Jim Koehler
02-19-2000, 03:07 PM
Hi guys,
The Dalton mag is a gradiometer - that is, it is sensitive to the
difference between the two sensors.The Geometrics unit Patrick is
referring to is, I believe, a single magnetometer only. The big
advantage of a gradiometer is that it is not affected by general
changes in the earth's magnetic field due to ionospheric currents.
At low magnetic latitudes, the local magnetic field varies fairly
slowly and not very much. At high magnetic latitudes, you can get
very rapid and very large changes in the magnetic field on the
ground. Up at Churchill, Manitoba, at the rocket range, I have seen
changes in the magnetic field (on the ground) of more than 1000 nT
over a time period of about 5 minutes. Using a single head mag. like
the Geometrics (or any other single sensor mag.), you can't tell if
the change you are seeing is caused by something you're going past
or by the ionospheric currents overhead.
I have seen a Dalton mag. owned by one of my neighbours down here in
Arizona and, in my opinion, it is not worth the money. It is
essentially identical to the gradiometer that Phil Barnes describes
with one small difference. Dalton passes a small DC current through
one of his sensors during the 'readout' phase so that you get a slow
beat even if there is no magnetic material in the vicinity. Changes
in the local magnetic gradient due to nearby magnetic materials then
cause a change in the period of the beat. He calls this method the
"delta-squared' method because he observes this 'change in the
difference' - the difference being due to this small DC current.
This helps but, in my opinion, only slightly. Unless you are going
to invest in a portable chart recorder, it is very difficult to
hear, through earphones, any meaningful change due to small
gradients in the field. I must add the caveat that I've never taken
the Dalton unit apart or looked at the insides. I've just seen the
unit, seen it operating and read the literature he provides. The
unit I saw was built more than three years ago and I have no idea
how much, if any, he has improved it since then.
All things considered, I'd just buy Phil Barnes excellent
construction CD (http://members.aol.com/phil770/index.htm)and build
the Phil Barnes unit. It works, has readily available components
(most of these can be bought at a local Radio Shack outlet), is
simple and inexpensive, and is essentially the same thing as the
Dalton unit. The sensitivity is, as I've described in my overall
document, about 25 nT, give or take. For a gradiometer of this type
where you are listening to the beat, you want the signal to decay
very slowly and hence probably want to use distilled water as the
liquid since it has a decay time constant of ~3 seconds. Kerosene
has a decay time of ~0.5 seconds so wouldn't be usable in this type
of mag. - the signal just fades out too rapidly - you need to
measure the frequency electronically over this short a period.
The 'stand-alone' unit I've described in my most recent addition to
my Web pages can use any liquid and gives a digital readout and has
a resolution of about 0.1 nT. However, it is a single sensor unit,
like the Geometrics, and has the failings of a single sensor unit.
For prospecting on the ground, you probably will want a gradiometer
and that requires two single magnetometer units. I've finished my
gradiometer but am making minor changes in it and am not ready to
release the plans till I'm sure all the i's have been dotted and the
t's crossed.
My 'stand-alone' mag. can probably be built for approx. $100 -
that's the cost of the parts and assuming that the labour, provided
by the builder, is free. The gradiometer requires two of these
units plus an additional few parts. I'd estimate a total cost of
about $300.
Jim
J.A. Koehler
3290 N. Koehler Rd.
Florence, AZ, 85232
Ph: (520) 868-1156
The Dalton mag is a gradiometer - that is, it is sensitive to the
difference between the two sensors.The Geometrics unit Patrick is
referring to is, I believe, a single magnetometer only. The big
advantage of a gradiometer is that it is not affected by general
changes in the earth's magnetic field due to ionospheric currents.
At low magnetic latitudes, the local magnetic field varies fairly
slowly and not very much. At high magnetic latitudes, you can get
very rapid and very large changes in the magnetic field on the
ground. Up at Churchill, Manitoba, at the rocket range, I have seen
changes in the magnetic field (on the ground) of more than 1000 nT
over a time period of about 5 minutes. Using a single head mag. like
the Geometrics (or any other single sensor mag.), you can't tell if
the change you are seeing is caused by something you're going past
or by the ionospheric currents overhead.
I have seen a Dalton mag. owned by one of my neighbours down here in
Arizona and, in my opinion, it is not worth the money. It is
essentially identical to the gradiometer that Phil Barnes describes
with one small difference. Dalton passes a small DC current through
one of his sensors during the 'readout' phase so that you get a slow
beat even if there is no magnetic material in the vicinity. Changes
in the local magnetic gradient due to nearby magnetic materials then
cause a change in the period of the beat. He calls this method the
"delta-squared' method because he observes this 'change in the
difference' - the difference being due to this small DC current.
This helps but, in my opinion, only slightly. Unless you are going
to invest in a portable chart recorder, it is very difficult to
hear, through earphones, any meaningful change due to small
gradients in the field. I must add the caveat that I've never taken
the Dalton unit apart or looked at the insides. I've just seen the
unit, seen it operating and read the literature he provides. The
unit I saw was built more than three years ago and I have no idea
how much, if any, he has improved it since then.
All things considered, I'd just buy Phil Barnes excellent
construction CD (http://members.aol.com/phil770/index.htm)and build
the Phil Barnes unit. It works, has readily available components
(most of these can be bought at a local Radio Shack outlet), is
simple and inexpensive, and is essentially the same thing as the
Dalton unit. The sensitivity is, as I've described in my overall
document, about 25 nT, give or take. For a gradiometer of this type
where you are listening to the beat, you want the signal to decay
very slowly and hence probably want to use distilled water as the
liquid since it has a decay time constant of ~3 seconds. Kerosene
has a decay time of ~0.5 seconds so wouldn't be usable in this type
of mag. - the signal just fades out too rapidly - you need to
measure the frequency electronically over this short a period.
The 'stand-alone' unit I've described in my most recent addition to
my Web pages can use any liquid and gives a digital readout and has
a resolution of about 0.1 nT. However, it is a single sensor unit,
like the Geometrics, and has the failings of a single sensor unit.
For prospecting on the ground, you probably will want a gradiometer
and that requires two single magnetometer units. I've finished my
gradiometer but am making minor changes in it and am not ready to
release the plans till I'm sure all the i's have been dotted and the
t's crossed.
My 'stand-alone' mag. can probably be built for approx. $100 -
that's the cost of the parts and assuming that the labour, provided
by the builder, is free. The gradiometer requires two of these
units plus an additional few parts. I'd estimate a total cost of
about $300.
Jim
J.A. Koehler
3290 N. Koehler Rd.
Florence, AZ, 85232
Ph: (520) 868-1156