Electrical resistivity tomography ERT is a method of imaging the electrical conductivity of the subsurface. Electrical conductivity is a useful metric for understanding the subsurface because it is governed by geomechanical and geochemical properties that drive subsurface systems. ERT works by injecting current into the subsurface across a pair of electrodes, and measuring the corresponding electrical potential response across another pair of electrodes. Many such measurements are strategically taken across an array of electrodes to produce an ERT data set. These data are then processed through a computationally demanding process known as inversion to produce an image of the subsurface conductivity structure that gave rise to the measurements. Data can be inverted to provide 2D images, 3D images, or in the case of time-lapse 3D imaging, 4D images.

Modern ERT data collection hardware can provide massive amounts of data in short periods of time. Owing to the computional demands of inverting ERT data to produce subsurface images, it is typically impossible to extract all of the resolution provided by these systems without the use of distributed memory parallel computing resources. FERM3D is the first (and to date only) fully parallel ERT inversion software available, and was developed specifically to address the computational demands of high resolution 3D and 4D subsurface imaging. All major computational efforts are fully parallelized in terms of both cpu effort and memory distribution, providing excellent scalability. Several new parallel algorithms were developed to address parallelization issues custom to the ERT/IPT inversion problem. The code utilizes highly flexible unstructured tetrahedral meshes, enabling advance imaging options such as the inclusion of known subsurface structures and advanced customized inversion constraints. In addition, F3D provides automonous 4D imaging capability. This capability was recently used to image a subsurface ammendment injection in 3D and in real-time, demonstrating the first real-time ERT imaging application in 2D or 3D.