Searching for wrecks with side scan sonar


#22

Hi @Walt_Holm

Looks like you are well down the path to having quite a nice system (and for the right price):+1::+1:

A couple of comments I would make about the towfish design

Basically good images come from 2 main things

  • Frequency - Given this in part is fixed by the hardware you purchase and so there is little you can do here – High frequencies give greater resolution (but at the loss of range ie High Frequency good picture but short range meaning lots of scanning lanes V Low Frequency lower resolution poorer images but longer range hence fewer scanning lanes)

  • Towfish stability - Similar to a still camera with a slow shutter speed, a sonar generates blurred, distorted and inaccurate images from a unstable towfish (Roll Pitch and Heave)

The two recommendations I would suggest (aside from a heavy unit to get it closer to the bottom) are

Long, lean and balanced towfish – Basically this is to try and reduce sway and movement of the fish so providing a better more stable base for the image to be produced. A depressor added to the fish also removes a lot of the heave and the roll (this was the best improvement I have seen to our system both in image quantity and better cable usage 1:2 depth ration from a 1:3 previously)

A Two Part tow system – This is to decouple sea state conditions and forces imposed on the boat effecting the position of the towfish in the water column

From this graph of a two-part tow system the weight is bouncing (Heave) over about 0.2m whist the side scan (towed vehicle) is bouncing over about 0.07m (0.2/0.07 = about a 2.8 times reduction in heave movement). The best way to think of this is like a float on an anchor in a big swell can help reduce the “bang” on the boat by absorbing part of the force.

Home made Depressor mounted up

Scott


#23

I have a StarFish 450F and a JW Fisher 600/100 kHz system, along with several hull-mounted units. Unfortunately, most areas where we side-scan here in Pensacola, FL are too deep for the hull-mounted units.

Here is a picture I took with my StarFish of a 250’ long wreck in 85’ of water. The cylindrical objects are boilers.


#24

Really nice looking image, its picking up great details (you can clearly see the small and ripples off the wreck) which is a great sign of a well tuned piece of equipment

It also looks like the towfish is running really smooth in the water, sharp crisp straight lines with very little signs of heave - was the ocean conditions flat? or was the towfish just that stable?

Does the Starfish allow you modify the (Time Varied Gain) TVG or is it fixed by the unit? It looks like it should be turned up a bit at the first bottom hit (basically the Nadir) and turned down a bit at the full range

All up really nice


#25

Thanks Scott.

The seas were calm when I shot that picture. I also picked a winter day since the wreck is obscured by a cloud of fish at other times of the year.

I love using the StarFish because it is very light. Unfortunately, it is not stable when there is much wave action. Adding a bunch of weight helps. I am also considering making a depressor wing for it.

Sadly, my StarFish software does not let me modify the TVG, as far as I can tell. It only has a global gain setting for each side. My other sonar fish does allow me to adjust the TVG. I guess I could implement TVG features myself by using the StarFish software development kit and writing a new front-end.

Bryan


#26

Greeting Watt,

I like what you did with the LSS transducer. I have a LSS 2 HD skimmer transducer and have been wanting to convert it to a tow behind. Were you able to find a cable that would work? Also, what about the the pressure on the connection fittings at 80 feet?


#27

that’s really wonderful, thanks for share…


#28

Hi @Hydrosurveyor:

I haven’t done any work on homebuilt sonar since last fall- Pretty much all my time is spent on Trident these days, and any spare time I have is to either use commercial sidescans, or play with some new homebuilt ROV designs.

So no, I haven’t found a suitable cable yet.

Once I find a suitable cable, I will probably not have a detachable connection on the fish itself- I’ll probably just have the cable soldered to the transducer. To use a LSS-2 with a Humminbird topside you need 8 wires- 4 for the 2 sonar bars, 2 for a downward-looking depth sounder, and 2 for a temperature sensor. I don’t know of any good, inexpensive 8-lead waterproof connectors right now.

If I were to try something, I would probably use 2 of the 4-pin LEDJump connectors that are easy to find on Amazon. When you pack them with dielectric grease they work pretty well.

-W


#29

If anyone wants to take a crack at an inexpensive metal detector for an ROV, this came up today: http://www.instructables.com/id/Eco-Friendly-Arduino-Metal-Detector/


#30

If you are looking for a cheap sonar the Raymarine’s Wi-Fish is the best choice. It is incredible what a $200 device can do. I think it is even better than my Imagenex Sportscan sidescan. I usually search for wrecks between 30 and 70m deep. Here is an example…

Grega


#32

Walt, (or anyone for that matter)

Have you used a LSS-2 transducer with a Humminbird? I have the 997C SI unit on a towfish but am looking for something better for a tow fish.


#33

@Jim_Scholz:

I tested the towfish that I posted above (with a LSS-2 transducer) with a Humminbird Helix 10 topside. It worked just fine, and gave noticeably better images than using the stock Humminbird transducers.

-W


#34

Thanks Walt. I will have to see if it will work with my 997C SI unit. I really like that unit but would like it better with greater detail.


#35

I envy you guys for having such a nice and complete equipment.