Cross borehole electrical resistivity tomography (ERT) was used to image the resistivity distribution before and during two infiltration experiments. In both cases water was introduced into the vadose zone, and the change in resistivity associated with the plume of wetted soil was imaged as a function of time. The primary purpose of this work was to study the capabilities and limitations of ERT to image underground structure and ground water movement in the vadose zone. A secondary goal was to learn specifics of unsaturated flow in a complex geologic setting. Tomographs of electrical resistivity taken before infiltration image coarser, well‐drained soils (sands and gravels) as more resistive zones, whereas finer grained soils (silts and clays), which hold more water by capillarity, are imaged as more conductive. Images of changes in resistivity during infiltration show growth of the water infiltration plume with time that is consistent with known geology. In the ERT images we see the effects of capillary barriers and infer differences between capillary‐driven flow through fine sediments and gravity‐driven flow through very permeable sediments. Images are consistent with numerical flow simulations using hydrological parameter values consistent with soil types inferred from well logs. ERT can be a useful tool to monitor movement of circuitous moisture fronts in a heterogeneous field setting that would go undetected by borehole measurements.

We used electrical resistance tomography (ERT) to map the subsurface distribution of a steam flood as a function of time as part of a prototype environmental restoration process performed by the Dynamic Underground Stripping Project. We evaluated the capability of ERT to monitor changes in the soil resistivity during the steam injection process using a dipole-dipole measurement technique to measure the bulk electrical resistivity distribution in the soil mass. The injected steam caused changes in the soil's resistivity because the steam displaced some of the native pore water, increased the pore water and soil temperatures and changed the ionic content of the pore water. We could detect the effects of steam invasion by mapping changes in the soil resistivity as a function of space and time. The ERT tomographs are compared with induction well logs, formation temperature logs and lithologic logs. These comparisons suggest that the ERT tomographs mapped the formation regions invaded by the steam flood. The data also suggest that steam invasion was limited in vertical extent to a gravel horizon at depth of approximately 43 m. The tomographs show that with time, the steam invasion zone extended laterally to all areas monitored by the ERT technique. us understand the heterogeneous subsurface environment, the factors controlling steam movement in situ, and the steam flow behavior induced by the process. An accurate understanding of the interaction between the steam process and the geologic environment is needed to assess the remediation effectiveness. This underground imaging technique substantially reduces the need for the number of boreholes that would otherwise be required to monitor the process. DESCRIPTION OF ERTTo image the resistivity distribution between two boreholes, we placed a number of electrodes in electrical contact with the soil in each borehole (Figure 1). Using an automatic data collection and switching system (shown schematically in Figure 2), we then applied a known current to any two electrodes and measured the resulting voltage difference between other pairs of electrodes. Each ratio of measured voltage and current is a transfer resistance. Next, we switched to two other electrodes, applied current between two other electrodes and again measured the voltage differences using electrode pairs not being used for the source current. We repeated this process until many combinations were measured which completely encircled the target area. For n electrodes there are n (n -3)/2 linearly independent transfer resistances. A complete set of linearly independent data contains the maximum information content about the target; any additional measurements collected are redundant. This formula does not count reciprocal measurements because a measurement and its reciprocal contain the same information and therefore are only counted as one by the formula. The reciprocal to any original transmitter-receiver pair is one where the original transmitter dipole becomes the receiver dipole and where the original receiver ...

We develop a Regional Seismic Travel Time (RSTT) model and methods to account for the first-order effect of the three-dimensional crust and upper mantle on travel times. The model parameterization is a global tessellation of nodes with a velocity profile at each node. Interpolation of the velocity profiles generates a 3-dimensional crust and laterally variable upper mantle velocity. The upper mantle velocity profile at each node is represented as a linear velocity gradient, which enables travel time computation in approximately 1 millisecond. This computational speed allows the model to be used in routine analyses in operational monitoring systems. We refine the model using a tomographic formulation that adjusts the average crustal velocity, mantle velocity at the Moho, and the mantle velocity gradient at each node. While the RSTT model is inherently global and our ultimate goal is to produce a model that provides accurate travel time predictions over the globe, our first RSTT tomography effort covers Eurasia and North Africa, where we have compiled a data set of approximately 600,000 Pn arrivals that provide path coverage over this vast area. Ten percent of the tomography data are randomly selected and set aside for testing purposes. Travel time residual variance for the validation data is reduced by 32%. Based on a geographically distributed set of validation events with epicenter accuracy of 5 km or better, epicenter error using 16 Pn arrivals is reduced by 46% from 17.3 km (ak135 model) to 9.3 km after tomography. Relative to the ak135 model, the median uncertainty ellipse area is reduced by 68% from 3070 km 2 to 994 km 2 , and the number of ellipses with area less than 1000 km 2 , which is the area allowed for onsite inspection under the Comprehensive Nuclear Test Ban Treaty, is increased from 0% to 51%.

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