Time-Lapse Ert Monitoring of Soil Moisture Dynamics from Simultanous Tl InversiÓn: a Case Study of Fupre, Southern Nigeria

Time-lapse electrical resistivity tomography (TL-ERT) is a promising monitoring technique since they provide large-scale subsurface information that complements surface observations. This study presents the result of joint inversion of 2D ERT data along six lines acquired in a proposed engineering site at two time-events (between September, 2023 and April, 2024). The study was aimed at monitoring fluctuation of the top of water table (TOW) and subsurface moisture dynamics using joint inversion of time-lapse 2D ERT data sets. The methodology employed involves measurement of apparent resistivity magnitudes along Six (6) profile lines at the study site at two distinct time-scales of September, 2023 and April, 2024. Time-lapse 2D ERT data along the Six (6) lines was collated using a 2D collation code and inverted simultaneously on a single scale using the method of least square active-time constraint resistivity weighting. This inversion method ensures that the resistivity weights were constant between the two survey periods for effective monitoring. A maximum depth of 17.1 m was imaged within the subsurface after about 2 to 7 iterations. The 2D ERT images showed characteristic apparent resistivity weights that are indicative of different subsurface features indicative of sandy zones with resistivity range of 500-632 ῼm, and the saturation zone (wet zone) with low resistivity range of 200-356 ῼm. In September, 2023 (a rainy season), the water table was observed to rise. The moisture content in the soil moves in response to the petrophysical characteristics within the subsurface. In April, 2024 (the end of the dry season and beginning of another rainy season cycle) the moisture content of the soil was observed to decreased in response to evapotranspiration. Eventually, the subsurface electrical resistivity (ER) distribution is expected to increase as moisture content decreased. As soil moisture content changes, the subsurface electrical conductivity (EC) along profiles Lx1, Lx2, Lx3, Lx5 and Lx6 changed from -50 to -100 % an indication of a reduction in water content in April, 2024, while EC along profile Lx4 changed from 0.10-25.1%. Therefore, the time-lapse inversion results provided monitoring of the subsurface moisture conditions and fluctuation in water table along the monitoring profiles in the study site. These data are invaluable for planning of groundwater exploration projects at the study area.