Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.  All Rights Reserved.
 

The design and density of surface geochemical sampling programs can significantly influence the interpretability of the survey. Testing hypotheses by purposeful sampling is the most straightforward application of surface geochemistry and the easiest to interpret but requires a priori geologic knowledge and provides limited information. Designing a spatial sampling program to produce a map is a much more difficult problem. Two common techniques are line profiles and areal surveys. The interpretation of these techniques, as a function of sampling density, is simulated by examining the relationships of artificial surveys to a known subsurface target. A deterministic model of hydrocarbon migration is used to constrain the randomized mixing of two nonoverlapping populations (anomalous and background). A high-resolution grid of simulated surface measurements is decimated to create lower resolution grids and line profiles. For regional high-grading, a grid with a minimum of two samples across the narrowest expected surface signal (minimum subsurface target width plus a dispersion zone) appears to be adequate. A higher density of four samples is suggested for prospect high-grading. In addition, the sampled area must include sufficient "background" measurements to recognize the existence of an anomaly. About 80% of the samples should be obtained outside the expected area of interest. The most cost-effective sampling program consist of two stages: a low-density regional survey to high-grade the area, followed by a higher density survey within the high-graded area. Undersampling is probably the major cause of ambiguity and interpretation failures involving surface geochemical studies.