Electrical Resistivity Imaging (ERI)

Electrical Resistivity Imaging (ERI) is a noninvasive vertical electrical sounding technique that produces profiles showing a virtual cross-section of soil, sediment and bedrock layers. It works well in electrically conductive materials such as clay and saltwater-saturated materials where GPR signals are rapidly absorbed. 

Transects may be laid in a straight line to generate a 2D cross-sectional image or in a grid pattern to generate a 3D image.  Test lines from about 54 to 1,800 feet (16.5 to 548 m) long.  Longer if a rollover survey is performed.  Depth of penetration is about 15 to 20% of line length, generally about 10 to 250 feet (3 to 76 m), in depth.  Good data to 100 feet is typical for 1,000 foot lines.

SEI pioneered a new method for ERI surveys with the creation of the Foundation Imager™.  This unique method allows for 3D ERI surveys around existing structures.  For the first time ever, soil conditions beneath an existing structure can be imaged and evaluated.

Applications

Use to map soil and rock thickness and elevation of subsurface interfaces.  Determine lateral continuity of soil and sediment layers.  Delineate geologic features such as buried sinkholes, lenses of organic soil, soil voids, caves, mineralized veins and fractures.  Can be used to map pollution plumes and saltwater intrusion.

SEI has been using the AGI resistivity imaging equipment since 1999 to evaluate sites in the Southeastern United States.  All operators have completed the manufacturer’s training course.

Equipment

Super Sting R8/IP with 121 electrical smart switches; Sting R1 Memory Earth Resistivity Meter with a Swift automatic switching device and a 28-electrode array; both units manufactured by Advanced Geosciences, Inc. (AGI), Austin, Texas.  Data is processed using RES2DINV and RES3DINV, and EarthImager software with finite element or finite difference models to produce colorized cross-sections.  Data can be processed in the field on a laptop personal computer.