| Geophysical techniques in unconsolidated sediments where shallow faulting, fracturing and channeling is probable and most likely affects groundwater flow are investigated. Barometric effects on soil gas sampling, as a supplement to geophysical investigation is also evaluated. Wenner and dipole-dipole resistivity, ground penetrating radar (GPR), and high resolution reflection seismic data are compared with cone penetrometer (CPT) information, existing monitoring well data, models of anticipated fault styles and outcrop examples of fracturing and jointing. Data from two free soil gas sampling programs are evaluated for the effects of barometric pumping over time on measured soil gas volumes. One program collected data from the trapped atmosphere immediately above two hazardous waste landfill areas and the second involved the collection of free soil gas samples on a rough grid with an approximate spacing of 1.6 kilometers over a 720 square kilometer area. These soil gas data may suggest the presence of fractures or faults that act as preferential pathways for gas migration from depth.; The results of these studies suggest that (1) the CPT study, combined with monitoring well data may suggest that faulting is present, (2) the addition of Wenner and dipole-dipole data may further suggest that offset zones exist in the shallow subsurface, (3) the high resolution seismic data will image faults to within a few meters of the surface but cannot identify faulting on the scale of our models, (4) offset 100 MHz and 200 MHz common mid-point (CMP) GPR will image features that may be fault planes and strands similar to the models, (5) three principal shallow subsurface features are readily detectable using GPR; paleochannels, joints or fractures, and faults, (6) the combination of all of the tools, particularly the GPR and seismic may allow for the mapping of shallow faults that may affect groundwater flow, and (7) soil gas data suggests that there is a predictable change in soil gas volume with a given change in barometric pressure, therefore, it may be possible to distinguish barometric soil gas fluctuations from geologically sourced fluctuations and map fractures or faults in unconsolidated sediments. |
