Seeing Through the Soil

The Fairfax County soil survey team got a preview of soil layers in the Lorton and Franconia areas before they even dug the first hole. The three-member team was testing ground penetrating radar (GPR) and electromagnetic induction (EMI).

GPR is a noninvasive geophysical tool that helps to detect large changes in physical characteristics of the soil. It responds to abrupt boundaries. GPR is used for determining depths to soil layers, water table, cobble, and bedrock.

In layman’s terms, the GPR system has two parts—an antennae and a receiver—and needs two operators. One person drags the antennae across the surface of the ground. Another carries the receiver.

The antennae is a bright orange box that measures about 2’ wide by 2 ft’ long by 1’ high. It connects with a cable to the receiver, which resembles a laptop computer. The antennae transmits and receives the radar signal, and then sends the information to the receiver. The receiver shows a line on the screen which represent the depth of soil layers. If a layer is consistently the same depth beneath the surface, the line remains straight. If a layer dips deeper, the lines on the screen dip as well.

Can the GPR replace soil scientists who dig holes to determine the different layers of soil and their characteristics? Not likely. The GPR shows only the changes between layers. It doesn’t identify what the changes are. Furthermore, there are many variables that limit the applicability of GPR. Generally, the radar depth of observation is shallow. The depth typically decreases as water content increases and as clay or salt content increases. The use of GPR is also interpreter dependent. The data mean little without a proper interpretation.

Like ground penetrating radar, electromagentic induction (EMI) has proven to be a valuable noninvasive tool, which allows observations without disturbing the soil. EMI transmits and receives a magnetic pulse that measures the conductivity of the soil, i.e., the weighted average of conductivity at 6 foot and 3 foot intervals. The apparent conductivity increases with the amount of clay, water, and salt in the soil.

For more information about either ground penetrating radar or electromagnetic induction, contact NRCS Soil Survey Project Leader David Harper via e-mail or call 703-324-1428.