PI MD

[DOOLEY]

HELLO GREEN,

i use a big glass coffee jar ( remove tinfoil in the lid !! )
builders sand,
sea salt from supermarket,
and tap water.

shake well...............

[green]

HELLO GREEN,

i use a big glass coffee jar ( remove tinfoil in the lid !! )
builders sand,
sea salt from supermarket,
and tap water.

shake well...............

http://www.geotech1.com/forums/showt...665#post208665

[The above thread suggests you need a large amount of wet sand. The wet sand time constant might be around 1usec.] The difference in readings between 6, 10, and 14usec delay should be about 50 to 1 for a 1usec TC if I did the math right. Just trying to learn what I can about wet salt sand acting as a target.

[6666]

Along with my tub of wet sand I also brought home several bottles of sea water to add to sand as it dried out.

[Ferric Toes]

I have not been able to replicate a wet beach with small containers of sea water or wet sand. The conductivity of sea water is about 10^6 times less conductive than stainless steel, so you need a very large quantity to generate an eddy current decay that falls within the normal sample delay periods. In the 1960's the head of department where I worked let us put salt in his private swimming pool of sufficient concentration to match the conductivity of sea water. We got no signal from it. With today's shorter sample delays you might have got a signal, but nothing compared to the beach.

Eric.

[green]

Along with my tub of wet sand I also brought home several bottles of sea water to add to sand as it dried out.

Could you detect the tub of wet sand? If yes, with what detector?

[green]

I have not been able to replicate a wet beach with small containers of sea water or wet sand. The conductivity of sea water is about 10^6 times less conductive than stainless steel, so you need a very large quantity to generate an eddy current decay that falls within the normal sample delay periods. In the 1960's the head of department where I worked let us put salt in his private swimming pool of sufficient concentration to match the conductivity of sea water. We got no signal from it. With today's shorter sample delays you might have got a signal, but nothing compared to the beach.

Eric.

Did you try a VLF detector with the swimming pool?

[Ferric Toes]

Did you try a VLF detector with the swimming pool?

Unfortunately not. This was 1967 when developing a diver held PI and there was little else available in the marketplace to compare it with. There were induction balance continuous wave machines at that time, but they were mainly high frequency (above 20kHz) and not what you would call VLF.

Eric.

[6666]

Could you detect the tub of wet sand? If yes, with what detector?

Yep could detect it with ace 250 so had to notch out 5 cents
I only used it till the sand went dry, but from what Eric has said
maybe it was just the minerals in the sand I was detecting.

[green]

Might have a chance to test the detector on a salt water beach. Want to try some tests to learn something about salt acting as a target. Including a possible test procedure. I want to try it in my back yard first. Don't have a chance to test in salt water very often so if anyone has other test suggestions they would be appreciated. Including a diagram of the detector now, similar to what I was thinking in first post. Looking at the test chart I need to add a column for target delay time. Any others and what should be entered in test conditions?

Tried some measurements in the back yard. Target delay 6usec, GEB off (Ballpark or approximate numbers) Coil on ground, lifting one end or the other +and- 250mv. Both ends 1 inch off ground, lifting one end or the other +and- 90mv. Both ends 2 inches off ground, lifting one end or the other +and- 40mv. Both ends 3 inches off ground, lifting one end or the other +and- 25mv. The reading increased as coil is tilted until a little over a 2 inch lift, fairly constant for another 2 inch lift and then decreases. The figure eight Rx reduces the ground signal but I still need GEB on to use the detector in the back yard even though the ground isn't very hot.

[green]

Tried some measurements in the back yard. Target delay 6usec, GEB off (Ballpark or approximate numbers) Coil on ground, lifting one end or the other +and- 250mv. Both ends 1 inch off ground, lifting one end or the other +and- 90mv. Both ends 2 inches off ground, lifting one end or the other +and- 40mv. Both ends 3 inches off ground, lifting one end or the other +and- 25mv. The reading increased as coil is tilted until a little over a 2 inch lift, fairly constant for another 2 inch lift and then decreases. The figure eight Rx reduces the ground signal but I still need GEB on to use the detector in the back yard even though the ground isn't very hot.

Some measurements with the coil on integrator. Coil on ground, lifting one end or the other +and- 2000mv(over full scale). Both ends 3 inches off ground, lifting one end or the other +and- 300mv. US quarter at 4inches, 60mv. US quarter at 4inches with coil off integrator (above reply conditions) 6usec delay, GEB off 260mv. I added the coil on integrator to indicate ferrous non ferrous. Worked good on the bench. With the ground in my yard causing over scale readings it's not working. Maybe on the beach or different ground.

