A.E. Hess

In situ measurements of fluid flow in DSDP Holes 395A and 534A: Results from the Dianaut Program

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.

High‐Resolution Flowmeter Logging Applications with the Heat‐Pulse Flowmeter

A number of recently‐developed high‐resolution flowmeter logging techniques are described in the well logging literature. These techniques are now or soon will be capable of measuring flows corresponding to less than 0.01 gallons per minute of borehole discharge. Such measurements have the potential to significantly improve the interpretation of geophysical logs in groundwater studies by showing how geological structure indicated on logs and tomographs is related to hydraulic properties of beds, solution openings and fractures. The potential benefits of such borehole measurements are illustrated by the results obtained with the U.S. Geological Survey heat‐pulse flowmeter. One useful application of high‐resolution borehole flow measurements is associated with the disturbance to aquifer hydraulics induced by the presence of an open borehole. This effect can be modeled and analyzed to provide a controlled test of aquifer response to disturbance. Flow measurements can also indicate the character of groundwate...

Applications of the Thermal-Pulse Flowmeter in the Hydraulic Characterization of Fractured Rocks

The U.S. Geological Survey has developed a thermal-pulse flowmeter capable of detecting borehole flows to as small as 0.04 L/min. This new flowmeter provides much greater sensitivity to slow vertical flow than that available using conventional spinner flowmeters. This paper describes five applications of the thermal-pulse flowmeter in the characterization of the hydrology of fractured rocks. These applications include measurement of flows in boreholes driven by ambient hydraulic-head differences, identification of fractures contributing flow during production tests, inference of fracture interconnections during aquifer tests, interpretation of water-quality contrasts in boreholes, and identification of fractures affected by hydraulic stimulation procedures. Each of the five applications is illustrated by specific examples selected from ongoing research activities at various fracture hydrology study sites.

Analysis of the borehole televiewer log from DSDP hole 395A: Results from the Dianaut Program

The French wireline re-entry expedition known as DIANAUT culminated in the successful completion of a series of downhole measurements in three Deep Sea Drilling Project (DSDP) holes in the north-central Atlantic Ocean. Among these measurements was a borehole televiewer (BHTV) log obtained in Hole 395A from bottom of casing at 112 meters below seafloor (mbsf) to a depth of 605 mbsf. In contrast with a BHTV survey previously conducted in this well during DSDP Leg 78B in 1981, televiewer data were not degraded by random cable oscillations due to ship heave and the quality of this log is excellent. These latest data were digitized and processed in terms of acoustic travel time and amplitude to compute hole size and shape, evaluate the structural integrity of the surrounding rock, and characterize intersecting fractures. The acoustic caliper and reflectivity logs correlate well with the lithologic column, particularly at the boundaries between major units which often are marked by breccias. Fractures in the lower part of the hole appear to be effectively sealed, in contrast with the open fractures identified in the upper sections. This supports a systematic pattern derived from complementary geophysical logs of increasing density, electrical resistivity, and elastic-wave velocity, and decreasing in situ permeability as a function of depth. Fractures intersecting the well exhibit a wide range of orientations marked by some clustering of dip azimuths approximately east-west. The presence of moderately dipping fractures striking subparallel to the ridge axis is consistent with both ridge-parallel topographic elongations observed in this area and with azimuthal seismic anisotropy reported elsewhere, suggesting that fractures such as those identified from the BHTV record contribute to these effects.