Usage of “Equivalent apertures” for rock fractures as derived from hydraulic and tracer tests

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Abstract
In the literature during the past several years there appear numerous references to the “equivalent aperture” of a rough‐walled rock fracture as derived from various hydraulic and tracer tests. However, the similar or even identical terms used by different researchers for “equivalent aperture” often do not have the same meaning. This has led to some confusion in the comparison of their results. In particular, there is a serious apparent contradiction in the claims of some authors that “equivalent apertures” derived from tracer tests are much larger than those derived from hydraulic tests, and the findings of others that apertures estimated from tracer tests are consistently smaller than those estimated from hydraulic tests. This apparent contradiction of the field results in fact arises from the different definitions of the so‐called “tracer aperture” as employed by different researchers. In this short technical note I have attempted to sort out the different definitions, denotations and usage of the various “equivalent apertures” and show that there are mainly three alternative definitions used in the literature. The meaning of each as related to experimental measurements is explained and their interrelationship discussed. It is shown that once the specific definition of “equivalent aperture” referred to by each researcher is identified, then the relative magnitudes of these “equivalent apertures” as reported by different groups of researchers are perfectly consistent with each other.