The Tx is constant rate at 6250 amps/second. What is a typical range of peak amps/second for VLF detectors?

[green]

Still trying to understand salt beach or salt water. crane replied in http://www.geotech1.com/forums/showt...685#post208685 about detecting salt when pumping detector at waist height. Something I can do when at the beach, don't know if the figure eight coil will tend to cancel salt response that far from target. Don't have to get to far from ground to not detect ground(few inches). If I turn the detector on in the garage the led is on almost all the time when the coil is moving even though the coil is a fair distance(few feet) from the car or the aluminum siding walls. Ground has a large area like the wall, why isn't it detected at distances similar to the car or aluminum siding? Recorded aluminum foil time constants awhile back. The time constant appears to be proportional to length for a square for a 1x1, 2x2 and 9x9 inch piece. Should I expect a piece 200x200 inches to be 200 times the 1x1 inch piece? The foil time constant doubles with two layers if still thin enough. The salt water depth changes with distance from shore. Would the time constant change with depth? Does any of this make sense? Are there any other questions?

[Tinkerer]

Still trying to understand salt beach or salt water. crane replied in http://www.geotech1.com/forums/showt...685#post208685 about detecting salt when pumping detector at waist height. Something I can do when at the beach, don't know if the figure eight coil will tend to cancel salt response that far from target. Don't have to get to far from ground to not detect ground(few inches). If I turn the detector on in the garage the led is on almost all the time when the coil is moving even though the coil is a fair distance(few feet) from the car or the aluminum siding walls. Ground has a large area like the wall, why isn't it detected at distances similar to the car or aluminum siding? Recorded aluminum foil time constants awhile back. The time constant appears to be proportional to length for a square for a 1x1, 2x2 and 9x9 inch piece. Should I expect a piece 200x200 inches to be 200 times the 1x1 inch piece? The foil time constant doubles with two layers if still thin enough. The salt water depth changes with distance from shore. Would the time constant change with depth? Does any of this make sense? Are there any other questions?

With foil, you look at the surface area and skin effect. With saltwater you look at a volume. In your garage, you are within a volume.
One way to look at the field of the coil, is as a volume, a sphere. Under 50ft of saltwater, the whole coil field is in the water, that is the target surrounds the coil field and fills the whole coil field.
On the beach, only half of the coil field sees the target.
With a 1"x1" target, only a fraction of the field(lines) hits the target and generates eddy currents.

[Ferric Toes]

I will have to look up some notes and reports on this as it is some years since I looked into this in detail. Tinkerer is right in that you are exciting a volume rather than a surface and the volume with depend on the TX search coil diameter. e.g. a 6in coil will excite a smaller volume than a 12in coil, which in turn will be less than a 15in coil, and so on. Unlike iron mineralised ground where the TC is independant of volume, sea water TC whether on a wet beach or in the sea itself will vary dramatically, which means it cannot be cancelled electronically in the same way as ground.

A coil above a wet beach or the sea surface is energising what is called a 'conductive half space'. A coil submerged in the sea at depth is in a 'conductive full space' and exciting the volume of a full sphere. We did some tests in 1967 with a 3ft diameter coil lowered on a line from a boat such that the coil was parallel to the water surface. We measured the RX signal amplitude with the coil just above the surface and then lowered it in 1ft steps. The amplitude increased to a maximum at 30ft and then stayed constant until the coil was near 30ft from the bottom when the amplitude reduced again to a value a bit above what it was at the surface. The difference was that the bottom still consisted of water saturated sand until it reached bedrock. On survey with a boat towed PI detector we found that it was quite accurate in plotting the depth of sand above bedrock; much like an echo sounder. On a chart recorder plot, metal signals stood out sharply against the more slowly undulating 'saturated sand signal'.

From memory a conductive half space obeys a t^-5/2 decay law.

Eric.

[Teleno]

Eric, do you recall what the samplig delay was at the time?

[Ferric Toes]

Eric, do you recall what the samplig delay was at the time?

With those early PI's the sample delay was 100uS. Even today with true diver operated PI detectors it is difficult to go shorter than 25uS with an 8in coil in, say, 50ft of sea water, otherwise the RX preamp saturates. A cancelling search coil may be the answer such as a fig.8 RX inside a round or oblong TX. Dynamic, or motion, type detectors are not favoured by divers as to keep sweeping side to side is tiring due to water resistance, so zeroing has to be manual.

Eric.