Comparison of 2D and 3D Electrical Resistivity Imaging Methods to Target High‐Yield Water‐Bearing Vertical Fracture Zones

Abstract

In bedrock terrain, groundwater predominantly flows through discrete fracture zones within a low permeability bedrock matrix. Fracture zones are typically long, linear, near‐vertical zones of increased fracture density found in most geologic settings. These zones are typically very narrow and are often expressed as natural topographic depressions such as straight stream valley segments, swales and sags in the land surface, or as linear tonal or vegetative alignments often referred to as lineaments or fracture traces. Often, however, fracture zones do not have surface expressions. In such cases, finding and intersecting fracture zones with a well is like finding the proverbial needle in the haystack. Electrical resistivity imaging (ERI), however, is changing all of this. ERI can be used to map the lateral and vertical variations in the electrical resistivity of the subsurface in a two‐dimensional (2D) profile beneath a survey line, or as a three‐dimensional (3D) image of the subsurface. Fracture zones are easily identified in ERI profiles because of the large contrast in electrical properties between porous fractured rock saturated with water (low resistivity) and surrounding dry unfractured rock (high resistivity). The use of electrical resistivity imaging by groundwater scientists has resulted in the development of many high yielding wells in areas typically characterized by low well yields. These successes are favorably changing our concept on the viability of using groundwater to meet future water supply needs of many local communities. Multiple 2D and 3D ERI surveys were completed over a known location of a bedrock fracture zone where a high yield well was successfully drilled. Although the well was originally located on the basis of one 2D ERI survey line, the completion of shallow and deep 3D ERI surveys offered significant additional detail and resolution on the nature (i.e., strike, dip, width) of this bedrock fracture zone. The 3D survey provided more accurate characterization of subsurface geology. The 2D profiles showed the vertical bedrock fracture zone but were also complicated by false anomalies projected into the profiles from areas off the survey line. The 3D ERI surveys are not limited to 2D representation and therefore represent more accurate images of the subsurface geology. 2D and 3D ERI results were compared and analyzed to develop a conceptual model of the bedrock fractures zone. This information was useful in analyzing aquifer test data and developing a 3D numerical groundwater flow model data for the site.