Loading ...
Sorry, an error occurred while loading the content.

[ai-geostats] variogram analysis

Expand Messages
  • Rajive Ganguli
    My question is general. What do you conclude if your variogram is wavy? Cyclic patterns? I have what appears to be high nugget, followed by a wavy pattern.
    Message 1 of 7 , Dec 7, 2004
    • 0 Attachment
      My question is general. What do you conclude if your variogram is
      wavy? Cyclic patterns? I have what appears to be high nugget,
      followed by a wavy pattern.

      If you wish, here is more info: an offshore placer platinum deposit,
      not too many boreholes - just 29 from decades ago spanning several
      square kilometers. The variogram (from GEOEAS) of the grade (ln) is
      given in:

      http://www.faculty.uaf.edu/ffrg/Variogram.zip

      The variogram is cyclic. Goes up and down. I tried various lags/directions.

      I will try to dig up the geological information and see what it says.
      --
      Rajive
    • Glover, Tim
      Usually when I ve seen a wavy semivariogram, it s because of a local feature superimposed over an existing field function - for instance, a release of
      Message 2 of 7 , Dec 7, 2004
      • 0 Attachment
        Usually when I've seen a "wavy" semivariogram, it's because of a local
        feature superimposed over an existing field function - for instance, a
        release of mercury in a field of soil with very low "natural" mercury
        content. The period of the waviness is related to the distance across
        the feature (the width of the spill, in this case). Of course, this is
        nothing particularly earth-shattering, but useful none the less.

        I've used semivariograms like this in the past to "guestimate" the
        approximate size of a plume based on sparse data. Not all geostatistics
        ends up in gridding and estimating at every point! Sometimes just
        looking at the semivariogram is very useful.

        Tim Glover
        Senior Environmental Scientist - Geochemistry
        Geoenvironmental Department
        MACTEC Engineering and Consulting, Inc.
        Kennesaw, Georgia, USA
        Office 770-421-3310
        Fax 770-421-3486
        Email ntglover@...
        Web www.mactec.com

        -----Original Message-----
        From: Rajive Ganguli [mailto:rajive.ganguli@...]
        Sent: Tuesday, December 07, 2004 4:50 PM
        To: ai-geostats@...
        Subject: [ai-geostats] variogram analysis

        My question is general. What do you conclude if your variogram is
        wavy? Cyclic patterns? I have what appears to be high nugget,
        followed by a wavy pattern.

        If you wish, here is more info: an offshore placer platinum deposit,
        not too many boreholes - just 29 from decades ago spanning several
        square kilometers. The variogram (from GEOEAS) of the grade (ln) is
        given in:

        http://www.faculty.uaf.edu/ffrg/Variogram.zip

        The variogram is cyclic. Goes up and down. I tried various
        lags/directions.

        I will try to dig up the geological information and see what it says.
        --
        Rajive
      • dwmccarn@aol.com
        Dear Rajive: I cannot conclude with only 328 pairs that the feature is wavy because I do not know how those pairs are distributed for each point in the
        Message 3 of 7 , Dec 7, 2004
        • 0 Attachment
          Dear Rajive:
           
          I cannot conclude with only 328 pairs that the feature is "wavy" because I do not know how those pairs are distributed for each point in the variogram.  Try different lag spacings, or create an "equal-n" lag variogram where each lag has the same number of pairs.  If that shows the same feature, then perhaps there is a repeating feature (faults, fractures, ore controls, etc.) occurring at regular intervals throughout the sampling domain.  I take it that you have 26 or so sample locations.
           
          Using "equal-distance" lags usually gives a large number of pairs to the first couple of lags, and then the n drops off rapidly, and the variogram is harder to interpret than with an "equal-n" type variogram.
           
          I wrote my variography codes to work both ways...
           
          Dan ii
           
          Dan W. McCarn, AIPG CPG #10245, Wyoming PG #3031, EurGeol #462
          10228 A Admiral Halsey NE; Albuquerque, NM 87111 USA
          Home: +1-505-822-1323; Cell: +1-505-710-3600

          The College of Santa Fe
          4501 Indian School NE Ste. 100; Albuquerque, NM 87110
          (505) 884-2732 fax (505) 262-5595
          dmccarn@...

          Institut f├╝r Geowissenschaften; Montanuniversit├Ąt Leoben
          Peter-Tunner-Strasse 5; A8700 Leoben, AUSTRIA
          Cell: +43-676/725-6622; Fax; +43-3842-402-4902; Office: +43-3842-402-4903
          In a message dated 12/7/2004 3:27:31 PM Mountain Standard Time, NTGLOVER@... writes:
          Usually when I've seen a "wavy" semivariogram, it's because of a local
          feature superimposed over an existing field function - for instance, a
          release of mercury in a field  of soil with very low "natural" mercury
          content. The period of the waviness is related to the distance across
          the feature (the width of the spill, in this case).  Of course, this is
          nothing particularly earth-shattering, but useful none the less.

          I've used semivariograms like this in the past to "guestimate" the
          approximate size of a plume based on sparse data.  Not all geostatistics
          ends up in gridding and estimating at every point!  Sometimes just
          looking at the semivariogram is very useful.  

