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.