Evaluation of New Geophysical Tools for Investigation of a Landfill, Camp Roberts, California

Characterization of material changes with depth (profiles) in many landfill sites can be problematic for some conventional geophysical methods. Localized anomalies within the landfill can complicate mapping of underlying layers, and layered-model techniques are inappropriate for imaging laterally discontinuous landfills. Recently-developed geophysical hardware and software tools provide the opportunity to image the vertical structure of a landfill and its geologic setting. In May, 2000 a sequence of geophysical data sets were acquired at a landfill site at Camp Roberts, CA to test the benefits of new hardware and software for characterizing the three-dimensional boundaries of the landfill and the geologic setting. Conventional magnetic and electromagnetic measurements provided a backdrop for these new methods. A Geometrics G-858 magnetic gradiometer equipped with a real-time GPS positioning system was used to map the areal extent of the landfill. Resistivity, seismic refraction, and electromagnetic data were acquired along profile lines to characterize the vertical extent of the landfill and geology. Seismic refraction data were processed with conventional time-delay methods, and with newer tomographic methods. The multielectrode resistivity data were compared with data acquired with the capacitively-coupled OhmMapper system The landfill boundaries that are defined in map view by the magnetic data are supported in profile by the seismic refraction data and multielectrode resistivity data. The seismic data are most effective in identifying trench locations when a tomographic inversion is used, instead of a conventional delay-time approach to interpretation. This shows a localized high-velocity zone that coincides with the trench boundaries that are defined by the magnetic data. The multielectrode resistivity data show a disruption of layering where trenching has occurred. Both the seismic data and the multielectrode resistivity data provide evidence that the shallow geology is laterally discontinuous and heterogeneous. The high electrical conductivity of the near surface imposed limitations on the penetration depth of both the OhmMapper and multielectrode resistivity systems. The multielectrode system was better suited for penetrating this zone than was the OhmMapper.