Assessment of ground condition changes by 4-D ERT imaging

To assess the ground condition changes, four-dimensional (4-D) or time-lapse imaging is commonly used in various application fields of geophysical methods. In these applications, accurate data measurement and proper 4-D imaging algorithms are the most essential components for the successful 4-D imaging. In this context, a new 4-D inversion algorithm was introduced recently, where measured resistivity data and geoelectric structure of subsurface is assumed to be 4-D (i.e., 3-D in space and varies with time) and inverted at a time by applying the regularization along time axis. Thanks to this sophisticated 4-D ERT imaging algorithm, we could provide much clear image of the ground condition change with much less inversion artifacts. In this study, we demonstrate two successful case of 4-D ERT imaging, where the target zones were affected by the change in the underground structure (case 1) and ground reinforcement in the subsidence area (case 2). By the 4-D ERT images, we could define the zones of change and explain their causes in these examples.