Bad Ground

[mikebg]

for me very bad ground in my area is small pieces of quartz which have rust deposits attached to them.pain in the butt

I received from radio amateurs two substances for specimen of BAD GROUND.

The first one is artificial black sand. It is made of brocken ferrite cores used for RF chokes (ferrite beads against interferences). Ferrite beads are made of lossy ferrite. However my experiment showed that it is lossy for high frequencies only. At metal detecting frequencies (untill 20 kHz), ferrite beads are not so lossy.

The second substance is a coarse powder used for producing radar-absorbing paint (stealth technology). I think the powder is fine iron grit covered by lossy ferrite and carbonyl iron. I have measured phase lead of this substance about 89.5 deg at frequency 20 kHz. However my measurement is not sure because the phase error is more than 0.5deg. It seems the anti-radar powder is not suitable for specimen of BAD GROUND.

Then I made an experiment with “lossless” ferrite cores used in short wave radio. I put several cores in microwave oven. The microwave heated them too quickly. In conclusion i reinvented that all ferrite substances are lossy for very high frequencies.

[taliesin]

i'm going to get some of that paint

put it on my car to avoid the police speed guns .Mike when you did the test did you have the s.u.t. covering a larger area than the search head ?

[mikebg]

put it on my car to avoid the police speed guns .Mike when you did the test did you have the s.u.t. covering a larger area than the search head ?

Taliesin, I have too small amount of BAD GROUND. For phase measurements I put the bad ground in a small plastic box 4 placed in the most sensitive area of differential coil configuration, as shown below.
1 is the first RX winding,
2 is oval nonconductive sheet (plastic, hardboard or plywood),
3 are consoles made of plastic,
4 is the plastic box containing bad ground,
5 is TX winding,
6 is the second RX winding and
7 are plastic nuts and bolts.

[taliesin]

i dont know mike to be honest i haven't tried it with larger amounts .it's just something i was thinking about.some of the bad bad ground i have been on is a larger surface area than the search head.

[mikebg]

Technical paper wanted

Qiaozhi, please change the title of this thread! The title should be "BAD GROUND".

TECHNICAL PAPER WANTED
I need the paper

M. Dabas and A. Jolivet and A. Tabbagh, “Magnetic susceptibility and viscosity of soils in a weak time varying field”, Geophys. J. Int., Vol. 108, pp. 101-109, 1992.

Also published online Geophys J Int 108(1):101-109 (2007)

ABSTRACT
We try in this paper to characterize the magnetic behaviour of soils both in the frequency and time domain. Our aim is to understand the anomalous responses we have observed in many EM surveys.

A wide range of soil samples have been analysed with two instruments specially designed for harmonic (80 Hz-10 kHz) and transient (8–100 μs after cut-off) EM low inducing fields (50 μT).

The most common spectrum observed could be explained with the well-dispersed single-domain grain theory (SD): a logarithmic decrease of the in-phase susceptibility with frequency and a constant quadrature susceptibility. Moreover, measurements in the frequency domain of the quadrature susceptibility and measurements of the coefficient of viscosity in the time domain have been compared and found to be in good accordance with theory. But other types of magnetic spectrum were encountered and could not be accounted for SD theories. This results in a non-logarithmic dependence of the in-phase susceptibility, a non-constant quadrature susceptibility and in the time domain a coefficient of viscosity S which is time dependent.

It is demonstrated that the response due to quadrature magnetic susceptibility in the frequency domain and the magnetic viscosity in the time domain is sufficiently strong to be responsible for the anomalous responses.

It is now possible to correct in an EM survey for the soil magnetic effect for a better determination of resistivity or conductivity.
(end of ABSTRACT)
______
NB.: It sounds interesting, but I hate to pay money for download a technical paper.

[Qiaozhi]

It is now possible to correct in an EM survey for the soil magnetic effect for a better determination of resistivity or conductivity.
(end of ABSTRACT)
______
NB.: It sounds interesting, but I hate to pay money for download a technical paper.

The paper you quoted does not seem to be available for free download, but the following paper makes reference to it.
http://adsabs.harvard.edu/full/1998GeoJI.133..201W

[Qiaozhi]

Here's another one.
http://www.eos.ubc.ca/ubcgif/uxo/PDF..._eudem2003.pdf

[taliesin]

one of these would be nice

www.geophyz.com/ anyone got a schematic

[mikebg]

www.geophyz.com/ anyone got a schematic

Qiaozhi, many thanks!
Taliesin, the circuit diagram of Magnetic Viscosity Meter MVM1 is designed by Eric Foster.
http://www.findmall.com/read.php?34,1030276,1031719
He has the schematic.

