Ted Dean writes about finding drilling targets with electrical resistivity imaging.

Resistivity imaging has a plethora of applications. It can be used to define soil bedrock interface, which is important to engineering investigations, especially in karst terrain. Karst terrain is notorious for having deep bedrock cutters and sharp pinnacles, which are easy to miss with conventional investigations using borings alone (Figure 1).

Resistivity decreases with increasing dissolved ions, and the method works well for detecting saltwater intrusion and some types of dissolved contaminant plumes. Figure 2 illustrates use to delineate a landfill leachate plume.

Figure 3 is a resistivity section from a location experiencing saltwater intrusion at a withdrawal well.

Subsurface voids are discernible using electrical resistivity imaging. Water- or mud- filled voids tend to have lower resistivity than surrounding rock (Figure 4).

Because air is completely resistant to electric current, air- filled voids are displayed as high-resistivity anomalies (Figure 5).

Find Drilling Targets With Electrical Resistivity Imaging

Electrical resistivity imaging is useful for finding fracture zones that are likely targets for high-yield wells. While fracture trace analysis reduces uncertainty over random drilling, electrical resistivity reduces uncertainty in that it confirms presence of a fracture zone or other drilling target. An added benefit of resistivity imaging is it finds fracture zones and other water-bearing features that have no evidence of their existence on the ground surface.

For example, Figure 6 illustrates a resistivity section from a community in the Blue Ridge province of Virginia. It is notoriously difficult to find high-yielding wells in the metamorphic rocks of the Blue Ridge. Twelve wells had previously been drilled in the community, several of which had been sited by a geologist using fracture trace analysis alone. Only six wells yielded sufficient water to bring into production, with yields ranging from 13,000 to 40,000 gallons per day (9 to 28 gpm). We conducted our own fracture trace analysis and suspected a topographic feature to be a fracture zone. We conducted electrical resistivity imaging and confirmed the topographic draw as a fracture zone as evidenced by vertical low-resistivity feature coincident with the valley. We recommended drilling on this feature, which resulted in a well yielding 114,000 gpd (79 gpm), almost three times the best yielding well located by fracture trace analysis alone.

At another site in the valley and Ridge province, a consultant had drilled three wells for a municipality on suspected fracture traces, none of which could be brought into production. We were subcontracted to conduct electrical resistivity imaging on other fracture traces and found a promising drilling target (Figure 7). Two wells were drilled on that fracture trace, one producing 2 million gpd (1,400 gpm), and the other producing 1,730,000 gpd (1,200 gpm).

Another benefit of using electrical resistivity imaging is it will find fracture zones or other drilling targets unrelated to topography. At a site in the Piedmont province of North Carolina, six wells had been drilled but only two had been brought into production with about 86,000-gpd yield each (60 gpm). Figure 8 illustrates a resistivity section that revealed a fracture zone that was completely independent of the local topography. In fact, the best drilling target on that section is under a local topographic ridge. Drilling at this location produced a well yielding 144,000 gpd (100 gpm). This example serves to illustrate why relying on fracture trace analysis alone can sometimes yield poor results. If drilling had taken place on this profile at the topographically low areas, the well would have been placed in high-resistivity bedrock. The high- resistivity values suggest there is low water saturation in these areas and wells drilled in these locations would produce little water.

In conclusion, this methodology combines traditional fracture trace techniques with modem geophysics. By integrating knowledge of the local geology, geomorphology and geophysics collected from the site, one may find high groundwater yields where random drilling or traditional methods alone have failed.