Pulse Delay

[Prospector Al]

Hi All,

I have seen several statements to the effect that signal gating pulse delays shorter than 10 uS are unusable on the beach, because the signal owing to the conductivity of the soil/water will swamp the signal from the target.

I tried to duplicate that condition by filling a container with 2%
saline (the approximate concentration of sea water.)

A sine-wave detector operating at 50 kHz responded to the container and did indeed overpower the detector.

A PI detector with a gate delay of 5 uS did not respond however, although it picked up small gold nuggets.

My question is this: Has anyone noticed if it is the salt water itself--obviously present in a larger quantity than my container--or is is it the saline-soaked soil that causes the signal?

Is it conceivable that it isn't solely the salinity but also other conductive ions in the soil, or perhaps even a dispersed "hot rock"
effect that is responsible for the signal?

Your responses would be greatly appreciated--it would save me a 100-mile trip to the ocean...

Prospector Al

 

[gatorguy]

My quess it is the salinity content which is a mineral and the saturation as Eric explained earlier is to much signal for that short of a Pulse Delay. Don

 

[Eric Foster]

Hi Prospector Al,

Definitely the salt water. I've been out in a boat and waved a coil over water that was in excess of 20ft deep. I couldn't get anywhere near it at 10uS, let alone 5uS. You also see the effect with a diver held PI with 30uS delay. Jump off a boat into 50ft of water and you get a signal at the surface, that has to be backed off. Dive down and the signal increase until you are about 15ft deep. The signal then stays constant, until you are 15ft from the bottom, and then it starts decreasing again. However, at the bottom, it is not quite as low as it was at the surface, because there is still a depth of water saturated sand below the coil.

The sea represents an infinite volume of conductive material to the coil field, which cannot be duplicated with containers of sea water, however big. To illustrate the scientific method we used at Oxford University, we dissolved a load of salt in our lab directors private swimming pool, until it was the same order of salinity as sea water. We then went in with an underwater PI. Result - no signal, although we did not have circuits, or coils, that would work at anything approaching 10uS in those days.

Eric.

 

[Prospector Al]

Hi Eric,

I understand the concept of a volume conductor where current paths further and further out contribute to the total conductance. I'm still amazed though, that you couldn't duplicate that effect in a swimming pool! The current paths further out contribute less and less
and theoretically, the the conductivity should approach a definite limit, asymptotically, as is the case with grounding stakes driven into the earth. Supposedly, the resistance between two stakes is about 33 ohms, no matter how far apart the stakes are...

The other amazing thing is the change of the signal with depth. Does
the temperature of the water have anything to do with it--that definitely changes with depth.

If conductivity is indeed the culprit, there is no significant reactive component to the signal--all the paths are in parallel, and the resultant inductance ought to be very low. Even so, the signal
from the sea water must have a time constant. Does anyone know what it is?

P. Al

 

[Prospector Al]

Hi Eric,

I used a 500 kHz sine wave detector to determine the time constant of 2% saline and got a value of ca. 0.3 uS. That applies to a small vessel only, of course.

I suspect that there is something else involved, though. Sea water contains many other ionic species and even some organic molecules which behave differently from the relatively small sodium and chlorine ions. I designed an electromagnetic flowmeter a few years ago and observed that the eddy currents circulating in the blood and the surrounding tissues had longer time constants than could be accounted for by the properties of saline.

There are several kinds of "polarization" going on under the influence of a varying magnetic field: Molecules stretch and rotate, etc.

When the field is removed, everything goes back to normal, but the absorbed energy is re-radiated, as is the case with hot rocks, and the
time constants involved are much longer than what can be attributed to inductance and conductivity alone.

The amount of organic material in sea water varies with depth--it become less prevalent when most of the light has been filtered out.

Perhaps that is the reason for the signal variation with depth you found?

If there were a way to eliminate this effect, one could use less delay for the gating pulse, and find smaller targets...

P. Al