Seepage-Induced Pore Pressure Variations Beneath an Earthen Levee Measured with a Novel Seismic Tool

Abstract

Shear-wave seismic reflection velocity-versus-depth models can complement our understanding of seepage pore pressure variations beneath earthen levees at locations between geotechnical sites. The seasonal variations of water level in the Mississippi River create pore pressure changes in the adjacent hydraulically connected aquifer on the protected side of artificial levees. Time-lapse shear-wave velocity analysis or repetition of seismic acquisition over an area is a non-invasive method that can detect seasonal pore pressure variations in shallow (<40 m) aquifers. The seismic reflection patterns during the seasonal pore pressure variations of the river show a distinctive change in the velocity semblance analysis, which is translated as a change in the average stress carried by the grain-to-grain contact, or simply the effective pressure. The seismic data show a greater variation of up to +140/−40 m/s or +700/−150 kPa in the confined aquifer zone, compared with the leaky confined aquifer zone of up to +46/−48 m/s or +140/−80 kPa. These relative effective pressure characteristics allow us to distinguish between confined and leaky aquifers and can be an optional approach for pressure prediction in floodplains along levees without the need to drill borings in the area to measure piezometric data.