117 3. Borehole geophysical methods Quick look or sophisticated quantitative interpretation methods (Boyer and Mari, 1997) based on relationships between measured physical parameters and petrophysical parameters are used to obtain petrophysical (such as porosity) and mechanical (such as Poisson’s ratio) parameters of geological formations. Figure 3.1 is an example of logs recorded in the molasse of the Swiss Plateau. The geological formation, constituted of argillaceous sandstone, is strongly altered. The caliper shows numerous caved zones in shaly beds (Figure 3.1a) strongly marked on the density and velocity logs. After correcting the logs according to the caliper, the logs are not perfect, but the values are close to those obtained on cores (Figure 3.1b). Archie (1942) has shown empirically that for water-saturated permeable formations, the relation between the true formation resistivity, Rt, and the resistivity, Rw, of the water impregnating the formation is given by: Rt/Rw = F = Φ–m (3.1) where F is the “resistivity formation factor”. Φ is proportional to the formation porosity and m is a “cementation factor”, that is a formation characteristic. An approximate value equal to 2 is generally adopted for the cementation factor. Wyllie et al. (1956) has established a linear relationship between the slowness Δt and the porosity Φ and shalyness Vsh of a water-saturated permeable formation: Δt = (1 – Φ – Vsh)Δtma + VshΔtsh + Φ ⋅ Δtf (3.2) where ma, sh and f represent respectively the matrix, the shales and the fluid. The same relationships are used for the neutron porosity ΦN and the density ρb. ΦN = (1 – Φ – Vsh)ΦNma + VshΦNsh + Φ ⋅ ΦNf (3.3) ρb = (1 – Φ – Vsh)ρma + Vshρsh + Φ ⋅ ρf (3.4) Logs are also recorded to add constraints in the processing and interpretation of geophysical models. The University of Poitiers (France) has developed a Hydrogeological Experimental Site (HES) for the sole purpose of providing facilities to perform long-term monitoring and experiments for a better understanding of fluid flow and transfers in fractured rocks (Bourbiaux et al., 2007). 35 boreholes, including two vertical and two inclined cored boreholes, were drilled on the site in two separate campaigns: 2002-2003 and 2004 (Figure 3.2a). All the boreholes are crossing completely the Dogger Aquifer (depth of boreholes = 125 m). A 3D survey has been designed to obtain a 3D interval velocity cube in depth (Mari and Porel, 2007). Figure 3.2b shows the resistivity log recorded in borehole M09 as well as the velocity distribution extracted from the 3D velocity block at the location of borehole M09.
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