Alluvial Sands 3D ERI
Estimating Sand & Gravel Deposit Heterogeneities, Basal Contact, Volume using ERI to 9 m Depth
Fig. 1. 3D subsurface resistivity from ERI of river-terrace gravelly sand over bedrock in Lincolnshire UK. High resistivities (orange-red) are confirmed moderately to well sorted gravelly sand. Low resistivities (blues) under the gravelly sand are confirmed mudstone. Moist, fine topsoil also exhibits low resistivities. Note spatial variability of gravelly sand resistivity. Dashed white line at ~ 9 m depth is the isosurface in the resistivity model that corresponds to the resistivity value in the basal gravel at an excavation to bedrock in field center.*
Fig. 2. Photograph of area (120 m x189 m) in Lincolnshire UK surveyed for 3D ERI showing the survey grid (blue lines) and location of two boreholes (red crosses). Survey lines parallel to the y-axis were separated by 6 m; lines parallel to the x-axis were separated by 12 m. Two additional survey lines were positioned at y= 6 and 186 m to improve resolution at the margins.*
Fig. 3. Resistivity values (black circles) and interpolated curve from the 3D ERI data at x=77m, y=100 m (location of excavation to bedrock) versus depth. Interface resistivity at ~9 m depth (147 Ωm) is the resistivity value that coincides with the basal gravel at the excavation to bedrock. Use of this value to pick deposit contact in resistivity model is the “known interface method” (KIM) for edge detection. Grey pts and curve give the sensitivity of the inverted resistivity model. Sensitivity of the model decreases with depth or increasing distance from surface electrodes.*
Fig. 4. Quarrying after the ERI survey revealed the actual bedrock surface over 40% of the ERI study area facilitating direct comparison of bedrock surface elevation and ERI-based estimates. (a) Bedrock surface elevation measured with GPS. (c) Bedrock surface elevation estimated by applying interface resistivity method (KIM) to ERI data. (e) bedrock surface elevation offset between observed ( a) and estimated from ERI using KIM (c) (GPS – KIM). Average offset = –0.3 ± 0.5 m (1 std dev). KIM works well in this study.*
* Chambers, J. E., Wilkinson, P. B., Penn, S., Meldrum, P. I., Kuras, O., Loke, M. H., and Gunn, D. A., 2013, River terrace sand and gravel deposit reserve estimation using three-dimensional electrical resistivity tomography for bedrock surface detection: Journal of Applied Geophysics, v. 93, p. 25-32.