Geostatistical Modelling

Origin of Nugget Effect

Usually the variogram will not tend to zero with distance. This occurs for a number of reasons, such as the presence of micro-structures or poor sample preparation and analytical precision.

In many deposits there is a superimposition of different scale mineralisations producing variograms of nested structure which consist of micro-structure and macro-structures. Should the scale of sampling be too coarse to fully define the micro-structure scaled variogram, the result will be that the macro-structure will be observed, with an apparent discontinuity at the origin representing the presence of micro-structure.

Some component of the nugget effect could also be due to sampling, sample preparation or analytical errors. If one could go back to sample the same site, the results will differ and the value of the semi-variogram at the origin would be equal to the mean squared sampling and assaying error. Should the sampling and assaying bias be due to a systematic error, this would not cause any harm in the variogram presentation.