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Aug 02 2011

Hydro Survey: Multi-beam vs. Single Beam (Part 1)

Dusty (my co-author on this blog) and I are fascinated with new technologies and finding faster and more efficient ways to do things. We love thinking about and discussing new directions and how they affect our A/E industry: How does social media affect civil engineering? When will we shift to the cloud? Can we have a UAV? (our capital purchase wish list each year is usually long and never cheap!) What new extensions for Microstation could be helpful? What is open source GIS and how can we move away from extremely expensive maintenance options? These are some of the questions we’re constantly asking. Multi-beam hydro survey is one of those areas we’ve been tracking and thinking about.

From total station survey and GPS to terrestrial scanning, from photogrammetry to LiDAR, and from single beam hydrographic survey to multi-beam bathymetry, there has been a technological shift occurring within the survey and remote sensing arenas.  LiDAR, terrestrial scanning, and multi-beam sonar are all essentially the same form of technology just applied in different environments. Multi-beam depth sounders do not utilize a laser but instead use a form of sonar, but it’s the “scanning” to get a high density of data that ties them together.

mb vs sb

You’ll see an example below that I’ve used in a previous post of what our single beam hydro survey results might look like before post-processing the data:

02_Raw Data

This is an example of what the results of a multi-beam survey look like:

image

This photo is actually from the upper Sacramento River near Redding, California. The initial differences are striking. Three things stand out to me:

Multi-beam: Denser Data

Just looking at the figure above you can see that multi-beam results are obviously a higher density of data. Even miniscule changes in the bed are captured. Single beam survey can capture some of the changes in the bed through a bend way but not with the same nuances that multi-beam does. You can see scour holes and underwater objects for that matter! There’s almost no way you could attempt to look at small debris piles or get a complete picture of scour with a single beam depth sounder. With multi-beam, you truly have a full picture of the channel bed.

Multi-beam surveys would not be significantly more useful for a 1-D model or even a relatively uniform channel with less erratic changes that you want to hydraulically 2D model. However, for different design scenarios, very precise monitoring, or when you’re trying to investigate what’s happening underwater (and conditions are prohibitive for divers), multi-beam simply crushes single beam surveying with very few exceptions. Check out the example below of how you might err in a monitoring situation using a single beam depth sounder:

clip_image002

Multi-beam: Minimal Post-Processing

There is still post-processing to be done after a multi-beam survey.  However, to a get a complete bathymetric surface the processing is minimal compared to single beam. There is a little more preparation needed on the front end for calibration but once your survey is complete, it’s just a matter of quality control and getting it into the mapping format needed.

Multi-beam: Spectacular Mapping Products

The figure above at the Golden Gate Bridge near San Francisco should be enough to demonstrate what you can do when you obtain data at the density and accuracy of a multi-beam system. You can make some nice mapping products using the process that I’ve walked through previously on this blog but only with much more post-processing and interpolation. Single beam surveys give you a great picture of the river but a generalized one that is best for hydraulic modeling, spotting significant changes, and cutting precise cross sections. Multi-beam surveys give you all of that and more.

Multi-beam the Decisive Winner? Not Quite Yet…

Have I wet your appetite for Multi-beam data yet? Why would I ever use single beam data after what I’ve just told you? Dusty and I have rallied to have a multi-beam system for awhile now. We even have it narrowed down to a few systems we think would be fantastic to utilize. We think it could save time (and thereby money), and provide a better product. But are there limitations of multi-beam versus the strengths of single beam? There certainly are.

There are 3 areas where I think single beam depth sounders still win: cost, data deluge, and surveying in shallow water. I’ll discuss those more in my next post.

Have you run into more benefits to multi-beam that I have not mentioned here?

2 pings

  1. » Our Top Posts of 2011: Year of the Flood Hydraulically Inclined

    […] Hydro Survey: Multi-beam vs Single Beam (Part 1) (August 2): In this post, I cover some of the differences between multi-beam and single beam […]

  2. Hydro Survey: Multi-beam vs. Single Beam (Part 2) » Hydraulically Inclined

    […] « Hydro Survey: Multi-beam vs. Single Beam (Part 1) […]

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