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| Matthews,
M. D., 1996, Importance of sampling design and density in target recognition,
in
D. Schumacher and M. A. Abrams, eds., Hydrocarbon migration and its near-surface
expression: AAPG Memoir 66, p. 243-253.
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Abstract
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. |
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