          Tim Glover
          Senior Environmental Scientist - Geochemistry
          Geoenvironmental Department
          MACTEC Engineering and Consulting, Inc.
          Kennesaw, Georgia, USA
          Office 770-421-3310
          Fax 770-421-3486
          Email ntglover@...
          Web www.mactec.com

          -----Original Message-----
          From: Rajive Ganguli [mailto:rajive.ganguli@...]
          Sent: Tuesday, December 07, 2004 4:50 PM
          To: ai-geostats@...
          Subject: [ai-geostats] variogram analysis

          My question is general.  What do you conclude if your variogram is
          wavy? Cyclic patterns?  I have what appears to be high nugget,
          followed by a wavy pattern.

          If you wish, here is more info: an offshore placer platinum deposit,
          not too many boreholes - just 29 from decades ago spanning several
          square kilometers.  The variogram (from GEOEAS) of the grade (ln) is
          given in:

          http://www.faculty.uaf.edu/ffrg/Variogram.zip

          The variogram is cyclic. Goes up and down.  I tried various
          lags/directions. 

          I will try to dig up the geological information and see what it says.
          --
          Rajive
           
          CONFIDENTIALITY NOTICE: The materials in this e-mail transmission (including all attachments) are private and confidential, and the property of the sender. The information contained in the materials is privileged and intended only for the use of the named addressee(s). If you are not the intended addressee, be advised that any unauthorized disclosure, copying, or distribution, or the taking of any action in reliance on the contents of this material is strictly prohibited. If you have received this e-mail transmission in error, please immediately notify the sender by sending an e-mail message, and thereafter destroy the e-mail you received and all copies thereof.
        • Isobel Clark
          Rajive I haven t read the other responses yet, so this may be redundant. Two possibilities: (1) anisotropy: if this is shallow marine data there should be a
          Message 4 of 7 , Dec 8, 2004
          • 0 Attachment
            Rajive

            I haven't read the other responses yet, so this may be
            redundant.

            Two possibilities:

            (1) anisotropy: if this is shallow marine data there
            should be a difference between longshore drift and
            off-shore deepening of sea-bed. You have an
            omni-directional semi-variogram. It is possible that
            the sampling grid is irregular enough to be
            highlighting directional differences??

            (2) mega-ripples: I have seen similar behaviour in
            off-shore marine diamonds which tend to hug the bottom
            of trenches or ripples. Major ocean beds have
            mega-ripples on the kilometre scale, which is what you
            are seeing here.

            More worrying, I would say, is the fact that your
            graph is dropping with distance. This suggests that
            you also have some underlying trend (non-stationarity)
            which is causing closely spaced samples to be 'more
            different' than those further apart.

            I notice you are using a log transform. What does your
            probability plot look like?

            Isobel
          • Digby Millikan
            Hole effect model, usually means your deposit has alternating high and low grade zones. Sorry I m not familiar with the geology of this deposit but examples of
            Message 5 of 7 , Dec 8, 2004
            • 0 Attachment
              Hole effect model, usually means your deposit has
              alternating high and low grade zones. Sorry I'm not
              familiar with the geology of this deposit but examples
              of this could be pods of high grade spaced apart from
              each other with waste or low grade halos between
              them. If only the high grade zones are economic you
              may want to seperate them out and model them seperately.

              Digby
              www.users.on.net/~digbym
            • Rajive Ganguli
              Isobel and others, Sorry for the delay. Since my posting, I have had an opportunity to discuss the geology (we found a mini report). Various types of
              Message 6 of 7 , Dec 13, 2004
              • 0 Attachment
                Isobel and others,

                Sorry for the delay. Since my posting, I have had an opportunity to
                discuss the geology (we found a "mini" report).

                Various types of deposits have been suggested: paleo-alluvial and
                paleofluvial channels, submerged strand lines, "false" bedrock
                combined with longshore current among others.

                Additionally, the holes were along a narrow (approx) NW-SE line.
                Omni-directional variogram should not have been done since, really,
                there was no lateral extent to layout of the holes.

                Data is lognormal. Click here
                (http://www.faculty.uaf.edu/ffrg/prelim_analysis.zip) for basic
                plots/varios. Wavy nature still present. There is big difference in
                the nature of the variogram in 135 deg direction vs. 150 deg.
                direction. Cannot explain that at this time except blame it on
                geology.

                We will be in that region doing geophyics and others next summer.
                Hopefully we will know more.

                Rajive

                On Wed, 8 Dec 2004 12:29:17 +0000 (GMT), Isobel Clark
                <drisobelclark@...> wrote:
                > Rajive
                >
                > I haven't read the other responses yet, so this may be
                > redundant.
                >
                > Two possibilities:
                >
                > (1) anisotropy: if this is shallow marine data there
                > should be a difference between longshore drift and
                > off-shore deepening of sea-bed. You have an
                > omni-directional semi-variogram. It is possible that
                > the sampling grid is irregular enough to be
                > highlighting directional differences??
                >
                > (2) mega-ripples: I have seen similar behaviour in
                > off-shore marine diamonds which tend to hug the bottom
                > of trenches or ripples. Major ocean beds have
                > mega-ripples on the kilometre scale, which is what you
                > are seeing here.
                >
                > More worrying, I would say, is the fact that your
                > graph is dropping with distance. This suggests that
                > you also have some underlying trend (non-stationarity)
                > which is causing closely spaced samples to be 'more
                > different' than those further apart.
                >
                > I notice you are using a log transform. What does your
                > probability plot look like?
                >
                > Isobel
                >


                --
                Rajive
              Your message has been successfully submitted and would be delivered to recipients shortly.