Groundwater Level Monitoring for Hydraulic Characterization of an Unconfined Aquifer by Common Mid-point Measurements using GPR

Abstract

Because of its limited bandwidth, ground penetrating radar (GPR) signal has an oscillating character that degrades the ability to distinguish reflection events and introduces an error in the velocity analysis. In this paper, we use the analytic signal to calculate the envelope velocity spectrum, which is void of the oscillating nature of a GPR wavelet. A comparison study on a synthetic common mid-point (CMP) gather demonstrates that the envelope velocity spectrum gives a more robust velocity estimation. We applied this technique to explore the potential of GPR for monitoring the pumping-induced groundwater level change, then estimating the hydraulic properties of an aquifer in Ulaanbaatar, the capital of Mongolia. A field survey was conducted along three different directions when a production well was under pumping and static conditions. In addition, a simple velocity picking scheme was suggested based on the maximum coherency curve. From the velocity picks, the vertical profiles of interval electromagnetic velocity and water content were estimated. The groundwater table was easily identified in these profiles. The groundwater level changes under the CMP sounding locations along the northeast and north survey lines were estimated to be 27 and 23 cm, respectively, whereas it failed to detect the groundwater level change on the northwest survey line because of the effect of strong scatterers near the groundwater table. The estimated groundwater level change was input into the unconfined aquifer model for estimating the hydraulic conductivity, which yields comparable results with a previous study. We conclude that the hydraulic parameter estimation using GPR is much easier and effective than the conventional hydraulic test methods.