There are many factors that can adversely affect a GPR survey. In unfavorable conditions, the data from a GPR survey may be essentially meaningless. Here are some examples of surveys that were conducted under less-than-optimal circumstances.
This project sought to prospect for a buried kiva and associated room blocks using different geophysical techniques. Unfortunately for the GPR technique, the amount of surface vegetation was a serious hindrance to collecting good data because the antenna continuously became uncoupled with the ground surface. The strings in the picture were established beforehand as "lanes" to survey in, which made maneuvering the equipment extremely difficult.
There has been a long-held belief that wet mud is a poor medium for conducting GPR surveys. It is true that most radar energy will only penetrate a couple of meters in these conditions. In this survey in Oregon, three meters of wet mud had to be cleared before the GPR antenna (200 MHz) could successfully resolve anything. Once the top few meters of mud were cleared, good reflections were achieved up to three meters.
This survey aimed to map bedrock beneath sand dunes. While we were mostly able to accomplish this, the survey had to be done in an "extreme" way. These circumstances are typically not preferred, as the antenna can frequently lose coupling to the surface, and the radar energy is angled almost vertical, which means it might not be able to reflect sections of bedrock. This type of survey is also very tiring!
Doing GPR surveys in urban centers can have potential downfalls. In this survey, radar was used on a busy street in Reno. Under the concrete street was metal mesh that made the reflection profiles quite confusing. While energy was still able to resolve features past this metal mesh, extra attention had to be given to interpreting these data.
Snow can be an excellent conductor of radar energy.
However, in very loose snow pack or areas where trees create void spaces in the snow, the energy may encounter air pockets and reflect these in the data. This can obscure and complicate the reflection profile, so extra care is needed during interpretation.
Water is often the most important factor in resolving cultural features in radar.
This is one example of how dramatic the water factor can be when surveying a site. The top slice maps are of a kiva in the American Southwest when ground conditions were dry. The bottom maps show the same kiva, surveyed after it rained. As you can see, the rain made the kiva almost invisible in the radar data.