ground balance capability

[Qiaozhi]

actually the soil affect the decay of preamp output like a small piece of iron or a small coin. maybe tomorrow i'll provide you a pictures illustrate this
my point was that u can't notice change in preamp output far away let us say after maybe 40 us

I notice from your scope image that there's a rather large DC offset of >2V at the preamp output. On an MPP that I have on the bench here, the offset is around 500mV.

[mohalothman]

I notice from your scope image that there's a rather large DC offset of >2V at the preamp output. On an MPP that I have on the bench here, the offset is around 500mV.

yeh i notice this dc offset which is dependent on the coil itself i mean if i install another coil with the same value the dc offset will change
i can't figure out why this occur !!!
any way this dc offset has no effect on soil response

[mohalothman]

something has my intention in SD2000 schematic here: http://www.geotech1.com/forums/showt...7844#post67844
look guys at transmit and sample timings scheme u see the delay time of the first sample of ground balance channel is 90us !!!
that doesn't assume the high mineralized soil could has a long time constant?! the same thing what i got with the soil here !!!

[Qiaozhi]

yeh i notice this dc offset which is dependent on the coil itself i mean if i install another coil with the same value the dc offset will change
i can't figure out why this occur !!!
any way this dc offset has no effect on soil response

What sort of response do you get to the soil sample with a different detector?

[mohalothman]

What sort of response do you get to the soil sample with a different detector?

u mean different coil?
i tested a large coil (30 cm diameter ) and small one (10 cm diameter) the response was almost the same
as well i tested my own work PIMD which is slightly different from MPP but i got the same response too

[kt315]

you experimnt on very metal infected place. thats not soil. you have to find clear place in desert at first.

[green]

A test with some ground from California. Hotter than ground from my back yard, don't know how it compares with other soil. The bag of ground was laying on the coil when recorded. I got .5volt change at 20usec with a gain of 4000 so should be able to see a change with gain of 1000(MPP). Interested if you can see a change with a no target scope vs a ground scope picture with the bag of dirt laying on the coil. Ground decays with a slope steeper than -1 with my detector(constant rate, 160usec Tx pulse). Ground looses signal fast at short delays but slower at longer delays. Signal at 100usec is 1/5 the signal at 20usec with a slope of -1.

[Digger429]

Q: If you were targeting objects with a quick delay such as gold, would it be worth taking the second sample immediately after the first sample finishes... thus being able to cancel out at least some of the ground interference while all of the gold signal should have gone by then.

eg. 15us of main sample to grab the gold, followed immediately by second sample.

[Qiaozhi]

u mean different coil?
i tested a large coil (30 cm diameter ) and small one (10 cm diameter) the response was almost the same
as well i tested my own work PIMD which is slightly different from MPP but i got the same response too

No - I meant different detector.
Anyway, you've tried both different coils and a different detector, with the same results. So it appears clearly that the soil is highly mineralized, and you will need to implement some form of ground balance circuit.

[Qiaozhi]

Q: If you were targeting objects with a quick delay such as gold, would it be worth taking the second sample immediately after the first sample finishes... thus being able to cancel out at least some of the ground interference while all of the gold signal should have gone by then.

eg. 15us of main sample to grab the gold, followed immediately by second sample.

That's more or less how ground balance on a PI is normally achieved. However, by changing the second sample timing to use for GB, you will lose the ability to eliminate the Earth field.
http://www.geotech1.com/forums/showt...660#post182660

[green]

http://www.geotech1.com/forums/showt...Ground-Balance

More information on MPP ground balance. Need to change the timing to add a ground balance sample and correct EF sample(EF sample time=ground sample time-target sample time or ground sample and target sample time control opposite switches for EF). Need to change or modify integrator.
Maybe someone could explain why you shouldn't be able to see ground signal with a longer sample delay time. I sampled ground change out of integrator with over 700usec delay(EF sample, target sample only). My integrator has a gain of 1 with a 10usec sample(target sample). 400uv change when laying the bag of ground on the coil. Changed the scales on the ground chart I posted reply #37 to include 1000sec. About 400uv at 700usec. Would be a larger change if ground decayed with a -1 slope as normally suggested.

chart slope should be -1 not 1.

[Qiaozhi]

Maybe someone could explain why you shouldn't be able to see ground signal with a longer sample delay time.

You will not be able to see the ground signal with a longer sample delay time because any eddy currents will have died away by the time the sample is taken. This is the same reason why you cannot detect small gold nuggets at longer delays, and also why beach detectors cannot operate below 15us on wet salty sand.

Note that the normal ground balance method (with a second sample) only works effectively if the ground matrix is homogeneous. i.e. the conductivity doesn't change significantly over a short distance.

[green]

You will not be able to see the ground signal with a longer sample delay time because any eddy currents will have died away by the time the sample is taken. This is the same reason why you cannot detect small gold nuggets at longer delays, and also why beach detectors cannot operate below 15us on wet salty sand.

Note that the normal ground balance method (with a second sample) only works effectively if the ground matrix is homogeneous. i.e. the conductivity doesn't change significantly over a short distance.

The small gold nuggets decay on a straight line on a linear X, log Y chart. Ground decays on a straight line on a log X, log Y chart so it takes a lot longer to decay to a not detectable value. I can see the ground signal at integrator out with a 10usec sample and 700usec delay reply #41. Do you see something wrong with the ground decay chart or my test method reply #41? Don't know how the signal strength from the zip lock bag of ground would compare with the coil laying on the ground where it was dug from.

[Qiaozhi]

The small gold nuggets decay on a straight line on a linear X, log Y chart. Ground decays on a straight line on a log X, log Y chart so it takes a lot longer to decay to a not detectable value. I can see the ground signal at integrator out with a 10usec sample and 700usec delay reply #41. Do you see something wrong with the ground decay chart or my test method reply #41? Don't know how the signal strength from the zip lock bag of ground would compare with the coil laying on the ground where it was dug from.

Is this soil highly mineralized?
If so, then I can understand why the eddy currents may persist for longer than a small gold nugget. Also, in that case you would also need to implement some form of ground balance.

[kt315]

crosslinked on the midnight http://www.geotech1.com/forums/showt...Balance-Theory
for all lovers of ground balance at highly mineralized soil conds