Many studies have shown that only a few of the fractures or solution openings intersecting boreholes and characterized as open or permeable by conventional geophysical logs actually form part of large-scale flow paths. Flow logging with a recently-developed high-resolution flowmeter provides a method for quickly and accurately identifying those individual fractures that are involved in conducting flow between observation boreholes and a pumped well. The flow measurement system is based on a thermal tag-trace technique, and is shown to detect flows as small as 0.01 gallon per minute. High-resolution flowmetering is applied to the investigation of secondary permeability flow systems by assuming the continuum of scales of real solution opening and fracture systems can be approximated by three scales: A regional scale of several thousand feet or more, an intermediate scale from 50 to 500 feet, and a small scale of 1–3 feet which is characteristic of conventional geophysical logs. Solution opening and fracture interconnections are investigated by measuring flows induced in observation boreholes by steady pumping at borehole separations typical of the regional scale, and by transient pumping at borehole separations typical of the intermediate scale. The application of multiple-scale borehole-to-borehole pumping and flowmeter measurements of vertical flow in the characterization of secondary permeability aquifer systems was tested in three geological environments:
solution openings and fractures in a sandstone and shale aquifer at Battle Creek, Michigan;
solution openings and fractures in dolomite near Niagara Falls, New York; and
fractures in metamorphic rocks near Dover, New Hampshire. In all three studies, the vertical flow logs indicated inflow and outflow at only a few of the many fractures identified.
Although horizontal solution openings can be projected across the observation boreholes in New York, the individual openings taking part in the flow were different in each borehole during pumping tests. In the fractured crystalline rock study in New Hampshire, most flow was transmitted along a few moderate to steeper-dipping fractures; many other similar fractures did not produce flow. These data are used to provide estimates of permeability along fracture flow paths between boreholes, and to constrain models for the connection between fractures that account for larger-scale pattern of flow in fractured rock aquifers.