After watching this thread for a couple of days, I think I might just make a couple of
comments relevant to the technical points.
* The gravity gradient components Gxy (=Gne in NED coordinates) and Guv ( =
0.5*(Gxx - Gyy) = 0.5*(Gnn - Gee)) are called differential curvature components rather
than "cross-horizontal gravity gradient". The name originates from the days of the
torsion balance and describes the relationship between these components and the local
curvature of the equi-potential surface.
* We usually (but not always) use a filter with a cutoff of about 280 m. After
Nyquist and a Russian gentleman whose name escapes me, this is equivalent to a station
spacing of half that amount, ie 140 m. The sampling density is thus not very
directional.
* We measure Gne and Guv, the differential curvature gradients, and we want to
be sure to measure those anomaly patterns that someone called "flowers". We call them
"cloverleafs" because they have 4 leaves. You can get a copy of my thesis from the
University of Western Australia if you want to know all about the anomaly patterns and
the various gravity gradient components and their physical meaning. (Dransfield, M.H.,
1994, 'Airborne Gravity Gradiometry', University of Western Australia)
* I don't think our resolution can be claimed to be better than 250 m.
Resolution is defined as the ability to distinguish two closely spaced sources and,
although related to wavelength it is not the same. It is definitely not the same as
the width of the source body.
* If people want to discuss the minimum diameter of pipe that Falcon can detect
and they don't mention the density contrast then they are certainly going to find grey
areas. We can definitely detect pipes with a density contrast of 2 gm/cc that are very
narrow but have real problems with ones of 400 m diameter when the density contrast is
negligible.
* There seems to be some vagueness about the differences between the width of a
source body (eg a pipe), the width of its gravity gradient anomaly, resolution and
wavelengths. We all (well me and lots of people I know) tend to use these terms
interchangeably but they are quite different in meaning. BHP have tried to be careful
in their published material to use the terms in a precise and correct manner. Very
narrow sources provide signal at a wide range of wavelengths and thus remain
detectable. Of course, one still must have enough resolution to distinguish the signal
from that due to other sources.
* Finally, most of the more technical discussions focus on numbers appropriate
for a single line of data whereas real data is collected on a grid. This changes some
things. For example, on the web site there is an image showing two sub-parallel linear
signals to demonstrate the resolution of Falcon data. Each of those signals is due to
a dike with a very narrow width (about 20 m). No-one would disagree that Falcon can
detect those dikes.
cheers
Mark Dransfield, project leader - Falcon Deployment and Processing
> -----Original Message-----
> From: Will Frangos [SMTP:[EMAIL PROTECTED]]
> Sent: Friday, 12 January 2001 03:33
> To: [EMAIL PROTECTED]
> Subject: Re: [SEGMIN]: "Flower Power"
>
>
> A quick, simple modeling exercise suggests that vertical cylindical bodies
> exhibit a "flowered" anomaly pattern in their cross-horizontal gravity gradient,
> e.g., dGx/dy, but not in their vertical gradients, dGz/dz, or in-line gradients,
> dGx/dx, though the latter may be somewhat more laterally extensive than other
> patterns. (No big surprise, eh?) Also, the sampling grid I see on the BHP web
> page is rather more dense in one dimension than the other, what with 100-m line
> spacing and (supposedly) a 250-m effective station interval. Could Misery have
> been "missed" for simple sparseness reasons?
>
> --Will Frangos
>
> Ken E Witherly wrote:>
>
> > Steve/all:
> >
> > On the BHP web site at:
> >
> > http://www.bhp.com/default.asp?page=951#
> >
> > a profile over a pipe is shown comparing Falcon data with ground data. The
> > lateral resolution for Falcon (as shown) is certainly better than 250m
> > (maybe some good old predictive filtering?).
> >
> > I don't see any 'flowering effect' in this example; the anomaly is wider
> > than the pipe but simply fits the width of the lake, the other major
> > (primary?) source of the observed density anomaly in these lake-pipe duet's
> > that are so common in the NWT.
> >
> > In the text at the top of the same page, there is also discussion regarding
> > not seeing a pipe of dimensions 140m by 100m, so there is seems to be a gray
> > area as to what can be resolved.
> >
> > Ken
> >
> > -----Original Message-----
> > From: [EMAIL PROTECTED]
> > [mailto:[EMAIL PROTECTED]]On Behalf Of Stephen Reford
> > Sent: Tuesday, January 09, 2001 11:42 AM
> > To: [EMAIL PROTECTED]
> > Subject: Re: [SEGMIN]: But what can it do?
> >
> > Terry,
> >
> > Apparently, kimberlite pipes result in a "flower"-type response in the
> > parameters derived from the Falcon data that have a much larger dimension
> > than the pipes themselves. I understand that BHP detected one pipe with a
> > 60 m diameter.
> >
> > Cheers! Stephen
> >
> > On Tue, 9 Jan 2001 [EMAIL PROTECTED] wrote:
> >
> > >
> > > In a message dated 1/9/01 6:36:58 AM, [EMAIL PROTECTED] writes:
> > >
> > > << Based on information that has appeared elsewhere, BHP states as
> > acquisition
> > >
> > > cost for Falcon of ~US$60 lkm (or $15/station since they as well state
> > they
> > >
> > > measure data at about a 250 sample interval). This is about airborne EM
> > >
> > > comes in at, so is not out of line. However, BHP's right to a 51% interest
> > >
> > > certainly looks like you've invited the proverbial 800 lb gorilla to
> > >
> > > dinner >>
> > >
> > > Sheesh, Ken, I think 250-meter gravity-gradient-sampling is downright
> > > terrible for kimberlite exploration.
> > >
> > > Very few economic pipes on the planet, if I remember right, have surface
> > > dimensions in excess of 250 meters. The giants like Mwadi, Orapa, Jwaneng
> > > (with grades less than 1 carat/tonne) probably would be nicely imaged with
> > > 250-meter gradient-stations. BUT, smaller (less than 20-million-tonne) but
> > > high-grade pipes (>4 carats/tonne) like Mir, International, and even
> > > Lac-De-Gras' Misery and A-154-South pipes could be trouble for any
> > > 250-meter-station technique. Even with the averaging/smearing inherent in
> > > airborne geophysical sampling--I'd be worried about "missing" the
> > high-grade
> > > but smaller kimb target.
> > >
> > > Comparing line-kilometer costs between HEM and Falcon is similar, but the
> > > high (<10-metre) sampling-rate of HEM probably still make it the superior
> > > airborne method for my kimberlite exploration programs. I believe Falcon
> > > probably has greater exploration potential in oil/gas exploration where
> > basin
> > > targets/structures are large and shallow density-contrasts are better
> > known
> > > thanx to 3D-seismic-static-corrections.
> > >
> > > Thanks for posting the BHP-Falcon promotional stuff, it is always
> > > interesting.
> > >
> > > Best Regards,
> > > Terry J. Crebs
> > > California Registered Geophysicist
> > > Lakewood, CO USA
> > >
> > >
> > >
> > > _______________________________________________________
> > > List archive: http://www.mail-archive.com/[email protected]
> > >
> >
> > _______________________________________________________
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> >
> > _______________________________________________________
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>
> _______________________________________________________
> List archive: http://www.mail-archive.com/[email protected]>
>
EOM
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