Estimation Of Apparent Transmissivity From Capacity Testing Of Boreholes In Bedrock Aquifers

Abstract

Several theoretical and semi-empirical studies indicate a near linear relationship between the specific capacity (F) of boreholes in bedrock aquifers and apparent transmissivity (T) of the form T=F/α, where α=c. 0.9. For several boreholes in Hvaler and Trondheim, Norway, the specific capacity of individual fractures has been determined by plotting Q_A (the yield of water from the aquifer during pumping or recovery) against s (drawdown). Individual apparent fracture transmissivities can be calculated, as can average apparent hydraulic conductivities for individual borehole sections.

The method presented is easily performed, requires modest amounts of data, is not mathematically difficult, and is believed to yield at least an order-of-magnitude estimate of transmissivity. Given that hydraulic conductivity in bedrock aquifers can range over seven or eight orders of magnitude, even such a coarse estimate can be of significant use. This is verified by comparison of hydraulic conductivities derived from testing of boreholes in Hvaler with conductivities deduced from water leakage into a nearby subsea tunnel in the same lithology.

Resume

La méthode proposée est facile à mettre en oeuvre; elle demande peu de données, n'est pas difficile mathématiquement et parait donner au moins un ordre de grandeur de la transmissivité. Etant donné que la conductivité hydraulique dans les aquifères de socle peut couvrir 7 ou 8 ordres de grandeur, une estimation, même grossière comme coll-ci, peut être significative. Ceci est vérifié en effectuant la comparasion entre les conductivités hydrauliques, obtenues en testant les forages de Hvaler, et celles déduites de l'étude de fuites dans un tunnel sous-marin voisin, dans les mêmes formations géologiques.