[taliesin]

i dont think he'll give it to me somehow

[mikebg]

Bad ground in FD

BAD GROUND IN FREQUENCY DOMAIN
From thermodynamics I know that the area inside a hysteresis loop equals the work done in the cycle. For ferromagnetic substances this work is dissipated as heat. It follows that energy loss in bad ground is proportional to number of cycles, ie the dissipated power is proportional to frequency (number of cycles per second).
This is illustrated below with straight lines in complex plane. The frequency response of an ideal (lossless) ferrite coincidents with Im axis because Re component is zero, ie the phase lead of received signal is allways in quadrature with TX current for all frequencies. The ferrite operates as if there is positive mutual inductance between RX and TX coil.
At lossy ferrite, there is Re component and it is proportional to frequency. Then the frequency response of bad ground is straight line, having phase angle less than 90 deg as shown below. When frequency increases, the amplitude also increases, but the phase angle “phi” remains the same.
Can we change the phase angle of bad ground?
Yes, if we change the area closed inside hysteresis loop. One method is to change the amplitude of sinusoidal excitation. The effect at CW metal detectors is illustrated below. Note that the sine wave excites bad ground with TX current in two directions and no DC component.
The other method is to involve DC component in TX field. This can decrease amplitude of GND signal and make bad soil transparent for TGT signal.
(Will be continued when I make new drawing for illustration)

[mikebg]

With suitable excitation we can reduce the GND signal from permeable soil having hysteresis properties. This is illustrated with attached figure. Let we analyse the following four cases:

A. Sine induction or bidirectional pulse induction.
TX current changes between +Im and -Im. The received signal is proportional to change of inductionB. The change is large in this case because is between +Bm and -Bm.

B. Conventional pulse induction.
We have unidirectional TX current which decays from +Im to zero. The change of induction noted as delta B is small because is between Bm and Br. The area closed in this partial hysteresis cycle is small.

C. NRZ (not return to zero) TX current or excitation with DC bias.
In this case TX current decays from Im to Imin. The area of hysteresis cycle becomes more small. The change of induction is between Bm and Bo, ie the GND signal diminishes. DC bias means steady magnetic field, which can be achieved with a powerful magnet and then we can operate with return to zero TX current (conventional PI excitation.

D. TX field with HF bias.
The applying of additional sinusoidal current with relatively high frequency is used at audio tape recording for erasing and better quality. "Erasing" of soil magnetization with HF field would increase GND signal because the change of induction appears between zero and Bm when electronic switch is closed. However this is not important if the sampling is made at open switch (conventional PI).

[mikebg]

I asked Eric Foster for operation with partial hysteresis cycle:
http://www.findmall.com/read.php?34,690903

He wrote (April 02, 2008 12:15PM):
"I have found that applying a steady magnetic field to Oz ironstone makes the ground signal worse, rather than better. However, you do get to a point where the ground signal does start to drop off, but I have only achieved this with a powerful ceramic magnet. Not practical in the field as all the loose ironstone would end up stuck to it. Rocks and soils from other places do immediately start to show a fall off in signal as a steady field is applied, so there must be something different in the Oz mineralogy."

The bad effect of steady magnetisation is known at audio tape recording. Only in low quality tape recorders is used permanent magnet for erasing.

What uses Minelab for bad ground? Here is an explanation
http://www.minelab.com/__files/f/5342/KBA04-1_Metal_Detecting_Terminology.pdf
"Dual Voltage Technology (DVT)
Minelab’s patented Dual Voltage technology (DVT) is implemented in the GPX series of metal detectors.
DVT transmits two different voltage levels from the search coil. This has the advantage of improved ground balance, increased sensitivity to small targets and increased detection depth."

[Bill512]

Mike , Minelab's primary method for ground balance is the MPS (the DVT helps further).

[Tinkerer]

Hi Mikebg,

thanks for the interesting analysis.
Would you say that a short burst of 1MHz, maybe 3us duration, high power oscillation can have a significant influence on the hot soil response?

Tinkerer