Borehole Geophysics Used To Characterize Vertical Fractures And Their Connections To Bedding Plane Aquifers In Dolomite

Geophysical logs in an array of six boreholes in argillaceous dolomite at a contamination site in northern Illinois<br>intersect four aquifers: a shallow bedding plane fracture, a vertical fracture, and two deeper permeable beds. The<br>vertical fracture aquifer appears to be representative of a set of such fractures dipping at about 85 degrees and<br>striking approximately northeast. Cross-borehole flow measurement techniques are used to investigate the<br>properties of the three horizontal aquifers, and to infer the character of the connection between the vertical fracture<br>and the deeper horizontal aquifers. The data indicate that the vertical fracture and the shallow bedding plane have<br>transmissivities nearly an order of magnitude greater than those of the two deeper horizontal aquifers in the<br>vicinity of the point where they intersect boreholes. The data also indicate that the deeper aquifers are<br>characterized by a storage coefficient of about 2x10”. The bedding plane and vertical fracture aquifers cannot be<br>modeled as confined aquifers, although relatively large storage coefficient values characterize models that most<br>closely approximate the variation of flow in the shallow bedding plane aquifer. The cross-borehole flow<br>experiments suggest that the vertical fracture is hydraulically connected with the deeper horizontal fractures, but<br>that fracture aperture decreases somewhat with depth.