Surf PI - Looked at decay curve when dunking coil in saltwater ... was not as dramatic as I thought

[green]

http://www.geotech1.com/forums/attac...3&d=1512769848 (from reply#1)

Comparing charts, .5M under vs 1M over sea. Looks like at 10usec .5M under maybe 1v difference, at 20usec .5M under maybe .05v difference. http://www.geotech1.com/forums/attac...8&d=1545261608 shows a similar change. No way could see straight line decay at 200usec delay unless amplifier was saturated for over 150usec. Been trying to generate a power slope with similar volts at 10 and 20usec that could see at 200usec. No success. Is there something other than signal increase causing saturation or slope changing to see salt water at 200usec delay? Any thoughts on what I'm missing?

[Polymer]

The sea is the target, no target would be away from the sea in the air. Charting a target on the bench is first, was thinking a 2inch square of regular aluminum foil would be a good place to start. Sample at 5usec/div for the foil. Maybe you could post a picture of your chart with 2inch square regular srength aluminum foil.

You need to be able chart linear-log and maybe log-log if the target doesn't chart straight line linear-log with sea as target. Just my thoughts maybe some knows what the slope is and if it changes with depth.

Great, that makes it way easier, the seawater being the target.
I'm not sure how to interpret "2inch square" - does that mean 2x2 inches which would be 4 square inches from my understanding
or 1.41x1.41 inches which would be 2 square inches?

I can't easily do anything mathematical with the old scope pics, as I only stored them graphically.
Now I know that I can store them numerically.

It will take a bit to set up a new experiment whilst still dealing with noisy boards and also waiting for a window of quiet weather.
I will find a spot where I can go much deeper than before without needing a boat.

[green]

Great, that makes it way easier, the seawater being the target.
I'm not sure how to interpret "2inch square" - does that mean 2x2 inches which would be 4 square inches from my understanding
or 1.41x1.41 inches which would be 2 square inches?

I can't easily do anything mathematical with the old scope pics, as I only stored them graphically.
Now I know that I can store them numerically.

It will take a bit to set up a new experiment whilst still dealing with noisy boards and also waiting for a window of quiet weather.
I will find a spot where I can go much deeper than before without needing a boat.

"2inch square" - does that mean 2x2 inches (yes)regular strength aluminum foil
Need to get good data with 2x2 inch foil before trying sea water.
Still not understanding fast decay and being able to detect at 200usec delay.

[Ferric Toes]

I've just been looking at some old papers and correspondence. For a PI RX/RX coil immersed in salt water, the decay obeys a t^-5/2 law. This is the same as geophysicists describe for a conductive half space. For the maths see Technical Note TN-7 from Geonics Ltd in Canada. Because TX energy is lost in a conductive medium, the range of metal objects will be reduced relative to that achieved in air. The only example I have from 1970 is that a brass plate 10 x 10 x 0.1cm would be detectable at 1.3m in air and between 0.6 and 0.8m in sea water. I don't have details of coil used but probably about than 1.0m diameter and a delay of 100uS.

For Polymer. Most of the problems with the 42" coil were mechanical rather than electrical. The first design sled turned over at the end of a traverse, so we had to design it to have runners top and bottom so that on the return parallel traverse the turning over did not matter. Original plastic pipe runners, although filled with resin, quickly wore out, so we had special nylon ones made that could be unbolted and changed when necessary. The only coil problem, as we found out the hard way, was that it had be always immersed in water to keep it cool and the prevent the ABS pipe from melting. Pulsing 50A definitely warms things up.

Eric.

[green]

Thanks Eric,

For a PI RX/RX coil immersed in salt water, the decay obeys a t^-5/2 law

Think I've seen the number before. Didn't think about what it means. If I'm doing it right the amplitude at 200usec would be 1/2000 of amplitude at 10usec. Detectable.

[green]

Charts comparing 3usec TC vs t^-5/2 law decay. Looks like possible to measure the difference.

[Polymer]

Green, I have made a quick in house experiment with the 2x2 inch foil as you described.

I am quite amazed at how much the PI reacts to that. The difference is definitely visible.

My thoughts on an expiriment by the sea are the following:

Bamboo pole 6m, measuring from 6m in the air above water and lowering in 1m increments to water level.
The same then from water surface to 6m underwater with at least 6m water headroom under the coil.
Still have to measure the exact water depth where I will be measuring. at least 13m of cable in all.
Hope it doesnt slow down coil too much - is this relevent for this experiment?

I will also be measuring the resonant frequency of the coil in the above steps, as I have measured a little with coil in bucket
and there are great changes, even though they are not specially visible on the output of the preamp.

Eric has described a formula - does this make this experiment obsolete? Would the results already be a known?
What I am trying to say is, is it better to spend the time doing a different experiment?

Anyway, here are my results from the in house foil test:

GIF Overlay


Here the two csv files. I have renamed the filetype to .pdf hoping I can upload these.
You will have to rename them back to .csv - hope this works

green_exp_no_foil_csv.pdf
green_exp_2by2in_foil_csv.pdf

The scilab software for data plotting is way to complex for me, I believe that would be something for Aziz's realm.
Have started playing with DatPlot and get nice curves from the numerical values, but have not found the log part yet.

"Happy New Year" by the way!

PICS of setup: