PNNL

Abstract

Spectral induced polarization (SIP) is a non-intrusive geophysical method that gleans information in the form of chargeability (the ability of a material to retain charge) in the time domain or its phase shift in the frequency domain. SIP is widely used to detect sulfide minerals, clay minerals, metallic objects, municipal wastes, hydrocarbons, and salinity intrusion. Although SIP is a temporal method, it cannot measure the dynamics of flow and solute/species transport in the subsurface over long times (often 10–100s of years). To capture long-term dynamics, data collected with the SIP technique need to be coupled with fluid flow and reactive-transport models. To our knowledge, there is no simulator in the open-source literature that couples fluid flow, solute transport, and SIP process models to analyze geoelectrical signatures in large-scale systems. PFLOTRAN-SIP was built to couple SIP data to fluid flow and solute transport processes. This framework built on the PFLOTRAN-E4D simulator that couples PFLOTRAN (a massively parallel multi-physics simulator for subsurface flow and transport) and E4D (a massively parallel geoelectrical simulator) without sacrificing computational performance. PFLOTRAN solves the coupled flow and solute transport process models to estimate solute concentrations, which were used in Archie’s model to compute bulk elec-trical conductivities at near-zero frequency. These bulk electrical conductivities were modified using the Cole-Cole model to account for frequency dependence. Using the estimated frequency-dependent bulk con-ductivities, E4D simulated the real and complex electrical potential signals for selected frequencies for SIP. These frequency-dependent bulk conductivities contain information relevant to geochemical changes in the system. The PFLOTRAN-SIP framework was demonstrated through a synthetic tracer-transport model sim-ulating tracer concentrations and electrical impedance for four frequencies. Later, SIP inversion estimated bulk electrical conductivities by matching electrical impedance for each of these frequencies. Estimated bulk electrical conductivities were consistent with the simulated tracer concentrations from the PFLOTRAN-SIP forward model. This framework allows practitioners of environmental hydrogeophysics and biogeophysics to monitor chemical, nuclear, and tracer transport sites as well as to detect sulfide minerals, metallic objects, municipal wastes, hydrocarbons, and salinity intrusion.