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