For a general question like “how deep can you see with ground penetrating radar (GPR)”, the answer is usually a range such as “2 to 10 feet” or “up to 18 inches”. However, for a client concerning his/her specific survey area and survey goal, this kind of answers often are not satisfying and sometimes can be misleading.
For example, you have been to many sites in a town and you know in this general area you can see about 4 to 5 feet deep with GPR in general. So you tell your client so, only to find out that you can not see anything with GPR after you arrive at the site – the survey area was at a low ground and saturated with water, making the GPR penetrating depth very shallow.
For a more specific question like “can you see through a 8-inch thick concrete pad with GPR?” or “the pipe is about 5 feet deep, can you see it with GPR?”, I hate to say “I don't know”, but it's the reality until it's tried at the survey location. Conditions vary from site to site, from area to area within the same site, and location to location within the same area. It's really hard to be sure how deep you can see with GPR without trying at the very survey location.
These two GPR data profiles shown in the picture here were collected over a concrete floor side by side about 1 foot apart from each other. It's for locating an electrical conduit crossing them. The electrical conduit was clearly visible in profile 1 but not in profile 2. The concrete in profile 1 was the original concrete while the concrete in profile 2 was newer (it's the replacement after the original concrete was removed for the access below). This conduit was actually first discovered pipe tracing with a cable/pipe locator. The portion of this conduit within this newer concrete area was about 5 to 6 feet long, and the GPR data collected within this area were all like profile 2. The electrical conduit was simply hard to see within this area but clearly visible once outside this area. Apparently, the floor is all concrete, but there are big differences in GPR penetrating depths across the floor.
Considering how GPR penetration depth is affected by individual subsurface layers, it's even harder to estimate it. Just like a sheet of paper right in front of your eyes can totally block your view, a thin layer of electrically conductive materials can totally block the GPR view. For example, sandy and silty soils are relatively “good” for GPR while clayey soils are not. You can say “the soils at the site are composed of MOSTLY sands and silts”, but a thin clay layer only a couple of inches thick at a shallow depth could severely block the GPR view and make the rest of soils, “MOSTLY sands and silts”, irrelevant. For another example, a layer of asphalt pavement usually is not a big problem for GPR unless it's really new or special. You can tell your client so, but how do you know there are not other layers of something like concrete beneath the asphalt pavement? Sometimes there could be a layer of gravel, and then another layer of asphalt beneath it. All these additional layers could impact the GPR penetration depth and/or performance in a negative way, and sometimes a single “bad” layer or layer interface could severely block the GPR view, regardless the rest of “good” subsurface materials.
In conclusion, it's really hard to answer the question “how deep can your see with GPR?”. In general, the question should not be answered without a context such as the site condition, the target characteristics and the GPR antenna frequenciesto be used, etc.
In most situations, no matter what the answer is, the bottom line is “I don't know until I try it”.