Surveys in Extremely Shallow Water

[Bum Luck]

Interesting looking stuff:

SES-2000 parametric sub-bottom profilers offer advantages at all water depths, but a unique feature is the possibility to work in extremely shallow waters as shown below. Sub-bottom profiling in very shallow water using a pinger, boomer, chirp or seismics is difficult due to reverberation and beam pattern issues (e.g. beam width and sidelobes). On the other hand working in areas with very shallow water is required for a wide variaty of applications like

• surveys in rivers and harbours
• surveys in coastal areas
• sediment investigation vor dredging projects
• pipeline and cable route surveys
• location and monitoring of buried pipelines and cables
• underwater archaeology

Using one of Innomar's shallow water SBP models (SES-2000 compact, SES-2000 light, SES-2000 standard) it is possible to get excellent sub-bottom data at water depths down to
less than 0.5 metre




SES-2000 compact parametric SBP echoprint data example from an extremely shallow river in Gambia
(water depth 1–2m; range 1–15m below sea level; sediment penetration more than 10m; frequency 10kHz; pulse width 0.2ms).



SES-2000 standard parametric sub-bottom profiler echoprint data example from a river cable route survey
(water depth 3–7m; range 2–14m below sea level; frequency 8kHz pulse width 0.13ms)

I've always thought that shallows were the biggest salvage challenge.

 

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[Salvor6]

The SES-2000 Compact is exactly what I have been looking for. Do you know the price? Do you know anyone that has one?

 

[Bum Luck]

The SES-2000 Compact is exactly what I have been looking for. Do you know the price? Do you know anyone that has one?

I don't, sorry.

 

[AUVnav]

Salvor,

The cost of a new unit is about $75K. There arent that many around (they just shipped unit #100 a few weeks ago) so you will not likely find any used ones on the market.

Here is a document on using SBP in shallow waters from a small boat... http://49.128.56.163/hydronav/templates/rhuk_milkyway/gg/pdf/an04_ses2000_small_boats.pdf

I would note that the Compact is a single freq unit (one at a time) so you may have to work the freq's to get the best results. The unit is not chirp, which you would want to locate and differentiate objects such as timbers and metals.

You may want to look into an Edgetech 3100 system. Not sure how much they are these days, but they used to be around $50K. Much more opps for used units.

 

[Salvor6]

Thanks AUVnav. I got an email from the U.S. distributor a couple days ago informing me of the price. And that's the cheap model!

 

[jktnet8]

The California drought (worst in 40 years) might provide a perfect situation for the use of such technology. Man-made lakes are evaporating to such an extent that long-lost gold rush towns are re-appearing above water. Underwater detecting is no longer needed, but this shallow water scanning apparatus could locate something exciting! Happy hunting! JK

 

[AUVnav]

Salvor6,

When looking at a unit, the shallower the water, the lower the freq.

(forgive me if you already know the following)

A single freq ping simply goes down and reflects off of the the different layers. As the sound is reflected, it returns. Signal freq is important for depth thru layers. (freq =wavelength) The system is based on the speed of sound, the system records the difference between the broadcast and return, thus as the sound penetrates the layers, it reflects back and the time difference is recorded as depth.

A chirp is just that, a chirp noise with a sweep from a lower to a higher freq, typically sweeping a 4 KHz range ( a whooop for lack of better explanation) perhaps from 1.5 to 5.5 kHz. (When looking for a shallow water unit, choose the one with the lower freq range)
The wavelength of the freq is thus changing, so accordingly, so does the return signal. This is how a chirp illuminates objects, rather than just reading a simple return.

This is similar to what the land based metal detectors do in the determination of iron based vs other. A land based metal detector will broadcast multiple single freq from 15 kHz to 100kHz

The issue with seawater (or fresh), is backscatter in the system. To help alleviate the backscatter at depth, you use a higher freq, and the software must filter between time and phase, and filter out the reflections, or backscatter from the depth. Near shore, it would be expected that there is far more vegetation and particulate in the water, hence more backscatter. More power creates more backscatter, but lower freq reduces backscatter.

With practice, much the same as a land based detector, the SBP chirp operator can use the parameters of the location, and determine what the freq reflection has illuminated. (ie, wooden timbers, ferrous, or other.) There is a myriad of site factors in this equation to consider, so one size does not fit all.

What is interesting is that this is really not difficult to build, broadcast a chirp and read the return. In MATLAB the signal is processed thru the FFT, then again, which shows the phase shift, and isolates time, time is the key, balanced against the speed of sound.
The trick for shallow water would be to have a seperate transmitter and receiver apart about 15 degrees from center, with a very short pulse chirp. Given the system relies on the speed of sound, the shallower you are, the pulse may not be finished before the first reflections, hence the noise fliter would potentially remove relevant data.
With an actuated accumulator to increase gain, this would be very interesting indeed.

(I am going to file a few patents before I say too much more!)

 

[Bum Luck]

Salvor6,

When looking at a unit, the shallower the water, the lower the freq.

(forgive me if you already know the following)

A single freq ping simply goes down and reflects off of the the different layers. As the sound is reflected, it returns. Signal freq is important for depth thru layers. (freq =wavelength) The system is based on the speed of sound, the system records the difference between the broadcast and return, thus as the sound penetrates the layers, it reflects back and the time difference is recorded as depth.

A chirp is just that, a chirp noise with a sweep from a lower to a higher freq, typically sweeping a 4 KHz range ( a whooop for lack of better explanation) perhaps from 1.5 to 5.5 kHz. (When looking for a shallow water unit, choose the one with the lower freq range)
The wavelength of the freq is thus changing, so accordingly, so does the return signal. This is how a chirp illuminates objects, rather than just reading a simple return.

This is similar to what the land based metal detectors do in the determination of iron based vs other. A land based metal detector will broadcast multiple single freq from 15 kHz to 100kHz

The issue with seawater (or fresh), is backscatter in the system. To help alleviate the backscatter at depth, you use a higher freq, and the software must filter between time and phase, and filter out the reflections, or backscatter from the depth. Near shore, it would be expected that there is far more vegetation and particulate in the water, hence more backscatter. More power creates more backscatter, but lower freq reduces backscatter.

With practice, much the same as a land based detector, the SBP chirp operator can use the parameters of the location, and determine what the freq reflection has illuminated. (ie, wooden timbers, ferrous, or other.) There is a myriad of site factors in this equation to consider, so one size does not fit all.

What is interesting is that this is really not difficult to build, broadcast a chirp and read the return. In MATLAB the signal is processed thru the FFT, then again, which shows the phase shift, and isolates time, time is the key, balanced against the speed of sound.
The trick for shallow water would be to have a seperate transmitter and receiver apart about 15 degrees from center, with a very short pulse chirp. Given the system relies on the speed of sound, the shallower you are, the pulse may not be finished before the first reflections, hence the noise fliter would potentially remove relevant data.
With an actuated accumulator to increase gain, this would be very interesting indeed.

(I am going to file a few patents before I say too much more!)

Well, count me in!