For several decades, induced polarization (IP) effects on transient electromagnetic (TEM) responses have been observed. These effects can manifest as late-time negative transients or as rapidly decaying curves and are usually associated with highly polarizable bodies. If neglected, IP-effects can lead to erroneous resistivity models. Recent work allows IP-effects to be incorporated into the inversion of TEM data on a more routine basis. In a recent field survey in western Tanzania, strongly IP-affected TEM signals were observed using a towed-transient electromagnetic (tTEM) system. The survey was carried out to locate drinking water resources in a weathered regolith setting. In these settings, inversion of tTEM data using a resistivity-only forward model (i.e., IP neglected) cannot fit the data and severely limits the value of the TEM data for hydrogeologic interpretation. To account for IP-effects, we applied a modified version of the Cole-Cole model, called the Maximum Phase Angle (MPA) model to invert IP-affected tTEM data. The MPA model incorporates four inversion model parameters, resistivity (ρ), maximum phase angle (φmax), relaxation time (τ) and frequency exponent (c). The MPA model fits the data well and improves the reliability of resistivity model. In much of the surveyed region, the inverted models using MPA display a three-layer system consisting of an upper resistive laterite layer of varying thickness, and an intermediate polarizable conductive unit overlying more resistive weathered basement rocks. The conductive polarizable layer is interpreted as a chemically weathered saprolite separating the surficial and deeper aquifers. Overall, tTEM inversion results provide a local understanding of groundwater systems, especially in such regions with very limited subsurface knowledge.
