WETLAND GEOPHYSICS: USING NON-INVASIVE GEOPHYSICAL IMAGING TO SUPPORT WETLAND RESTORATION PROJECTS IN NORTHWEST OHIO

Wetlands are currently being restored across Northwest Ohio to reduce nutrient loading into Lake Erie. Most of the restoration sites are however old farm fields with farming legacies including drainage tiles with limited records of their locations and depths. An effective pre- and post-restoration assessments of these wetlands will require locating the tiles and understanding variations in their soil properties. Such assessments often rely on in-situ probing and sampling approaches or using existing soil and utility maps which provide limited information. In this study, we show the use of multiple geophysical techniques for supporting wetland restoration projects by delineating drainage tiles and characterizing the variations in soil properties including texture, moisture, and organic matter content at a large spatial scale. We used 250 MHz antennas, EM-38 MK2 conductivity meter, and a Supersting R8 resistivity meter with Cu-CuSO₄ non-polarizing electrodes to acquire ground penetrating radar (GPR), electromagnetic (EM) and self-potential (SP) measurements along parallel transects. Both GPR and EM where towed behind a utility terrain vehicle equipped with an RTK differential GPS system. The ray traces, 2-way travel time and amplitudes of reflected GPR waves were analyzed for similar successive hyperbolas in parallel transects indicative of tile locations. In- and Quad-phase EM data were interpolated using statistical kriging to obtain a spatial distribution of apparent electrical conductivity (ECa) and magnetic susceptibility while natural potential from SP (in mV) were combined with conductivity model to obtain a 2D distribution of current density. Soil samples collected at selected locations were analyzed for soil properties estimates. Both GPR amplitudes and interpreted hyperbolas show locations of tiles spaced 5 – 15 m in a parallel sequence at depths of 0.6 to 1 m. Conductivity distributions correlate with existing soil maps and show soil variations at a higher spatial resolution. Hence, such derived conductivity maps could be used to guide subsequent soil sampling for routine monitoring. Measured ECa and natural potential (mV) also show strong correlation with soil moisture and organic matter making them a proxy for estimating temporal and spatial variations in soil properties within wetlands.