Ultrasonic imaging of organic liquid contaminants in unconsolidated porous media

Laboratory experiments were conducted to measure the change in the ultrasonic wave signature as a function of the fraction of nonaqueous-phase liquid (NAPL) contaminants in initially water-saturated sand samples. This represents a fundamental step in the application of high-frequency seismic measurements to detect and delineate NAPL contamination in groundwater. The NAPLʹs used are n-dodecane, iso-octane and Freon 113®. P-wave velocity changes almost linearly as a function of the fraction of pore space occupied by NAPL. At a given NAPL fraction, the velocities rank with the bulk moduli of the NAPLʹs. The percent change in velocity at residual water saturation relative to the water-saturated medium ranges from 15% for n-dodecane to 31% for Freon 113®. Velocity changes are significant relative to the ability of seismic methods to detect changes on the order of 5%. These data are described by a model previously derived by G.T. Kuster and M.N. Toksöz of P-wave transmission through a fluid matrix with spherical inclusions that is modified to account for a two-fluid-phase matrix. Amplitude data as a function of NAPL fraction are described by assuming the distribution of NAPL in the column and using attenuation coefficients for the water-saturated and residual-water medium. Amplitudes are shown to be sensitive to both the fraction of NAPL and its distribution, whereas velocity is only a function of NAPL fraction.