Hi, Mark, Donald Donald Myers final sentence opened a door for me: "There is no definitive answer to the question since it depends on the question, i.e., what is the data to be used for? In this perspective, I would line to raise several issues. My background and experience is mainly that of a mining geologist who is familiar with geostatistics, and has had the opportunity to work with several geostatisticians. Forgive me for broadening the problem, but such are geology and minng. In my opinion, the final purpose of aampling is estimation leading to mining extraction that achieves optimal metal recovery and minimal dilution. Estimating/ modeling of the variogram and kriging are interim objectives. Geological mapping, interpretation and modeling are also essential steps with whom sampling methods, sampling grid dimensions and geostatistical aspects should be integrated in the determination of global and local continuity (Sinclair and Vall�e, 1994). The basic problem of sampling grid size should be viewed in three dimensions. Our usual sampling grids are planned for efficiency, using a stratified drilling pattern perpendicular to the plane of apparent structural / geological continuity. So far, so good! However, once we have achieved a first delineation and can calculate a semivariogram along the drill hole axis, too often we neglect or forget to verify if these continuity parameters apply are present in the other two perpendicular directions. This problem has been detected by Michel David and described in a sampling paper titled "What happens if?" given at a sampling symposium in Australia in 1976 where he describes the problem and recommends sampling specifically designed to verify continuity parameters in the second and third dimensions. Sadly, there is only one sentence in "Geostatistical Ore Reserve Estimation (which was already in print at the time this paper was prepared and presented) that refers to this problem (around page 200). Journel and Huijbregts in "Mining Geostatistics" also describe this problem (1978) and recommend a few simple tools, for instance to lay out a cross of more closely spaced drill holes within the main grid. I consider, based on my mining experience, that sampling of rock in place should be targeted stratified sampling, not random sampling. Regionalized variables require regionalized sampling When the exploration or mining geologist halves the sampling grid dimension, he/she is basically using a similar strategy. I know the results of this effort are viewed by geostatisticians as the "clustering" problem! For the mining geologist, the objective is local estimation and close determination of ore limits, whether they are grade contacts or sharp contacts both for planning and extraction. What is the ideal sampling grid dimension for the mining geologist and the mining engineer? My answer: the one that allows to plan and develop and extract the ore (from stopes or open pit) efficiently as described above. I understand the student/researcher cannot rely on similar amounts of funds for drilling. Another important consideration in the Davil paper and in Geosatistical Ore Estimaion is that of sample preparation and assaying quality control. Quoting from the paper" " ... (frequently) it is the sanple preparation procedure whick generates the nugget effect rather than the real mineralization which generates the nugget effect" [underlined in the text]. This subject makes up almost a chapter of Geostatistical Ore Reserve Estimation. A higher "induced" nugget effect, that is a higher sampling/ assaying variance of the values used for a selectin decision will reduce the accuracy of LOCAL estimation and mine selection. What is the final grid dimension (the one we call measured resource, proven reserve). This will vary depending on orebody configuration and mining method selected. In a open pit, with a fairly sizeable, not too complex and not too nuggetty ore body, you generally can get away with a wider grid for proven ore than yo can in the majority of cases underground, In an open pit you can sample more systematically, from bench to bench than you can in most underground mine. Also one has more flexibility for selection by adjusting blast limits based on test drilling on the pit floor and sampling of blast holes. For those this may interest, here are references of papers, including some I was involved in that deal with these subjects and perspectives. Best Regards Marcel Vall�e Eng. Geol. G�oconseil Marcel Vall�e Inc. 706 Routhier St Sainte-Foy, Qu�bec G1X 3J9 Canada Tel: (1) 418 652 3497 Fax: (1) 418 652 9148 Email: [EMAIL PROTECTED] David M., 1976, What Happens If? A few remarks on Useful Geostatistical Concepts for the Design of Sampling Patterns. The Aus. I.M.M Melbourne Branch, Sampling Symposium, Sept. 1976, preprint of proceedings, 16 p. (A major sampling paper) Postolski, T. A., Sinclair, A. J. (1998) Geology as a Basis for Refining Semivariogram Models for Porphyry-Type Deposits. Exploration and Mining Geology, Vol. 7, N� 1-2, p. 45-50. Vall�e, M, Dagbert, M, & C�te, D. (1993) Quality control requirements for more reliable mineral deposit and reserve estimates. CIM Bulletin, vol. 86. No 969, p. 65-74. (the ideas presented in this note have been developed here) Sinclair, A.J. and Vall�e, M. (1994) Reviewing continuity: An essential element of quality control for deposit and reserve estimation. Exploration and Mining Geology, Vol.3, N� 2, pp. 95-108. (Determination of continuity is at the core of estimation!) Vall�e, M. (1998) Sampling Quality Control. Exploration and Mining Geology, Vol, 7, N� 1-2, p. 107-116. (Some material on sampling strategy; sampling and assay variance work against good semivariograms and good estimation) Vall�e, M. (1992) Guide to the evaluation of gold deposits. CIM Special Volume 45, Canadian Institute of Mining, Metallurgy and Petroleum, Montr�al, Canada, 299 p. (A general perspective of the work of the geologist in estimation and mining. It was consdidered by Canadian miners, at the time of publication, to be oriented too much toward the use of geostatistics!) Vall�e M. Draft Standards for Exploration and Resource/Reserve Estimation. Developed and expanded by the author from a report sponsored by the Qu�bec Department of Natural Resources following the Bre-X affair, 110 pages, 800 k WordPerfect file. (This on-going work presents an ISO-9000:2000 compatible Quality Management and Continuous Improvement system that intends to integrate the geological, geostatistical and minng characteristics and constraints of mineral development.) The Volume 7, No 1-2 issue of Exploration and Mining Geology is a special issue on "Quality Assurance, Continuous Improvement and Standards in Resource Estimation. Abstracts of the papers can be viewed on the CIM web site (www.cim.org) at the Geological Society and/or Publications pages. Eight of the nineteen papers deal with sampling and assaying QA/QC. <30> -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and "unsubscribe ai-geostats" followed by "end" on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
