Structure and reactivity of water at biomaterial surfaces

Tutorial : 2-D and 3-D electrical imaging surveys

Two-dimensional electrical imaging surveys are now widely used in engineering and environmental surveys to map moderately complex structures. In order to adequately resolve such structures with arbitrary resistivity distributions, the regularised least-squares optimisation method with a cell-based model is frequently used in the inversion of the electrical imaging data. The L 2 norm based least-squares optimisation method that attempts to minimise the sum of squares of the spatial changes in the model resistivity is often used. The resulting inversion model has a smooth variation in the resistivity values. In cases where the true subsurface resistivity consists of several regions that are approximately homogenous internally and separated by sharp boundaries, the result obtained by the smooth inversion method is not optimal. It tends to smear out the boundaries and give resistivity values that are too low or too high. The blocky or L 1 norm optimisation method can be used for such situations. This method attempts to minimise the sum of the absolute values of the spatial changes in the model resistivity. It tends to produce models with regions that are piecewise constant and separated by sharp boundaries. Results from tests of the smooth and blocky inversion methods with several synthetic and field data sets highlight the strengths and weaknesses of both methods. The smooth inversion method gives better results for areas where the subsurface resistivity changes in a gradual manner, while the blocky inversion method gives significantly better results where there are sharp boundaries. While fast computers and software have made the task of interpreting data from electrical imaging surveys much easier, it remains the responsibility of the interpreter to choose the appropriate tool for the task based on the available geological information.

/pdf/a-comparison-of-smooth-and-blocky-inversion-methods-in-2-d-17iu0jl2na.pdf

A comparison of smooth and blocky inversion methods in 2-D electrical imaging surveys

Numerical simulations are used to compare the resolution and efficiency of 2D resistivity imaging surveys for 10 electrode arrays. The arrays analysed include polepole (PP), pole-dipole (PD), half-Wenner (HW), Wenner-α (WN), Schlumberger (SC), dipole-dipole (DD), Wenner-β (WB), γ -array (GM), multiple or moving gradient array (GD) and midpoint-potential-referred measurement (MPR) arrays. Five synthetic geological models, simulating a buried channel, a narrow conductive dike, a narrow resistive dike, dipping blocks and covered waste ponds, were used to examine the surveying efficiency (anomaly effects, signal-to-noise ratios) and the imaging capabilities of these arrays. The responses to variations in the data density and noise sensitivities of these electrode configurations were also investigated using robust (L1-norm) inversion and smoothness-constrained least-squares (L2-norm) inversion for the five synthetic models. The results show the following. (i) GM and WN are less contaminated by noise than the other electrode arrays. (ii) The relative anomaly effects for the different arrays vary with the geological models. However, the relatively high anomaly effects of PP, GM and WB surveys do not always give a high-resolution image. PD, DD and GD can yield better resolution images than GM, PP, WN and WB, although they are more susceptible to noise contamination. SC is also a strong candidate but is expected to give more edge effects. (iii) The imaging quality of these arrays is relatively robust with respect to reductions in the data density of a multi-electrode layout within the tested ranges. (iv) The robust inversion generally gives better imaging results than the L2-norm inversion, especially with noisy data, except for the dipping block structure presented here. (v) GD and MPR are well suited to multichannel surveying and GD may produce images that are comparable to those obtained with DD and PD. Accordingly, the GD, PD, DD and SC arrays are strongly recommended for 2D resistivity imaging, where the final choice will be determined by the expected geology, the purpose of the survey and logistical considerations.

/pdf/a-numerical-comparison-of-2d-resistivity-imaging-with-10-1kfer57nmn.pdf

A numerical comparison of 2D resistivity imaging with 10 electrode arrays

Recent developments in the direct-current geoelectrical imaging method

[1] Analytical solutions to the heat transport equation in porous media have been developed in the past to estimate surface water-groundwater interactions. These solutions, however, are based upon simplifying assumptions that are frequently violated in natural systems. A nonvertical flow field, inherent to most field settings, can violate the one-dimensional (1-D) flow assumption and lead to erroneous velocity estimates. In this study, we have developed a 2-D heat and mass transport finite element-based numerical model for a stream aquifer cross section experiencing flow-through. Synthetic multilevel streambed temperature time series were generated with the model using a sinusoidal temperature boundary. The temperature data was used to quantify the vertical flow velocity with a 1-D analytical solution based on the amplitude decay and phase shift of temperature with depth. Results demonstrate that erroneous vertical components of fluid velocity can be obtained by the 1-D analytical solution when the true vertical velocity approaches zero and the flow regime becomes almost horizontal. The results also illustrate that the amplitude ratio method performs quite poorly on the gaining side of the stream where the only reliable method is phase shift. On the losing side of the stream, both methods can be employed but a better estimation is obtained from the amplitude ratio method. In general, amplitude ratio and phase shift data should be used in conjunction to maximize the information of the system.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012WR011918

Use of heat as tracer to quantify vertical streambed flow in a two‐dimensional flow field

/pdf/spatially-dense-drip-hydrological-monitoring-and-5f8uywa1xr.pdf

Spatially dense drip hydrological monitoring and infiltration behaviour at the Wellington Caves, South East Australia

Retention of the Gram-negative bacterium SW8 on surfaces under conditions relevant to the subsurface environment: Effects of conditioning films and substratum nature

Small but significant supplies of groundwater are found in weathered crystalline rocks. In areas where weathering is actively occurring as a result of tectonic movement, the weathering zones show major lateral variation. Electrical techniques can be used to detect zones of deep weathering because of the significant resistivity contrast between the different weathering zones. Bulk resistivity decreases as weathering intensity increases. Groundwater reserves are located in the intermediate weathering grades between fresh rock (high resistivity) and clay dominated (low resistivity) weathered material. The determination of accurate thicknesses and resistivities for intermediate zones is complicated by the problems of equivalence and suppression. A review of the use of constant separation traverses (CST), vertical electrical sounding (VES), electromagnetic profiling and electrical imaging is presented from a small research catchment close to Bauchi in Northern Nigeria. Results from each of these metho...

The electrical image method compared with resistivity sounding and electromagnetic profiling for investigation in areas of complex geology: A case study from groundwater investigation in a weathered crystalline rock environment

Ian Acworth

Papers

A comparison of smooth and blocky inversion methods in 2-D electrical imaging surveys

Use of heat as tracer to quantify vertical streambed flow in a two‐dimensional flow field

Spatially dense drip hydrological monitoring and infiltration behaviour at the Wellington Caves, South East Australia

Retention of the Gram-negative bacterium SW8 on surfaces under conditions relevant to the subsurface environment: Effects of conditioning films and substratum nature

The electrical image method compared with resistivity sounding and electromagnetic profiling for investigation in areas of complex geology: A case study from groundwater investigation in a weathered crystalline rock environment