Delineation of Capillary Transition Zone from Ground Penetrating Radar Data
Authors: Nguyen Buu-Long , Bruining Johannes , Slob Evert C. , Hopman Victor 
Published: December 2007

Spilled petroleum products spread laterally in the capillary transition zone above the water table. Ground Penetrating Radar is a technique that can potentially delineate the capillary transition zone as a varying water content strongly influences the reflected signal through its effect on the electrical properties. The vertical water distribution is determined on the pore scale by the grain size distribution and on the reservoir scale mainly by the permeability distribution. We use a fractional Brownian motion model to generate a permeability field that shows correlation over all scales. We derive the integral equations for the reflected signal from a inhomogeneous slab with a depth-dependent dielectric constant and electrical conductivity. The numerical solution of the equations allows us to compute the signal reflected from different sand profiles. The results show that the reflected signal is strongly influenced by both the grain size distribution and the variation in permeability. A strong frequency dependence is also observed. We conducted an experiment in a model tank to compare the result with the derived theories. All of these results are considered of importance for the interpretation of Ground Penetrating Radar data involving the capillary transition zone.

Introduction

Petroleum products are continually introduced into the subsurface environment as a consequence of petroleum operations, through the practice of land application of refinery wastes and inadvertent depositions of refined petroleum products caused by pipelines and storage tank failure. The movement of spilled petroleum products must be quantified to rationally assess the impact on underlying groundwater resources. Furthermore, predicting of the spreading of petroleum contaminants in the subsoil is critical for the design of effective remediation action. Experiments show that the capillary transition zone (CTZ) above the groundwater table strongly influences the initial spreading of petroleum contaminants. When introduced into the subsurface, gravity causes the latter to migrate downward through the vadose zone as a distinct liquid. Due to the effect of capillary forces this vertical migration is accompanied by lateral spreading above the capillary fringe, rather than on the water table as usually supposed.

Ground penetrating radar (GPR) is an electromagnetic technique related to seismic reflection; both methods are based on wave propagation and provide very detailed and continuous images of the subsurface. The two methods are complementary: the deep subsurface is the domain of seismic reflection and is beyond the reach of GPR, but in the shallow zones GPR offers an attractive and cost-effective alternative. GPR has been successfully used in geotechnical investigations for over a decade, generally for the depth of a few meters. Recent developments in digital instrumentation and low-frequency antennas have extended the GPR domain in radar-favorable areas (low soil conductivities) to depths of several tens of meters, making the method suitable for geological applications. Ref. 8 describes geological applications of GPR in Canada, and Ref. 9 gives hydrogeological examples from the USA. Recently attempts have been made to remotely detect and delineate subsurface zones of hydrocarbons and gasoline using GPR.

Present interpretation techniques consider the subsurface as being constituted of entities of different shapes e.g. lenses and layers of sediments with a certain dielectric constant or permittivity. The success of GPR depends therefore on the contrast in dielectric permittivity in the subsurface. In hydrogeological investigations, for example, the large contrast in between dry and water-saturated sands makes the water table a strong reflector of radar waves.

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