When there is a good penetration depth for ground penetrating radar (GPR), GPR can be used to estimate thicknesses of subsurface layers. However, caution has been taken as such estimation may not be accurate without references or ground truth information.
As EM waves from GPR travel down and bounce back from a subsurface interface between two layers, the travel time can be determined. In order to calculate the depth to the interface, the wave travel velocity needs to be known. Although there are indirect ways to estimate the wave velocity and depths such as using the common midpoint (CMP) method, the most direct and accurate way is to get the ground truth. The ground truth collected at one or more locations can be used as the references. If the subsurface materials are relatively homogenous, the subsurface layer thicknesses can be accurately and efficiently estimated using the GPR data collected in a large area.
One difficulty for using GPR to estimate subsurface layer thickness is the shallow GPR penetrating depth. For example, it's not uncommon that we can't see the bottom of a concrete slab with GPR, especially if the concrete is thick or relatively new. This is why we always bring both high and low frequency antennas for such projects. A high frequency antenna as in a concrete GPR system has high resolution making estimation more accurate, but it might not have enough penetration depth to begin with. In the other hand, the low frequency antenna might work where the concrete GPR fails, but with low resolution and accuracy.
There are other complications such as subsurface layers with similar electrical properties. In such cases, careful data processing might be needed to distinguish them. If you have questions regarding a project, please feel free to consult our GPR experts.