Acoustic detection of immiscible liquids in sand

Abstract

Laboratory cross-well P-wave transmission at 90 kHz was measured in a 61 cm diameter by 76 cm tall water-saturated sand pack, before and after introducing a non-aqueous phase organic liquid (NAPL) (n-dodecane). In one experiment, NAPL was introduced to form a lens trapped by a low permeability layer; a second experiment considered NAPL residual trapped behind the front of flowing NAPL. The NAPL caused significant changes in the travel time and amplitude of first arrivals, as well as the generation of diffracted waves arriving after the direct wave. The spatial variations in NAPL saturation obtained from excavation at the end of the experiment correlated well with the observed variations in the P-wave amplitudes and travel times. NAPL residual saturation changes of 4% were detectable and the 40 to 80% NAPL saturation in the NAPL lens was clearly visible at acoustic frequencies. The results indicate that small NAPL saturations may be more easily detected with amplitude rather than travel time data, but that the relationships between the amplitude changes and NAPL saturation may be more complex than those for velocity.