DW sound propagation model measurements

[Dr.Sid]

This is results of my measurements of DW sound propagation model (SPM). It contains raw data, methods how they were obtained and my conclusions about how the DW SPM works.

First the raw data
Sea state 1 for all SSPs. The target is surface car carrier.

X axis shows distance in nautical miles.
Left Y axis shows track's SNR (signal to noise ration) as it is displayed on FFG's broadband station.

There are these SNR graphs here:
- gray line shows how SNR changes with distance with no layer, with bottom limited conditions. It is here just for reference. Note that it is not straight line. More about this later.
- blue line shows SNR in surface duct conditions how it was measured above the layer. Note that it is almost straight line, the most simple situation, spherical spreading.
- red line shows SNR below the layer. Up to distance about 5.5 nm layer just makes the signal little weaker (about 10%). It is the same for convergence zones conditions, so that line is not displayed.
- purple line shows convergence zones conditions above the layer. It is very similar to the surface duct, except for the peak at 30 nm. There is another peak at 60 nm but it is not measurable with SNR (it does not get over 0). However it can be seen nicely on the BB or NB display. Note that convergence zone is really narrow, it takes seconds to cross it.

Right Y axis shows depth (in ft) to show SHADOW ZONE shape.
- Shadow zone shape is shown as dashed line, for both surface duct and convergence zones conditions.
For surface duct, with distances greater then 5.5 nm when listening from below the layer, you will hear nothing. Surface target will be lost. And also surface listener will not be able to hear you, nor ping you. This is the shadow zone.
Between 5.5 nm and 11 nm this shadow zone is not bottom limited, but it has it's lower border. It starts at 5.5 nm at layer depth and goes down really fast. I guess it goes down even further but I could not measure it with FFG and these depths are not accessible for subs either. In distances greater then 11 nm you can consider shadow zone to be bottom limited.
For convergence zones conditions the shadow zone has different depth because the layer depth is different, but otherwise the shape is the same.




Now how did I measure this, just fot your info

- I used custom made mission with good weather and only target over deep water.
- I started at 1 nm going parallel with the target and moved array into different depths, grasping the situation at first. I launched BT buoy to get real SSP and layer depth. I also saved the game because you can't reproduce layer depth in any other way.
- Then I made 1 run for each graph line. I put array into depth I want, then I went away from the target, keeping the array at constant depth (more or less). Any time SNR changed I paused, jumped to navmap and used R tool to measure distance from ARRAY TIP (beginning of LOS) to target. It's not much exact method, but lines came out quite straight, so I guess it is OK.
- Then I made little more measurements around shadow zone beginning to know for sure where it starts and how does the bottom border of the shadow zone look like, same for convergence zones.
- Then I made some experiments when both target and listener where submerged to see how it behaves. I did no graph since I only found when the signal is weakened by the layer and when it is not weakened.
- then with some papers covered with numbers I put it all into Excel.

Conclusion

There is no measurable difference if you change array depth, until you cross the layer or shadow zone border. There are simply 4 cases:

Above - target is above the layer, if there is any. In bottom limited conditions consider all situations as 'Above layer'.
Under - target is under the layer, but closer than about 6nm, so it is not in the shadow zone. This apply same for both convergence zones and surface duct.
Shadow - target is under the layer and beyond 6nm, inside the shadow zone.
CZ - convergence zone - target is right at 30 nm or 60nm, above the layer, and this applies only to convergence zones situation.

This gives these combinations. Note that transmission is symmetric. If A can hear B, B can hear A with same quality.

Above-above - signal is affected only by distance, let's call this 'normal transmition'.
Above-under (or under-above) - signal is somewhat weaker. For subs it can complicate ID, but you will be detected.
Above-shadow (or shadow-above) - signal is completely lost. Both passive and active is useless.
Above-CZ - slight signal increase. Allows very short detection of target. Signal is too weak for ID.
Under-under - signal is weaker, by the same amount as in Above-under situation. So even if both target and listener are bellow the layer, the signal is weakened. Anyway there is no shadow zone when both listener and target are below the layer, so target is lost depending only on distance.

Now some other informations and ideas:

- in DW, SNR is just signal strength. It has nothing to so with noise. It does not change with my speed, it stays high even near array washout, when you simply can't see anything on BB display. However I did not test if it is affected by weather (I guess it must be, because I know even in DW bad weather makes targets harder to track). With good weather SNR looks simply like signal strength in decibels. The logarithmic scale is obvious because spherical spreading looks like straight lines.

- with bottom limited conditions (gray line) I tried to get far enough to lost the signal. I gave it up at 50 nm (I've got even farther away when measuring convergence zones, signal still there even over 60 nm). There was no line visible on the display but the TAG line was still there and reacting correctly. There is little trick here with these tags. When signal is lost they vanish about 2 minutes after that happens. But SNR changes seem to be right on time. Signal lost is best judged on single beam NB display.

- note that surface duct and convergence zones conditions give better signal transition above the layer than bottom limited conditions. This is correct, because the signal is trapped in shallow depths, even the amount of the effect seems right.

- The bulge on the bottom limited gray line could be surface shadow. With SSP used here rays would bend down, creating something similar. I guess it should be much more prominent, but I have no other explanation what it should be.

Bottom type influence:
Bottom type affects general detection ranges. Rock gives best sound transfer, mud about double transmission loss, sand about triple transmission loss. Bottom type affects sound propagation over all possible depths, shallow and deep.

Other links:
Sonoboy made measurements to better describe shape of the shadow zone. You can find it in this topic http://www.subsim.com/radioroom/showthread.php?t=124973
Or if just the image is enough for you, here it is: http://img228.imageshack.us/my.php?image=layergk9.jpg