In many drilling programs, the first if often unstated question is: how many holes will it take to create an accurate map of the subsurface? This critical question is driven by the constraints of time, money, and program goals. Geostatistical methods of estimating subsurface parameters based on drilling offer not only objectivity but error estimation as well thus a means of optimizing the number of drillholes in an investigation. The step from drilling to contour maps of subsurface horizons is no longer a paper and pencil excercise but one conducted with the appropriate software using a site plan that has been divided into an underlying regular grid. In a multi-stage drilling program, each round of drilling is followed by estimation of the depth to the target horizon at each grid cell. The two most commonly used geostatistical techniques are the inverse distance method and the krige method. Subseqent to grid estimation, a contour map of the depth to, surface elevation of, or thickness of the target horizon is drawn and layered on top of the site plan. In addition, a second contour map of the estimation error can serve as a guide to selection of additional drill locations. When project error tolerances are met as measured by estimation error maps, the drilling stage of the investigation is complete.
