Abstract

Soil moisture at shallow depths (<2 m) is a key parameter for understanding several processes associated with the growth and development of natural and intervened ecosystems, including crop irrigation, growth in sclerophyll forest, wetland characterization, and climate change assessment. Non-invasive geophysical electromagnetic techniques, such as Frequency Domain Electromagnetic (FDEM) methods, are able to address the limitations of traditional techniques, particularly conventional methods, including punctual and invasive observations. However, FDEM requires an adequate calibration and is more limited when single-frequency instruments are used. We present an approximate and quick technique to calibrate and invert data acquired with a single-frequency FDEM equipment. This technique includes: (1) a calibration approach to improve the accuracy of apparent electrical resistivity values, based on vertical electrical sounding measurements, and (2) a nearexhaustive grid search approach to invert the data based on a simple model with a layer and half space. We assessed the performance of this approach against electrical resistivity tomography (ERT) measurements at seven sites located in Central Chile, which span a wide range of soil electrical resistivity (1-1000 Omega m) and moisture conditions. Our analysis indicates that non-corrected FDEM data can produce apparent electrical resistivity values that are biased by more than 100% compared to ERT data in these environments. By contrast, electrical resistivity models derived from corrected FDEM data are comparable to ERT inversion results in the range of 0.5-1.0 m soil depth. Our findings suggest that the proposed methodology represents an adequate tool for fast mapping of soil resistivity under different vegetation and soil types, with application for humidity estimates in large agricultural fields or climate change assessments.