GPR Survey With Snow Or Ice Cover On The Ground
"The ground is snow covered, is it OK for the GPR survey?" This is a very common question from our clients. Ground penetrating radar (GPR) actually works well through snow or ice. Below is a GPR profile showing an underground storage tank (UST) with ground covered by about 4 to 6" of snow. The located UST was marked on-site as in the picture to the right.
Although snow or ice on the ground surface wouldn't block GPR signals, it's not a favorable working condition for some types of GPR surveys, such as those searching for underground utilities. A GPR designed for underground utility locating typically has a ground-coupled antenna, meaning it works the best when it's close to the ground surface so the energy can be efficiently "coupled" into subsurface materials. When the ground is covered by certain thickness of snow or ice, the antenna is effectively lifted off the ground, reducing the effective energy transmitting into the ground. As a result, the effective GPR penetration depth is reduced. However, in GPR-friendly soils such as dry sands, it's generally not a serious problem when the snow/ice cover is thin. You get reduced by still very good penetration depth for underground utility locating.
When a snow or ice cover reaches a certain thickness, a "ringing" effect due to multiply reflections from the ground surface could occur and it could severely impact the visibility of reflections from interested targets. Below is a GPR profile over a steel underground storage tank with 10' diameter. A typical reflection from a tank has the shape of a hyperbola as shown in the GPR profile above, but in the GPR profile shown below only the portion without the snow cover is clearly visible.The ringing effect dominates the signals where the snow cover is. Although the ringing effect could be reduced with data processing techniques such as deconvolution, it's not very practical for certain applications which need data interpretation on the fly. Note that the GPR profile shown below is collected diagonally across the tank area so diameter of the tank appears to larger than it actually is.