Estimating porosity with ground‐penetrating radar reflection tomography: A controlled 3‐D experiment at the Boise Hydrogeophysical Research Site

Abstract

[1] To evaluate the uncertainty of water-saturated sediment velocity and porosity estimates derived from surface-based, ground-penetrating radar reflection tomography, we conducted a controlled field experiment at the Boise Hydrogeophysical Research Site (BHRS). The BHRS is an experimental well field located near Boise, Idaho. The experimental data set consisted of 3-D multioffset radar acquired on an orthogonal 20 Â 30 m surface grid that encompassed a set of 13 boreholes. Experimental control included (1) 1-D vertical velocity functions determined from traveltime inversion of vertical radar profiles (VRP) and (2) neutron porosity logs. We estimated the porosity distribution in the saturated zone using both the Topp and Complex Refractive Index Method (CRIM) equations and found the CRIM estimates in better agreement with the neutron logs. We found that when averaged over the length of the borehole, surface-derived velocity measurements were within 5% of the VRP velocities and that the porosity differed from the neutron log by less than 0.05. The uncertainty, however, is scale dependent. We found that the standard deviation of differences between ground-penetrating-radar-derived and neutron-log-derived porosity values was as high as 0.06 at an averaging length of 0.25 m but decreased to less than 0.02 at length scale of 11 m. Additionally, we used the 3-D porosity distribution to identify a relatively high-porosity anomaly (i.e., local sedimentary body) within a lower-porosity unit and verified the presence of the anomaly using the neutron porosity logs. Since the reflection tomography approach requires only surface data, it can provide rapid assessment of bulk hydrologic properties, identify meterscale anomalies of hydrologic significance, and may provide input for other higherresolution measurement methods.

Capacitive conductivity logging and electrical stratigraphy in a high-resistivity aquifer, Boise Hydrogeophysical Research Site

“…However, other adjacent wells do not show these sequences, indicating they occur in a limited contiguous region. This type of patchy or lenslike distribution is expected for units 2 and 4 ͑Barrash and Clemo, 2002; Barrash and Reboulet, 2004;Bradford et al, 2009͒. Furthermore, the capacitive-conductivity logs provide an independent and convincing rationale for identifying the contact between units 3 and 2 where ambiguous or gradational porosity trends occur, such as in wells B2 and B5 ͑Figure 12c and b, respectively͒.…”

“…As noted, the neutron porosity log has been used to define the stratigraphy at the BHRS ͑Barrash and Clemo, 2002͒, and this stratigraphy has been recognized as consistent with independent data sets, including core ͑Reboulet and Barrash, 2003;Barrash and Reboulet, 2004͒, radar reflection profiles and volumes ͑e.g., Clement et al, 2006;Bradford et al, 2009͒, and crosswell seismic tomography ͑Moret et al, 2006͒. In Figures 10-12, however, porosity stratigraphy is compared to the stratigraphy determined from the capacitive-conductivity logs.…”

Reflection waveform inversion of ground-penetrating radar data for characterizing thin and ultrathin layers of nonaqueous phase liquid contaminants in stratified media

“…Here, we take thin layers to be those whose thickness is ≤ 1∕2λ and ultrathin layers to be those with thickness of ≤ 1∕8λ. In such cases, measuring layer thickness (d) or effective dielectric permittivity (ε ef ) using conven-tional velocity analysis is impossible; we must turn to other techniques if we seek to quantify thin-layer parameters (Bradford et al, 2009). Other available tools for the GPR practitioner facing a thinlayer problem include attribute analysis, amplitude variation with offset (AVO) analysis, and inversions.…”

Estimating Debye Parameters from GPR Reflection Data Using Spectral Ratios

“…The BHRS is the site of numerous previous borehole and surface GPR experiments (e.g. Bradford et al, 2008;Clement et al, 1999). Well defined radar reflections are observed at all major unit boundaries with the exception of the Unit 3/Unit 2 boundary which is gradational (Figure 4).…”