[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 * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
[ai-geostats] variogram analysis
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 * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
RE: [ai-geostats] variogram analysis
Rajive, Cyclic variograms indicate that your attribute of interest also fluctuates. I encountered this when working with time-series of water levels, in which case the fluctuations were related to seasonality. I am not sure what it would mean in the case of platinum deposits. Such variograms can be modeled using the hole effect model, but 2-dimensional semivraiogram modeling when you have anisotropy to account for, can be tricky with a hole effect because you cannot apply a hole effect model in more than one direction. It may be better to work with a residual, i.e. to find a correlated cyclic variable, remove the cyclicity for semivariogram and kriging purposes and add the kriged residual back in at the end. If you do want to model such a variogram, e.g. if you only encounter the cyclicity in one direction, and you are working with GSLib, then you may have to modify the kriging code, as the dampening factor (if the cyclicity diminishes with lag)is not specified in the parameter file. I don't know what other programs allow you to do with the hole effect model, though ... Noémi -Original Message- From: Rajive Ganguli [mailto:[EMAIL PROTECTED] Sent: Tuesday, December 07, 2004 4:50 PM To: [EMAIL PROTECTED] 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 * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
RE: [ai-geostats] variogram analysis
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 [EMAIL PROTECTED] Web www.mactec.com -Original Message- From: Rajive Ganguli [mailto:[EMAIL PROTECTED] Sent: Tuesday, December 07, 2004 4:50 PM To: [EMAIL PROTECTED] 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 * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
Re: [ai-geostats] variogram analysis
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 #46210228 A Admiral Halsey NE; Albuquerque, NM 87111 USAHome: +1-505-822-1323; Cell: +1-505-710-3600The College of Santa Fe4501 Indian School NE Ste. 100; Albuquerque, NM 87110(505) 884-2732 fax (505) 262-5595[EMAIL PROTECTED]Institut für Geowissenschaften; Montanuniversität LeobenPeter-Tunner-Strasse 5; A8700 Leoben, AUSTRIACell: +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, [EMAIL PROTECTED] writes: Usually when I've seen a "wavy" semivariogram, it's because of a localfeature superimposed over an existing field function - for instance, arelease of mercury in a field of soil with very low "natural" mercurycontent. The period of the waviness is related to the distance acrossthe feature (the width of the spill, in this case). Of course, this isnothing particularly earth-shattering, but useful none the less.I've used semivariograms like this in the past to "guestimate" theapproximate size of a plume based on sparse data. Not all geostatisticsends up in gridding and estimating at every point! Sometimes justlooking at the semivariogram is very useful. Tim GloverSenior Environmental Scientist - Geochemistry Geoenvironmental DepartmentMACTEC Engineering and Consulting, Inc.Kennesaw, Georgia, USAOffice 770-421-3310Fax 770-421-3486Email [EMAIL PROTECTED] Web www.mactec.com-Original Message-From: Rajive Ganguli [mailto:[EMAIL PROTECTED] Sent: Tuesday, December 07, 2004 4:50 PMTo: [EMAIL PROTECTED]Subject: [ai-geostats] variogram analysisMy question is general. What do you conclude if your variogram iswavy? 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 severalsquare kilometers. The variogram (from GEOEAS) of the grade (ln) isgiven in:http://www.faculty.uaf.edu/ffrg/Variogram.zipThe variogram is cyclic. Goes up and down. I tried variouslags/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. * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
Re: [ai-geostats] variogram analysis
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 * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
