Long Electrode Resistivity
Electrical resistivity surveying typically employs small stainless steel (or other material) electrodes to transmit electrical current and receive voltage. These small electrodes are between 1 and 2 feet long and up to ¾-inch in diameter, and are hammered in the ground up to 1 foot in depth. In some circumstances, it may be desirable to increase the sensitivity of the resistivity survey within deeper parts of the subsurface; this can be accomplished by use of long electrode resistivity. A long electrode could be something as simple as a well or even a horizontally oriented pipe. HGI has pioneered the use of long electrode ERT (or LE-ERT) to image plumes in industrialized and infrastructure rich areas, to monitor changing subsurface conditions, and to map deep reservoirs for enhanced recovery of oil.
HGI has pioneered the use of long electrode resistivity in industrial settings to map reservoirs, plumes, and changing subsurface conditions.
HGI first used LE-ERT (also called well-to-well resistivity in some publications) to image historical leaks from underground storage tanks at the Hanford Site in Washington. Each tank has upwards of 8 steel wells, used for geophysical borehole logging, and tanks are grouped together in tank ‘farms’. The T-tank farm was known for a massive leak on tank T-106 and drilling indicated a southeastern trajectory. HGI used 110 wells of different lengths to acquire a set of pole-pole data. The image below was produced with the modeling code RES3DINVx64, which models the wells as a vertical set of conductive cells. There are also other codes that can accommodate LE-ERT (e.g., EarthImager3D, BERT, E4D) as these developers have quickly adapted to the rising popularity of the method. The well-to-well resistivity results show a low resistivity feature south and southeast of tank T-106 and confirm other leaks in tanks T-101 and T-103. T-111 is also suspected of leaking. Confirmation of the method was conducted by drilling in the tank farm and analyzing groundwater data below disposal waste sites outside of the farm.
Another example of the application of LE-ERT is reservoir monitoring of a water/surfactant flood into the Shannon Formation at the Rocky Mountain Oil Testing Center (RMOTC). HGI monitored the floods by transmitting and receiving electrical data on each well over a 122 day period. Installation included 50,000 feet of wire draped along the surface to connect each of the 22 monitoring wells to the HGI monitoring trailer. The sampling occurred approximately every 23 seconds on a 30-channel resistivity acquisition system specifically designed for the experiment. The data below show two snapshots from the long electrode resistivity monitoring of injection on well 63S. The injected solution is shown to be growing, moving south, and bound by the northern fault.