Geophysical methods used to image landfill interiors in Northern Illinois have evolved from the relatively simple one-dimensional (1D) resistivity soundings, used in the 1970s and 1980s, to more sophisticated two-dimensional (2D) resistivity and seismic refraction tomography profiles, augmented by ground-penetrating radar (GPR) and geophysical well logging. To illustrate this evolution of methods, geophysical imaging is described at two sites: the Mallard North landfill in Hanover Park, IL, and the Orchard Hills landfill, near Davis Junction, IL.
Resistivity soundings from Mallard North were inverted for true resistivity. The resistivity of municipal solid waste (MSW), mixed with clay soil, was between 9 and 19 Ω-m (unsaturated) and between 2 and 7 Ω-m (leachate saturated). Refuse thickness and leachate levels were also accurately predicted.
Orchard Hills 2D resistivity surveys showed most MSW had resistivities between 20 and 130 Ω-m (unsaturated). MSW resistivity near the leachate recirculation lines fell by as much as 3% after leachate injection started. Elastic properties of the waste were assessed using seismic wave velocities from seismic refraction tomography. The final tomographic model consisted of hundreds of cells ranging in P-wave velocity from 350 to 643 m/s, with an average value of 484 m/s. Two separate regions were identified: an upper zone consisting of a lower velocity (380–460 m/s), 3–6 m thick, and a deeper region of higher velocity (505–560 m/s). Poisson’s ratio ranged from 0.38 to 0.44 with the deeper waste exhibiting higher Poisson’s ratios, suggesting less rigidity.
