Laboratory Tests and Field Surveys to Explore the Optimum Frequency for GPR Surveys in Detecting Qanats

In this paper, we discuss the results of laboratory tests and field surveys using ground penetrating radar (GPR) method to detect qanats at the main campus of Shahid Bahonar University of Kerman (SBUK), Iran. The main purpose of laboratory experiments was to explore the optimum frequency of GPR surveys to detect qanats for the subsoil in the study site. We performed a variety of laboratory tests with a 3 GHz antenna to detect qanats (simulated using dielectric empty targets) hosted by sand with volumetric water content (VWC) values in the range 1.5–8%. The depth to each target was progressively increased until either approaching the edges of the sandbox or modelling a qanat depth for which GPR data could not detect the target anymore. The scaling factors were calculated for each test to estimate the maximum depth of detecting qanats as a function of the scaled GPR frequency. The results showed that in areas where the subsoil is dominated by sand, medium-frequency GPR antennas can penetrate to depths of a few tens of meters, but the penetration depth considerably decreases when the soil moisture and/or clay content of the medium increase. Based on the results of laboratory simulations, qanats are detectable at a maximum normalized depth of about 15–17 times of the wavelengths in very dry sands with VWC less than 5% while the detectable range rapidly drops down to less than 3 or 4 times of the wavelengths in more humid sands with VWC of about 8%. We also discuss the results of a few field GPR surveys that were measured using antennas with the 50 MHz and the 250 MHz frequencies in the northwestern part of the study area. The processed GPR images could detect a qanat in the position compatible with the results of previous remote sensing studies performed in the area. The depth to the detected qanat is 13.5 m, which is a little bit beyond the maximum limit predicted by the laboratory tests.