Evaluating the applicability of transient electromagnetic (TEM) data to characterize aquifer geometry in urban areas

Understanding subsurface properties within urban areas is critical for an adequate management of groundwater, for instance to delineate the migration of pollutants, artificial recharge systems or geothermal collectors. Information is available from extraction wells, yet the resolution of the information is limited to the locations where wells are available. Geophysical methods offer an alternative to gain subsurface information. However, asphalted roads and limited accessibility might reduce the applicability of electrical methods for investigations beyond a few meters, whereas vibrations due to traffic and railroads might hinder the application of seismic methods. In this work, we investigate the use of the transient electromagnetic (TEM) method to resolve the geometry of aquifers in urban areas. We propose the use of relative small loops to gain separation from buried structures and increase data quality in late times as required to reach a depth of investigations of ca. 40 m. Measurements were conducted in gardens located within cities deploying single-loop as well as in-loop geometries using two different instruments. Additionally, we evaluate our small loop configuration in a quasi noise-free site through comparison to larger loops and electrical methods. Analysis of the data demonstrate that relative small loops (12.5 m x 12.5 m) may be a possible solution to gain information in urban areas down to a depth of 30 m, yet a minimal separation to anthropogenic structures of ca. 5 m is required. Information at such depth can not be easily gain with refraction seismic or electrical resistivity tomographic measurements in such small areas. Moreover, our results reveal the possibility to gain similar information with smaller loops (6.25 m x 6.25 m), offering the possibility to increase the separation to sources of noise (i.e., buried infrastructure) and increase the data quality. The inversion of TEM measurements collected along a 100 m profile permitted to obtain vertical and lateral variations in aquifer geometry with a maximal depth of investigation of ca. 40 m, while DC-resistivity measurements in the same profile were limited to less than 10 m depth. Stochastic inversion of the data permitted to investigate the uncertainty in the obtained model parameters (resistivity and thickness of the resolved layers, i.e., aquifer).