Fluvial Sediment 3D ERI
Mapping Complex Sediment Contacts Using 3D ERI, Calibrated With Two Cores, to 20 m Depth.
Fig. 1. 3D resistivity block models of two adjacent areas connected by a through-flowing river in south-central UK. This is an oblique rendering with view toward the northeast. Data collection for the resistivity profiles over the southern block follows gridlines shown in Fig. 2. In the model, material with resistivities above 150 Ωm (solid orange-red volumes) are confirmed sandy gravel (note channel forms). Material with resistivities less than 50 Ωm (solid blue volumes) are confirmed peat. Limestone bedrock (Chalk Fm) consistently has resistivities of 100-140 Ωm. Weathered limestone and alluvium exhibit moderately low (resistivity (50-80 Ωm). This 3D resistivity model, calibrated to the geology with a few cores, provides the basis for 3D geologic and hydrogeologic models. *
Fig. 2. Profile lines and electrode positions used to collect resistivity data for the 3D resistivity model of the southern block in Fig. 1. The area is 175 m x 125 m. Two core locations are shown in white.*
Fig. 3. Horizontal slices through the resistivity model in Fig. 1 (southern block). Resistivity color scale is same as Figs. 1 and 4. Left-side is surface resistivity which shows lateral variations in alluvium and peat . Center (resistivity at 5 m below surface) shows variations in sandy gravel resistivity that reflect porosity variations. Right-side (resistivity at 10 m depth) emphasizes variations in the 2-4 m thick layer of weathered (putty) limestone that grades downward into unweathered limestone (chalk). White line across center is the line of section for Fig.4. *
Fig. 4. W-E vertical section through the 3D resistivity model in Fig 1. that shows one core at 81 m horizontal distance used for calibration. Fig. 3 shows section location. Lateral resistivity variations in the gravels (red) indicate lenticular channel forms. Lateral resistivity variations in the weathering profile on the limestone (chalk) reflect variations in clay content which influence permeability. *
Fig. 5. Vertical variations in resistivity extracted from the 3D resistivity model compared to lithostratigraphy in two cores. Black lines with filled circles depict resistivity from the model. Colored vertical bars represent core geology as indicated in the key.
Horizontal solid and dashed lines on the resistivity profiles show several ways to pick deposit contacts which are listed in the key. The best method in this study is labelled KIM (known interface method) and displayed in grey. The KIM method draws interfaces in the resistivity model at the same resistivity values as coincide with the lithologic contacts in cores. These interfaces are the best estimates for deposit contacts in a 3D geologic model. In the southern block, the top and bottom of the gravel units were picked at 70 Ωm and 165 Ωm, respectively, using core BHS1.*
* Chambers, J. E., Wilkinson, P. B., Uhlemann, S., Sorensen, J. P. R., Roberts, C., Newell, A. J., Ward, W. O. C., Binley, A., Williams, P. J., Gooddy, D. C., Old, G., and Bai, L., 2014a, Derivation of lowland riparian wetland deposit architecture using geophysical image analysis and interface detection: Water Resources Research, v. 50, no. 7, p. 5886-5905.