Borehole‐to‐borehole hydrologic response across 2.4 km in the upper oceanic crust: Implications for crustal‐scale properties

Abstract

Subseafloor hydrologic observatories (CORKs) were installed in four boreholes in young seafloor on the eastern flank of the Juan de Fuca ridge to evaluate the hydrogeology of the upper oceanic crust. Two CORKs installed at Site 1301 were incompletely sealed, allowing cold bottom water to flow into basement at 2–5 L/s and causing a pressure perturbation in a preexisting sealed CORK at Site 1027, which was 2.4 km away. The pressure perturbation at Site 1027 is analyzed using conventional aquifer test methods, yielding transmissivity of T = 0.5 to 1.2 × 10−2 m2/s and bulk permeability of k = 0.7 to 2 × 10−12 m2 in the upper 300 m of basement. Storativity (a parameter that includes fluid and aquifer compressibilities, porosity, and layer thickness) is S = 1 to 3 × 10−3, corresponding to a crustal aquifer (matrix) compressibility of βm = 3 to 9 × 10−10 Pa−1. The inferred basement permeability is consistent with, but at the low end of, permeabilities calculated from single‐hole packer experiments at this and other young oceanic crustal sites; it is much less than values estimated from numerical models, analyses of formation response to tidal pressure oscillations, or pressure responses to coseismic strain events. The relatively low permeability indicated by the cross‐hole response may result from the basement aquifer in this area being azimuthally anisotropic, with a preferential flow direction oriented subparallel to the abyssal hill topography and tectonic structural fabric created at the ridge axis; this hypothesis will be tested during future experiments.