A comparison of the use of radar images and neutron probe data to determine the horizontal correlation length of water content

Estimation of the lateral correlation structure of subsurface water content from surface‐based ground‐penetrating radar reflection images

“…In both cases, the rather weak assumption was made that the gray scale tones in the photograph were representative of sediment grain size, with the further assumption that finer-grained sediments would be associated with higher water contents. Knight et al [2007] also observed similarities between the horizontal correlation statistics of GPR data and neutron probe measurements. However, Knight et al [2004] and Oldenborger et al [2004] both noticed that the lateral correlation structure of a GPR image is significantly affected by its vertical resolution, which is controlled by the GPR antenna frequency.…”

“…What this means is that both the vertical and lateral correlation properties will, in general, be significantly different between a water content distribution and the corresponding GPR image. In our view this is a key aspect that has so far not been properly accounted for in previous efforts to estimate the lateral correlation statistics of water content from GPR data [e.g., Rea and Knight, 1998;Dafflon et al, 2006;Knight et al, 2007].…”

“…To this end, geophysical methods offer the potential to bridge a gap in terms of resolution and coverage between traditional hydrological characterization techniques such as borehole log and core analyses and pumping and tracer tests [e.g., Rubin and Hubbard, 2005]. Of particular interest in the recent past has been the idea of relating the lateral correlation statistics of surface-based reflection groundpenetrating radar (GPR) data to those of the water content distribution of the probed region [e.g., Rea and Knight, 1998;Dafflon et al, 2006;Knight et al, 2007]. The primary reasons for this interest are that (1) changes in water content are widely recognized as the predominant cause of changes in the radar wave velocity, and thus of GPR reflections, in the subsurface; (2) in electrically resistive media, surfacebased GPR reflection surveys provide a quick and noninvasive means of imaging the shallow subsurface with unsurpassed resolution; and (3) geostatistical information on subsurface heterogeneity is particularly useful for many hydrological applications [e.g., Gelhar, 1993].…”

“…[3] A number of studies have explored the relationship between the lateral correlation structure of seismic and radar velocity fields and those of the corresponding seismic and GPR reflection data [e.g., Gibson, 1991;Hurich, 1996;Pullammanappallil et al, 1997;Line et al, 1998;Rea and Knight, 1998;Bean et al, 1999;Knight et al, 2004;Oldenborger et al, 2004;Dafflon et al, 2006;Carpentier and Roy-Chowdhury, 2007;Knight et al, 2007]. Although the seismic and GPR communities have to date worked largely independently of one another, it is important to note that the results obtained are mutually transferrable because of the strong mathematical analogies that exist between seismic and electromagnetic wave propagation [e.g., Carcione and Cavallini, 1995;Belina et al, 2009].…”

Geostatistical inversion of seismic and ground‐penetrating radar reflection images: What can we actually resolve?

“…This means that we estimate parameters describing the geostatistical nature of the heterogeneity, rather than a detailed distribution of material properties. To this end, estimation of the geostatistical properties of subsurface velocity heterogeneity from surface-based seismic and ground-penetrating radar (GPR) reflection images has been a long-standing problem of significant interest [e.g., Holliger et al, 1992;Hurich, 1996;Pullammanappallil et al, 1997;Rea and Knight, 1998;Bean et al, 1999;Poppeliers and Levander, 2004;Carpentier and Roy-Chowdhury, 2007;Knight et al, 2007]. Of particular interest has been the estimation of the lateral statistics of a subsurface velocity field from those of the corresponding reflection image, as this information cannot be obtained from borehole log or core analysis.…”