Monitoring the Maximum Depth of Drainage in Response to Pumping Using Borehole Ground Penetrating Radar

Abstract

There are many methods available for nondestructive measurement of volumetric water content. However, no current method can monitor rapidly to great depths with high spatial resolution over large sample volumes and with minimal need for medium-specific calibration. Borehole ground penetrating radar (BPGR) may provide this capability. It is well established that BGPR can make rapid measurements to great depth. Like time domain reflectometry (TDR), BGPR infers the water content from dielectric permittivity measurements, which show a robust correlation with volumetric water content. The goal of this investigation was to determine whether BGPR water content measurements made with a vertical sampling interval that is smaller than the antenna length could be used to measure the water content profile with high spatial resolution. Repeat water content profiles measured with BGPR during a pumping test and under static conditions 1 yr later are presented. The results show that BGPR measurements are highly repeatable, allowing for differencing of profiles to determine the water content change profile. However, this high repeatability required calibration over a depth range below the water table due to instrument drift and operator inconsistencies. Although critical refractions obscure the water content profile near the ground surface, there is no evidence that refracted waves have deleterious effects on travel time profiles collected across the water table, allowing for determination of the maximum depth of drainage from the water content change profiles. There is good agreement between the patterns of maximum depth of drainage and water table depth during pumping and recovery. However, the maximum depth of drainage, referenced to the middle of the BGPR antennae, is consistently 50 cm deeper than the water table. The results demonstrate that the primary limitation on the achievable resolution of water content monitoring with BGPR is the user-selected measurement sample interval, which can be much smaller than the antenna length. However, the cause of the near constant downward offset of the BGPR measurements compared with the water table depth must be studied further to allow for direct use of BGPR to track water movement during pumping and recovery.