2. Borehole geophysical logging for 3-D hydrogeologic Characterization at a contaminant release site
Environmental investigations of a hydrocarbon contaminate release site in southern California were performed, and remediation systems designed, by several different consultants at different times. Consequently, different interpretations were superimposed on older conceptual models and certain important details became obscured. As time went on, the remediation of the site was failing and the interpretations of the subsurface conditions by the various consultants were contradictary. Therefore, it was necessary to re-evaluate the conceptual model of the subsurface conditions and re-design the remediation system.
Borehole geophysical logging was performed in numerous existing monitoring wells to determine the vertical lithologic variations within the wells that had been geologically logged and interpreted by the various consultants. Additionally, the geophysical logging provided a reliable means for correlating between the various wells that previously had conflicting interpretations and correlation was difficult.
Induction resistivity and natural gamma logs were obtained in numerous cased monitoring wells since these logs do not require an open hole. The gamma ray and electrical resistivity data are sensitive to changes in the material grain size and permeability. The respective logs from each well were analyzed and lithostratigraphic boundaries were defined based on the log response.
The logs showing the lithostratigraphic boundaries from all of the wells were digitized and plotted in three-dimension to show the general concept of site variations.
Next, the digitized 3-D logs were used to create a preliminary 3-D lithostratigraphic zone model of lateral and vertical variations between the various wells based on the log response and boundaries.
The geophysical data from the zone model was then gridded and further 3-D computer processed to provide a model of the entire site. This model allows for the extrapolation of the data to other locations of the site. This information can be used to generate cross-sections and isopach maps through the site and provide a qualitative means for predicting potential fluid flow/contaminant transport and the re-assessment of the remediation system.