salt response

[Tinkerer]

So how about an analogy with a bell... you may strike it front, back, or sideways, and it responds with the same timbre. Now, what if we should observe the full available thickness of an object up to the skin depth - if not obstructed?
I'm wrapping my head around this one and it seem plausible.

The skin depth makes sense on the beach. But how does it work for a submersed coil?
The varying magnetic field generates eddy currents.
The eddy currents travel at a finite speed through the conductor.
The ocean is an infinitely large conductor.
Then there is the magnetic field set up by the eddy currents. It would travel together with the eddy currents. How far did it travel between pulses?

The analogy of the bell. The timber is always the same, but the amplitude changes very much depending on the size of the striker.

It would seem that if the target signal of saltwater persists after 50us, it has to have a Tau of more than 10us.

[Davor]

PI detectors with long sample periods observe a large bandwidth spanning down to low very frequencies. Skin depth gets thicker with lower frequencies, and my comprehension is that it is a source of long taus for thick bulk objects in general. When a target is thin, the whole object is thinner than skin for a certain frequency and below, and it reduces the effective tau, just like in case of a foil.

[Tinkerer]

PI detectors with long sample periods observe a large bandwidth spanning down to low very frequencies. Skin depth gets thicker with lower frequencies, and my comprehension is that it is a source of long taus for thick bulk objects in general. When a target is thin, the whole object is thinner than skin for a certain frequency and below, and it reduces the effective tau, just like in case of a foil.

The attached pdf has some information on seawater.

On a traditional PI we have 2 TX Tau's. During TX ON, as we usually do not flat-top the TX ramp a long Tau, and a short Tau with the switch OFF transient.

Tomorrow I will post LTSpice simulations showing the influence of a long and a short TX ramp on targets that have different Tau. This clearly shows the advantage of not carrying over the TX ON eddy currents.

[Tinkerer]

Improved TX

The traditional PI TX pulse is about 100us long and peaks near 500mA. Note the target amplitude.
Target Tau:
10us
100us
500us

Now look at the target response with the improved TX pulse.
The Very long TX pulse is 2000us long at about 500us peak. See how the long Tau target amplitude response increased.

The short TX pulse is about 100us long and peaks at about 500mA. Note the increased target response amplitude compared with the traditional PI.

[green]

The traditional PI TX pulse is about 100us long and peaks near 500mA. Note the target amplitude.
Target Tau:
10us
100us
500us

Now look at the target response with the improved TX pulse.
The Very long TX pulse is 2000us long at about 500us peak. See how the long Tau target amplitude response increased.

The short TX pulse is about 100us long and peaks at about 500mA. Note the increased target response amplitude compared with the traditional PI.

Interesting. What does the improved Tx current look like when reversing polarity?

[Tinkerer]

Interesting. What does the improved Tx current look like when reversing polarity?

You mean making a square wave TX?

[green]

You mean making a square wave TX?

The traditional pi looks at target response with zero coil current. Looks to me the coil current might be changing during target decay for your last two examples.

[Tinkerer]

The traditional pi looks at target response with zero coil current. Looks to me the coil current might be changing during target decay for your last two examples.

Correct.
This accelerates the decay of the target response and thus enhances the measurable difference for long Tau targets.

Next I will show the comparison of the actual "kick" of the TX.

[Tinkerer]

Bi-polar continuous current_target response

And here is the bi-polar continuous current target response for comparison.