Description
Production rates of water wells drilled in fractured crystalline bedrock vary widely. For two wells of the same depth, one that does not intersect any significant fractures will produce less than a few gallons per minute, but one that intersects a fracture may produce thousands of gallons per minute. This study compares the results of three geophysical methods conducted at four sites where wells have intersected significant fractures. Two of the methods, VLF and Ground Conductivity Meters (GCM), have a previous history of use for well siting. The third method, self-potential (SP) or natural potential, is untested for this purpose. The VLF and GCM methods both consistently located many conductive anomalies at four sites. At one site, the source clearly appeared to be a water filled fracture at depth. Most other locations showed evidence of a strong response to conductivity changes in the shallow subsurface. The data from some locations indicated that conductivity contrasts may also extend deeply into the subsurface. The linear nature of the features defined by the anomalies suggests the conductive bodies are associated with fractures. The presence or lack of a quadrature response, and the width and shapes of some of the total field peaks and in-phase cross-overs was the best indication of the source of the anomalies detected with the VLF. The relationships of the conductivity troughs in the horizontal loop mode, the presence and width of conductivity peaks in the vertical coil mode; and two-dimensional model results were used to interpret the source of the anomalies detected using GCM. The SP results displayed fewer anomaly locations. At two sites, sharp negative anomalies, coincident with EM anomaly locations, were present in the data. A review of other data from the sites, an evaluation of factors producing SP responses, and modeling of general SP sources indicate that water infiltrating along fractures in the shallow subsurface is the source of these anomalies. Some broader negative anomalies were also present in the data. The source here may be deeper fracture flow or thickened overburden. The responses were low relative to the noise levels of the lines and, given the variety of factors that influence SP data, other possible sources cannot be eliminated. Many fracture locations and orientations were identified by the VLF and GCM methods at these sites, but the relationships between the orientations and locations of the fractures, the lineaments, and productive wells, are quite variable. Exact siting of wells may be improved by further analysis of fracture patterns present at site. Fractures located by these methods may not necessarily have any permeability at depth, but these methods appear to be quick and reliable indicators of fractures. The SP shows some promise of locating fractures, and may aid in identifying good locations for wells. Further study would be necessary to determine if it is possible to obtain more consistent and reliable fracture locations, with the SP method.
