the EdgeTech MPES transducer design incorporates two dedicated full-length transmit and receive channels to obtain the side scan records. This allows the system to retain the high-quality and high-resolution imagery normally associated
with EdgeTech dual-frequency side scan sonars without interfering with the bathymetry signals.

In addition to the 10 bathymetry receive elements, the EdgeTech MPES transducer design incorporates two dedicated full-length transmit and receive channels to obtain the side scan records. This allows the system to retain the high-quality and high-resolution imagery normally associated with EdgeTech dual-frequency side scan sonars without interfering with the bathymetry signals [...].

shallow SSS and MBES data from Edgetech 6205

Conclusions
Increased demand for full-coverage, shallow-water mapping has led to the development of a new technology to improve the efficiency and accuracy of shallow-water surveys. The unique MPES, or hybrid approach, takes the benefits of both phase and beamforming methods [...].

Lisa N. Brisson, Dr. Tom Hiller

 

Read the entire article on: EdgeTech.com


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