Crossline resistivity tomography was developed to find out anomalous seepage pathways in an embankment dam. By applying crossline tomography to the investigation of embankment dams, leakage pathways can be effectively located because the crossline tomogram presents resistivity distribution in the horizontal plane of an embankment dam. To test the effectiveness of crossline tomography, we applied it to data from an experiment designed to delineate anomalous seepage pathways in the embankment dam. The method yields relatively accurate geoelectric structure of the dam when applied to synthetic data. In the crossline resistivity tomogram, abrupt discontinuities of a low resistivity band corresponding to the core of the dam can be interpreted as leakage pathways. Application to real data obtained from an embankment dam in Korea yields the result which accurately depicts two anomalous seepage pathways. The identified pathways were consistent with low resistivity zones in the dipole-dipole resistivity section obtained on the crest of the dam. One pathway was confirmed by visual inspection of the dam, and afterward, by trenching.

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The self-potential (SP) method is widely used in seepage evaluation hydrological studies to monitor the integrity of infrastructure such as dams, sea dikes, and other types of flood control devices because the electric signals that are measured are directly related to seepage rate. At leaking areas along sea dikes, large SP anomalies can be generated by the rising and falling of tides. Unfortunately, SP data are often contaminated with several types of noise, such as that from drifting electrodes, telluric disturbances, and external electrical noise. Furthermore, SP signals can have high levels of spatial variability due to heterogeneity in lateral resistivity at the locations where the electrodes are installed. Because of these issues, it is very difficult to correlate the measured SP voltages with the streaming potentials associated with groundwater flows at particular points in time. To alleviate these problems, we developed a simple but effective interpretation method for SP monitoring data that involves subtracting consecutive SP voltages collected at different time points from a particular monitoring station. This subtracting procedure is able to effectively reduce spurious SP anomalies caused by electrode drift, change in resistivity, and other types of interference. Therefore, any changes observed in SP measurements over certain time frames were interpreted as resulting primarily from temporal changes in seepage flow. To demonstrate the performance of this method, we analysed SP monitoring data measured at a sea dike located on the southern coast of Korea. Our results confirmed that the SP interpretation method is able to explain changes in streaming potentials depending on the tide change over time and to detect the horizontal location of anomalous seepage zones along the sea dike.

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Measuring resistivity is a potentially powerful method of monitoring leakage zones that have developed in a dam, and their expansion over time. Generally, for embankment dams, two-dimensional (2D) resistivity data have been measured along the dam crest for the detection of leakage zones. However, the three-dimensional (3D) effects created by specific dam geometry and fluctuations in reservoir water levels significantly distort the 2D resistivity data measured at the dam crest. This study evaluates the 3D effects through 3D resistivity modelling software, which was developed to calculate apparent resistivity data for geometries and material distributions for embankment dams. These modelling results demonstrated that the 3D effect from the dam geometry and variations in water level is significant. Especially, in the case of monitoring, changes in 3D effects from water level fluctuations cause a spurious near-surface layer when time-lapse inversion is applied with a cross-model constraint. To overcome this problem, we introduced a combined reference model constructed from the independent inversion of both time-lapse data and original reference data. The combined reference model was able to effectively suppress the spurious near-surface layer and to clearly image the damaged zone when the change in water level was small. However, a time-lapse inversion using the combined reference model also failed to identify the damaged zone when the change in water level was large. Finally, by using the resistivity monitoring system devised for dam surveillance to a test dam site, resistivity monitoring data were acquired. From the time-lapse inversion of two data sets showing a large change in water level between two measurements, it was confirmed that the variation of water levels produces the occurrence of a spurious near-surface layer due to a strong 3D effect. remain in time-lapse inversion of resistivity monitoring data for more detailed and quantitative interpretation.Internal erosion is an inevitable by-product of ageing for all embankment dams. This internal erosion generally develops into piping over a long time through a combination of backward erosion and concentrated leaks, a process that eventually leads to dam failure. Thus internal erosion and piping are major causes of embankment dam failure. Internal erosion initially results in increased porosity due to the loss of fine particles in the core. Resistivity is known to be very sensitive to changes in porosity in embankment dams. Especially in embankment dams with clay cores, the resistivity of a damaged core increases since saturated clay generally exhibits lower resistivity than that of fresh ground water. On the other hand, the saturated downstream shell shows a decrease in resistivity with the increase of water and clay content. The expansion of the leakage zone into surrounding areas results in an increase in leakage, leading to changes in resistivity.