Velocity variations and water content estimated from multi‐offset, ground‐penetrating radar

Abstract

The common midpoint (CMP) processing technique has been shown to be effective in improving the results of ground‐penetrating radar (GPR) profiling. When radar data are collected with the CMP multioffset geometry, stacking increases the signal‐to‐noise ratio of subsurface radar reflections and results in an improved subsurface image. An important aspect of CMP processing is normal‐moveout velocity analysis. Our objectives are to show the effect of multiple velocity analyses on the stacked radar image and particularly, to demonstrate that this velocity information can also be used to determine subsurface water content. Most GPR surveys are very limited in spatial extent and assume that within the survey range, radar velocity structure in the shallow subsurface can be adequately approximated by a single velocity function in data processing. In this study, we show that variation in radar velocity can be quite significant and that the stacked profile improves as the number of velocity analysis locations is increased. Interval velocities can be calculated from the normal moveout velocities derived in the CMP velocity analysis. With some reasonable assumptions about subsurface conditions necessary for radar propagation, interval velocity can be converted to an estimate of volumetric water content. Therefore, by collecting GPR data in the multioffset CMP geometry, not only is the radar profile improved but it also allows for an interpretation of subsurface variation in water content. We show the application of these techniques to multioffset GPR data from the Chalk River test area operated by Atomic Energy of Canada Limited.

Three‐dimensional spatial and temporal monitoring of soil water content using electrical resistivity tomography

“…By comparing the GPR velocity before and after forced injection of salt water at a test site, Eppstein and Daugherty [1998] visualized soil moisture patterns in three dimensions. Greaves et al [1996] related the calculated radar interval velocities to the water content and calculated a water content cross section at depth. Sheets and Hendrickx [1995] investigated the feasibility of soil water content measurement using electromagnetic induction in an arid region of southern New Mexico and found that a linear relationship exists between bulk soil electrical conductivity and total soil water content in the top 1.5 m of a profile.…”

Multi-offset ground-penetrating radar imaging of a lab-scale infiltration test

“…Traditional multi-offset GPR survey techniques, i.e., CMP or WARR, are appealing strategies for monitoring water content changes associated with one-dimensional infiltration as they are well established in the literature (Berard and Maillol, 2007;Fisher et al, 1992;Greaves et al, 1996;Grote et al, 2005) and can be easily put into practice with widely available commercial GPR systems. Analysis of the data from these surveys typically relies on normal-moveout (NMO) corrections (Fisher et al, 1992), however, which assumes idealized, locally continuous reflector geometries.…”

Forward modeling of ground‐penetrating radar data using digitized outcrop images and multiple scenarios of water saturation

“…The potential of the method for gaining information about subsurface structure arises from the apparent correlation between material type and electrical properties, in which contrasts cause reflections of electromagnetic waves. Successful applications of GPR are as far ranging as agriculture [Freeland et al, 1998], archeology [Tohge et al, 1998], and hydrological analyses [Greaves et al, 1996;Rubin et al, 1998;Chen et al, 2001]. While many GPR investigations have been carried out above the groundwater table, the sensitivity of GPR to the presence of pore water in the unsaturated zone is well known [Aspdon, 1998].…”