Radar Tomograms At Mirror Lake, New Hampshire: 3-D Visualization And A Brine Tracer Experiment

In this paper we examine radar tomograms produced from data acquired in the FSE well field<br>at the fractured rock hydrology site at Mirror Lake, Grafton County, New Hampshire. We first<br>show an example of 3-dimensional tomogram visualization with superimposed projections of<br>hydraulically significant fractures. We find a generally good, but not one-to-one, correlation<br>between electromagnetic velocity tomograms and fracture projections from individual wells. One<br>reason why the velocity tomograms do not always provide unambiguous indications of flow paths<br>is that the tomograms reflect rock electrical properties which vary because of lithologic<br>heterogeneity at the site. Another is that hydraulic permeability in fractured media may be<br>controlled by small-aperture “choke points” that are probably not well imaged by the tomograms.<br>In addition, hydraulic flow may be through a network of interconnected fractures, many of which<br>may be oriented differently from any particular fracture observed at a borehole.<br>We then show a difference attenuation tomogram produced from a brine tracer experiment carried<br>out at the Mirror Lake site in 1995. An objective of this experiment was to distinguish between<br>the mere presence of water and hydraulic permeability. When interpreting tomograms between<br>two hydraulically connected wells, an unrecognized assumption is often made that hydraulic flow<br>paths are in the plane of the wells. This assumption may be invalid, especially in crystalline rock<br>where flow is dominantly through fractures, rather than through the rock matrix. Our difference<br>attenuation tomogram shows that the primary flow path left the plane of the injection and pumped<br>wells.