WORK PLANS - Laserfiche WebLink

Home Browse Search

2.3 Interpretation of 2-D Resistivity Results A 2D electrical resistivity survey was conducted along two profiles as part of a general site characterization of the former drum disposal area. The purpose of the electrical resistivity survey was to determine if electrical imaging methods could successfully image a subsurface contact between native soils and the fill material. Resistivity data is typically presented as a "smoothed" model, as the measurements include a bulk resistivity of all materials the current encounters between the potential electrodes. Data obtained during a resistivity survey indicates the electrical properties of a soil or rock, which may change within a particular unit. Therefore, sharp, easily definable geologic contacts are not always observed in DC resistivity data. Interpretation of resistivity models is always improved if subsurface geologic information is available. Color-enhanced 2-D cross sections for RL-1 and RL-2 are attached as Figures 2 and 3, respectively. The location of each line are shown in Figure 1. The color bar indicates the amplitude of the calculated apparent resistivity output from the least-squares inversion. Magenta, red, and orange represent zones of relatively high apparent resistivity, while the cyan regions indicate areas of low apparent resistivity (more conductive). Two distinct layers are observed in the electrical resistivity data. There is a surficial soils / fill layer characterized by relatively low apparent resistivity values with variable thicknesses between 10-20 ft thick. This layer is underlain by more resistive sediments interpreted as coarser-grained sediments. There does not appear to be an obvious contrast between the fill material within the expected pit and the fill material outside of the disposal area. However, there is a resistive surficial soils unit on the northern portion of RL-2 interpreted as being caused by fill material containing less fines. There was also evidence of concrete buried in the northern half of RL-2. While, there is no obvious structure or electrical contrast between the disposal pit and surrounding fill, the resistivity allows for a "limiting depth" interpretation at the contact between the fill material and native soils. The contact between the fill material layer and native soils beneath RL-1 is interpreted as being approximately 12 ft deep under the central portion of the pit, and 15 ft at the ends of the profile. The contact between the fill material layer and native soils beneath RL-2 is interpreted as being approximately 11 ft deep under the central portion of the line, 14 ft beneath the intersection with RL-1 and the center of the pit area, and as deep as 15 ft beneath the eastern edge of the profile. 6243 Versar 4 July 6, 2006