GPR can see a lot more than just metal objects. GPR sees the contrast of materials. In this example, you can see several distinct contrast objects. The top layer is the air above the concrete the first hard line is the top of a slab-on-grade. The distortion near the red line is wire mesh and the red line represents the bottom on the slab and the top of the grade. In this case, you can very clearly see the slab thicken. Objects A and B are metal ducts embedded under the surface of the slab. Reading GPR scan graphs come over time. To well seasoned GPR techs looking at scan graphs might as well be high-resolution pictures of the subsurface they are looking into.