Chris and Richard
If you are using the mag sus to try and constrain an inversion then I
reckon that a consistent 1.7 factor does matter. I've tried a number
of ways of creating bounds files varying from the measured max-min for
the given voxel through to the median value +/- some factor, which I
generally start at half an order of magnitude. Despite using an order of
magnitude range, if everything is 1.7 times too low then unless you make
your factors really sloppy you end up with a model that looks like Swiss
cheese with holes around all the drill holes. Same as you get if you
have remanence in the direction of the earth's field. If you make your
factors too sloppy then you essentially end up with an unconstrained
inversion.
Once you combine the constraints from mag sus with down hole 3C mag you
really make it hard and both the down hole mag and the mag sus data have
to be first class or not worth using. To date I haven't had a lot of
luck combining the two (perhaps because the prospects failed) but using
the down hole 3C mag in a well drilled area certainly adds a lot of
definition to the model at depth and I have been able to steer holes
into potassic altered zones, just not more ore unfortunately. Using
drill hole magsus and just airmag does improve the model at depth as
long as you are prepared to live with Swiss cheese.
I'm talking here about pre-mining resources with hundreds of holes or
deep porphyry exploration with 20 or so 1-2km holes into a 1km radius
cylinder, not a few shallow holes which are not going to add anything to
an inversion.
Cheers
Kim
On 15/09/18 19:53, Chris Wijns via SEGMIN wrote:
Hi Richard, my philosophy has long been that the core correction factor is not
worth worrying about for exactly the reason you mention. I am interested in an
interval that is 20 times as magnetic as the surrounding intervals, not 1.7
times...
Regards,
Chris
Original Message
From: Richard Smith via SEGMIN
Sent: Saturday, 15 September 2018 19:46
To: [email protected]
Reply To: [email protected]
Cc: Richard Smith
Subject: Re: [SEGMIN] Magnetic susceptibility meter
Hi Kim,
I notice you got my post. I did not, so sent it again. It seems that the new
SEGMIN might not sent it to the sender sometimes.
I asked the student if he had used the core radius correction and he said he
had, but as you say, it is not clear it is was set routinely each time the
meter was turned on. If the data downloaded from the meter had an additional
value, the correction applied or an uncorrected value, then that would assist
in the QC.
There are aspects of these magnetic susceptibility meters which are problematic
in addition to the core radius correction. Another is how they deal with
negative susceptibilities for diamagnetic rocks like quartzite. Table 2 in the
paper I attached in the last message shows that the instruments return values
that are sometimes zero, sometime positive and sometimes negative. The
crossplots produced in figure 2 are on a log scale, so these negative values
are ignored.
Concerning the plot you attached. My comment is that the values are generally
within a factor of two of each other. My experience of most magnetic
susceptibility values on heterogeneous rocks is that a factor of two is less
than the variability of the rock itself, so the readings are essentially the
same. The core correction factor is generally a factor of two or so, so
perhaps it is not significant in the grand scheme of things!
RegardsRichard Smith
-----Original Message-----
From: Kim Frankcombe via SEGMIN <[email protected]>
To: segmin <[email protected]>
Cc: Kim Frankcombe <[email protected]>
Sent: Sat, Sep 15, 2018 12:04 am
Subject: Re: [SEGMIN] Magnetic susceptibility meter
Richard
When I reviewed the abstract for this as a SEG presentation I asked
whether the non KT series meter readings had been corrected for sample
size. I didn't get an answer at the time so perhaps you could clarify.
As an additional piece of info attached or attached and scrubbed is a
plot comparing SM-30, Magrock and KT5 against sample values returned
from Don Emerson. We accepted Don's value as the truth and adjusted each
reading for sample size using the manufacturer's tables. One of the
samples was 1/4 NQ core so the sample size correction was a fiddle. I
can't recall which one and would have to start a virtual machine to open
the windows format spreadsheet sorry. Also not sure which mouse ate the
bottom of the K in the axis titles - put it down to my usual scapegoat -
microsoft. I suspect that when all thee meters cluster away from the
truth that we have use a poor estimate for sample size correction
Cheers
Kim
On 15/09/18 09:31, Richard Smith via SEGMIN wrote:
Attached is a recent review of mag susc meters. A few years out of date. I
hear the GDD is cheaper now.
Note that for attachments on the new SEGMIN list there is a link at the bottom
of the message, or you can check the archive
RegardsRichard Smith
-----Original Message-----
From: Aline T. Melo via SEGMIN <[email protected]>
To: segmin <[email protected]>
Cc: Aline T. Melo <[email protected]>
Sent: Fri, Sep 14, 2018 3:36 pm
Subject: [SEGMIN] Magnetic susceptibility meter
Hello,
Does anyone has experience with KT-20
susceptibility/conductivity/ip-resistivity meter (
http://www.terraplus.ca/products/pdf/KT-20.pdf)?
Any pros and cons?
I am looking for recommendations to buy a susceptibility meter, and
appreciate any help.
Thanks!
Aline
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