What is the effect that 1 dielectric value has on your project? How off will your depth estimates be if you miscalculated by ONLY 1 dielectric value? Can the consequences be catastrophic?
The effect of 1 dielectric value CAN have significant implications for your project. On a recent project I was working on, 1 dielectric value change increased the estimated depth to a target by 4 inches (10 cm). 4 inches can be the difference between damaging a pipe or not damaging a pipe! Disturbing an unmarked grave or not disturbing an unmarked grave!
How can you be off in your estimates? What can be the sources of your miscalculations? I think there are three. First, you use the preconditioned, suggested values of general soil types that are on your instrument. You think you are in “wet soil” and you click that option. The GPR uses a value of, ohhhh, 20…but, your soils are not as wet as you thought and you over estimate the dielectric value and underestimate the depth.
Second, you use the hyperbola matching option and you only match one hyperbola as a representative of the entire site. In reality, your site consists of a range of dielectric values and your estimate consists of a VERY SMALL sample. Try to model more than 1 hyperbola.
Finally, you mismatch the hyperbola and your model is either too narrow or too broad. That’s a problem. In these cases, be as precise as possible with your match so your estimates are as accurate as possible.
So can 1 dielectric value have an impact on your project. Yes! Do many people overestimate or underestimate by more than 1 dielectric value? YES!! Being off by 1 has an effect, but being off by 3 or 5 can have an enormous effect on depth estimates by a foot or more in the shallow subsurface.
Be careful and explain the limitations of the technology and site to your clients and customers.
LearnGPR.com is an online training platform for Ground Penetrating Radar that helps civil engineers, utility locators, and others learn about GPR through fun and engaging courses and coaching.