The DIANAUT program provided the first opportunity to directly measure vertical fluid flow in ocean boreholes by means of a high resolution thermal flowmeter. Measurements of volumetric flow rate were obtained in DSDP (Deep Sea Drilling Project) Holes 395A and 534A. These results identified a total flow of 2300 L/hr of seawater entering Hole 395A from the seafloor that diminished to about 550 L/hr at a depth of 251 meters below seafloor (mbsf), indicating that approximately 3/4 of the original downward flow had exited the borehole and entered the open formation across the upper 140 m of basement. This information allows the upper oceanic crust at this site to be delimitated into three hydrologic units, with basalt permeabilities of 3.0 × 10−14 m2 near the sediment/basement interface decreasing sharply as a function of depth to values much less than 10−16 m2 below 440 mbsf. It is estimated that approximately 108 L of seawater have entered this well since it was drilled in 1975. Quantitative flow measurements in Hole 534A were inconclusive because of technical problems with the flowmeter packer. Nevertheless, results showed that borehole fluid was moving upward and out into the open ocean at a rate on the order of a few hundred liters per hour, roughly one order of magnitude less than that determined for Hole 395A and moving in the opposite direction. There is good correlation between these field measurements and the attendant temperature logs from each well, and the results provide strong evidence of important mass‐transport processes associated with the diverse submarine hydrologic systems in the upper oceanic crust.
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)