ground balance capability

[mohalothman]

hi everyone
recently i was working on miniPulse plus rev-D, i build the circuit and did air test every thing was just fine except i encountered an expected problem when i did soil test. the problem was the detector have strong response to the soil. of course this problem is not issue when i move the search coil parallel with the soil surface.
i have done an investigation found to solve this problem you should do a ground balance means eliminate soil effect. i tried increase sample delay and it worked but i lost significant depth even for big target (large constant time target).
do anyone encounter this problem?! and try solve it? any idea ?
thanks

[kt315]

you got first result, thats good, and must go afar. you have to learn what is 'second strobe/sample' and how it is used.
you are on the way.

[mohalothman]

kt315 thanks for your replay
actually the second sample in MPP is used to cancel out the signal from the earth's magnetic field.
the second sample have no role achieving ground balance!

[kt315]

ohhhh.... nooottt....

did you read this guide for full down's?

http://www.geotech1.com/forums/forum...Published-2015

[mohalothman]

yes i read it all
the book talk about ground balance and suggest "taking two sample fairly close to each other and subtracting a properly amplified second sample from the first sample"
this technique i guess it doesn't work well in MPP for tow reason:
the first: the detector pick up a signal due to the earth's magnetic field if the gain of the two sample are not identical
the second important reason is the MPP sense the change of the target signal instead of invariant signal due to the high pass filter after the integrator
adjusting the second sample gain add somehow a fixed signal (voltage) that will be eliminated by the high pass filter like when i fix a target front of the search coil

[Davor]

What bothers you is most probably a shielding problem, and not a ground balance problem. Only at harshest Australian terrains will you face the real problem with the ground.

[Qiaozhi]

Davor is correct. It's most likely a coil shielding issue. What sample delay were you using when you encountered the soil problem?

Also, please note that the MPP does not have a ground balance circuit. The second sample is used to provide Earth field elimination. You can check that this is working properly by waving a fridge or whiteboard magnet a few inches away from the coil.

[mohalothman]

Davor, Qiazhi
i used a shielded coil but the problem is still existing.
the MPP still response to the soil until approximately 80 us delay time that means i will lose a considerably depth

[Davor]

So where are you from?
My vote still goes for poor shielding. Is your shield connected to the circuit ground, and how?
BTW, I did develop a GB circuit for Minipulse, but sadly my Minipulse is still waiting its fairy godmother to breathe life into it, and I did not test it myself. Beside the GB timing circuit, you need an analogue absolute value circuit to be able to detect the taus beyond a hole. Simple PI GB schemes convert the all-positive signal to part positive, part negative, and a hole in the middle.

[Qiaozhi]

Davor, Qiazhi
i used a shielded coil but the problem is still existing.
the MPP still response to the soil until approximately 80 us delay time that means i will lose a considerably depth

A target response with an 80us delay cannot be caused by light soil mineralization. In an air test, try waving a largish chunk of ferrite in front of the coil. Unless your sample delay is ridiculously low, there should be no response.

It still sounds like a shielding issue to me. Are you sure the shield is actually connected?
I once made a silly mistake and connected the cable shield to the hot line, and vice versa. The coil was unusable.

Any photos of your setup?

[mohalothman]

i m from syria
and of course i connect the shield to the ground
i ll provide you photos of my coil tomorrow
davor forgive me your idea "Simple PI GB schemes convert the all-positive signal to part positive, part negative, and a hole in the middle" confuse me a bit
btw i did a air test with a ferrite peice and obtain no response

[kt315]

yes i read it all - vs - MPP does not have a ground balance circuit

[Wahyu]

http://www.geotech1.com/forums/showt...8us#post184096

simple GB work for me, eliminate redbrick and iron stone

[mohalothman]

thanks Wahyu it is very helpful

[eclipse]

Maybe it's best to try and narrow down what's wrong when working on the field.
What I would do is go on the field and start working with the device. When you start seeing erratic behaviour and false signals - try to dig and see if it's actually a rock,
or if it's black sand / rocks / or soil. Whatever is giving signal you need to be aware, take it out in a bag - go home and see what you're dealing with.