Topic: Sonar physics -------- 1.Reflection

[Sea-Rover]

If an old wooden boat lies somewhere at the bottom and it seems almost no signal
then it could be because it consists of a soft wood.
If the wreck is also quite a few decades down, then the timber as water is transparent, almost invisible.
It is not a fault of the device or a special misadjustment an acoustic
Problem.
If what is sought, one should first think about what material to make is the object.

Material         Acoustic Impedance           Reflection Coefficient %

Air                   0.000428                                           99.9
Castor Oil         1.45                                                   0.09
Water fresh      1.48                                                   0.04            
Water sea        1.54                                                ---------
Pine                  1.57                                                   0.009
Oak                   2.90                                                   9.4
Concrete           8.0                                                   46.0
Steel                47.0                                                   88.0

f the wood is very long in the water and is not protected, the poorer will be the reflection of sound.

[Rickard]

I have seen on several occasions how transparent objects can be. The image shows a small sailing boat that is made of pine. The bottom of the hull seems to be missing, but it's not. Sound (455 kHz) is travelling through the 1/2" pine boards, hits the extremely soft sea floor, bounces back through the hull again and back to the transducer. One can seen shadows from the ribs and the thicker parts of the hull.
Rickard

[Sea-Rover]

Hi Rickard,
this is already correct if the transducer to the object is very flat. This achieves a good silhouette.
Even when your picture is grainy, the ship with the sediment.
Without a shadow of the object is easily overlooked. So with a better Towfish when moored the boat.
The sonar should be adjusted when using the geological conditions.
Incidentally, a very nice sonar image.

[Roddy]

Sea-Rover,
DO you have a spec/mil number of the typf of your tow fish?
Nice physics class!

[SonarPro]

In regards to the 455 kHz scanning sonar image of sail boat

Did you dive the target or video the boat via an ROV?

My experience with sonars of this frequency with soft pine boats in cold fresh water does agree with the pine adsorbing the sonar pulse, however the pulse does not have the strength to pass through. My best best interpretation of that image is your boat is missing some planks.

respectively

Vince

[Rickard]

Vince,
No, I haven't dived or filmed the boat so your interpretation may be correct. Diving is awkward and filming is almost impossible because of low visibility. But I have scanned the wreck from several angles and there are no traces from debris or planks close the hull. The boards in this model are fastened with copper rivets that last rather long. Still, I can't be sure in this particular case, but this is not the only wreck I have observed with `transparent´ hull. At least they look transparent.
The images show an 8 m long wreck at 455 kHz (darker image) and 262 kHz (lighter image) with the transducer at exactly the same position. Transparency is, as expected, better at the lower frequency. No diving or filming in this case either.
Rickard