Evaluation of Time Lapse Acoustic Monitoring of Immiscible Fluid Flows in Near Surface by Attenuation Examination Method

Velocity information alone, in acoustic monitoring of fluid fronts and interfaces when non-aqueous spills occur in the near surface unconsolidated sediments or water table/aquifers, and of those associated with consequent remediation, may not describe such immiscible flows and displacement fronts adequately. Attenuation characteristics examination, for its greater sensitivity to subsurface fluids, can offer better resolution in describing such subsurface processes. Characterization of controlled immiscible fluid displacement process signifying examination of attenuation characteristics, by comparing spectral curves, so, is demonstrated as extended results of an experimental study. In the experiment oil is displaced with brine, flowed through a saturated pack of ceramic beads against gravity under controlled constant head flow conditions. P-wave data were acquired employing pulse transmission with 1MHz sensors. Pertinent concise review of well studied wave attenuation mechanisms and associated anelastic and viscoelastic models is provided for connective lucidity of presented results and interpretation. Application of the method following a benchmarked time-lapse type workflow for data analyses, results presentation and plausible inferring/interpretation is effectively evaluated. Cited method unambiguously resolved, oil from brine and the interfacial mixing and evolution. Subtle insight instilling and interpretation enhancing frequency dependent combined effects related to fluid density, viscosity and flow rate were observed, too.