Seismic and Resistivity Measurements for Real-Time Monitoring of Microbially Induced Calcite Precipitation in Sand

A variety of biogeochemical processes, from inorganic mineral precipitation to bio-film formation to bio-gas generation, are being investigated as alternative methods to improve soil properties. Every process applied in a geotechnical application requires the ability to monitor the progression of treatment, preferably in real time. While monitoring of the biogeochemical processes is necessary to properly apply and manage the treatment process, ultimately verification that the treatment is improving the engineering soil properties as desired is necessary. Because direct measurements of soil properties (e.g., strength tests) during treatment are infeasible, the use of indirect non-destructive measurements during treatment is desirable. Development of these real-time, non-destructive measurements would increase the “certainty of execution” of bio-treatment methods. To this end, seismic velocity, and resistivity measurements are examined herein to assess their ability to monitor the extent and spatial distribution of microbially induced calcite precipitation (MICP) in sands. Shear wave velocity (S-wave) test results are used to develop a generalized correlation to the precipitated calcite mass; this in turn enables prediction of changes in void ratio (porosity), density, and shear modulus during treatment. Compression wave velocity (P-wave) measurements are determined under different saturation conditions and used in combination with S-wave measurements to observe how the Poisson’s ratio evolved during treatment. The applicability of resistivity measurements for monitoring the MICP treatment process is also examined. Finally, the seismic properties of MICP treated sand are compared with other conventional materials and the implications of these results for real-time monitoring during future field-scale applications discussed.