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Carl,
Since this is your house I'll let you have the last word. Have a nice day. Jungle Jim
Since this is your house I'll let you have the last word. Have a nice day. Jungle Jim
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Re: LRL Statement
Carl,
Since this is your house I'll let you have the last word. Have a nice day. Jungle Jim -
Re: My opinion of the Dalton unit.
That is what I meant to say.
The sensor on the Geometrics I have is a 3-axis sensor. Kind of
like 3 sensors at different angles. If anyone wants me to send a
small photo of the sensor to the list, let me know. I would assume
that some of you have never even seen what one looks like.
Thanks Jim!
Patrick
Jim Koehler wrote:
The Proton Mag ForumHi 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.
JimJ.A. Koehler
3290 N. Koehler Rd.
Florence, AZ, 85232
Ph: (520) 868-1156Comment
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