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Correcting Water Content Measurement Errors Associated with Critically Refracted First Arrivals on Zero Offset Profiling Borehole Ground Penetrating Radar Profiles

Department of Hydrology and Water Resources, University of Arizona, P.O. Box 210110, Tucson, AZ 85721

View larger version (32K): [in a new window]   Fig. 1. (A) Raypaths of electromagnetic waves below the ground surface. Critically refracted raypaths are identified within the high water content layers. A direct raypath is identified within the low water content layer. (B) Zero offset profiling borehole ground penetrating radar travel times for a continuous measurement (thin line) and discrete measurement (black circle). dz is the vertical sampling interval, and zrtd is the refraction termination depth.

View larger version (33K): [in a new window]   Fig. 2. Two three-layered geometries demonstrating the travel times of the first-arriving energy in the subsurface. (A) Thick middle layer, in which hmin is less than the layer thickness. (B) Relatively thin middle layer that appears hidden; that is, the layer thickness is less than hmin. The direct and refracted travel times are identified by solid blue lines and dashed green lines, respectively.

View larger version (40K): [in a new window]   Fig. 3. (A) Flow diagram showing the steps involved to invert first arrival travel time profile to water content profile considering critical refraction. (B) A spreadsheet designed to implement the process of inverting first-arrival travel time to water content with critical refraction.

View larger version (23K): [in a new window]   Fig. 4. (A) Water content profile of a coarsely layered soil system derived from the database of Carsel and Parrish (1988) under hydrostatic conditions, (B) the velocity profile determined from Eq. [7], and (C) first-arriving travel time profile calculated from ray tracing. Discretization for all plots is 0.05 m, with travel time down-sampled to dz = 0.25 m to simulate a zero offset profiling borehole ground penetrating radar survey. Selected slopes (ns m–1) are shown in suspected areas of critical refraction.

View larger version (22K): [in a new window]   Fig. 5. (A) Water content profile of a finely layered soil system derived from the database of Carsel and Parrish (1988) under hydrostatic conditions, (B) the velocity profile determined from Eq. [7], and (C) first-arriving travel time profile calculated from ray tracing. Discretization for all plots is 0.05 m, with travel time down-sampled to dz = 0.25 m to simulate a zero offset profiling borehole ground penetrating radar survey. Selected slopes (ns m–1) are shown in suspected areas of critical refraction.

View larger version (24K): [in a new window]   Fig. 6. A comparison showing the inversion of first-arriving travel time calculations to obtain water content for both (A) coarsely layered and (B) finely layered soil profiles. The thin blue lines represent the actual water contents; dashed green lines are the water contents obtained assuming all first-arriving travel times are direct; thick red lines are the water contents after considering critical refraction. The error in Lw is reported.

View larger version (28K): [in a new window]   Fig. 7. Zero offset profiling borehole ground penetrating radar travel time data obtained before and after infiltration. The water content axis on the bottom assumes that all first arrivals are direct. The gray area shows increase in travel time (water content) due to the addition of water.

View larger version (32K): [in a new window]   Fig. 8. Inversion of travel time data from infiltration experiment corrected for critical refraction. The two gray areas represent the change in inferred water content due to the consideration of critical refraction. The total correction for both areas is Lw = 0.38 m. After correction, the total change in Lw was 0.81 m compared with an applied length of water of 0.898 m. Black dots on the water content profiles represent depths for the velocity calculations.