ULTRASONIC-IMAGING OF ORGANIC LIQUID CONTAMINANTS IN UNCONSOLIDATED POROUS-MEDIA

Laboratory experiments were conducted to measure the change in the ult rasonic wave signature as a function of the fraction of nonaqueous-pha se liquid (NAPL) contaminants in initially water-saturated sand sample s. This represents a fundamental step in the application of high-frequ ency seismic measurements to detect and delineate NAPL contamination i n groundwater. The NAPL's used are n-dodecane, iso-octane and Freon 11 3(R). P-wave velocity changes almost linearly as a function of the fra ction of pore space occupied by NAPL. At a given NAPL fraction, the ve locities rank with the bulk moduli of the NAPL's. The percent change i n velocity at residual water saturation relative to the water-saturate d medium ranges from 15% for n-dodecane to 31% for Freon 113(R). Veloc ity changes are significant relative to the ability of seismic methods to detect changes on the order of 5%. These data are described by a m odel previously derived by G.T. Kuster and M.N. Toksaz of P-wave trans mission through a fluid matrix with spherical inclusions that is modif ied to account for a two-fluid-phase matrix. Amplitude data as a funct ion of NAPL fraction are described by assuming the distribution of NAP L in the column and using attenuation coefficients for the water-satur ated and residual-water medium. Amplitudes are shown to be sensitive t o both the fraction of NAPL and its distribution, whereas velocity is only a function of NAPL fraction.