SP Monitoring at a Sea Dike

ABSTRACT 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.