Investigation of integrated geophysical methods to characterize near surface formations for environmental engineering

Near surface nondestructive imaging for is aimed at evaluating unconsolidated to moderately consolidated porour or granular sediments. Usual targets of interest are depth to bedrock, depth of water table and mapping of subsurface morphology and evolution of toxic fluid (spills) flows such as oils, concentrated brines and their interactivity. Investigative propositions of interest span vast realm of geo-environmental engineering. This includes such applications as structural health and aseismic monitoring, hydraulic/hydrogeological characterization, geotechnical investigations including critical zones/sites identification in areas of public and industrial infrastructure development, groundwater management and natural resources exploration and exploitation associated activities such as mining and those termed unconventional in oil and gas industry. Conventional field methods include P- and S-wave surveys, and electrical resistivity (resistance) measurements. The presented results are outcomes and findings of two laboratory experimental studies designed, owing to their inherent amenability, on field scale concepts of the said methods, to advantage. In the first of two studies S-wave polarized propagation characteristics are exploited to quantitatively evaluate the combined architectural, rheological and fluid transport properties effect of fractured porous media (sandstone specimen) upon acquired or recorded (ultrasonic) S-wave signature when through transmitted with controlled source pulsing. Various time and frequency based analyses unambiguously delineate fracture geometry, stress effects of fracture stiffness and density, amplitude effects of fracture aperture size against the stationary effects of azimuth variability. Critical findings include the characteristic stop-band artifact in transmitted bandwidth signature of fracture planes and direct correlation of S-wave velocity anisotropy with permeability anisotropy. The second study involved spatio-temporal imaging of an immiscible fluid displacement (oil with brine) through an unconsolidated granular sediment analogue (glass-beads-pack) under controlled constant head flow conditions against gravity, using P-, S-wave and electrical resistance data, acquired as integrated. Dry and saturated granular material characteristics with different saturant (state) effects were evaluated post analyzed integrated offering fresh insights. Peculiar repeatable artifacts in imaged data not only unambiguously discriminated oil from brine, a consistent and significant "attenuation" signature of evolving fluid-fluid interface was discovered in ultrasonic data verifiable form electrical resistance observations. Ultrasonic velocity variation and frequency dependence could be understood employing usual anelastic/visco-elastic models of wave propagation, typically as Biot theory. Gassman and Biot theory applied to validate the results within a zero frequency and megahertz range appear to offer plausible analytical estimates of P-wave velocities. S-wave velocities are underestimated due to ambiguities surrounding the practiced procedures and methods of quantification of critical parameters, with no consideration to the nature of chraracteristics S-wave propagation being different compared to P-wave.