Complementing GPR Detection of Tree Roots in Urban Areas

Trees provide valuable environmental services, particularly in urban areas, which can be affected by the urban heat island effect and higher levels of pollution. However, roots can also be disruptive to infrastructure as their roots can cause subsidence, cracks, and displacements. Geophysical methods can help to detect and map the location of roots in relation to these systems, which can inform maintenance and repair decisions and help to prevent costly and disruptive repairs. Ground Penetrating Radar (GPR) has emerged as the most promising method that can help to identify the presence and extent of roots, but while much of the literature focuses on the potential of this application, its limitations and possible complements are less discussed. This is particularly relevant in urban soils, where permittivity contrast between the soil and some roots may not be very strong. The goal of the paper is to describe alternative methods that can be used in conjunction with GPR for the purpose of identifying urban tree roots. Here, we discuss how other methods (most notably, electrical resistivity imaging) could be used to complement GPR in the detection and mapping of urban tree roots. We present case studies when GPR alone could not confidently confirm the position of tree roots in an urban area and describe how electrical surveys can be a complementary method and can identify individual roots when the distance between electrodes is small enough.