Nature of the Signal - White's TM808

[audreamer]

I am trying to understand how the signal from this machine behaves. According to the manual the machine detects underground metallic objects when the front coil is positioned directly above the object. But now that I have been using the machine for a while, it seems that the transmission and reception of the VLF signal is not so straight forward. Especially when attempting to scan the target while at the bottom of a vertical pit a few feet underground. Hopefully someone out there knows more about how the signal can change based on ground/environmental conditions.

The first question I had was about signal strength. I had a target that was giving a very strong signal that sent the "intensity meter" past 100. The intensity of the signal remained the same until the hole reached 3-4 FT deep. At that depth I was still getting the signal over the same area but the intensity dropped to 40. It has remained at 40 all the way down to what is now a depth of 6 FT. My only guess is that the receiving coil on the back is now being blocked by more soil than it was at the surface and as a result it is now receiving a much weaker signal from the underground object. Can anyone comment on whether or not this diagram is accurate?

View attachment 1661243

The next question I had was about the form of the signal that's being transmitted by the front coil. Since I'm lacking in the necessary terminology I'll just post another diagram.

View attachment 1661244

The last question I had was about the possible ways the underground object could be detected while scanning at the bottom of a deep pit. Especially if I am only a few feet away from the object. Is it possible that the object could be received by the transmitted signal from the back coil even if the front coil is not directly above the object? Here is another diagram to better illustrate what I mean.

View attachment 1661245

Thanks for any info.

 

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[Tom_in_CA]

Here's what I would do, if I was you : Take that out to the clean dry sand beach (like back in the sand dunes). Or out to a farmer's field section that know is sterile (devoid of any targets, no past habitations or junk, etc...).

Bury various objects like: Soda can, tool box, toaster, etc..... Then pass over those known locations. Approach from all different directions, etc....

 

[Carl-NC]

The rear coil is TX and front coil is RX. Field lines look like this:



Depth & position depend a lot on the character of the target. If you were looking for a gigantic coin then the ones pictured represent where they would have a peak signal, and how they would need to be tilted to achieve a peak signal. Note the flat-buried coin would have a peak signal slightly behind the RX coil. The '808 takes a lot of experience to get good with it.

Also see my reply in the White's forum.

 

[audreamer]

The rear coil is TX and front coil is RX. Field lines look like this:

View attachment 1661840

Depth & position depend a lot on the character of the target. If you were looking for a gigantic coin then the ones pictured represent where they would have a peak signal, and how they would need to be tilted to achieve a peak signal. Note the flat-buried coin would have a peak signal slightly behind the RX coil. The '808 takes a lot of experience to get good with it.

Also see my reply in the White's forum.

OH man... This is PRICELESS! Really appreciate the info. I've been scratching my head trying to visualize the form of the field for a long time now. This will vastly improve my understanding of the unit.