Ground Penetrating Radar Soil Suitability Maps

Ground penetrating radar’s effectiveness is greatly dependent on site-specific soil type. The soil type of a survey area will determine the depth of penetration of the ground penetrating radar signal.  One of the most difficult soil types for GPR equipment to penetrate is wet clay. As an example, where a mid-level frequency GPR system may be able to penetrate up to 16 feet in sand or gravel based soils, the same system would find it difficult to penetrate less than 4 feet in wet clay. Depending on the survey application (e.g. geophysical or utility locate) the soil type may determine the feasibility of using this technology.

The reason for this is because soils that have high electrical conductivity will rapidly attenuate radar energy which restricts the depth of penetration of the GPR signal. The main culprit for this is water, soluble salt contents and clay which will increase the electrical conductivity of the soil. Clays can hold more water than sand and silt soils.  Areas which have soils with more than 35% clay may be restrictive regarding the depth of penetration for deeper targets.

During our process of evaluating the scope for any project in the United States we first confirm the level of soil suitability for that area. We start by checking a SOIL SUTIABLITY MAP provided on the USDA web site. They provide a map with an overall overview of the entire country (see sample below) and more detailed maps for most states. Unfortunately, we have not found a similar resource for Canada and other countries.

GPR suitablity map
These maps give you a good idea of the soil suitability of a general area. However, for specific site conditions soil samples or local knowledge are helpful. In many cases the depth of penetration may not be conclusive until the ground penetrating radar equipment is properly calibrated and tested on the project site. At the very least these maps allow us to inform our clients of any potential limitations for their site before we start the project.

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